Physics – Optics
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996asu..rept.....c&link_type=abstract
Technical Report, Arizona State Univ. Tempe, AZ United States
Physics
Optics
X Ray Binaries, X Ray Sources, X Ray Optics, Galaxies, Galactic Clusters, Magellanic Clouds, Rosat Mission, Black Body Radiation, Brightness Temperature, Cooling, Coronas, Low Temperature, Lyra Constellation, Milky Way Galaxy, Optical Properties, Ratios
Scientific paper
This ROSAT project was based on the idea that accurate X-ray positions were needed to optically identify and hence further study the already known X-ray source population in the Magellanic Clouds. ROSAT-HRI images were obtained for a large number of previously unidentified X-ray sources in these nearby galaxies. Based on these data, we were then able to find the optical counterparts and further study both their X-ray and optical properties. One of the surprising results of the project was the high level of variability among these stellar sources. Over 50% of the sources were found to be variable, some by large factors. In fact, more than two dozen Einstein sources were not detected at all, presumably because they had dropped below the detection limit of our ROSAT observation. However, an equal number of serendipitous sources were found which should have been previously detected in other surveys, if they were not variable. Another new result is that only a small fraction of the X-ray sources in the direction of the Magellanic Clouds are actually members of these galaxies. A large percentage of the sources seen projected against these galaxies are either foreground coronal sources or background AGN. We even found two cases of background galaxy-cluster cooling flows, with one of the clusters not previously known! (Presumably searches for clusters of galaxies have avoided the crowded regions around the Magellanic Clouds). Within the LMC and SMC themselves, the very luminous massive X-ray binaries are represented in about the numbers expected, based on a comparison with the Milky Way. However, the low-mass X-ray binaries (LMXB) which are so common in our own Galaxy (with over 150 known) are hardly present. Only two such systems are found. Since we know the LMXB in the Galaxy represent a very old stellar population, the suggestion is that this old population is missing in the much younger Magellanic Clouds. However, this is not a completely satisfactory solution, since old stars such as RR Lyrae are well known in the LMC. ROSAT observations have lead to the discovery of a whole new class of objects known as the 'supersoft X-ray sources', which are recognized by their very low blackbody temperatures (kT approx.30-60 eV). These are readily found in the Magellanic Clouds because of the low column density of hydrogen in the direction of these galaxies. We have used our ROSAT data to aid in the identification and understanding of the physical properties of several of these interesting new systems. A large data paper containing information on all of the ROSAT positions, fluxes, source variability, optical identification, and discussion of each of the individual sources is nearing completion and should be submitted for publication within the next month.
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