XMM-Newton discovers truly diffuse emission in M31
14 December 2000The Andromeda galaxy (M31), only 2.6 million light years away, is an ideal field of study for X-ray astronomy. XMM-Newton has observed its galactic centre, revealing many new point sources and the probable presence of a very hot diffuse gas which contributes to the overal X-ray luminosity.
Sources in M31 can be observed at a nearly uniform distance and because the galaxy is inclined, there is less intervening matter and the sources are less obscured than those in the Galactic plane. Andromeda is in many respects similar to our own Milky Way.
Observations of this region using the EPIC-MOS and EPIC-pn cameras have detected more than a hundred discrete point sources, more than two dozen of which had never been seen but which have been registered due to XMM-Newton's improved sensitivity.
"XMM-Newton has allowed us to go further than just detecting sources in M31" says Dr. Robert Shirey of the University of California. " We are able to study the individual properties of many of the X-ray binary systems. The differences in the EPIC spectra of the "reddest" and "bluest" sources we detect are striking."
Amongst the binary systems is one transient source with a neutron star. Others may hide black holes. Another object is a transient source which is still active, eight months after being first detected by NASA's Chandra X-ray observatory. Periodic oscillations are revealed in yet another source detected by XMM-Newton. (These will be the subject of a paper by Julian Osborne of the University of Leicester, UK, which will focus on the properties of these fascinating binary X-ray sources).
During its nearly 10 hour-long exposure, XMM-Newton's observation of Andromeda has also provided new insights into the galaxy's central region and particularly its so-far unresolved soft-X ray emission. With its improved spatial resolution and much greater X-ray collecting area, XMM-Newton shows that part of this emission is featureless. Its spectra do not reveal emission lines, and the emission clearly corresponds to even more unseen point sources, probably low-mass X-ray binary stars.
But 10-20% of this X-ray glow is of a totally different nature. XMM-Newton has for the first time identified the component of a truly diffuse emission, containing multiple emission lines highlighting the presence of various chemical elements. This suggests and corresponds to models of a thin, extremely hot plasma - in excess of a million degrees - a kind of interstellar broth that is quite distinct from the typical emission spectra of point sources.
"It is likely that much of this hot gas is produced in supernova explosions" says Shirey. "By measuring how much of this gas is present, we should be able to estimate the rate of supernovae in M31."
The significance of the presence of this hot plasma goes far beyond the case of Andromeda, as this galaxy's central region is often considered as the prototype for the population of early-type X-ray galaxies.
Two more XMM-Newton observations of Andromeda's central region are scheduled as part of the Guaranteed-Time programme, as are five other fields along its disk. It is expected they will give even finer insights into its population of X-ray sources and its interstellar soup.
Acknowledgements to the first author of the paper to be published in Astronomy and Astrophysics Letters:
Robert E.Shirey (Dept.of Physics, University of California US) - "The central region of M31 observed with XMM-Newton: Group Properties and Diffuse Emission "