The central region of M31 observed with XMM-Newton. II. Variability of the individual sources

We present the results of a study of the variability of X-ray sources in the central 30 of the nearby Andromeda Galaxy (M31) based on XMM Performance Verification observations. Two observations of this field, with a total exposure time of about 50 ks, were performed in June and December of 2000. We found 116 sources brighter than a limiting luminosity of 6 x 10(35) erg/s (0.3--12 keV, d=760 kpc). For the ~60 brightest sources, we searched for periodic and non-periodic variability; at least 15% of these sources appear to be variable on a time scale of several months. We discovered a new bright transient source ~2.9 from the nucleus in the June observation; this source faded significantly and was no longer detected in December. The behaviour of the object is similar to a handful of Galactic LMXB transients, most of which are supposed to harbor black holes. We detected pulsations with a period of ~865 s from a source with a supersoft spectrum. The flux of this source decreased significantly between the two XMM observations. The detected period is unusually short and points to a rapidly spinning magnetized white dwarf. The high luminosity and transient nature of the source suggest its possible identification with classical or symbiotic nova, some of which were observed earlier as supersoft sources.

XMM-Newton and Chandra Observations of the Central Region of M31

The archival XMM-Newton data of the central region of M31 were analyzed for diffuse X-ray emission. Point sources with the 0.5--10 keV luminosity exceeding $sim 4 times 10^{35}$ erg s$^{-1}$ were detected. Their summed spectra are well reproduced by a combination of a disk black-body component and a black-body component, implying that the emission mainly comes from an assembly of luminous low-mass X-ray binaries. After excluding these point sources, spectra were accumulated over a circular region of $6arcmin$ (1.2 kpc) centered on the nucleus. In the energy range above 2 keV, these residual spectra are understood mainly as contributions of unresolved faint sources and spill-over of photons from the excluded point sources. There is in addition a hint of a $sim 6.6$ keV line emission, which can be produced by a hot (temperature several keV) thin-thermal plasma. Below 2 keV, the spectra involve three additional softer components expressed by thin-thermal plasma emission models, of which the temperatures are $sim 0.6$, $sim 0.3$, and $sim 0.1$ keV. Their 0.5--10 keV luminosities within 6$arcmin$ are measured to be $sim 1.2 times 10^{38}$ erg s$^{-1}$, $sim 1.6 times 10^{38}$ erg s$^{-1}$, and $sim 4 times 10^{37}$ erg s$^{-1}$ in the order of decreasing temperature. The archival Chandra data of the central region of M31 yielded consistent results. By incorporating different annular regions, all the three softer thermal components were confirmed to be significantly extended. These results are compared with reports from previous studies. A discussion is presented on the origin of each thermal emission component.

XMM-Newton observations of the M31 northern disk: Properties of selected X-ray sources and diffuse emission

We present the results of XMM-Newton survey of the northern part of the disk of M31. The results of a spectral and timing analysis of the thirty seven brightest sources are presented. Combining the results of X-ray analysis with available data at other wavelengths, we were able to classify ~50%, or 19 out of 37 sources. Two sources in our sample were previously unknown: the hard X-ray source XMMU J004415.8+413057 and a transient supersoft source XMMUJ004414.1+412206. We report the discovery of possible X-ray pulsations from the source XMMUJ004415.8+413057 with a period of 197 s. The spectral and timing properties of XMMU J004415.8+413057 make it first accreting X-ray pulsar candidate detected in M31. We report on the first unambiguous detection of the soft unresolved X-ray emission from the disk of M31. The unresolved emission follows the pattern of the spiral arms and can be traced up to distance of ~0.5 deg (~7 kpc at 760 kpc) from the center of the galaxy. The spectrum of the unresolved emission shows dominant soft thermal component which can be fitted with a ~0.3 keV optically thin thermal plasma emission models. We suggest that significant part of this diffuse soft X-ray component may represent hot diffuse gas in the spiral arms of M31 and emission from normal stars in the disk of M31.

XMM-Newton survey of the central region of M31. Spectral properties and variability of bright X-ray sources and source classification

(Abridged) We present the results of the systematic survey of X-ray sources in the central region of M31 using the data from XMM-Newton observations performed in the years 2000-2004. The spectral properties and variability of 123 bright X-ray sources with apparent luminosities between ~10^{36} and ~5x10^{38} ergs/s were studied in detail. The spectral distribution of M31 X-ray sources, based on the spectral fitting with a power law model is clearly bimodal with a main peak corresponding to a photon index Gamma ~ 1.75 and a shoulder at Gamma ~ 2.0-2.2 extending to the soft spectral region, and shows clear evolution with source luminosity. The distribution of absorbing columns towards M31 sources derived from spectral analysis has a peak at N_H~1.2x10^{21} cm^{-2} extending up to 1.3x10^{22} cm^{-2}, with an average value of (1.52 +/- 0.02)x10^{21} cm^{-2}. More than 80% of sources observed in two or more observations show significant variability on the time scales of days to years. About 50% of the sources in our sample are spectrally variable. The spectral evolution of a number of sources is correlated with the level of their X-ray flux, while some sources demonstrate complex patterns of evolution on the hardness-intensity diagram. Based on the similarity of the properties of M31 X-ray sources and their Galactic counterparts, we expect most of the X-ray sources in our sample to be accreting binary systems with neutron star and black hole primaries. A total of 44 X-ray sources can be identified as probable X-ray binaries. Combining the results of X-ray analysis with available data at other wavelengths, we classify 7% and 24% of sources in our sample as, respectively, probable black hole and neutron star candidates.

The detection of diffuse emission in HCG 16 with XMM-Newton

We report results obtained from analysis of the XMM-Newton observation of the compact group of galaxies HCG 16. It is a peculiar system composed of 7 spirals, 6 of which are active, and its nature as a bound system has been much debated. The EPIC camera observations give new insights into the X-ray parameters describing the physical status of the group. We detect diffuse X-ray emission with a rather elliptical morphology which extends to at least a radius of 135 h^{-1}_{50} kpc from the group centre. The spectrum within this region is well modelled by a thermal plasma with a temperature of 0.49+/-0.17 keV, and a non-zero metallicity. We measure a bolometric X-ray luminosity of 9.6 10{^40} h^{-2}_{50} erg/s which may be only a small fraction of the total luminosity because of the limit in spatial detection arising from the high background level. Despite its low temperature and luminosity, HCG 16 obeys the Lx-T relation obtained for brighter galaxy groups even if it lies in a very extreme position. The properties of the diffuse emission confirm the bound nature of HCG 16 even if the gas trapped in the potential well may not yet be virialized. This reopens the debate about the real nature of spiral-dominated galaxy groups, and on their role in a more general cosmological context.