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[Waspie_Dwarf]

XMM-Newton gives new insight into neutron stars

8 January 2008

In a neutron star (left), the quarks that comprise the neutrons are confined inside the neutrons. In a quark star (right), the quarks are free, so they take up less space and the diameter of the star is smaller.

Credits: NASA/CXC/M.Weiss

XMM-Newton has given astronomers and physics a valuable new insight into the most exotic stars in the Universe. Known as neutron stars, the composition of these extremely dense stellar objects has always been something of a puzzle. Now, XMM-Newton has revealed that they almost certainly resemble over-sized atomic nuclei.

Natalie Webb and Didier Barret, Centre d’Etude Spatiale des Rayonnements, Toulouse, France, have used XMM-Newton’s EPIC camera to find three previously undiscovered neutron stars and accurately measure the quantity of various X-rays coming from the their surfaces. They were then able to compare their results with theoretical predictions to deduce the internal composition of the neutron stars. All three neutron stars lie in globular clusters of stars that orbit the centre of our galaxy.
Neutron stars are the exotic remains of exploded stars. Whilst most of a massive star’s outer layers are thrown off into space, the dead heart at the centre of the former star implodes. Astronomers call this small heart a neutron star and it has some amazing properties. Although it is the size of an asteroid, usually about 10–12 kilometres across, it contains more mass than our Sun, meaning its composition must be highly dense - so dense, in fact, that it cannot be made of normal atoms.

Initially astronomers believed that the neutron star was composed almost exclusively of neutrons, all squeezed together and resembling a giant atomic nucleus. Doubts crept in when observations started showing that some neutron stars appeared to have higher masses than expected, up to double the mass of the Sun, whilst others appeared to have even smaller radii of just 6-8 kilometres, so astronomers postulated exotic models containing uncommon particles such as pions, kaons or quarks.  

ESA's XMM-Newton is the most sensitive X-ray telescope ever built. Its high-technology design uses over 170 wafer-thin cylindrical mirrors spread over three telescopes.
Its orbit takes it almost a third of the way to the Moon, so that astronomers can enjoy long, uninterrupted views of celestial objects.

This unique X-ray observatory was launched by Ariane 5 from the European spaceport at Kourou in French Guiana on 10 December 1999. It derives its name from its X-ray multi-mirror design and honours Sir Isaac Newton.

Credits: ESA

Determining the mass and radius of a neutron star is a big challenge because they are such small objects they cannot be seen in detail. Instead, astronomers must collect the light coming from the neutron star and then use computer models to converge on a solution that reveals the neutron star’s size and mass.

“Knowing the distance to the neutron star accurately is crucial in this work,” says Webb. This is why the researchers looked for neutron stars in the spherical clusters of stars surrounding the Milky Way. Known as globular clusters, these well-studied objects have accurate distance estimates, which can be used for any star found in them. The team identified a likely neutron star in three different clusters: Omega Centauri, M13 and NGC 2808.

The neutron stars were all found to be orbiting other stars and emitting X-rays. These X-rays are expecting to pass through an atmosphere of hydrogen gas surrounding the neutron star. Webb and Barret compared their results to those produced by new theories of possible neutron star interiors. The computer codes that used these theoretical models were only released into the astronomical community by other astronomy groups about a year ago.

Webb and Barret’s new analysis shows that astronomers had previously been under-estimating the mass and over-estimating the radii of some neutron stars. They conclude that neutron stars can have masses up to 2.4 solar masses and radii that start from 8 km.

They discovered that despite all the musing over highly complicated particle interiors, still the most likely composition of a neutron star is what astronomers first suspected forty years ago: neutrons. They found only one exotic solution that remained feasible, an interior made of quarks. These particles are constituents of neutrons and would be able to squeeze together more densely.

They hope to extend the work to other neutron stars.


Note:

Constraining the Equation of State of Supranuclear Dense Matter from XMM-Newton Observations of Neutron Stars in Globular Clusters by Natalie A Webb and Didier Barret is published in The Astrophysical Journal (Volume 671, Issue 1, pp. 727-73)3.


