Integral Gamma Ray Observatory

[Waspie_Dwarf]

Integral sees a GRB out of the corner of its eye

This artist's impression provides a schematic of how the imager on-board ESA's Integral satellite (IBIS) can reconstruct images of powerful events like gamma-ray bursts (GRB) using the radiation that passes through the side of Integral’s imaging telescope.
IBIS uses two detector layers, one on top of the other, while most gamma-ray telescopes contain just a single detector layer. In IBIS, the higher energy gamma rays trigger the first detector layer (called ISGRI), losing some energy in the process, but they are not completely absorbed. This is known as Compton scattering. The deflected gamma rays then pass through to the layer below (called PICSIT) where they can be captured and absorbed by the PICSIT crystals because they have given up some energy in their passage through the first layer. The blue-shaded part of the image describes the fully coded field of view of the instrument.

IBIS can see around corners because gamma rays from the most powerful GRBs would pass through the lead shielding on the side of the telescope, then through the first detector layer before coming to rest in the second layer. The scatter locations in the two detecor layers and the energy deposits can then be used to determine the direction of the GRB.

Credits: ESA/C.Carreau

16 June 2006
Thanks to a clever piece of design and a sophisticated piece of analysis by European astronomers, Integral - ESA’s orbiting gamma ray observatory - can now make images of the most powerful gamma-ray bursts even if the spacecraft itself is pointing somewhere completely different.

Scientists know that once every day or two, a powerful gamma ray burst (GRB) will take place somewhere in the Universe. Most will last between 0.1 and 100 seconds, so if your telescope is not pointing in exactly the right place at the right time, you will miss taking an image of it – unless that telescope is Integral. The satellite can now take images round corners, if the gamma-ray blast is strong enough.
When GRB 030406 exploded unexpectedly in early April this year, Integral was observing another part of the Universe, about 74 times the diameter of the full Moon away. Nevertheless Dr Radoslaw Marcinkowski, Space Research Center, Warsaw, Poland, and colleagues have reconstructed an image of the event using the radiation that passed through the side of Integral’s imaging telescope.

The key is that the Imager on-Board Integral Satellite (IBIS) uses two detector layers, one on top of the other. Most gamma-ray telescopes contain just a single detector layer. In IBIS, the higher energy gamma rays trigger the first detector layer, losing some energy in the process, but they are not completely absorbed. This is known as Compton scattering. The deflected gamma rays then pass through to the layer below where they can be captured and absorbed because they have given up some energy in their passage through the first layer.

"In this way, we are able to capture and analyse the higher energy gamma rays," says Marcinkowski. IBIS can now see around corners because Marcinkowski realised that gamma rays from the most powerful GRBs would pass through the lead shielding on the side of the telescope, then through the first detector layer before coming to rest in the second layer. The scatter locations in the two detector layers and the energy deposits can then be used to determine the direction of the GRB.


Marcinkowski had heard of Integral registering a solar flare in this way even though the satellite wasn’t pointing at the Sun. He thought that if it worked with solar flares, it must work with the most powerful GRBs. On 6 April 2003, his hunch was proved correct, Integral provided an accurate location for GRB 030406 even though it was not looking in the burst’s direction.

Until now, the science teams have been forced to rely on luck that the satellite was pointing to the right place at the right time because GRBs are unpredictable. At present, they image about one a month. The Compton scattering technique could raise the number of Integral catches by 50 percent. "We believe that using this method we can image between 2 and 5 more bursts per year," says Marcinkowski.


The team now hope to fully automate the analysis routine that recognises the signals and localises them. This would mean that the software could run automatically at the Integral Science Data Centre (ISDC) in Geneva, Switzerland and automatically alert astronomers to its gamma-ray catches when they occur.


Notes:

The original paper, "GRB030406 – an extremely hard burst outside of the Integral field of view", by R Macinkowski et al. (2006), is published in Astronomy and Astrophysics (452, 113-117, doi: 10.1051/0004-6361:20064811)

Source: ESA - News

[Waspie_Dwarf]

Where are the supermassive black holes hiding?

This artist's impression shows the thick dust torus that astronomers believe surrounds many supermassive black holes and their accretion discs. When the torus is seen edge-on as in this case, much of the light emitted by the accretion disc is blocked, creating a "hidden" black hole.
However, the sharp gamma-ray and X-ray eyes of Integral can peer through the thick dust and identify the black hole within. An Integral survey of the local universe found few hidden black holes, implying that they must have existed earlier (deeper) in the universe.

Credits: ESA / V. Beckmann (NASA-GSFC)

26 July 2006
European and American scientists, on a quest to find super-massive black holes hiding in nearby galaxies, have found surprisingly few. Either the black holes are better hidden than scientists realised or they are lurking only in the more distant universe.

Scientists are convinced that some super-massive black holes must be hiding behind thick clouds of dust. These dusty shrouds allow only the highest energy X-rays to shine through. Once in space, the X-rays combine into a cosmic background of X-rays that permeates the whole of space.
The search for hidden black holes is part of the first census of the highest-energy part of the X-ray sky. Led by Loredana Bassani, IASF, Italy, a team of astronomers published results in The Astrophysical Journal Letters in January this year. They show the fraction of hidden black holes in the nearby Universe to be around 15 percent, using data from ESA’s orbiting gamma-ray observation, the International Gamma Ray Astrophysics Laboratory (Integral).

Now astronomers from NASA Goddard Space Flight Center in Greenbelt, Maryland, and the Integral Science Data Centre near Geneva, Switzerland, have found an even smaller fraction using nearly two years of continuous data, also from Integral. The work shows that there is clearly too few hidden black holes in the nearby Universe to create the observed X-ray background radiation.

"Naturally, it is difficult to find something we know is hiding well and which has eluded detection so far," says Volker Beckmann of NASA Goddard and the University of Maryland, Baltimore County, lead author of the new report to be published in an upcoming issue of The Astrophysical Journal. "Integral is a telescope that should see nearby hidden black holes, but we have come up short," he says.

The X-ray sky is thousands to millions of times more energetic than the visible sky familiar to our eyes. Much of the X-ray activity is thought to come from black holes violently sucking in gas from their surroundings.


Recent breakthroughs in X-ray astronomy, including a thorough black hole census taken by NASA's Chandra X-ray Observatory and Rossi X-ray Timing Explorer, have all dealt with lower-energy X-rays. The energy range is roughly 2 000 to 20 000 electron-volts (optical light, in comparison, is about 2 electron-volts). The two Integral surveys are the first glimpse into the largely unexplored higher-energy, or 'hard', X-ray regime of 20 000 to 300 000 electron-volts.

"The X-ray background, this pervasive blanket of X-ray light we see everywhere in the universe, peaks at about 30 000 electron volts, yet we really know next to nothing about what produces this radiation," says Neil Gehrels of NASA Goddard, a co-author.

The theory is that hidden black holes, which scientists call Compton-thick objects, are responsible for the 30 000 electron-volts peak of X-rays in the cosmic X-ray background. Integral is the first satellite sensitive enough to search for them in the local universe.


This all-sky map shows regions of ionized hydrogen gas in the local universe. The hidden black holes detected in the INTEGRAL survey of high-energy X-ray sources are located within the diamond-shape marks. Many sources were detected through the line of sight of the dusty Milky Way galactic plane, which is the bright area stretching across the center of the entire image from left to right.

Credits: D. Finkbeiner / ESA, INTEGRAL, V. Beckmann, NASA-GSFC

According to Beckmann, of all the black hole galaxies that Integral detected less than 10 percent were the heavily shrouded 'Compton thick' variety. That has serious implications for explaining where the X-rays in the cosmic X-ray background come from.

"The hidden black holes we have found so far can contribute only a few percent of the power to the cosmic X-ray background," says Bassani. This implies that if hidden black holes make up the bulk of the X-ray background, they must be located much further away in the more distant universe. Why would this be? One reason could be that in the local universe most super-massive black holes have had time to eat or blow away all the gas and dust that once enshrouded them, leaving them revealed.

This would make them less able to produce X-rays because it is the heating of the gas falling into the black hole that generates the X-rays, not the hole itself. So, if the black hole had cleared its surroundings of matter there would be nothing left to produce X-rays.


Conversely, another possibility is that perhaps the hidden black holes are more hidden than astronomers realised. "The fact that we do not see them does not necessarily mean that they are not there, just that we don’t see them. Perhaps they are more deeply hidden than we think and so are therefore below even Integral's detection limit," says Bassani.

