arXiv subjects

M. Gilfanov

Publications and source records attributed to M. Gilfanov.

At least 91 records · Page 5Linked to original sources

arXiv2004

A Hard X-ray survey of the Galactic Center with INTEGRAL/IBIS. Catalog of sources

During the period Aug.23-Sept.24 2003, the INTEGRAL observatory performed an ultra deep survey of the Galactic Center region with a record sensitivity at energies higher than 20 keV. We have analized images of the Galactic Center region obtained with the ISGRI detector of the IBIS telescope (15-200 keV) and present here a catalog of detected sources. In total, 60 sources with a flux higher than 1.5 mCrab have been detected. 44 of them were earlier identified as Galactic binary systems, 3 are extragalactic objects. 2 new sources are discovered.

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arXiv2003

Doppler broadening of the iron line and R-Gamma correlation in black hole binaries

RXTE/PCA observations of several black hole X-ray binaries in the low spectral state revealed a tight correlation between spectral parameters and characteristic frequencies of variability of X-ray flux. In particular, the amplitude of reflection increases and the slope of Comptonized radiation steepens as the noise frequencies increase. The data also suggest that there is a correlated increase of the width of the Fe fluorescent line, probably related to the Doppler broadening of the reflection features. Moreover, the width of the line seems to follow DeltaE/E ~ nu_qpo^{1/3} law. If confirmed with higher energy resolution observations, this result will have significant impact on the models of the accretion flow and on our understanding of the nature of characteristic frequencies of variability in X-ray binaries. In particular, it will lend support to the truncated disk picture, confirming that the spectral variations are indeed related to changes of the position of the inner disk boundary and that characteristic variability frequencies are proportional to the Keplerian frequency at the inner boundary of the accretion disk.

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arXiv2003

Low mass X-ray binaries as a stellar mass indicator for the host galaxy

Using results of Chandra observations of old stellar systems in eleven nearby galaxies of various morphological types and the census of LMXBs in the Milky Way, we study the population of low mass X-ray binaries and their relation to the mass of the host galaxy. We show that the azimuthally averaged spatial distribution of the number of LMXBs and, in the majority of cases, of their collective luminosity closely follows that of the near-infrared light. Considering galaxies as a whole, we find that in a broad mass range, log(M)~9-11.5, the total number of LMXBs and their combined luminosity are proportional to the stellar mass of the host galaxy. Within the accuracy of the light-to-mass conversion, we cannot rule out the possibility of a weak dependence of the X/M ratio on morphological type. However, the effect of such a dependence, if any, does not exceed a factor of ~1.5-2. The luminosity distributions of LMXBs observed in different galaxies are similar to each other and, with the possible exception of NGC1553, are consistent with the average luminosity function derived from all data. The average XLF of LMXBs in nearby galaxies has a complex shape and is significantly different from that of HMXBs. It follows a power law with a differential slope of ~1 at low luminosities, gradually steepens at log(Lx)>37.0-37.5 and has a rather abrupt cut-off at log(Lx)~39.0-39.5. This value of the cut-off luminosity is significantly, by an order of magnitude, lower than found for high mass X-ray binaries.

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arXiv2003

Lx-SFR relation in star forming galaxies

We compare the results of Grimm et al. (2003) and Ranalli et al. (2003) on the Lx-SFR relation in normal galaxies. Based on the Lx-stellar mass dependence for LMXBs, we show, that low SFR (SFR<1 Msun/year) galaxies in the Ranalli et al. sample are contaminated by the X-ray emission from low mass X-ray binaries, unrelated to the current star formation activity. The most important conclusion from our comparison is, however, that after the data are corrected for the ``LMXB contamination'', the two datasets become consistent with each other, despite of their different content, variability effects, difference in the adopted source distances, X-ray flux and star formation rate determination and in the cosmological parameters used in interpreting the HDF-N data. They also agree well, both in the low and high SFR regimes, with the predicted Lx-SFR dependence derived from the parameters of the ``universal'' HMXB luminosity function. This encouraging result emphasizes the potential of the X-ray luminosity as an independent star formation rate indicator for normal galaxies.

