* Astronomy

La collaboration H.E.S.S. (High Energy Stereoscopic System), une équipe internationale essentiellement européenne d'astrophysiciens, a découvert une émission gamma aux très hautes énergies (THE; E>100 GeV) en provenance de Centaurus A, une des galaxies actives les plus proches, et le plus proche émetteur radio extragalactique puissant. Cette découverte démontre la présence d'accélération de particules à de telles énergies dans cette source et suggère que l'émission gamma aux très hautes énergies est un phénomène commun des noyaux actifs de galaxies.

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Astronomers studying a nearby galaxy with NASA's Chandra X-ray Observatory have detected a rare type of star system with a black hole that has begun glowing with a new X-ray source.
Usually, when astronomers study the galaxy, called Centaurus A, it's the giant X-ray jets emanating from its heart that steal the show, according to Gregory Sivakoff of Ohio State University.

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Astronomers studying a nearby galaxy have spied a rare type of star system -- one that contains a black hole that suddenly began glowing brightly with X-rays.
Though this type of star system is supposed to be rare, its the second such system discovered in that galaxy, called Centaurus A.
The discovery suggests that astronomers have more to learn about the lives and deaths of massive stars in galaxies such as our own.
Normally when astronomers study Centaurus A, its the giant X-ray jets emanating from the heart of the galaxy that steal the show. The jets extend from the galaxy for 13,000 light years in different directions.
But when his team studied Centaurus A with NASAs Chandra X-ray Observatory starting in March 2007, they saw a new X-ray source -- much smaller than the X-ray jets, but still glowing brightly. The source wasnt there during the last survey of the galaxy in 2003, but it shined throughout the time of the new observations, from March to May of 2007.
Because it hadnt been seen before, the astronomers classified the object as a transient X-ray source, meaning that the object had been there before 2007, but had only recently brightened enough to stand out.
Gregory Sivakoff, a postdoctoral researcher in astronomy at Ohio State University. discussed the results in a press briefing Wednesday, January 9, 2008 at the American Astronomical Society meeting in Austin, Texas.

The newly bright object, dubbed CXOU J132518.2-430304, is most likely a binary star system, the researchers concluded. The two stars likely formed at the same time, with one much more massive than the other. The more massive star evolved more quickly, and collapsed to form a black hole. It is now slowly devouring its companion. Such binary systems are thought to be extremely rare.
But this is the second bright, transient X-ray binary system discovered in Centaurus A -- and thats the problem, Sivakoff said.

When we look at other galaxies like Centaurus A, we dont see these bright, transient X-ray binaries. But now weve found two such objects in Centaurus A, and the implication is that we may not understand these objects as well as we thought we did. So right now, our discovery is actually pointing to a puzzle rather than a solution - Gregory Sivakoff.

Because Centaurus A is near to our galaxy, astronomers have long hoped to use it as a Rosetta stone for studying other galaxies with black holes.
As astronomers piece together an explanation for the existence of the newly-discovered binary system, they may gain a better understanding of how black holes form from massive stars and how binary systems evolve.

These binary systems are signposts of the massive stars that once existed in galaxies like Centaurus A. To understand the massive stars, we must first know how to read the signs - Gregory Sivakoff.

Sivakoff and Ralph Kraft of the Harvard-Smithsonian Centre for Astrophysics led the study; their collaborators were from NASA Goddard Space Flight Centre, Oak Ridge Associated Universities, University of Hertfordshire, University of Virginia, University of Bristol, McMaster University, and the University of Birmingham.

Source

Title: The Centaurus Group and the Outer Halo of NGC 5128: Are they Dynamically Connected?
Authors: Kristin A. Woodley

NGC 5128, a giant elliptical galaxy only ~ 4 Mpc away, is the dominant member of a galaxy group of over 80 probable members. The Centaurus group provides an excellent sample for a kinematic comparison between the halo of NGC 5128 and its surrounding satellite galaxies. A new study, presented here, shows no kinematic difference in rotation amplitude, rotation axis, and velocity dispersion between the halo of NGC 5128, determined from over ~340 of its globular clusters, and those of the Centaurus group as a whole. These results suggest NGC 5128 could be behaving in part as the inner component to the galaxy group, and could have begun as a large initial seed galaxy, gradually built up by minor mergers and satellite accretions, consistent with simple cold dark matter models. The mass and mass-to-light ratios in the B-band, corrected for projection effects, are determined to be (1.3 ±0.5) X 10^12 solar masses and 52 ±22 solar masses/solar luminosity for NGC 5128 out to a galactocentric radius of 45 kpc, and (9.2 ±3.0) x 10^12 solar masses and 153 ±50 solar masses/solar luminosity for the Centaurus group, consistent with previous studies.

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