R. Supper

The second ROSAT PSPC survey of M 31 and the complete ROSAT PSPC source list

This paper reports the results of the analysis of the second ROSAT PSPC survey of M 31 performed in summer 1992. We compare our results with those of the rst survey, already published in Supper et al. (1997). Within the10: 7d eg 2 eld of view, 396 individual X-ray sources are detected in the second survey data, of which 164 are new detections. When combined with the rst survey, this result in a total of 560 X-ray sources in the eld of M 31. Their (0.1 keV{2.0 keV) fluxes range from 7 10 15 erg cm 2 s 1 to 7:6 10 12 erg cm 2 s 1 , and of these 560 sources, 55 are tentatively identied with foreground stars, 33 with globular clusters, 16 with supernova remnants, and 10 with radio sources and galaxies (including M 32). A comparison with the results of the Einstein M 31 survey reveals 491 newly detected sources, 11 long term variable sources, and 7 possible transient sources. Comparing the two ROSAT surveys, we come up with 34 long term variable sources and 8 transient candidates. For the M 31 sources, the observed luminosities range from 4 10 35 erg s 1 to 4 10 38 erg s 1 . The total (0.1 keV{2.0 keV) luminosity of M 31 is (3:4 0:3) 10 39 erg s 1 , distributed approximately equally between the bulge and disk. Within the bulge region, the luminosity of a possible diuse component combined with faint sources below the detection threshold is (2:0 0:5) 10 38 erg s 1 . An explanation in terms of hot gaseous emission leads to a maximum total gas mass of (1:0 0:3) 10 6 M.

ROSAT PSPC observation of M31

This article reports on the analysis of the first M31 survey with the ROSAT PSPC performed in July 1991. The spectral characteristics of the 396 individual x‐ray sources detected will be discussed as well as their positional correlation with the 108 individual x‐ray sources detected by the Einstein observatory. The optical identifications of the ROSAT x‐ray sources are the result of a MIT‐MPE‐UvA collaboration. A detailed analysis of the integral flux distribution shows that just two thirds of the detected x‐ray sources in the field of M31 must be background objects. The discovery of a significant absorption of background radiation is discussed, as well as an upper limit for the luminosity of a diffuse emission component. Finally a comparison of the luminosity function of 29 globular clusters sources with the one in our own galaxy reveals a good agreement.

Globular Cluster X-ray Sources in M31

The giant spiral galaxy M31 is close enough for a detailed study of accreting X-ray sources with luminosities comparable to those in the Milky Way. With the Einstein observatory Trinchieri’ and Fabbiano discovered 108 individual point sources within N 100 ksec IPC and N 200 ksec HRI observations, which had a limiting sensitivity of approximately lo3’ erg/s. Long2 and van Speybroeck detected 117 sources within these observations. A search for optical counterparts of these 117 sources by Crampton3 et al. revealed 23 candidates for X-ray G C sources. The identifications were done by spatial correlation of the X-ray sources with G C lists of Hodge4, Sargent’ e t al., Battistini‘ et al., and van den Bergh’. Among these 23 possible G C sources 19 were considered as being extremely strong candidates. From these results Battistinis e t al. calculated a luminosity distribution of the X-ray G C sources in M31 and compared it to that one of the Milky Way. They found a significant difference between both luminosity distributions and raised the question whether the G C sources in M31 are more luminous and/or more numerous compared with those in the Milky Way. Because these distributions are a function of X-ray G C sources as well as a function of optical GC sources, new X-ray and optical observations were expected t o reveal more detailed results about the discrepancy of both luminosity distributions. Within two ROSAT PSPC deep surveys of M31 563 individual X-ray sources have been discovered. The observations add up t o N 400 ksec of total exposure with a limited sensitivity of N erg/s. Within these observations 38 X-ray G C sources have been identified by spatial correlations t o optical GC lists of Battistini6’’ e t al. and Magnier”. The last one is the result of a new deep optical survey of M31. In the following the spectral properties of the brightest of these 38 X-ray G C sources will be discussed showing typical spectra for low mass X-ray binaries (LMXB). A new luminosity distribution will be presented and also a comparison t o that one of the Milky Way, revealing that both distributions are comparable.