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arXiv2003

A Flux Limited Atlas of Galaxy Cluster Temperature Maps

An atlas of gas temperature maps is presented for a flux limited catalog of galaxy clusters. The sample of clusters is based on the Edge et al. (1990) sample, with the inclusion of five additional clusters, all with fluxes f_X(2-10 keV)>= 1.7x10**-11 ergs sec**-1 cm**-2, drawn from a variety of other sources. The temperature maps are derived from ASCA GIS observations using a common methodology to correct for the Point-Spread Function and calculate the local projected gas temperature in such a way so as to make each cluster directly comparable to all others in the sample. Variations in the temperature distribution, when present at > 90% confidence, are characterized by their severity and extent. We find that 70% of the clusters in our sample have significant variations in the projected gas temperature. The presence of these variations increases with increasing luminosity, as does the spatial scope and severity within a cluster. For a more limited sample we find that one third of clusters with temperature structure have radio halos. The high rate of occurance of structure emphasizes the need for caution when using clusters to measure cosmological parameters.

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arXiv2003

The spectrum of the cosmic X-ray background observed by RTXE/PCA

We have analyzed a large set of RXTE/PCA scanning and slewing observations performed between April 1996 and March 1999. We obtained the 3-20 keV spectrum of the cosmic X-ray background (CXB) by subtracting Earth-occulted observations from observations of the X-ray sky at high galactic latitude and far away from sources. The sky coverage is approximately ~22600 sq.deg. The PCA spectrum of CXB in 3-20 keV energy band is adequately approximated by a single power law with photon index Gamma~1.4 and normalization at 1 keV ~9.5 phot/s/cm2/keV/sr. Instrumental background uncertainty precludes accurate RXTE/PCA measurements of the spectrum of cosmic X-ray background at energies above 15 keV and therefore we can not detect the high energy cutoff observed by HEAO-1 A2 experiment. Deep observations of the 6 high latitude points used to model the PCA background provide a coarse measure of the spatial variation of the CXB. The CXB variations are consistent with a fixed spectral shape and variable normalization characterized by a fractional rms amplitude of ~7% on angular scales of ~1 square deg.

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arXiv2003

HMXB, ULX and star formation

Based on recent X-ray observations of the Milky Way, Magellanic Clouds and nearby starburst galaxies we study population of high mass X-ray binaries, their connection with ultra-luminous X-ray sources and relation to the star formation. Although more subtle SFR dependent effects are likely to exist, the data in the lg(Lx)~36-40.5 luminosity range are broadly consistent with existence of a ``universal'' luminosity function of HMXBs, which can be roughly described as a power law with differential slope of ~1.6 and a cutoff at lg(Lx)~40.5. The ULX sources found in many starburst galaxies occupy the high luminosity end of this single slope power law distribution, whereas its low luminosity part is composed of ``ordinary'' high mass X-ray binaries, observed, e.g. in the Milky Way and Magellanic Clouds. As the normalization of the ``universal'' luminosity function is proportional to the star formation rate, the number and/or the collective X-ray luminosity of HMXBs can be used to measure the current value of SFR in the host galaxy. Distant (unresolved) starburst galaxies observed by Chandra at redshifts of z~0.2-1.3 obey the same Lx-SFR relation as local galaxies, indicating that the ULXs at these redshifts were not significantly more luminous than those found in nearby galaxies.

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arXiv2003

Boundary layer, accretion disk and X-ray variability in luminous LMXBs

Using Fourier frequency resolved X-ray spectroscopy we study short term spectral variability in luminous LMXBs. With RXTE/PCA observations of 4U1608--52 and GX340+0 on the horizontal/normal branch of the color-intensity diagram we show that aperiodic and quasiperiodic variability on ~ sec-msec time scales is caused primarily by variations of the luminosity of the boundary layer. The emission of the accretion disk is less variable on these time scales and its power density spectrum follows Pdisk(f) ~ 1/f law, contributing to observed flux variation at low frequencies and low energies only. The kHz QPOs have the same origin as variability at lower frequencies, i.e. independent of the nature of the "clock", the actual luminosity modulation takes place on the neutron star surface, The boundary layer spectrum remains nearly constant in the course of the luminosity variations and is represented to certain accuracy by the Fourier frequency resolved spectrum. In the considered range Mdot ~ (0.1-1)Mdot_Edd it depends weakly on the global mass accretion rate and in the limit Mdot ~ Mdot_Edd is close to Wien spectrum with kT ~ 2.4 keV (in the distant observer's frame). The spectrum of the accretion disk emission is significantly softer and in the 3--20 keV range is reasonably well described by a relativistic disk model with a mass accretion rate consistent with the value inferred from the observed X-ray flux.

