Observations of M 31

As part of the Chandra GTO program we are monitoring and surveying M31 using the HRC and ACIS cameras. These observations have resolved the nuclear X-ray source into five separate sources, one of which is very soft and may (or may not!) be associated with the central super-massive black hole. In addition, the superb spatial resolution and low scattering of the Chandra telescope allows us to unambiguously resolve the diffuse emission from the point sources. This emission is clearly softer than the point sources, and also increases with temperature radially. The monitoring nature of the observations allows detailed study of the variability of the point sources. Introduction: M31 has been imaged with Chandra monthly as part of the AO1 GTO program. Each month five HRC observations provide a complete image of the galaxy to a sensitivity of 10 37 erg s −1. Transient sources are re-observed with the ACIS camera, typically yielding ∼ 100 counts per source. Thus the known (Galactic) types of accretion powered x-ray transients are being detected, their spectra measured, and their lightcurves studied. In the first observation we discovered a bright transient 26 ′′ to the west of the nucleus (Murray et al 1999), which may be associated with a stellar mass black hole. The next 8 ACIS images were centered on the nucleus, to follow the evolution of this transient. More recently a second bright transient was discovered in the HRC images which included M32 (Garcia et al 2000a). The most recent ACIS imaging has therefore been centered on this transient, which is likely in M32. During AO2 the GTO monitoring program will be cut in ∼ 1/2, but there are two GO programs which also will image M31. A program lead by R. Di Ste-fano will use the ACIS to repeatedly image the area surrounding three super-soft sources, and a program lead by P. Kaaret will use the HRC to study the variability of sources in the nuclear region. These studies (and others likely to come) should allow much of what has been considered 'galactic X-ray astronomy' to be carried out in our nearest neighbor galaxy, M31. The Nucleus: The central object seen with the ROSAT HRI is clearly resolved into 5 sources by Chandra (Figure 1). The celestial location of these sources can be determined to ∼ 1 ′′ based on the Chandra aspect solution alone. To verify the Chandra coordinates we registered …