Meanwhile, the NASA team is now planning to extend his search for hidden black holes further out into the universe. "This is just the tip of the iceberg. In a few more months we will have a larger survey completed with the Swift mission. Our goal is to push this kind of observation deeper and deeper into the universe to see black hole activity at early epochs. That’s the next great challenge for X-ray and gamma-ray astronomers," concluded Beckmann.

Note:

The findings appear in The Astrophysical Journal, in an article titled "Integral IBIS Extragalactic survey: Active Galactic Nuclei Selected at 20-100 keV", by L. Bassani et al., published on 10 January 2006 (vol. 636, pp L65-L68).

The other scientific paper on which this story is based is "The Hard X-ray 20-40keV AGN Luminosity Function" by V. Beckmann et al., accepted for publication in a future issue of The Astrophysical Journal. A pre-print of the paper can be downloaded at: arxiv.org/abs/astro-ph/0606687.

Source: ESA - News

[Waspie_Dwarf]

NASA Scientists Conduct Census of Nearby Hidden Black Holes

Scientists on a quest to find hidden black holes in the local universe have found surprisingly few.


Image above: This illustration shows the thick dust torus that astronomers believe surrounds supermassive black holes and their accretion discs. When the torus is seen edge-on’ as in this case, much of the light emitted by the accretion disc is blocked. However, the sharp X-ray and gamma-ray eyes of INTEGRAL can peer through the thick dust and locate "hidden" black holes. INTEGRAL's survey of the local universe searched for hidden black holes but found few, which implies these kinds of black holes are largely in the more distant (earlier) universe.
+ High resolution TIF image (6 MB)
Credit: ESA / V. Beckmann (NASA-GSFC)

The observation implies that if these hidden black holes exist---and most scientists are convinced they do---they must be from the more distant, earlier universe, a concept that has interesting implications for galaxy evolution.

This work constitutes the first census of the highest-energy part of the X-ray sky, where the most dust-enshrouded black holes are thought to shine. A team from NASA's Goddard Space Flight Center in Greenbelt, Md., conducted the census, comprised of nearly two years of continuous data from the European Space Agency's International Gamma Ray Astrophysics Laboratory, or INTEGRAL, satellite.

"Naturally it is difficult to find something we know is hiding well and which has eluded detection so far," said Volker Beckmann of Goddard and the University of Maryland, Baltimore County, lead author on a report in an upcoming issue of The Astrophysical Journal. "INTEGRAL is a telescope that should see nearby hidden black holes, but we have come up short."

The X-ray sky is thousands to millions of times more energetic than the visible sky familiar to our eyes. Much of the X-ray activity is from black holes violently sucking in gas from their surroundings.


Image above: This all-sky map shows regions of ionized hydrogen gas in the local universe. The hidden black holes detected in the INTEGRAL survey of high-energy X-ray sources are located within the diamond-shape marks. Many sources were detected through the line of sight of the dusty Milky Way galactic plane, which is the bright area stretching across the center of the entire image from left to right.
Click image to enlarge.
Credit: D. Finkbeiner (hydrogen) / ESA, INTEGRAL, V. Beckmann, NASA-GSFC (gamma ray)

Recent breakthroughs in X-ray astronomy, including a thorough black hole census with NASA's Chandra X-ray Observatory and Rossi X-ray Timing Explorer, have all dealt with lower-energy X-rays. The energy range is roughly 2,000 to 20,000 electron-volts. Optical light, in comparison, is about 2 electron volts.

The INTEGRAL survey is the first of its kind to glimpse into the largely unexplored higher-energy, or "hard," X-ray regime of 20,000 to 40,000 electron-volts.

"The X-ray background, this pervasive blanket of X-ray light we see everywhere in the universe, peaks at about 30,000 electron volts, yet we really know next to nothing about what produces this radiation," said co-author Neil Gehrels of Goddard.

The theory is that hidden black holes, which scientists call Compton-thick objects, are responsible for the peak at 30,000 electron volts. These X-rays are so energetic that they would penetrate even the most dust-enshrouded black holes yet remain beyond the range of powerful lower-energy X-ray observatories such as Chandra.

High-energy light in general is harder to focus than optical and lower-energy (longer-wavelength) forms of light. As a result, INTEGRAL doesn't have the resolution to make sharp images like Chandra and Hubble can.

"Basically, the higher you go in energy, the harder it is to detect faint sources," said Chris Shrader of Goddard, another co-author. "This is why no hard X-ray mission has been able to study many individual objects in the distant universe. That would require a next-generation telescope. But INTEGRAL is now the first to resolve the local universe."

INTEGRAL can obtain an unbiased count of black holes in the local universe by virtue of seeing even those that are hidden. Of all the black hole galaxies that INTEGRAL detected---that is, galaxies with supermassive black holes in their cores actively accreting gas---about 40 percent were unobscured black hole galaxies, called Seyfert 1 galaxies. About 50 percent were somewhat obscured black hole galaxies called Seyfert 2 galaxies. And less than 10 percent were the heavily shrouded "Compton thick" variety.

This implies that if hidden black holes make up the bulk of the X-ray background, they aren't local. Why? One reason could be that, in the modern local universe, these black holes have had time to blow away the gas and dust that once enshrouded them, leaving them unobscured. This liberation of gas and dust would have its consequences; it would blow away to influence star and galaxy formation elsewhere.

"This is just the tip of the iceberg," Beckmann said. "In a few more months we will have a larger survey completed with the Swift mission. Our goal is to push this kind of observation deeper and deeper into the universe to see black hole activity at early epochs. That's the next great challenge for X-ray and gamma-ray astronomers."

Simona Soldi and Nicolas Produit of the INTEGRAL Science Data Centre near Geneva, Switzerland, also participated in this result.

Christopher Wanjek
Goddard Space Flight Center

Source: NASA - Exploring the Universe - Stars and Galaxies

[Waspie_Dwarf]

ESA steps towards a great black hole census

This all-sky image was obtained by ESA's Integral gamma-ray observatory during four years of operations and provides an important step towards estimating how many black holes there are in the Universe.
The sources in this image were artificially blurred and the colour map was stretched to make both strong and weak sources readily visible. The concentration of sources along the mid plane of the image is due to neutron stars and stellar mass black holes in our Galaxy, while the majority of sources located far away from the Galactic plane are super-massive black holes in other galaxies. The Cosmic X-ray Background is composed from the emission of tens of millions of similar objects much further away from us.

Superposed is an Earth image by ESA/EUMETSAT’s Meteosat satellite. Using the Earth as a shield to block the emission from the population of distant black holes astronomers precisely gauged the X-ray and gamma-ray background. An angular size of the Earth as seen from Integral during actual observations was smaller than shown in the image.

Credits: ESA/IKI Integral team

7 September 2006
Astronomers using ESA's orbiting gamma-ray observatory, Integral, have taken an important step towards estimating how many black holes there are in the Universe.

An international team, lead by Eugene Churazov and Rashid Sunyaev, Space Research Institute, Moscow, and involving scientists from all groups of the Integral consortium used the Earth as a giant shield to watch the number of tell-tale gamma rays from the distant Universe dwindle to zero, as our planet blocked their view.
"Point Integral anywhere in space and it will measure gamma rays," says Pietro Ubertini, INAF, Italy and Principal Investigator on Integral's gamma-ray imager. Most of those gamma rays do not come from nearby sources but from celestial objects so far away that they cannot yet be distinguished as individual sources. This distant gamma-ray emission creates a perpetual glow that bathes the Universe.

Most astronomers believe that the unseen objects are supermassive black holes, millions or billions of times heavier than the Sun and each sitting at the centre of a galaxy. As the black holes swallow matter, the swirling gases release X-rays and gamma rays. Accurately measuring the glow, known as the X-ray and gamma-ray background, is the first step towards calculating how many black holes are contributing to it and how far away in the Universe they are located.

The new Integral observations were made during January and February 2006 and provide highly accurate data on the gamma-ray background. The key to success was using the Earth as a shield.

Allowing the Earth to enter Integral's field of view goes against the standard set of nominal observations for the satellite, because the optical devices needed to determine the spacecraft’s attitude would be blinded by the bright Earth. So, this operation required remarkable efforts from the ISOC/MOC teams operating the mission, who had to rely on alternative spacecraft control mechanisms. But the risk was worth it: by measuring the decrease of the gamma-ray flux once the Earth had blocked Integral's view and by making a model of the Earth’s atmospheric emission, the astronomers precisely gauged the gamma-ray background.