For more information:

Natalie Webb, Centre d'Etude Spatiale des Rayonnements, France
Email: Natalie.Webb @ cesr.fr

Didier Barret, Centre d'Etude Spatiale des Rayonnements, France
Email: Didier.Barret@cesr.fr

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: Norbert.Schartel @ esa.int


Source: ESA - Space Science - News

[Waspie_Dwarf]

Cosmic engines surprise XMM-Newton

7 April 2008
XMM-Newton has been surprised by a rare type of galaxy, from which it has detected a higher number of X-rays than thought possible. The observation gives new insight into the powerful processes shaping galaxies during their formation and evolution.

Scientists working with XMM-Newton were looking into the furthest reaches of the universe, at celestial objects called quasars. These are vast cosmic engines that pump energy into their surroundings. It is thought an enormous black hole drives each quasar.

As matter falls into the black hole, it collects in a swirling reservoir called the accretion disc, which heats up. Computer simulations suggest that powerful radiation and magnetic fields present in the region eject some of gas from the gravitational clutches of the black hole, throwing it back into space.

This outflow has a profound effect on its surrounding galaxy. It can create turbulence in the gas throughout the galaxy, hampering star formation. Thus, understanding quasars is an important step to understanding the early history of galaxies.  

BAL quasar, top view

Credits: ESA (Image by C. Carreau)

However, the structure surrounding a quasar is difficult to see because they are so distant. The light and X-rays from them takes thousands of millions of years to reach us.

About 10-20% of quasars are of a special type called BAL quasars. The BAL stands for ‘broad absorption line’ and seems to indicate that a thick cocoon of gas surrounds the quasar.

Most researchers believe that gas flows away from a BAL quasar along the equatorial direction of the accretion disc. These quasars show little X-ray emission, indicating that there is enough gas to absorb most of the X-rays given out from the region near the black hole.

But some BAL quasars appear to be spewing material out along their polar axes, at right angles to the accretion discs.

JunXian Wang, Center for Astrophysics, University of Science and Technology of China, Hefei, and his colleagues including Tinggui Wang and Hongyan Zhou, used XMM-Newton to target four such polar BAL quasars, identified by them previously. They were investigating whether the X-rays were being absorbed strongly.

BAL quasar, side view

Credits: ESA (Image by C. Carreau)

XMM-Newton observed the quasars at specific times during 2006 and 2007. Two of them emitted more X-rays than the researchers anticipated, indicating that there is no veil of absorbing gas surrounding these particular quasars. “Our results can help refine the computer simulations of how these quasars work,” says Wang.


It may mean that BAL quasars are more complicated than originally thought. “Perhaps there can be both equatorial outflows and polar outflows simultaneously from these objects,” says Wang. Maybe, the outflows are even produced by similar means.

Computer simulations suggest that the polar outflows, like the gas ejected from the accretion disc, are also material falling in, turned away by fierce radiation before it comes near the black hole.


Wang and colleagues are now following this work up. They hope to monitor more BAL quasars over a longer period of time. “We need more data so that we can look into the details of the X-ray emission,” says Wang.

It seems that the more astronomers look into the distant universe, the more complex it becomes.


Notes:

The findings appear in ‘XMM observations of BAL Quasars with polar outflows’ by J. Wang, P. Jiang, H. Zhou, T. Wang, X. Dong, and H. Wang, published in the Astrophysical Journal Letters on `20 March 2008.


For more information:

Junxian Wang, Center for Astrophysics
University of Science and Technology of China, Hefei
Email: Jxw @ ustc.edu.cn

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: Norbert.Schartel @ esa.int


Source: ESA - Space Science - News

[Waspie_Dwarf]

XMM-Newton discovers part of missing matter in the universe

6 May 2008
ESA’s orbiting X-ray observatory XMM-Newton has been used by a team of international astronomers to uncover part of the missing matter in the universe.

