arXiv subjects

M. Gilfanov

Publications and source records attributed to M. Gilfanov.

At least 109 records · Page 6Linked to original sources

arXiv2000

Soft state of Cygnus X-1: stable disk and unstable corona

Two component X-ray spectra (soft multicolor black body plus harder power law) are frequently observed from accreting black holes. These components are presumably associated with the different parts of the accretion flow (optically thick and optically thin respectively) in the vicinity of the compact source. Most of the aperiodic variability of the X-ray flux on the short time scales is associated with the harder component. We suggest that drastically different amplitudes of variability of these two components are simply related to the very different viscous time scales in the geometrically thin and geometrically thick parts of the accretion flow. In the geometrically thin disks variations of viscosity or mass accretion rate occurring at large radius from the black hole on the local dynamical or thermal time scales do not cause any significant variations of the mass accretion rate at smaller radii due to a very long diffusion time. Any variations on the time scales shorter than the diffusion time scale are effectively dampened. On the contrary such variations can easily survive in the geometrically thick flows and as a result the mass accretion rate in the innermost region of the flow will reflect modulations of the mass accretion rate added to the flow at any distance from the black hole. Therefore if primary instabilities operate on the short time scales then the stability of the soft component (originating from the geometrically thin and optically thick flow) and variability of the hard component (coming from the geometrically thick and optically thin flow) are naturally explained.

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arXiv2000

Comptonization, reflection and noise in black hole binaries

Based on the analysis of a large sample of RXTE/PCA observations of several black hole binaries in the low spectral state we show that a correlation exists between the spectral parameters and characteristic noise frequency. In particular, the amplitude of reflection increases and the slope of Comptonized radiation steepens as the noise frequency increases. We consider possible implications of these results on the accretion flow models and discuss a possible observational test aimed to discriminate between different geometries of the accretion flow.

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arXiv2000

Frequency resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state

Using the RXTE/PCA data we study the fast variability of the reflected emission in the soft spectral state of Cyg X-1 by means of Fourier frequency resolved spectroscopy. We find that the rms amplitude of variations of the reflected emission has the same frequency dependence as the primary radiation down to time scales of <30-50 msec. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behavior differs notably from the hard spectral state, in which variations of the reflected flux are significantly suppressed in comparison with the primary emission, on time scales shorter than ~0.5-1 sec. If related to the finite light crossing time of the reflector, these results suggest that the characteristic size of the reflector -- presumably an optically thick accretion disk, in the hard spectral state is larger by a factor of >5-10 than in the soft spectral state. Modeling the transfer function of the disk, we estimate the inner radius of the accretion disk R_in~100R_g in the hard and R_in<10R_g in the soft state for a 10M_sun black hole.

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arXiv1999

RXTE broad band observations of X-ray Nova XTE J1755-324

The properties of X-ray Nova XTE J1755-324, observed with RXTE and GRANAT observatories in 1997 are reported. The lightcurve of the source was typical for X-ray Novae, except for somewhat shorter decay time scale and the time elapsed between primary and subsequent secondary and tertiary outbursts. At the peak of the lightcurve the source had two-component spectrum, typical for the bright X-ray Novae, with disk blackbody temperature Tin~0.8 keV and photon index of the hard power law tail α~ 2.0. The peak luminosity can be estimated as L~10^{38} erg/s (0.1-100 keV, assuming 8.5 kpc distance). A notable and peculiar feature of the spectral evolution of the source was a short, ~10 days long, episode of increased hardness, occurred shortly before the tertiary maximum of the light curve. During the last pointed observations ~100 days after the primary outburst the source has been found in the hard spectral state with luminosity ~fewX10^{36} erg/s. The overall pattern of the temporal and spectral evolution of XTE J1755-324 resemble in general that of ``canonical'' X-ray Novae (e.g. Nova Muscae 1991) and suggests that the compact object in the binary system is a black hole

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arXiv1999

The 1-12 Hz QPO and dips in GRS 1915+105: tracers of Keplerian and viscous time scales?

We analysed 9 RXTE/PCA observations of GRS 1915+105 in the flaring state, when the hardness of the source spectrum was changing on the time scales from few seconds up to 1000 seconds. The quasi periodic oscillations (QPOs) with the frequency varying between ~ 1 and 12 Hz are associated with the episodes of harder source spectrum. In each observation we found tight correlation between the duration of the hard episode and the characteristic QPO frequency. For a half of the observations this correlation matches the relation between the viscous time scale and the Keplerian frequency, when both quantities are evaluated for various radii in the radiation pressure dominated accretion disk. Assuming that the QPO frequency is proportional to the Keplerian frequency at the boundary between an optically thick accretion disk and a hot comptonization region, the changes of the QPO frequency can then be understood as due to variations of this boundary position on the viscous time scales.