Another bonus of the Integral observations is that the observatory's complementary instruments allowed the strength of both X-rays and gamma rays to be measured simultaneously. In the past, different satellites have had to measure the different energies of X-rays and gamma rays, leaving astronomers with the task of having to piece the results together like the pieces of a jigsaw puzzle.

It is not just the overall glow that Integral has seen. Before the satellite's launch, only a few dozen celestial objects were observed in gamma rays. Now Integral sees about 300 individual sources in our Galaxy and around 100 of the brightest supermassive black holes in other galaxies. These are the tip of the iceberg. Astronomers believe there are tens of millions of active black holes spread throughout space, all contributing to the gamma-ray background. From earlier observations in the softer X-ray band it is known that the soft background radiation is almost entirely populated by Active Galactic Nuclei (AGN). So it is highly likely that these objects are also responsible here at higher Integral energies, even if this is not proven yet.

The next step is for astronomers to programme computer models to calculate how the emission from this unseen population of black holes merges to give the observed glow. These computer models will predict the number and distance of the black holes, and provide insights into the way they behave at the centre of young, middle-aged and old galaxies. Meanwhile, the Integral team will continue to refine their measurements of the perplexing gamma-ray background.


Notes:

For more information on these findings see: "Integral observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth" (http://www.arxiv.org/abs/astro-ph/0608250), by Churazov et al.; "Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations" (http://www.arxiv.org/abs/astro-ph/0608253), by Sazonov et al.; "Earth X-ray albedo for CXB radiation in the 1-1000 keV band" (http://www.arxiv.org/abs/astro-ph/0608252), by Churazov et al. .

Source: ESA - News

[Waspie_Dwarf]

Integral catches a new erupting black hole

This animation consists of a sequence of two images taken by the IBIS/ISGRI instrument
on board ESA’s Integral gamma-ray observatory, and it shows the galactic centre region
in hard X-rays.

In one frame, the bright X-ray nova IGR J17497-2821 (indicated by the arrow) is seen
during its discovery outburst, while the other frame is the same field as observed prior
to the nova event. The other variable sources are well known X-ray binaries. The two
separate images can be downloaded here: image 1, image 2

Credits: ESA/ISDC/IBIS/R.Walter

27 November 2006
ESA's gamma-ray observatory, Integral, has spotted a rare kind of gamma-ray outburst. The vast explosion of energy allowed astronomers to pinpoint a possible black hole in our Galaxy.

The outburst was discovered on 17 September 2006 by staff at the Integral Science Data Centre (ISDC), Versoix, Switzerland. Inside the ISDC, astronomers constantly monitor the data coming down from Integral because they know the sky at gamma-ray wavelengths can be a swiftly changing place.

"The galactic centre is one of the most exciting regions for gamma ray astronomy because there are so many potential gamma-ray sources," says Roland Walter, an astronomer at the ISDC, and lead author of these results.

To reflect the importance of this region, Integral is now running a Key Programme, in which almost four weeks of its observing time is given over to the study of the galactic centre. This is allowing astronomers to understand the gamma-ray characteristics of the galactic centre and its celestial objects, better than ever before.


This X-ray image was obtained by ESA's XMM-Newton satellite during the night of
22-23 September 2006, and shows the intense X-ray emission of the X-ray nova IGR
J17497-2821. The X-ray nova was first spotted by ESA’s Integral gamma-ray
observatory.

Credits: ESA/EPIC/ISDC

It was during one of the first of these observations that astronomers saw the outburst take place. An unexpected event of this kind is known as a 'target of opportunity'. At first they did not know what kind of eruption they had detected. Some gamma ray outbursts last for only a short period of time and so they immediately alerted other observatories around the world of the outburst’s position, allowing them to target the explosion, too. Fortunately, Integral has the capability to pinpoint the position of such a very bright event incredibly accurately.

In this case, the outburst continued to rise in brightness for a few days before beginning a gradual decline that lasted for weeks. The way the brightness of an outburst rises and falls is known to astronomers as a light curve. "It was only after a week that we could see the shape of the light curve and realised what a rare event we had observed," says Walter.


Artist's representation of an X-ray nova. The compact object on the right - a neutron
star or a black hole – ‘swallows’ gas from a companion star. The gas swirls in a disk
around the compact object at very high velocity (close to the speed of light) and emits
X-rays.

Credits: ESA

Comparing the shape of the light curve to others on file revealed that this was an eruption thought to come from a binary star system in which one component is a star like our Sun whereas the other is a black hole.

In these systems, the gravity of the black hole is ripping the Sun-like star to pieces. As the doomed star orbits the black hole, it lays down its gas in a disc, know as an accretion disc, surrounding the black hole.

Occasionally, this accretion disc becomes unstable and collapses onto the black hole, causing the kind of outburst that Integral witnessed. Astronomers are still not sure why the accretion disc should collapse like this but one thing is certain: when it does collapse, it releases thousands of times the energy than at other times.

Because such active star–black hole binaries are thought to be rare in the Galaxy, astronomers expect Integral to see such an outbursts only once every few years. That makes each and every one a precious resource for astronomers to study.


This image shows the field around the new X-ray nova IGR J17497-2821 (circled as
source number 1) obtained by the Leonhard Euler (ground) telescope on 21 September
2006.

This source was found to have brightened by about 1 magnitude compared to earlier
observations of this region.

Credits: Geneva Observatory

Thanks to the quick reactions of the astronomers at ISDC, observations were taken with satellites and observatories all around the world. ESA's XMM-Newton X-ray observatory, NASA's Chandra and Swift space telescopes, numerous ground-based telescopes captured the elusive radiation from this cataclysmic event. Now astronomers are hard at work, understanding what it all means.


Notes:

The results, described in the article "IGJ 717497-2821: A new X-ray Nova", by Roland Walter et al., will appear in Astronomy and Astrophysics. An on-line version with images can be found at http://isdc.unige.ch/Science/news/061123/.

For more information about how Integral's astronomers deal with targets of opportunity see the November 2006 issue of the ESA Bulletin at: http://www.esa.int/SPECIALS/ESA_Publications/index.html

Source: ESA - News

[shun]

Interesting, of course. Maybe, the star's path is kicked into an elliptical orbit, upon approach to the BH. At the apoapsis, the accretion disk would recieve less gas, the various heating mechanisms within the disk loose energy, and then contraction and collapse into the sch. radius of the remainder material in a short time frame.

[Waspie_Dwarf]

Integral sees the Galactic centre playing hide and seek

From February 2005, ESA’s Integral gamma-ray observatory began to regularly monitor
the centre of our Galaxy, and its immediate environment, known as the Galactic bulge.

This movie is composed of the sequence of individual observations performed about
every three days from February 2005 to April 2006, during the Galactic bulge monitoring
by the IBIS/ISGRI instrument on board Integral.

According to the Integral observations in April 2006, the high-energy rays from a handful
of sources closest to the galactic centre, most of which are X-ray binaries, all faded
temporarily.

The fortuitous dimming allows astronomers to set new limits on how faint these X-ray
binaries can become. It also allows a number of new investigations to be undertaken with
the data.

The movie covers a sky area of 5.3x4.1 degrees, centred on the Galactic Centre. In this
version of the movie a few frames have been removed to facilitate the download. The
complete (high-resolution) version can be obtained by clicking here.

Credits: ESA/ISDC

18 January 2007
ESA's gamma ray observatory Integral has caught the centre of our galaxy in a moment of rare quiet. A handful of the most energetic high-energy sources surrounding the black hole at the centre of the Galaxy had all faded into a temporary silence when Integral looked.

This unusual event is allowing astronomers to probe for even fainter objects and may give them a glimpse of matter disappearing into the massive black hole at the centre of our galaxy.

The Galactic centre is one of the most dynamic places in our Galaxy. It is thought to be home to a gigantic black hole, called Sagittarius A* (pronounced 'A star'). Since the beginning of the Integral mission, ESA's gamma ray observatory has allowed astronomers to keep watch on this ever-changing environment.


Integral has discovered many new sources of high-energy radiation near the galactic centre. From February 2005, Integral began to regularly monitor the centre of the Galaxy, and its immediate environment, known as the Galactic bulge.

Erik Kuulkers of ESA's Integral Science Operations Centre, ESAC, Spain, leads the Galactic bulge monitoring programme. Integral now keeps its high-tech eyes on about 80 high-energy sources in the galactic bulge. "Most of these are X-ray binaries," says Kuulkers.

