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Register a new userXMM-Newton and Chandra Observations of the Central Region of M31
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Added by
Hiromitsu Takahashi
Publication date
2004
fields
Physics
and research's language is
English
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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.
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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.
(Abridged) We present the results of M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 sq.arcmin of M31. We detected 43 X-ray sources coincident with globular cluster candidates from optical surveys. The estimated isotropic X-ray luminosities of GC sources lie between ~10e35 and ~10e39 erg/s in the 0.3 - 10 keV energy band. The spectral properties and variability of M31 GC X-ray sources are consistent with that derived for the LMXBs in the bulges of M31 and Milky Way. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on a time scales from days to years. The X-ray luminosity function of GC sources is found to be significantly different from that of the point sources in the bulge and disk of M31 and that of the Galactic GC X-ray sources. GC sources make dominant contribution to the bright source counts in the areas of M31 covered by the survey: ~40% of the total number of sources with luminosities above 10e37 ergs/s reside in GCs with fraction of GC sources rising to 67-90% for the luminosities above 10e38 ergs/s. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way galaxy, but surprisingly close to that of the early-type galaxies. The brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that globular clusters hosting bright X-ray sources are optically brighter and more metal rich than the rest of M31 globular clusters. The brightest sources with luminosities above ~10e38 ergs/s show tendency to reside in more metal poor clusters.
We present the results of a {it Chandra} observation of the central region of Abell 3112. This cluster has a powerful radio source in the center and was believed to have a strong cooling flow. The X-ray image shows that the intracluster medium (ICM) is distributed smoothly on large scales, but has significant deviations from a simple concentric elliptical isophotal model near the center. Regions of excess emission appear to surround two lobe-like radio-emitting regions. This structure probably indicates that hot X-ray gas and radio lobes are interacting. From an analysis of the X-ray spectra in annuli, we found clear evidence for a temperature decrease and abundance increase toward the center. The X-ray spectrum of the central region is consistent with a single-temperature thermal plasma model. The contribution of X-ray emission from a multiphase cooling flow component with gas cooling to very low temperatures locally is limited to less than 10% of the total emission. However, the whole cluster spectrum indicates that the ICM is cooling significantly as a whole, but in only a limited temperature range ($geq 2$ keV). Inside the cooling radius, the conduction timescales based on the Spitzer conductivity are shorter than the cooling timescales. We detect an X-ray point source in the cluster center which is coincident with the optical nucleus of the central cD galaxy and the core of the associated radio source. The X-ray spectrum of the central point source can be fit by a 1.3 keV thermal plasma and a power-law component whose photon index is 1.9. The thermal component is probably plasma associated with the cD galaxy. We attribute the power-law component to the central AGN.
(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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