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Register a new userXMM-Newton survey of the central region of M31. Spectral properties and variability of bright X-ray sources and source classification
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Authors
Sergey Trudolyubov
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(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.
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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.
We present the results of the ongoing XMM-Newton survey of M31. 17 X-ray sources detected in the survey have bright radio counterparts, and 15 X-ray sources coincide with SNR candidates from optical and radio surveys. 15 out of 17 sources with radio counterparts, not SNR candidates, have spectral properties similar to that observed for background radio galaxies/quasars or Crab-like supernova remnants located in M31. The remaining two sources, XMMU J004046.8+405525 and XMMU J004249.1+412407, have soft X-ray spectra, and are associated with spatially resolved H-alpha emission regions, which makes them two new SNR candidates in M31. The observed absorbed X-ray luminosities of SNR candidates in our sample range from 1e35 to 5e36 ergs/s, assuming the distance of 760 kpc. Most of the SNR candidates detected in our survey have soft X-ray spectra. The spectra of the brightest sources show presence of emission lines and can be fit by thermal plasma models with kT~0.1-0.4 keV. The results of spectral fitting of SNR candidates suggest that most of them should be located in a relatively low density regions. We show that X-ray color-color diagrams can be useful tool for distinguishing between intrinsically hard background radio sources and Crab-like SNR and thermal SNR in M31 with soft spectra.
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.
We present here a detailed X-ray spectral analysis of the AGN belonging to the XMM-Newton bright survey (XBS) that comprises more than 300 AGN up to redshift ~ 2.4. We performed an X-ray analysis following two different approaches: by analyzing individually each AGN X-ray spectrum and by constructing average spectra for different AGN types. From the individual analysis, we find that there seems to be an anti correlation between the spectral index and the sources hard X-ray luminosity, such that the average photon index for the higher luminosity sources (> 10E44 erg/s) is significantly flatter than the average for the lower luminosity sources. We also find that the intrinsic column density distribution agrees with AGN unified schemes, although a number of exceptions are found (3% of the whole sample), which are much more common among optically classified type 2 AGN. We also find that the so-called soft-excess, apart from the intrinsic absorption, constitutes the principal deviation from a power-law shape in AGN X-ray spectra and it clearly displays different characteristics, and likely a different origin, for unabsorbed and absorbed AGN. Regarding the shape of the average spectra, we find that it is best reproduced by a combination of an unabsorbed (absorbed) power law, a narrow Fe Kalpha emission line and a small (large) amount of reflection for unabsorbed (absorbed) sources. We do not significantly detect any relativistic contribution to the line emission and we compute an upper limit for its equivalent width (EW) of 230 eV at the 3 sigma confidence level. Finally, by dividing the type 1 AGN sample into high- and low-luminosity sources, we marginally detect a decrease in the narrow Fe Kalpha line EW and in the amount of reflection as the luminosity increases, the so-called Iwasawa-Taniguchi effect.
Although absorbed quasars are extremely important for our understanding of the energetics of the Universe, the main physical parameters of their central engines are still poorly known. In this work we present and study a complete sample of 14 quasars (QSOs) that are absorbed in the X-rays (column density NH>4x10^21 cm-2 and X-ray luminosity L(2-10 keV)>10^44 ergs/s; XQSO2) belonging to the XMM-Newton Bright Serendipitous Survey (XBS). From the analysis of their ultraviolet-to-mid-infrared spectral energy distribution we can separate the nuclear emission from the host galaxy contribution, obtaining a measurement of the fundamental nuclear parameters, like the mass of the central supermassive black hole and the value of Eddington ratio, lambda_Edd. Comparing the properties of XQSO2s with those previously obtained for the X-ray unabsorbed QSOs in the XBS, we do not find any evidence that the two samples are drawn from different populations. In particular, the two samples span the same range in Eddington ratios, up to lambda_Edd=0.5; this implies that our XQSO2s populate the forbidden region in the so-called effective Eddington limit paradigm. A combination of low grain abundance, presence of stars inwards of the absorber, and/or anisotropy of the disk emission, can explain this result.
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