X-ray binaries are made up of two stars in orbit around one another. One star is a relatively normal star; the other is a collapsed star, such as a white dwarf, neutron star or even a black hole. If the stars are close enough together, the strong gravity of the collapsed star can pull off gaseous material from the normal star. As this gas spirals down around the collapsed star, it is heated to over a million degrees centigrade and this causes it to emit high energy X-rays and gamma rays. The amount of gas falling from one star to the other determines the brightness of the X-ray and gamma-ray emission.


This mosaic image, built with exposures obtained by the IBIS/ISGRI instrument on board
ESA’s Integral gamma-ray observatory in April 2006, shows the Galactic centre region,
an area of the sky supposed to host a gigantic black hole and characterised by the
presence of a variety of hard X-ray and gamma-ray point sources. Due to the variability
these sources possess on all time scales, the region never looks exactly the same.

Surprisingly, the sources were ‘off’ around the time of the observation (including the
normally bright well-known black-hole candidate and micro-quasar 1E 1740.7-2942),
displaying an unusually ‘quiet’ galactic centre. This is a very different view from those
obtained on the long-term average.

The sources and the positions marked in white are almost permanently visible, while
those marked in red are known ‘transient’ sources, that is sources more often ‘off’ than
‘on’.

The source called 1E 1740.7-2942 is normally the brightest source in the Galactic Centre
region. It is a well-known black-hole candidate, as well as a micro-quasar source. The
massive black hole at the very centre of our Galaxy, Sagittarius A* (or Sgr A*), is very
close to source marked as ‘1’, corresponding to IGR J17456-2901.

The image covers a sky area of 4.3x2 degrees, and it is centred on (0, -0.5) degrees in
Galactic coordinates.

According to the Integral observations in April 2006, the high-energy rays from about ten sources closest to the galactic centre all faded temporarily. Kuulkers excludes the possibility that a mysterious external force is acting on all the objects to drive them into quiescence. "All the sources are variable and it was just by accident or sheer luck that they had turned off during that observation," he says with a smile.

The fortuitous dimming allows astronomers to set new limits on how faint these X-ray binaries can become. It also allows a number of new investigations to be undertaken with the data.


This mosaic image, built with exposures obtained by the IBIS/ISGRI instrument on board
ESA’s Integral gamma-ray observatory, provides another view of the Galactic centre region.

Differently from the previous image, this view was built with all exposures taken during
the 1.5 years of Integral’s Galactic bulge monitoring, thus providing an ‘average’ view
of the area.

By comparing this view with the previous one, it is possible to observe the highly
variable nature of the sources.

In particular, the sources and the positions marked in white are almost permanently
visible, while those marked in red are known ‘transient’ sources, that is sources more
often ‘off’ than ‘on’.

The source called 1E 1740.7-2942 is normally the brightest source in the Galactic
Centre region. It is a well-known black-hole candidate, as well as a micro-quasar source.
The massive black hole at the very centre of our Galaxy, Sagittarius A* (or Sgr A*), is
very close to source marked as ‘1’, corresponding to IGR J17456-2901.

The image covers a sky area of 4.3x2 degrees, and it is centred on (0, -0.5) degrees in
Galactic coordinates.

"When these normally bright sources are faint, we can look for even fainter sources," says Kuulkers. These could be other X-ray binaries or the high-energy radiation from giant molecular clouds interacting with past supernovae. There is also the possibility of detecting the faint high-energy radiation from the massive black hole in our Galaxy's centre.

Integral's Galactic bulge monitoring programme will continue throughout this year. The data is made available, within a day or two of being collected, to the scientific community via the Internet from a dedicated webpage at the Integral Science Data Centre (IDSC), Geneva, Switzerland. This way, anyone interested in specific sources can watch for interesting changes and trigger follow up observations with other telescopes in good time.


Notes:

The findings are accepted for publication in the Astronomy & Astrophysics magazine, in the article titled: "The INTEGRAL Galactic bulge monitoring program: the first 1.5 years", by E.Kuulkers et al.

Source: ESA - News

[Waspie_Dwarf]

Integral points to the fastest spinning neutron star

This artist's impression shows a spinning neutron star (pulsar) approximately 10 kilometres
in diameter. When a neutron star orbits another star, its strong gravitational field can
pull gas from the other star. This coats the surface of the neutron star. When the
coating reaches a height of between 5-10 metres, the gas ignites in a thermonuclear
explosion. This massive release of energy generally lasts from between several seconds
to several minutes and a burst of X-rays is released.

Credits: NASA/Dana Berry

16 February 2007
Astronomers using ESA's gamma-ray observatory, Integral, have detected what appears to be the fastest spinning neutron star yet. This tiny stellar corpse is spinning 1122 times every second. If confirmed, the discovery gives astronomers the chance to glimpse the insides of the dead star.

The neutron star, known by its catalogue number XTE J1739-285, was discovered during one of its active phases on 19 October 1999 by NASA's Rossi X-Ray Timing Explorer (RXTE) satellite. In August 2005, while Integral was monitoring the bulge of the Galaxy, XTE J1739-285 started to come back to life. About a month later Integral discovered the first short bursts of X-rays from the object.

Erik Kuulkers of the ESA Integral Science Operations Centre, Spain, who leads the Galactic bulge monitoring programme, informed Philip Kaaret, University of Iowa, via email that things were still hotting up near the end of October. Kaaret arranged for the RXTE satellite to observe XTE J1739-285 between 31 October and 16 November. Together the two satellites recorded about twenty bursts between September and November.


Just because a star dies, it doesn't mean its life is over. A neutron star is the tiny heart of a collapsed star. Measuring about 10 kilometres across, yet containing something like the mass of the Sun, the interior of a neutron star is the most exotic realm that astronomers can imagine. According to their calculations a thimbleful of neutron star material weighs a hundred million tonnes.

When a neutron star orbits another star, its strong gravitational field can pull gas from the other star. This coats the surface of the neutron star. When the coating reaches a height of between 5-10 metres, the gas ignites in a thermonuclear explosion. This massive release of energy generally lasts from between several seconds to several minutes and a burst of X-rays is released.

Previous observations of other neutron stars have shown that the X-rays emitted during bursts display oscillations that correspond to the rotation rate of the neutron stars. So the team began analysing the XTE J1739-285 bursts for oscillations. What they found was astounding. In the brightest burst, which RXTE recorded on 4 November, there were indeed oscillations but they were nearly twice as fast as any previously observed.

"It was quite a surprise to us," admits Kuulkers. However, after running a series of checks, the team satisfied themselves that the oscillations were indeed taking place 1122 times a second (1122 Hz).


Previously, the fastest neutron stars were known to spin with frequencies between 270-619 Hz. This had led some astronomers to estimate, using statistical arguments, that the fastest a neutron star could spin was 760 Hz. If the new observations are confirmed, XTE J1739-285 smashes this limit.

"Our detection is just above the level where we think there is something real. We definitely need more observations. If we see the signal again, then everyone will believe it," says Kuulkers.

This doesn't mean that neutron stars can spin as fast as they like. If the spin is too fast, even the crushing gravity of the star will be unable to hold matter to the surface and the star will break up. The exact break-up speed depends on the internal conditions of a neutron star and as yet, astronomers do not know these precisely.


"Our putative 1122 Hz detection places a serious constraint on neutron star models. If we can find more stars that spin in this range, it will certainly allow us to exclude some models of their interior structure," says Kuulkers.

So, now it is just a matter of patience. The astronomers will keep watch, not only for XTE J1739-285 to burst again, but also for other fast-spinning X-ray neutron stars.


Notes

"Evidence for 1122Hz X-Ray burst Oscillations from the Neutron-Star X-Ray Transient XTE J1739-285", by P. Kaaret et al., will be published in The Astrophysical Journal on 10 March 2007.

Source: ESA - Integral

[Waspie_Dwarf]

Integral expands our view of the gamma-ray sky

The upper image shows the sky distribution of four of the main soft gamma-ray source
populations observed in the third Integral/IBIS survey catalogue. This newly-released
catalogue contains 421 sources. Of the known systems, the low-mass X-ray binaries
(LMXB) are old systems mainly populating the galactic bulge, the high-mass X-ray
binaries (HMXB) are younger systems seen along the galactic plane, and the active
galactic nuclei (AGN) are extragalactic sources seen over the whole sky. Around one
out of four of the sources seen by Integral are unidentified, and their distribution is
also shown.