10 years ago, scientists predicted that about half of the ‘ordinary’ or normal matter made of atoms exists in the form of low-density gas, filling vast spaces between galaxies.

All the matter in the universe is distributed in a web-like structure. At dense nodes of the cosmic web are clusters of galaxies, the largest objects in the universe. Astronomers suspected that the low-density gas permeates the filaments of the web.

The low density of the gas hampered many attempts to detect it in the past. With XMM-Newton’s high sensitivity, astronomers have discovered its hottest parts. The discovery will help them understand the evolution of the cosmic web.

Only about 5% of our universe is made of normal matter as we know it, consisting of protons and neutrons, or baryons, which along with electrons, form the building blocks of ordinary matter. The rest of our universe is composed of elusive dark matter (23%) and dark energy (72%)s.  

Composite optical and X-ray image of galaxy clusters Abell 222 and Abell 223. The cluster pair is connected by a filament permeated by hot X-ray emitting gas.

The optical image was obtained by SuprimeCam at the Subaru telescope, the X-ray image showing the distribution of the diffuse hot gas (yellow to red) was obtained by XMM-Newton.

Credits: ESA/ XMM-Newton/ EPIC/ ESO (J. Dietrich)/ SRON (N. Werner)/ MPE (A. Finoguenov)

Small as the percentage might be, half of the ordinary baryonic matter is unaccounted for. All the stars, galaxies and gas observable in the universe account for less than a half of all the baryons that should be around.

Scientists predicted that the gas would have a high temperature and so it would primarily emit low-energy X-rays. But its very low density made observation difficult.

Astronomers using XMM-Newton were observing a pair of galaxy clusters, Abell 222 and Abell 223, situated at a distance of 2300 million light-years from Earth, when the images and spectra of the system revealed a bridge of hot gas connecting the clusters.

"The hot gas that we see in this bridge or filament is probably the hottest and densest part of the diffuse gas in the cosmic web, believed to constitute about half the baryonic matter in the universe," says Norbert Werner from SRON Netherlands Institute for Space Research, leader of the team reporting the discovery.

This is a model of the cosmic web. Clusters of galaxies are expected to develop at the intersections of the web.

Credits: Springel et al., Virgo Consortium

“The discovery of the warmest of the missing baryons is important. That’s because various models exist and they all predict that the missing baryons are some form of warm gas, but the models tend to disagree about the extremes,” adds Alexis Finoguenov, a team member.

Even with XMM-Newton’s sensitivity, the discovery was only possible because the filament is along the line of sight, concentrating the emission from the entire filament in a small region of the sky. The discovery of this hot gas will help better understand the evolution of the cosmic web.

"This is only the beginning. To understand the distribution of the matter within the cosmic web, we have to see more systems like this one. And ultimately launch a dedicated space observatory to observe the cosmic web with a much higher sensitivity than possible with current missions. Our result allows to set up reliable requirements for those new missions." concludes Norbert Werner.


ESA’s XMM-Newton Project Scientist, Norbert Schartel, comments on the discovery, “This important breakthrough is great news for the mission. The gas has been detected after hard work and more importantly, we now know where to look for it. I expect many follow-up studies with XMM-Newton in the future targeting such highly promising regions in the sky.”


For more information:

Norbert Werner, SRON Netherlands Institute for Space Research
Email: N.Werner @ sron.nl

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: Norbert.Schartel @ esa.int


Notes:

The article 'Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223', by N. Werner, A. Finoguenov, J. Kaastra, A. Simionescu, J. Dietrich, J. Vink and H. Böhringer' has been published in the Astronomy & Astrophysics Letters on 17 March 2008.

The team of astronomers that made the discovery includes N. Werner (SRON, Netherlands Institute for Space Research), A. Finoguenov (MPE, Germany), A. Simionescu, H. Böhringer (MPE, Germany), J. Kaastra (SRON, Netherlands Institute for Space Research and Utrecht University, Netherlands), J. Dietrich (ESO, Germany) and J. Vink (Utrecht University, Netherlands).


Source: ESA - Space Science - News