Supersoft X-ray sources in M31: II. ROSAT-detected supersoft sources in the ROSAT, Chandra and XMM eras

摘要

We have performed Chandra observations during the past 3 years of 5 of theM31 supersoft X-ray sources (SSS) discovered with ROSAT. Surprisingly, only oneof these sources has been detected, despite a predicted detection of about20-80 counts. This has motivated a thorough check of the ROSAT M31 survey Idata, including a relaxation of the hardness ratio requirement used to selectSSS. This increases the number of SSS identified in survey I by 7. We thencarried out a comparison with the ROSAT M31 survey II dataset which hadhitherto not been explicitly investigated for SSS. We find that most of theROSAT survey I sources are not detected, and only two new SSS are identified.The low detection rate in the ROSAT survey II and our Chandra observationsimplies that the variability time scale of SSS is a few months. If the majorityof these sources are close-binary SSS with shell hydrogen burning, it furtherimplies that half of these sources predominantly experience large mass transferrates.