The lower picture is a false colour image of the central region of our galaxy. This is a
composite image based on all-sky IBIS/ISGRI maps in three energy windows
(between 17 and 100 keV) and represents the true 'X-ray colours' of the sources.
Red sources are dominated by emission below 30 keV, while blue sources have harder
spectra, emitting strongly above 40 keV.

Credits: IBIS survey team

20 February 2007
Integral's latest survey of the gamma-ray universe continues to change the way astronomers think of the high-energy cosmos. With over seventy percent of the sky now observed by Integral, astronomers have been able to construct the largest catalogue yet of individual gamma-ray-emitting celestial objects. And there is no end in sight for the discoveries.

Integral is the European Space Agency's latest orbiting gamma-ray observatory. Ever since Integral began scientific operations in 2003, the project team has been devoting a substantial proportion of its observing time to a survey of the gamma-ray universe.

"The gamma-ray sky is notoriously variable and extremely unpredictable," says Anthony Dean, University of Southampton, UK, one of the original proposers of the Integral mission. Hence, the need for Integral's constant vigilance and an accurate catalogue of all gamma-ray sources. With this, astronomers can target individual gamma-ray objects for more detailed, study.


The task of Integral, ESA's International Gamma-Ray Astrophysics Laboratory, is to
gather the most energetic radiation that comes from space. The spacecraft was launched
in October 2002 and it is helping to solve some of the biggest mysteries in astronomy.
Gamma rays are even more powerful than the X-rays used in medical examinations.
Fortunately, Earth's atmosphere acts as a shield to protect us from this dangerous cosmic
radiation. However this means that gamma rays from space can only be detected by
satellites. Integral is the most sensitive gamma-ray observatory ever launched. It detects
radiation from the most violent events far away and from processes that made the
Universe habitable.

Credits: ESA (Medialab)

For the past three and a half years, Integral has been collecting survey data. At the end of every year, the data has been turned into a catalogue of sources.

During the first year, it concentrated on the regions close to the centre of our galaxy and found more than 120 sources. During the following year, Integral broadened its reach and found almost 100 more sources.

Now Integral has observed over 70 percent of the sky, with a total exposure time of 40 million seconds. A European team of astronomers led by Antony Bird, University of Southampton, UK, have turned all three years' worth of data into the third Integral catalogue of gamma-ray sources. It contains a total of 421 gamma-ray objects. Most have been identified as either binary stars in our Galaxy containing exotic objects such as black holes and neutron stars, or active galaxies, far away in space. But a puzzling quarter of sources remain unidentified so far.

"I think many of these will turn out to be either star systems enshrouded in dust and gas, or cataclysmic variable stars," says Dean. Integral observes in the gamma-ray band so it can see through the intervening material. It has demonstrated that it can discover sources obscured at other wavelengths.

One surprise has been the efficiency with which Integral has detected just one minor subclass of cataclysmic variable stars (CVs), the so-called intermediate polars. Initially astronomers were not sure that CVs would emit gamma rays. Indeed, Integral has already shown that only about one percent of them do. "At the moment, the reason why this should be is totally mysterious," says Dean.

As its surveys points further from our Galaxy, so Integral increasingly sees the active galaxies. These represent about a tenth of all galaxies and each one has some kind of extraordinary activity taking place in its core. It is widely accepted that this activity is driven by a gigantic black hole sucking matter out of existence.

Roman Krivonos, Max-Planck-Institute für Astrophysik, Germany, and colleagues have used the Integral survey to show that AGN are concentrated in the same places that ordinary galaxies are found. Whilst this is not an unexpected result, it is the first time such an AGN distribution has been seen at high-energies.

"Integral represents a milestone in gamma-ray astronomy," says Dean. Thirty years ago, NASA’s Einstein observatory produced a catalogue of X-ray sources that became the standard reference document for all X-ray observatories – including ESA's XMM-Newton. "Integral is doing the same for gamma-ray astronomy," says Dean.

"We are in a golden age of gamma-ray astronomy," agrees Bird. And ESA's Integral is at the forefront of this brave new universe.

Source: ESA - Integral

[Waspie_Dwarf]

Radioactive iron, a window to the stars

25 June 2007

The distribution of massive stars in our Galaxy, which are the likely sources of the observed gamma-rays from the isotope iron-60, is best traced by the radioactivity of aluminium-26 shown in this image from NASA's COMPTON osbervatory. Integral has made detailed observations of aluminium-26's gamma-rays. The emission from iron-60 is too faint for making images with current gamma-ray telescopes.

Credits: MPE 2001 (data: NASA COMPTON)

ESA’s orbiting gamma-ray observatory, Integral, has made a pioneering unequivocal discovery of radioactive iron-60 in our galaxy that provides powerful insight into the workings of massive stars that pervade and shape it.

Found drifting in space, the radioactive isotope has been sought for long. All past reported sightings of iron-60 have been subject to controversy. Now Integral has provided unequivocal evidence.

Since late 2002, Integral has been collecting data from across the galaxy. It shows an enhancement in gamma rays at two characteristic energies, 1173 and 1333 kilo electron Volts. These are produced by radioactive decay of iron-60 into cobalt-60.

Roland Diehl of the Max-Planck-Institut für extraterrestrische Physik, headed the work and believes it is a major step forward. “These gamma-ray lines have been detected before with some dispute. Integral, the only instrument capable of doing this, shows that iron-60 does exist in interstellar space in our Galaxy,” he says.

More than a curiosity, its presence opens a door into the very heart of the most massive stars in the cosmos. The majority of chemical elements are built inside stars from raw ingredients present during star formation from an interstellar gas cloud. In addition to hydrogen and helium produced during the Big Bang, the gas contained enrichments, known to astronomers as ‘metals’, from previous generations of stars and their nuclear reactions.

Until this detection, astronomers had only one radioactive isotope to probe into the current build-up of chemical elements in stars and their distribution with respect to future star formation. That was the radioactive isotope aluminium-26, first discovered in 1978. “The study of aluminium-26 has developed into its own branch of astronomy,” says Diehl.

The task of Integral, ESA's International Gamma-Ray Astrophysics Laboratory, is to gather the most energetic radiation that comes from space. The spacecraft was launched October 2002 and will help to solve some of the biggest mysteries in astronomy. Gamma rays are even more powerful than the X-rays used in medical examinations. Fortunately, the Earth's atmosphere acts as a shield to protect us from this dangerous cosmic radiation. However this means that gamma rays from space can only be detected by satellites. Integral is currently the most sensitive gamma-ray observatory ever launched. It detects radiation from the most violent events far away and from processes that made the Universe habitable.

Credits: ESA. Illustration by D. Ducros

Iron-60 gives astronomers valuable new insight - although produced in the same stars as aluminium-26, its production differs markedly. Iron-60 is synthesised both later in a star’s life and deeper inside.

As massive stars age, they develop a layered structure in which different chemical elements are fused together. While aluminium-26 is one rung on the ladder of nuclear reactions, iron-60 is produced from pre-existing stable iron isotopes by a process called ‘neutron capture’ in the respective layers where helium and carbon atoms are undergoing fusion.

“Iron-60 provides the entry into studying neutron capture in stars through contemporaneous radioactivity,” says Diehl. It has also prompted a number of particle accelerators to begin more detailed studies of how easily iron captures neutrons.

Unlike aluminium-26, iron-60 is only expelled into space when the star explodes at the end of its life. It then decays with a half-life of 1.5 million years, producing the gamma rays that Integral detected.

The new data pins down the ratio of iron-60 to aluminium-26, which has a half-life of 740 000 years. Previous predictions have fallen anywhere between 10 and 100 percent. Integral shows it to be 15 percent, which agrees well with current theoretical estimates. But theoreticians and nuclear physicists have been stimulated by Integral’s results to strive for more precise predictions.

Although Integral clearly sees the telltale gamma rays, they are too faint for it to map out enhancements and paucities across the Galaxy. “Mapping the distribution of iron-60 is a job for the next generation of gamma-ray instruments,” says Diehl.

Nevertheless, the team will continue observing with Integral for as long as they can, in the hope of gaining some coarse ideas about the isotope’s spread across the Galaxy.