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arXiv2003

IGR J16318-4848: an X-ray source in a dense envelope?

The hard X-ray source IGR J16318-4848 was recently discovered by the INTEGRAL observatory (Courvoisier et al.) and subsequently uncovered in archival data of ASCA observations in 1994 (Murakami et al.). We present results of a detailed analysis of the ASCA data. The spectrum of the source in the 0.5--10 keV band is extraordinarily hard and is virtually unobservable below 4 keV because of strong photoabsorption NHL L>4e23 cm^-2. The 4--10 keV emission is dominated by a Kalpha line of neutral or weakly ionized iron with an equivalent width of \~2.5 keV. There is also an indication for a second line at ~7 keV. Our analysis of archival observations of the IGR J16318-4848 infrared counterpart, discovered by Foschini et al., shows that the point source is detected at different wavelengths in the 1--15 micron range. The available data suggest that IGR J16318-4848 is an X-ray binary system enshrouded by a dense envelope. It is possible that the source is a wind-fed high-mass X-ray binary similar to GX 301-2. We argue that IGR J16318-4848 might be the first representative of a previously unknown population of highly absorbed galactic X-ray sources, which remained undetected with X-ray missions before INTEGRAL.

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arXiv2002

High Mass X-ray Binaries as a Star Formation Rate Indicator in Distant Galaxies

Based on CHANDRA observations of nearby starburst galaxies and RXTE/ASM, ASCA and MIR-KVANT/TTM studies of high mass X-ray binary (HMXB) populations in the Milky Way and Magellanic Clouds, we propose that the number and/or the collective X-ray luminosity of HMXBs can be used to measure the star formation rate (SFR) of a galaxy. We show that, within the accuracy of the presently available data, a linear relation between HMXB number and the star formation rate exists. The relation between SFR and collective luminosity of HMXBs is non-linear in the low SFR regime, $L_X\propto \SFR^{\approx 1.7}$, and becomes linear only for sufficiently high star formation rate, when the total number of HMXB sources becomes sufficiently large. Such behaviour is caused by the fact, that we measure collective luminosity of a population of the discrete sources. Although more subtle SFR dependent effects are likely to exist, the data are broadly consistent with the existence of a universal luminosity function of HMXBs which can be roughly described as a power law with a differential slope of $\sim 1.6$, a cutoff at $L_X \sim few \times 10^{40}$ erg/sec and a normalisation proportional to the star formation rate. We apply our results to (spatially unresolved) starburst galaxies observed by CHANDRA at redshifts up to $z\sim 1$ in the Hubble Deep Field North and show that the calibration of the collective luminosity of HMXBs as a SFR indicator based on the local sample agrees well with the SFR estimates obtained for these distant galaxies with conventional methods.

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arXiv2002

The Milky Way in X-rays for an outside observer: Log(N)-Log(S) and Luminosity Function of X-ray binaries from RXTE/ASM data

