Abstract. The initial results from XMM-Newton observations of the rich cluster of galaxies Abell 1795 are presented. The spatially-resolved X-ray spectra taken by the European Photon Imaging Cameras (EPIC) show a temperature drop at a radius of ∼200 kpc from the cluster center, indicating that the ICM is cooling. Both the EPIC and the Reflection Grating Spectrometers (RGS) spectra extracted from the cluster center can be described by an isothermal model with a temperature of ∼4 keV. The volume emission measure of any cool component (< 1 keV) is less than a few % of the hot component at the cluster center. A strong O viii Lyman α line was detected with the RGS from the cluster core. The O abundance and its ratio to Fe at the cluster center is 0.2-0.5 and 0.5-1.5 times the solar value, respectively.

We present the results of an M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering $6100 arcmin 2 of M31. We detected 43 X-ray sources coincident with GC candidates from various optical surveys. The inferred isotropic X-ray luminosities of GC sources lie between $10 35 and $10 39 ergs s À1 in the 0.3-10 keV energy band. The spectral properties of the 31 brightest sources in our sample were found to be similar to those of the low-mass X-ray binaries (LMXBs) located in the bulge and the GCs of the Milky Way. The spectral distribution of the M31 GC X-ray sources is consistent with that derived for the bulge of M31 and other nearby galaxies of different morphological type. Several sources demonstrate a correlation between the level of X-ray flux and the hardness of their energy spectrum reminiscent of the Galactic Z and atoll sources. We found that $80% of the M31 GC sources with multiple flux measurements available show significant variability on timescales from days to years. The X-ray source RX J0043.2+4127, coincident with GC Bo 163, has been found to show recurrent transient outbursts with peak luminosities of $10 38 ergs s À1 . Several sources in our sample show significant variability on a timescale of individual observations, ranging from aperiodic fluctuations to regular dipping. 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. The luminosity distribution of M31 GC sources has $10 times higher peak luminosity and a much higher fraction of bright sources than the Milky Way GC distribution. Six persistent sources in our sample (or $14% of the total number) have luminosities exceeding 10 38 ergs s À1 during all observations, and three other sources occasionally exceed that luminosity level. Our observations indicate that GC sources make the 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 10 37 ergs s À1 reside in GCs, with the fraction of GC sources rising to 67%-90% for luminosities above 10 38 ergs s À1 . The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way but surprisingly close to that of early-type galaxies. We found that the brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that GCs hosting bright X-ray sources are optically brighter and more metal-rich than the rest of the M31 GCs, in agreement with previous studies. The brightest sources, with luminosities above $10 38 ergs s À1 , show a tendency to reside in more metal-poor clusters. The remarkable similarities between the properties of the M31 GC X-ray sources and those of Galactic neutron star LMXBs lead us to expect most of the persistent M31 GC X-ray sources to be LMXB systems with neutron star primaries. However, the current X-ray spectral and timing data c...

Using the data of 5 years of RXTE observations we investigate the X-ray spectral and timing properties of GRS 1915+105 during the hard steady states. The broad-band energy spectrum of the source during these periods is dominated by extended hard component with characteristic cut-off/ break at ∼ 10 − 120 keV. The power density spectrum of the source rapid aperiodic variability shows dominant band-limited white noise component breaking at few Hz accompanied by a group of strong QPO peaks and in some cases an additional high frequency noise component with characteristic cutoff at ∼ 60 − 80 Hz. According to the results of our simultaneous X-ray spectral and timing analysis the behavior of the source during the hard steady states can be reduced to a couple of major distinct types. i)Type I states: The dominant hard component of the energy spectrum has characteristic quasi-exponential cutoff at 50 − 120 keV. The broad-band power density spectrum of the source shows significant high frequency noise component with a cutoff at ∼ 60 − 80 Hz. ii)Type II states: The hard spectral component has a break in its slope at ∼ 12 − 20 keV. The high frequency part of the power density spectrum fades quickly lacking significant variability at frequencies higher than ∼ 30 Hz. These two types of the X-ray hard states are also clearly distinguished by their properties in the radio band: while during the type I observations the source tends to be 'radio-quiet', the type II observations are characterized by high level of radio flux ('plateau' radio states). In this work we demonstrate aforementioned differences using the data of 12 representative hard steady state observations. We conclude that the difference between these two types can be probably explained in terms of different structure of the accretion flow in the immediate vicinity of the compact object due to presence of relativistic outflow of matter.