Notes:

The findings will appear in the article titled, “SPI observations of the diffuse 60Fe emission in the Galaxy”, by W. Wang , M. Harris, R. Diehl, H. Halloin, B. Cordier, A.W. Strong, K. Kretschmer, J. Knödlseder, P. Jean, G.G. Lichti, J.P. Roques, S. Schanne, A. von Kienlin, G. Weidenspointner, C. Wunderer, accepted for publication on 26 April 2007 in the Astronomy and Astrophysics journal.

Source: ESA - Space Science - Integral

[Waspie_Dwarf]

Gamma-ray lighthouse at the edge of our universe

3 October 2007

This is an artist's impression of a blazar. A blazar is a very compact and highly variable energy source associated with a supermassive black hole. It is also characterized by a relativistic jet that is pointing in the general direction of the Earth. Blazars are among the most violent phenomena in the universe and are an important topic in extragalactic astronomy.

Credits: Boston University - Cosmovision

There is a gamma-ray lighthouse shining from the edge of our universe. Astronomers have discovered it using ESA’s orbiting gamma-ray observatory, Integral. Now, they must work hard to understand it.

The object, known only by its catalogue name IGR J22517+2218, was discovered this year, but its nature was unknown. This is not an unusual situation. Around 30% of the sources discovered by Integral remain unidentified so far. All astronomers know for certain, is that there are celestial sources out there, pumping gamma rays into space. However, the identification of the sources with individual celestial objects will have to wait for more detailed observations in other wavelengths.

In fact, this was the case for IGR J22517+2218. It came as a surprise when NASA’s Swift satellite recorded the object in X-rays, giving its position within much more precision than can be achieved in gamma-rays. IGR J22517+2218 was identified with the already known active galaxy MG3 J225155+2217. This galaxy is so distant that it is the furthest celestial object ever to be recorded by Integral.

All active galaxies are powered by supermassive black holes. These celestial monsters contain between a million and several thousand million times the mass of the Sun.

They generate a gravitational field so large that they swallow any matter passing nearby, releasing enormous amounts of energy in the process. In the case of IGR J22517+2218, the Integral observations show that it is a gargantuan powerhouse, throwing out stupendous quantities of gamma rays.

“It is gobbling up an entire solar system every few days and hurling the energy out in gamma-rays,” says Loredana Bassani, IASF-Bologna/INAF, Italy, who together with colleagues has investigated this distant galaxy.

The Integral observations show that the galaxy is one of a special kind of active galaxy, known as a blazar. These are the most energetic of the active galaxies. However, the Integral data does show some curiosities.

“This is a very peculiar object. We have been able to classify it as a blazar but it has some strange characteristics,” says Bassani.

Integral, ESA's International Gamma-Ray Astrophysics Laboratory, is gathering some of the most energetic radiation that comes from space. The spacecraft was launched in October 2002 and is helping to solve some of the biggest mysteries in astronomy.

Gamma rays are even more powerful than the X-rays used in medical examinations. Fortunately, Earth's atmosphere acts as a shield to protect us from this dangerous cosmic radiation. However this means that gamma rays from space can only be detected by satellites.

At time of launch, Integral was the most sensitive gamma-ray observatory ever put into space. It detects radiation from the most violent events far away and from processes that made the Universe habitable.

Credits: ESA

Blazars tend to have two major peaks of emission. In objects similar to IGR J22517+2218, one peak occurs in infrared wavelengths and is produced by the radiation given off by electrons spiralling around the magnetic field lines. The other peak occurs at high-energy gamma-ray wavelengths and is produced by those same electrons colliding with photons of light.

In the case of IGR J22517+2218, the object appears to have only one peak. This occurs in neither of the conventional wavelength ranges but, in fact, in the low-energy gamma-ray band instead. Either the infrared peak has been moved up in energy, or the high-energy gamma-ray peak has been moved down.

Either way, when the team can work out what this means, it will doubtlessly tell them a lot about active galaxies, and blazars in particular. “Whatever we discover, this object will stretch our understanding of the blazars,” says Bassani.

The team hope to continue observing this object at all wavelengths in an effort to build up a full picture of the radiation given out by this celestial object. In this way, they will be able to piece together the manner in which the supermassive black hole at the heart of IGR J22517+2218 is devouring its surroundings.


Notes:

The findings appear in ‘IGR J22517+2218=MG3 J225155+2217: A new gamma-ray lighthouse in the distant universe’ by L. Bassani, R. Landi, A. Malizia, M. Fiocchi, A. Bazzano, A. J. Bird, A. J. Dean, N. Gehrels, P. Giommi and P. Ubertini. It has been accepted for publication in the Astrophysical Journal Letters on 18 September 2007.


For more information:

Loredana Bassani, INAF IASF, Bologna, Italy
Email: Bassani @ iasfbo.inaf.it

Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int

Source: ESA - Space Science - News

[Waspie_Dwarf]

Science with Integral – five years on

17 October 2007

Integral, ESA's International Gamma-Ray Astrophysics Laboratory, is gathering some of the most energetic radiation that comes from space. The spacecraft was launched in October 2002 and is helping to solve some of the biggest mysteries in astronomy.

Gamma rays are even more powerful than the X-rays used in medical examinations. Fortunately, Earth's atmosphere acts as a shield to protect us from this dangerous cosmic radiation. However this means that gamma rays from space can only be detected by satellites.

At time of launch, Integral was the most sensitive gamma-ray observatory ever put into space. It detects radiation from the most violent events far away and from processes that made the Universe habitable.

Credits: ESA

With eyes that peer into the most energetic phenomena in the universe, ESA’s Integral has been setting records, discovering the unexpected and helping understanding the unknown over its first five years.

Integral was launched on 17 October 2002. Since then, the satellite has helped scientists make great strides in understanding the gamma-ray universe - from the atoms that make up all matter, giant black holes, mysterious gamma-ray bursts to the densest objects in the universe.


The atoms that make us

The centre of the galaxy shines brightly in gamma rays with a specific energy of 511 keV. This is the energy released during an encounter between an electron and its antimatter counterpart, the positron. It is not yet known what creates the antimatter particles.

The image shows the full sky at 511 keV.

Credits: ESA/ J. Knödlseder et al.

Surveying the entire galaxy looking for the radioactive isotope aluminium 26 with Integral, scientists have been able to calculate that a supernova goes off in our galaxy, once every 50 years.

According to Integral, something is creating a lot of gamma rays at the centre of our galaxy - the suspect is positrons, the antimatter counterparts of electrons. Scientists have been baffled as to how vast numbers of such particles can be generated every second and how these sources would be distributed over the sky to match the gamma-ray map.

The densest objects in the universe

The red and green boxes represent dense objects discovered by Integral in our galaxy. The blue spots are supermassive black holes in the distant universe. The yellow spots are so-far unidentified sources of gamma-rays. Their distribution suggests that they are probably dense objects within the Milky Way.

Credits: ESA/ IBIS Survey team (A Bird et al.)

Within months of operation, Integral solved a thirty-year-old mystery by showing that the broadband gamma-ray emission observed towards the centre of the galaxy was produced by a hundred individual sources. A catalogue of close to 500 gamma-ray sources from all over the sky, most of them new, was then complied.

Scientists now know that a rare class of anomalous X-ray pulsars, or magnetars, generates magnetic fields a thousand million times stronger than the strongest steady magnetic field achievable in a laboratory on Earth. These sources show, unexpectedly, strong emission in the Integral energy range.

Integral revealed that a sub-class of X-ray binary stars, called super-giant fast X-ray transients, previously thought to be extremely rare, is actually common in our galaxy. The satellite has also discovered a completely new class of high-mass X-ray binaries, called highly absorbed X-ray binaries.

Giant black holes

This all-sky map shows regions of ionized hydrogen gas in the local universe. The hidden black holes detected in the INTEGRAL survey of high-energy X-ray sources are located within the diamond-shape marks. Many sources were detected through the line of sight of the dusty Milky Way galactic plane, which is the bright area stretching across the center of the entire image from left to right.

Credits: D. Finkbeiner / ESA, INTEGRAL, V. Beckmann, ISDC Geneva

Integral has seen about 100 of the brightest supermassive black holes, the main producers of gamma radiation in our universe, in other galaxies. But while looking for them in nearby galaxies, surprisingly few have been found.

They are either too well-hidden or are only present in the younger galaxies which populate the more distant universe.