We study the Log(N)-Log(S) and X-ray luminosity function in the 2-10 keV energy band, and the spatial (3-D) distribution of bright, log(L_X) > 34-35 erg/s, X-ray binaries in the Milky Way. In agreement with theoretical expectations and earlier results we found significant differences between the spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The volume density of LMXB sources peaks strongly at the Galactic Bulge. HMXBs tend to avoid the inner 3-4 kpc of the Galaxy, HMXBs are more concentrated towards the Galactic Plane and show clear signatures of the spiral structure in their spatial distribution. LMXB sources have a flatter Log(N)-Log(S) distribution and luminosity function than HMXBs. The integrated 2-10 keV luminosities of X-ray binaries, averaged over 1996--2000, are 2-3 * 10^39 (LMXB) and 2-3 * 10^38 (HMXB) erg/s. Normalised to the stellar mass and the star formation rate, respectively, these correspond to 5 * 10^28 erg/s/M_sol for LMXBs and 5 * 10^37 erg/s/(M_sol/yr) for HMXBs. Due to the shallow slopes of the luminosity functions the integrated emission of X-ray binaries is dominated by the 5-10 most luminous sources which determine the appearance of the Milky Way in the standard X-ray band for an outside observer. In particular variability of individual sources or an outburst of a bright transient source can increase the integrated luminosity of the Milky Way by as much as a factor of ~2. Although the average LMXB luminosity function shows a break near the Eddington luminosity for a 1.4 M_sol neutron star, at least 11 sources showed episodes of super-Eddington luminosity during ASM observations. We provide the maps of distribution of X-ray binaries in the Milky Way in various projections, which can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.

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arXiv2002

Super-Eddington outburst of V4641 Sgr

X-ray transients provide unique opportunity to probe accretion regimes of at a vastly different accretion rates. We analyze a collection of the RXTE observations (Galactic Center scans, ASM monitoring and a pointed observation) of enigmatic transient source high mass X-ray binary V4641 Sgr and argue that they broadly support the hypothesis that giant September 1999 outburst was associated with an episode of super-Eddington accretion onto the black hole. During the outburst an extended optically thick envelope/outflow has been formed around the source making the observational appearance of V4641 Sgr in many aspects very similar to that of SS433. These results suggest that objects like V4641 Sgr and SS433 indeed represent the class of objects accreting matter at a rate comparable or above Eddington value and the formation of an envelope/outflow is a generic characteristic of supercritical accretion. When the accretion rate decreased the envelope vanished and the source short term variability and spectral properties started to resemble those of other galactic black hole candidates accreting at a rate well below the Eddington value. Interestingly that during this phase the source spectrum was very similar to the Cygnus X-1 spectrum in the low state inspite of more than order of magnitude larger X-ray luminosity.

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arXiv2001

Reflection and noise in the low spectral state of GX339-4

We analyze RXTE/PCA observations of GX339-4 in the low spectral state from 1996--1997 and show that the pattern of its spectral and temporal variability is nearly identical to that of Cyg X-1. In particular, a tight correlation exists between the QPO centroid frequency and the spectral parameters. An increase of the QPO centroid frequency is accompanied with an increase of the amplitude of the reflected component and a steepening the slope of the underlying power law. Fourier frequency resolved spectral analysis showed, that the variability of the reflected component at frequencies higher than ~1-10 Hz is suppressed in comparison with that of the primary emission.

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arXiv2001

V4641Sgr - Super-Eddington source enshrouded by an extended envelope

An optical spectroscopy of an unusual fast transient V4641 Sgr constrains its mass to 8.7-11.7M_sun (9.6M_sun is the best fit value) and the distance to 7.4--12.3 kpc (Orosz et al. 2001). At this distance the peak flux of 12 Crab in the 2--12 keV energy band, measured by ASM/RXTE, implies the X-ray luminosity exceeding 2-3e39 erg/s, i.e. near or above the Eddington limit for a 9.6M_sun black hole. An optical photometry shows that at the peak of the optical outburst the visual magnitude increased by Delta m_V > 4.7^m relative to the quiescent level and reached m_V < 8.8^m. An assumption that this optical emission is due to irradiated surface of an accretion disk or a companion star with the the black body shape of the spectrum would mean that the bolometric luminosity of the system exceeds L>3e41 erg/s > 300 L_Edd. We argue that the optical data strongly suggest presence of an extended envelope surrounding the source which absorbs primary X-rays flux and reemits it in optical and UV. The data also suggests that this envelope should be optically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgr at the peak of an optical flare are very similar to those of SS433. This envelope is likely the result of near or super Eddington rate of mass accretion onto the black hole and it vanishes during subsequent evolution of the source when apparent luminosity drops well below the Eddington value. Thus this transient source provides us direct proof of the dramatic change in the character of an accretion flow at the mass accretion rate near or above the critical Eddington value as predicted long time ago by the theoretical models.