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arXiv1999

Reflection and noise in Cygnus X-1

We analyzed RXTE/PCA observations of Cyg X-1 from 1996-1998. We found a tight correlation between the characteristic noise frequencies (e.g. the break frequency nu_br) and the spectral parameters in the low spectral state. The amplitude of reflection increases and the spectrum of primary radiation steepens as the noise frequency increases (nu_br changes by a factor of ~15 in our sample). This can be understood assuming that increase of the noise frequency is associated with the shift of the inner boundary of the optically thick accretion disk towards the compact object. The related increase of the solid angle, subtended by the disk, and of the influx of the soft photons to the Comptonization region lead to an increase of the amount of reflection and steepening of the Comptonized spectrum. The correlation between the slope of primary radiation and the amplitude of reflection extends to the soft spectral state. A similar correlation between reflection and slope was found for the frequency resolved spectra in the 0.01-15 Hz frequency range. Such a correlation could appear if the longer time scale variations are associated with emission originating closer to the optically thick disk and, therefore, having steeper Comptonized spectra with larger reflection.

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arXiv1999

Frequency resolved spectroscopy of Cyg X-1: fast variability of the Fe K_alpha line

We studied the frequency resolved energy spectra of Cyg X--1 during the standard low (hard) spectral state using the data of the Rossi X-Ray Timing Explorer. We found that the relative amplitude of the reflection features -- the iron fluorescent line at ~6.5 keV and the smeared edge above ~7 keV -- decreases with the increasing frequency. In particular we found that the equivalent width of the iron line decreases above ~1 Hz and drops twice at frequency of ~10 Hz. An assumption that such behavior is solely due to a finite light crossing time of the reflecting media, would imply the characteristic size of the reflector ~5E8 cm, corresponding to ~150 Rg for a 10Msun black hole. Alternatively lack of high frequency oscillations of the reflected component may indicate that the short time scale, ~50-100 msec, variations of the primary continuum appear in geometrically different, likely inner, part of the accretion flow and give a rise to a significantly weaker, if any, reflected emission than the longer time scale events.

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arXiv1999

Deep SIGMA observations of the central square degree of the Galaxy

The Center of the Milky Way emits radiation through all the electromagnetic spectrum due to the presence of several astrophysical phenomena. Various scientifical questions remain unsolved in the picture that emerge from observations, the main one being the presence and activity of a supermassive black hole at the Galactic Center. Also in the X-ray band a strong diffuse emission is detected up to 22 keV, it has been suggested that its origin is linked to past high energy activity of the Galactic Center. The French SIGMA soft gamma ray telescope on board the Russian Granat satellite observed the Galacti Center (GC) for abour 9 X 10^6 seconds from 1990 to the end of 1997. Its unique imaging capabilities coupled with the unprecedented exposure already allowed us to set preliminary constraints on the persistent high energy emission from Sgr A*. Here we present a new analysis of the complete set of observations of this region taking into account the possible presence of diffuse emission in the low energy channels. This allows us both to test current advection models of high energy emission from Sgr A* and to investigate on past high energy activity.

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arXiv1999

Compact and Diffuse Sources in the Galactic Center Region

The 10 by 10 degrees region around dynamic center of our Galaxy is known to host a large number of bright X-ray sources, most of which are low mass binary systems. While high luminosity Z-sources are especially bright in the standard X-ray band (e.g. 2-10 keV), lower luminosity systems are dominating the flux above 30 keV. The two hardest sources in the region, i.e. 1E1740.7-2942 and GRS 1758-258 are thought to be black hole candidates (BHC) based on the similarity of spectral properties and short-term variability to that of dynamically proven BHCs. However, while most of the other galactic BHCs are known to be transients, these two objects could be considered persistent though variable sources. The persistent behavior of these sources implies some constraints on the parameters of the binary systems. Both sources were detected in the first GRANAT observation of the GC region. They are the only two persistent BHCs seen in the 35-75 keV SIGMA image after collecting several million seconds of exposure time over eight years, indicating that they are perhaps the only two persistent BHCs in the region. Another peculiar object in the field is of course a putative supermassive black hole (Sgr A*) at the dynamic center of our Galaxy. The present day's X-ray luminosity from Sgr A* is very small ($\la 10^{36}$ erg/s). However an indication that Sgr A* might have been much brighter in the past was found while studying diffuse X-ray emission in the region. We speculate here on how future X-ray observatories may verify the hypothesis of the violent activity of Sgr A* in the past.