ESA's Integral gamma-ray observatory has been observing Earth during a period spanning from 24 January to 9 February 2006. The main purpose of the observations has been to study the high-energy diffuse background radiation known as 'cosmic X-ray background' (CXB), by analysis of the decrease of its isotropic flux (not varying with distance or direction) while Earth passes in front of Integral's field of view. High-energy emission from the atmosphere, due to reflections of the CXB, interaction with cosmic rays and aurorae, have also been observed.

Credits: ESA

Galaxies throughout the universe are believed to be responsible for creating the diffuse background glow of gamma rays, observed over the entire sky. Integral used the Earth as a giant shield to disentangle this faint glow. Making the measurements possible was a technological and operational feat.

The data will help understand the origin of the highest energy background radiation and possibly, provide new clues to the history of growth of supermassive black holes since the early epochs of the Universe.

Mysterious bursts

This artist's impression illustrates how a gamma-ray burst can flare dramatically over a short time period (gamma ray bursts usually last between a hundredth of a second to a hundred seconds). The bursts can occur as often as several times a day. There is no way to predict when or where they will next occur.

ESA missions such as XMM-Newton, Integral and Ulysses study these mysterious, powerful bursts.

Credits: ESA (Illustration by AOES Medialab)

Although not designed to be a gamma-ray-burst ‘watchdog’, scientists realised that Integral could perform this task if assisted by sufficiently powerful software. ESA set a new record for speed and accuracy with the Integral Burst Alert System on 3 December 2003 when a burst was detected, localised and astronomers were alerted in 18 seconds.

The event, called GRB 031203, was faint and close, in cosmological terms, which suggests that an entire population of low energy gamma-ray bursts has so far gone unnoticed.


On 27 December 2004 Integral was hit by the strongest flux of gamma rays ever measured by any spacecraft and it even measured radiation that bounced off the Moon. The culprit was a magnetar, SGR 1806-20, located 50 000 light years away on the other side of our Milky Way. Thanks to this outburst, astronomers now think that some gamma-ray bursts might come from similar magnetars in other galaxies.

Integral has also been able to take images of gamma ray bursts, while the telescope was not pointed in the right direction. This was done using radiation that passed through the side of Integral’s imaging telescope and struck the detector.

Christoph Winkler, ESA’s Integral Project Scientist says “Integral has indeed played a major role in modern gamma-ray astronomy. So much has happened in the span of five years but much more is still to come.”


For more information:

Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int

Source: ESA - Space Science - News

[Waspie_Dwarf]

New scientific riches from Integral

7 November 2007

This image shows the soft gamma-ray sky, as revealed by Integral during its first five years of operation. Soft gamma rays each have energy somewhere between 20-100 keV. Most sources of celestial gamma ray in the Universe emit in this region of the gamma ray spectrum. This image is a mosaic of data taken with the Imager on Board the Integral Satellite (IBIS).

Credits: ESA/ Integral/ IBIS Survey Team

Astronomers from around the world have been discussing the extraordinary scientific riches that have flowed from ESA’s orbiting gamma-ray observatory, Integral. Here we present the gist of some of the astonishing ones.

When Integral was launched in 2002 it was intended to take detailed gamma-ray images and spectra of the universe and to look for new types of sources in this relatively unexplored field. In all these objectives, it has excelled.

“Integral discovers about 100 new gamma-ray sources a year,” says one of the conference organisers, Angela Bazzano, Istituto di Astrofisica Spaziale e Fisica Cosmica, Roma. These objects are spread throughout the Universe.

Its instruments have been fine-tuned to the so-called soft gamma-ray band, where the majority of celestial gamma rays are emitted. It has used these instruments to identify some of the most puzzling sources of celestial gamma rays. These Tera electron Volt (TeV) sources were first seen from telescopes on Earth looking for the brief flash of light that occurs when highly energetic gamma rays enter the atmosphere.

Integral, ESA's International Gamma-Ray Astrophysics Laboratory, is gathering some of the most energetic radiation that comes from space. The spacecraft was launched in October 2002 and is helping to solve some of the biggest mysteries in astronomy.

Gamma rays are even more powerful than the X-rays used in medical examinations. Fortunately, Earth's atmosphere acts as a shield to protect us from this dangerous cosmic radiation. However this means that gamma rays from space can only be detected by satellites.

At time of launch, Integral was the most sensitive gamma-ray observatory ever put into space. It detects radiation from the most violent events far away and from processes that made the Universe habitable.

Credits: ESA

Integral identified the TeV sources based upon the much lower energy gamma rays that they emit. It discovered that most are a compact object called a pulsar surrounded by a nebula of gases. Each pulsar is the tiny core of a once giant star. Although just 15 km across, each pulsar contains more mass than the Sun, and generates an intense magnetic field.

The Integral results suggest that these particular pulsars give off a ‘wind’ of electrons that collides with the surrounding nebula and is enormously accelerated by the strong magnetic field. These ultra-fast electrons give off the gamma rays seen by Integral and ground-based telescopes.

Sources almost invisible to X-ray telescopes, the strongly obscured supergiant binaries have been revealed. Multiwavelength observations show that in these systems gigantic stars, each containing many tens of times the mass of the Sun coexist with neutron stars. A dense envelope of gas, a cocoon, shed from the giant star engulfs the neutron star.

In 40 years of X-ray astronomy, only a dozen supergiant X-ray systems had been found. Integral has already doubled this number and shown that most of them are persistent emitters of X-rays and gamma-rays, most probably slow pulsars.

The transient sources are a special breed as well. For reasons still under debate, every now and again, they flare up brightly for a few hours, up to a day, and then disappear again from sight. Without the sensitivity and observing strategy of Integral, these short bursts of gamma rays would have escaped detection and this class of celestial object would remain undiscovered.

This is an artist's impression of Integral’s investigation of high-mass X-ray binaries. Integral has revealed two different types of stars. In the first example (top panel), the neutron star companion is buried deeply in the extended outer envelope of the star. This results in persistent X-ray emission. In the second example (bottom panel), the neutron star only occasionally swallows material from the envelope, causing a sudden brightening of its gamma rays.

Credits: University of Paris (S. Chaty)

Integral has been surveying gamma rays that are given out by radioactive isotopes created inside stars and spilled out into space at the end of the star’s life. By measuring the amount of these isotopes from the brightness of the gamma rays they emit, models of stellar interiors and their nuclear energy sources can be checked. Integral has been looking at radioactive forms of aluminium and iron.

The results have been surprising and are forcing a rethink of the details about how those high-mass stars work. “Theorists have been alerted to review the complexities of convection in stellar interiors, and new experiments have been started to understand the relevant nuclear reaction rates,” says Roland Diehl, Max Planck Institut für extraterrestrische Physik.

He adds that Integral’s astronomers will be checking their own data on more observations hoping to converge on a better understanding of stellar interiors, and of the dynamics of hot gas in the inner regions of our Galaxy.

Integral observations have posed another puzzle to astrophysicists by mapping the gamma rays from annihilations of positrons, a form of antimatter produced in various ways including radioactive decay, plasma jets around neutron stars, and probably by the decay of some varieties of dark matter. The galaxy's disc is fainter than expected, the inner part of the Galaxy outshines it by far - a puzzle still unsolved.

Integral has opened the gamma-ray universe to astronomers like never before. “Integral is now a major, worldwide observatory,” says Pietro Ubertini, Director of Istituto di Astrofisica Spaziale e Fisica Cosmica, Roma.


Notes:

These results were presented at the ‘Five years of Integral’ workshop held from 17-19 October 2007 in Sardinia, Italy. It was attended by about 130 scientists from around the world.


For more information :

Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int

Source: ESA - Space Science - News

[Waspie_Dwarf]

Integral discovers the galaxy’s antimatter cloud is lopsided

9 January 2008

This is an artist’s impression of ESA’s orbiting gamma-ray observatory, Integral.

Credits: ESA

The shape of the mysterious cloud of antimatter in the central regions of the Milky Way has been revealed by ESA’s orbiting gamma-ray observatory Integral. The unexpectedly lopsided shape is a new clue to the origin of the antimatter.

The observations have significantly decreased the chances that the antimatter is coming from the annihilation or decay of astronomical dark matter.
Georg Weidenspointner at the Max Planck Institute for Extraterrestrial Physics and an international team of astronomers made the discovery using four-years-worth of data from Integral. The cloud shows up because of the gamma rays it emits when individual particles of antimatter, in this case positrons, encounter electrons, their normal matter counterpart, and annihilate one another.

One signature of positron-electron annihilation is gamma rays carrying 511 thousand electron-volts (keV) of energy. There has been a vigorous debate about the origin of these positrons ever since the discovery of the 511 keV emission from the centre of the galaxy by gamma-ray detectors flown on balloons during the 1970s.