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arXiv2001

Merging Binary Clusters

We study three prominent bi-modal X-ray clusters: A3528, A1750 and A3395. Using observations taken with ROSAT and ASCA, we analyze the temperature and surface brightness distributions. We also analyze the velocity distributions of the three clusters using new measurements supplemented with previously published data. We examine both the overall cluster properties, as well as the two sub-cluster elements in each. These results are then applied to the determination of the overall cluster masses, and demonstrate excellent consistency between the various methods used. While the characteristic parameters of the sub-clusters are typical of isolated objects, our temperature results for the regions between the two sub-clusters clearly confirm the presence of merger activity. These three clusters represent a progression of equal-sized sub-cluster mergers, starting from initial contact to immediately before first core passage.

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arXiv2001

New class of low frequency QPOs: signature of nuclear burning or accretion disk instabilities?

We report the discovery of a new class of low frequency quasi--periodic variations of the X-ray flux in the X-ray bursters 4U1608-52 and 4U1636-536. We also report an occasional detection of a similar QPO in Aql X-1. The QPOs, associated with flux variations at the level of percents, are observed at a frequency of 7-9 mHz. While usually the relative amplitude of flux variations increases with energy, the newly discovered QPOs are limited to the softest energies (1-5 keV). The observations of 4U1608-52 suggest that these QPOs are present only when the source X-ray luminosity is within a rather narrow range and they disappear after X-ray bursts. Approximately at the same level of the source luminosity, type I X-ray bursts cease to exist. Judging from this complex of properties, we speculate that a special mode of nuclear burning at the neutron star surface is responsible for the observed flux variations. Alternatively, some instabilities in the accretion disk may be responsible for these QPOs.

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arXiv2001

On the X-ray time lags in the black hole candidates

It is shown that the energy dependence of the time lags in Cygnus X-1 excludes any significant contribution of the standard reflected component to the observed lags. The conclusion is valid in the 0.1--10 Hz frequency range where time lags have been detected with high enough significance. In fact the data hint that reflected component is working in opposite direction, reducing the lags at energies where contribution of the reflected component is significant. We argue that the observed logarithmic dependence of time lags on energy can be understood as due to the small variations of the spectrum power law index in a very simple phenomenological model. We assume that an optically thin flow/corona, emitting a power law like spectrum, is present at a range of distances from the compact object. The slope of the locally emitted spectrum is a function of distance with the hardest spectrum emitted in the innermost region. If perturbations with different time scales are introduced to the accretion flow at different radii the observed X-ray lags naturally appear due to the inward propagation of perturbations on the diffusion time scales.

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arXiv2000

High frequencies in the power spectrum of Cyg X-1 in the hard and soft spectral states

We analyzed a large number of RXTE/PCA observations of Cyg X-1 in the hard and soft spectral states with total exposure time of ~190 and ~10 ksec respectively and time resolution better than ~250us in order to investigate its variability down to a few milliseconds time scales. The model of modifications of the power density spectra due to the dead time effect was tested using RXTE observations of an extremely bright source - Sco X-1. The results of these tests demonstrated, that although some problems still remain, for sources like Cyg X-1 (which is an order of magnitude less bright than Sco X-1) present knowledge of the instrument is sufficient to reliably analyze the power spectra in the kHz frequency range. In both spectral states of Cyg X-1 we detected statistically significant variability up to ~150-300 Hz with a fractional rms amplitude above 100 Hz at the level of ~2-3%. The power spectrum in the hard state shows a steepening at frequency of ~40-80 Hz with the power law slope changing from ~1.7 to ~2.3-2.4. In the soft state the slope of power spectrum changes from ~1 to ~2 at the frequency of ~15-20 Hz without any evidence of further steepening up to \~100-150 Hz. These break frequencies represent the highest characteristic frequencies detected in the power density spectrum of Cyg X-1 so far.

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