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arXiv1998

Restrictions on the parameters of boundary layer and accretion disk of X-ray bursters in the low state

Using ASCA observations of several X-ray bursters in the low spectral state Lx~1-5x10^{36} erg/s) restrictions on the parameters of the accretion disk and the boundary layer were obtained. The low state spectra of X-ray bursters observed with ASCA often can be well described by a power law with photon index of 1.7--2.2 and are not consistent with significant contribution of the soft spectral component to the total luminosity. For a blackbody spectrum with temperature of 0.2--2 keV the upper limits correspond to <10--20% of the total 0.5--10 keV luminosity. On the other hand, theoretical calculations predict that in the case of a Keplerian accretion disk around a slowly rotating neutron star (with radius of 3Rg) ~2/3 of the total gravitational energy released in the system can be released in the boundary layer between the inner part of the accretion disk and the neutron star surface. More accurate analysis of the ASCA spectra shows that: (1) The inner radius of the standard geometrically thin optically thick part of the accretion disk exceeds $R_{in} > 10-25 R_g$. (2) The characteristic temperature in the inner part of the accretion flow including the boundary layer exceeds $\sim$2 keV independently upon the assumption about the density in the accretion flow. The electron scattering gives the dominant contribution to the opacity in this region.

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arXiv1998

1996/1997 low luminosity state and state transitions of GRS1915+105: RXTE observations

The results of X-ray observations of Galactic superluminal jet source GRS 1915+105 during its low luminosity state and state transitions in October, 1996 - April, 1997 with PCA and HEXTE instruments aboard Rossi X-ray Timing Explorer (RXTE) are reported. Except some peculiarities, the major spectral and temporal properties of the source during this period were similar to that of the Galactic black hole candidates in the intermediate state (corresponding to the transition between the canonical high and low states). The power law component of the spectrum gradually hardens with decrease of the source flux in the 3 - 150 keV energy band. The 2 - 10 Hz QPOs were found to be a generic feature of the power density spectrum in this state. The changes of the QPO centroid frequency are strongly correlated with the changes of spectral and timing parameters (in particular parameters of the soft spectral component). This type of correlation holds on the wide range of time scales (from seconds to months). Overall trend of the spectral and temporal properties suggests that with further drop of the source flux (below the values observed during the reported period) they would match the canonical values for a low state of the Galactic black hole candidates.

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arXiv1998

A Hot Spot in Coma

We study the temperature structure of the central part (r<18' ~0.7 h50**-1 Mpc) of the Coma cluster of galaxies using ASCA data. Two different analysis methods produce results in good agreement with each other and reveal the presence of interesting structures in the gas temperature distribution. Globally, the average temperature in the center of the cluster is 9.0 +/- 0.6 keV in good agreement with previous results. Superimposed on this, we find a cool area with temperatures of 4-6 keV associated with a filament of X-ray emission extending southeast from the cluster center detected by Vikhlinin and coworkers. We also find a hot spot with a temperature of around 13 keV displaced north from the central peak of emission. The distribution of the gas temperatures and relative specific entropies suggests that the cool features are most likely gas stripped from a galaxy group centered on NGC 4874 falling toward the core from outside, while the hot spot located ``ahead'' of this in-falling gas is due to shock heating. Thus our results suggest that we are observing Coma during a minor merger with a small group of galaxies associated with NGC 4874 shortly before the initial core passage.

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arXiv1998

The millisecond X-ray pulsar/burster SAX J1808.4-3658: the outburst light curve and the power law spectrum

The X-ray light curve and broad band spectral properties of the millisecond X-ray pulsar/burster SAX J1808.4-3658 are reported. In the course of RXTE observations in April--May 1998 the 3--150 keV luminosity of the source decreased by a factor of ~100 from the peak value of ~9E36 erg/s (for a 4 kpc distance). However, the spectrum was remarkably stable and maintained a roughly power law shape with a photon index of ~2 without a strong high energy cut-off below 100 keV, similar to that sometimes observed in the low spectral state of X-ray bursters. An approximation of the averaged spectrum with an exponentially cut-off power law with a superimposed reflected component yields the 90% lower limit on the e-folding energy ~270 keV. We speculate that Comptonization on the bulk motion in the radiation dominated shock might be a possible mechanism of spectral formation. The decaying part of the X-ray light curve was cut off abruptly at luminosity ~few 1E35 erg/s. Such behavior might be due to centrifugal inhibition of accretion (transition to the ``propeller'' regime) as well as to disk instability. In either case an upper limit on the neutron star magnetic field strength is B < few 1E7 Gauss.