Some astronomers have suggested that exploding stars could produce the positrons. This is because radioactive nuclear elements are formed in the giant outbursts of energy, and some of these decay by releasing positrons. However, it is unclear whether these positrons can escape from the stellar debris in sufficient quantity to explain the size of the observed cloud.

The left-hand panel shows the glow of 511 keV gamma rays coming from the annihilation of electrons by their antimatter counterparts, the positrons. The map shows the entire sky, with the galactic centre at the middle. The emission can be seen extending towards the right-hand side of the map. The right-hand panel shows the distribution of hard low mass X-ray binary stars. This stellar population has a distribution that matches the extent of the 511 keV map.

Credits: ESA/ Integral/ MPE (G. Weidenspointner et al.)

Other astronomers wondered whether more exotic processes were at work. From earlier results using much less data, the positron cloud seemed to be spherical and centred on the centre of the galaxy. Such a shape and position corresponds to the expected distribution of dark matter in the centre of our galaxy, so it was suggested that dark matter was annihilating or decaying into pairs of electrons and positrons, which then annihilated to produce the gamma rays.
The trouble with this idea, however, was that the dark matter particles needed to be much less massive than most theories were predicting.

The new results give astronomers a valuable new clue and point away from dark matter as the origin of the antimatter. Beyond the galactic centre, the cloud is not entirely spherical. Instead it is lopsided with twice as much on one side of the galactic centre as the other. Such a distribution is highly unusual because gas in the inner region of the galaxy is relatively evenly distributed.

Equally importantly, Integral found evidence that a population of binary stars is also significantly off-centre, corresponding in extent to the cloud of antimatter. That powerfully suggests these objects, known as hard (because they emit at high energies) low mass X-ray binaries, are responsible for a major amount of antimatter.

This computer model of the 511 keV gamma rays coming from the central region of the galaxy matches the asymmetry around the galactic centre. The gamma rays from the galactic centre are symmetrical and the only way to fit the observations was to assume that the asymmetry was caused by gamma rays coming from the inner disc region of the galaxy, not throughout the galaxy. This ties in with the observed distribution of hard low mass X-ray binaries.

Credits: ESA/ Integral/ MPE (G. Weidenspointner et al.)

A low mass X-ray binary (LMXB) is a celestial system in which a relatively normal star is being eaten alive by a nearby stellar corpse, either a neutron star or a black hole. The gravitational field of the stellar corpse is so strong that it rips gas from the normal star. As this gas spirals down towards that object, it is heated so much that positron-electron pairs can be spontaneously generated in the intense radiation field, although the 511 keV emission is probably too weak to be detected from individual LMXBs by Integral.

“Simple estimates suggest that about half and possibly all of the antimatter is coming from the X-ray binaries,” says Weidenspointner. The other half could be coming from a similar process around the galaxy’s central black hole and the various exploding stars there. He points out that the lop-sided distribution of hard LMXBs is unexpected, as stars are distributed more or less evenly around the galaxy. More investigations are needed to determine whether the observed distribution is real.

Integral is currently the only mission that can see both the 511 keV radiation and the hard LMXBs. Weidenspointner and colleagues will be watching keenly to see whether it discovers more LMXBs and, if so, where they are located.

“The link between LMXBs and the antimatter is not yet proven but it is a consistent story,” says Weidenspointner. It has real astrophysical importance because it decreases the need for dark matter at the centre of our galaxy.


Notes:

‘An asymmetric distribution of positrons in the galactic disk revealed by gamma rays’ by Georg Weidenspointner et al. is being published today, 10 January, in the journal Nature.


For more information:

Georg Weidenspointner, Max Planck Institute for Extraterrestrial Physics
Email: Georg.Weidenspointner @ hll.mpg.de

Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int

Gerald K. Skinner, Astrophysics Science Division, NASA/GSFC
Email: skinner @ milkyway.gsfc.nasa.gov

Source: ESA - Space Science - News

[Waspie_Dwarf]

Integral: Stellar winds colliding at our cosmic doorstep

20 February 2008

ESA’s Integral has made the first unambiguous discovery of high-energy X-rays coming from a rare massive star at our cosmic doorstep, Eta Carinae. It is one of the most violent places in the galaxy, producing vast winds of electrically-charged particles colliding at speeds of thousands of kilometres per second.

The only astronomical object that emits gamma-rays and is observable by the naked eye, Eta Carinae is monstrously large, so large that astronomers call it a hypergiant. It contains between 100–150 times the mass of the Sun and glows more brightly than four million Suns put together. Astronomers know that it is not a single star, but a binary, with a second massive star orbiting the first.

It has long been suspected that such massive binary stars should give off high-energy X-rays, but until now, the instruments required for the observations were lacking. Recently, Integral has conclusively shown that Eta Carinae gives off high-energy X-rays, more or less in agreement with theoretical predictions.

This is an image of the region around Eta Carinae, as seen by Integral in the high-energy X-ray range. The distance between Eta Carinae and the Integral point source IE 1048.1-5937 is 45 arcminutes.

Credits: ESA/ Integral (Leyder et al.)

“The intensity of the X-rays is a little lower than we expected, but given that this is the first-ever conclusive observation, that’s okay,” says Jean-Christophe Leyder of the Institut d’Astrophysique et de Géophysique, Université de Liège, Belgium.

The high-energy X-rays come from a vast shockwave, set up and maintained between the two massive stars. The shockwave is produced when the two stars’ stellar winds collide, creating a system that astronomers term a colliding-wind binary. Massive stars are constantly shedding particles that are ‘blown’ away into space by the effect of light and other radiation given off by the star.

This starlight is so fierce that the stellar winds can reach speeds of 1500–2000 km/s. With two massive stars in close proximity, as they are in the Eta Carinae system, the winds collide and set up fearsome shockwaves where temperatures reach several thousand million degrees Kelvin. “It’s a very tough environment,” says Leyder.

This is an artist’s impression of ESA’s orbiting gamma-ray observatory, Integral.

Credits: ESA

Electrically-charged particles called electrons get caught in the magnetic environment of the shockwaves, bouncing back and forth and being accelerated to huge energies. When they finally burst out of the shockwave, they collide with low-frequency photons and give them more energy, creating the emission that Integral has seen.

Understanding this emission is important because astronomers believe that it lies at the heart of many diverse phenomena in the universe. Stellar winds have profound implications on the evolution of stars, the chemical evolution of the universe and as a source of energy in the galaxy.

Massive stars are rare, so two in a binary system is even rarer. “In our galaxy, there are probably only 30-50 colliding-wind binaries that display a clear signature of wind-wind collision,” says Leyder. A year ago, ESA’s XMM-Newton saw X-rays from the colliding wind binary, HD 5980, situated in the neighbouring galaxy, the Small Magellanic Cloud.

This is an image of the Carina Nebula as seen by the Hubble space telescope. The location of Eta Carinae is indicated.

Credits: NASA, ESA, UCB (N. Smith), STScI/AURA (The Hubble Heritage Team)

Integral covers a different, higher energy range in X-rays than that covered by XMM-Newton. This is why it was able to detect the more energetic X-rays emitted by Eta Carinae. Based on observations, scientists have learnt that the Eta Carinae system loses one Earth mass per day, which is roughly 140 times higher than the mass loss rate in HD 5980.

To have a rare, massive binary star such as Eta Carinae virtually at our cosmic doorstep at 8000 light-years, close enough to be observable in detail, is a stroke of luck. Now that they know what to look for, astronomers will continue searching for other examples of colliding wind binaries emitting high-energy X-rays further afield.


Notes:

‘Hard X-ray Emission from Eta Carinae’ by J-C. Leyder, R. Walter and G. Rauw has been accepted for publication in the journal Astronomy and Astrophysics.

For more information on XMM-Newton’s Colliding Wind Binary observation, see: First X-ray detection of a colliding-wind binary beyond the Milky Way


For more information:

Jean-Christophe Leyder, Institut d’Astrophysique et de Géophysique, Université de Liège, Belgium
Email: leyder @ astro.ulg.ac.be

Roland Walter, Integral Science Data Centre, Observatoire de Genève, Switzerland
Email : Roland.Walter @ obs.unige.ch

Christoph Winkler, ESA Integral Project Scientist
Christoph.Winkler @ esa.int


Source: ESA - Space Science - News