Abstract. We present the results of a study of the variability of X-ray sources in the central 30 of the nearby Andromeda Galaxy (M 31) based on XMM-Newton 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 6×10 35 erg s −1 (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 our analysis of the high-energy radiation from black hole (BH) transients, using archival data obtained primarily with the Rossi X-ray Timing Explorer (RXTE), and a comprehensive test of the bulk motion Comptonization (BMC) model for the high-soft state continuum. The emergent spectra of over 30 separate measurements of GRO J1655-40, GRS 1915+105, GRS 1739 1630-47 XTE J1755-32, and EXO 1846-031 X-ray sources are successfully fitted by the BMC model, which has been derived from basic physical principles in previous work. This in turn provides direct physical insight into the innermost observable regions where matter impinging upon the event horizon can effectively be directly viewed. The BMC model is characterized by three parameters: the disk color temperature, a geometric factor related to the illumination of the black hole (BH) site by the disk and a spectral index related to the efficiency of the bulk motion upscattering. For the case of GRO J1655-40, where there are distance and mass determinations, a self consistency check of the BMC model has been made, in particular, the assumption regarding the dominance of gravitational forces over the pressure forces within the inner few Schwarzschild radii. We have also examined the time behavior of these parameters which can provide information on the source structure. Using our inferred model parameters: color temperature, spectral index and an absolute normalization we present new, independently derived, constraints on the black hole mass, mass accretion rate and the distance for the aforementioned sources. Also notable is the relationship between the color temperature and flux, which for GRO J1655-40 is entirely distinct from a simple T 4 dependence, and strikingly consistent with the disk model we have invoked -standard Shakura-Sunyaev's disk with the modification to the electron scattering. This -3provides insight into the origin of the seed soft photons, and allows us to impose an important estimation of the hardness parameter, T h , which is the ratio of the color temperature to the effective temperature -we find T h ≃ 2.6, higher than previous estimates used in the literature. Subject headings: accretion -black hole physics -binaries: close -radiation mechanisms: nonthermal -Compton and inverse Compton-relativitystars: individual (GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE J1755-32, and EXO 1846-031)

We present an analysis of the RXTE observations of the recently discovered Galactic microquasar XTE J1748−288 during its 1998 outburst. General spectral and temporal properties of the source and their evolution were very typical for the Galactic black hole candidates (BHC) and, in particular, black hole X‐ray novae. The spectral evolution of the source during the outburst can be considered a sequence of qualitatively distinct states. During the first observations, corresponding to a maximum of the X‐ray flux, the spectrum of the source consisted of a dominating hard power‐law component and a soft thermal component, which can be described by a model for the multicolour‐disc emission. The hard component contributed ≥80 per cent to the X‐ray luminosity in the 3–25 keV energy band. Overall the two‐component spectral shape is an attribute of the very high state observed previously in BHC, but the domination of the hard component is unusual. Later on, as the X‐ray source faded, its energy spectrum qualitatively changed, showing high and then low states, both typical for black hole binaries. As the energy spectrum changed, the fast variability also evolved dramatically. Initially the power‐density spectrum was formed by a dominating band‐limited noise component, quasi periodical oscillation features at 20–30 Hz and at ∼0.5 Hz, and a very low‐frequency noise component. After a significant decrease of the contribution of the hard spectral component, the amplitude of the fractional variability decreased by an order of magnitude and the power‐density spectrum adopted a power‐law shape with a broad quasi periodical oscillation peak around 0.03 Hz. When the system switched to the low state, the power‐density spectrum shape changed again and the quasi periodical oscillations have not been detected since. When the source was observed in the very high state, a clear correlation between quasi periodical oscillation parameters and X‐ray flux was seen. Such a correlation gives an insight into our understanding of the accretion process in X‐ray black hole binaries.

We present an analysis of the RXTE observations of 4U 1630-47 during its outburst of 1998. The light curve and the spectral evolution of the outburst were distinctly different from the outbursts of the same source in 1996 and in 1999. Special emphasis of our analysis was on the observations taken during the initial rise of the flux and during the maximum of the outburst. The maximum of the outburst was divided into three plateaus, with almost constant flux within each plateau, and fast jumps between them. The spectral and timing parameters are stable for each individual plateau, but distinctly different between the plateaus. The variability detected on the first plateau is of special interest. During these observations the source exhibits quasi-regular modulations with period of ~10 - 20 s. Our analysis revealed significant differences in spectral and temporal behavior of the source at high and low fluxes during this period of time. The source behavior can be generally explained in the framework of the two-phase model of the accretion flow, involving a hot inner comptonization region and surrounding optically thick disk. The variability and spectral evolution of the source were similar to what was observed earlier for other X-ray Novae. We show that 4U 1630-47 has a variety of properties which are typical for Galactic black hole binaries, both transient and persistent. We argue that this system may be an intermediate case between different groups of black hole candidates.Comment: 14 pages, 12 figures, submitted to MNRA

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