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arXiv1998

SIGMA and XTE observations of the soft X-ray transient XTEJ1755-324

We present observations of the X-ray transient XTEJ1755-324 performed during summer 1997 with the XTE satellite and with the SIGMA hard X-ray telescope onboard the GRANAT observatory. The source was first detected in soft X-rays with XTE on July 25 1997 with a rather soft X-ray spectrum and its outburst was monitored in soft X-rays up to November 1997. On September 16 it was first detected in hard X-rays by the French soft gamma ray telescope SIGMA during a Galactic Center observation. The flux was stronger on September 16 and 17 reaching a level of about 110 mCrab in the 40-80 keV energy band. On the same days the photon index of the spectrum was determined to be alpha =-2.3 +/- 0.9 (1 sigma error) while the 40-150 keV luminosity was about 8 x 10^{36} erg/s for a distance of 8.5 kpc. SIGMA and XTE results on this source indicate that this source had an ultrasoft-like state during its main outburst and a harder secondary outburst in September. These characteristics make the source similar to X-Nova Muscae 1991, a well known black hole candidate.

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arXiv1998

Evidence for Merging in the Centaurus Cluster

We present a two dimensional map of the gas temperature distribution in the Centaurus cluster, based on ASCA observations derived using a novel approach to account for the energy dependent point spread function. Along with a cool region, centered on NGC4696, asymmetric temperature variations of moderate amplitude are detected. The hottest region roughly coincides with the position of the second brightest galaxy NGC4709, known to be the dominant galaxy of one of the subgroups in the Centaurus cluster. ROSAT images show faint surface brightness emission also centered on this galaxy. The imaging and spectral results suggest that a subcluster, centered on NGC4709 is merging with the main cluster centered on NGC4696, in agreement with the earlier suggestion by Lucey et al. (1986) that these two systems are located at the same distance despite their different line-of-sight velocities.

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arXiv1998

The 6.4 keV Fluorescent Iron Line from Cluster Cooling Flows

For the inner region of the cooling flow (e.g. within the radius of $\sim$50--100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as $\sim 0.01$. Assuming that the cooling time in the inner region is few times shorter than the life time of the cluster, the Thomson depth of the accumulated cold gas can be higher accordingly (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4 keV iron fluorescent line, with an equivalent width proportional to $τ_T$. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than few $10^{23} cm^{-2}$. Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and it's distribution) can be measured.

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arXiv1997

Temperature Structure in Abell 1367

We study the temperature structure of the rich cluster of galaxies Abell 1367 using two independent methods of correcting for the ASCA PSF. The results of the two methods are in excellent agreement and give solid evidence of a strong and localized shock in the intracluster medium (ICM). Our analysis suggests that we are observing the merger of two subclusters. We find that the lower luminosity subcluster, located to the northwest, has a higher temperature of 4.2 +/- 0.3 keV, compared to the more luminous southeast subcluster, whose temperature is 3.2 +/- 0.1 keV. An analysis of the ROSAT surface brightness profiles of both subclusters is also presented. The data agree well with predictions from numerical simulations of mergers between subcluster sized masses, in particular, with early stages of the merger, prior to the first core passage.

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arXiv1997

SIGMA observations of X-ray Nova Velorum 1993 (GRS 1009-45)

We report on hard X-ray observations of X-ray Nova Velorum 1993 (GRS 1009-45) performed with the SIGMA coded mask X-ray telescope in January 1994. The source was clearly detected with a flux of about 60 mCrab in the 40-150 keV energy band during the two observations with a hard spectrum (alpha ~ - 1.9) extending up to ~ 150 keV. These observations confirm the duration of the activity of the source in hard X-rays over 100 days after the first maximum and suggest a spectral hardening which has already been observed in Nova Muscae. These and other characteristics found in these observations strengthen the case for this Nova to be a black hole candidate similar to Nova Muscae.

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