arXiv subjects

M. Revnivtsev

Publications and source records attributed to M. Revnivtsev.

At least 91 records · Page 5Linked to original sources

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

An upper limit on the X-ray luminosity of the black hole - microlens OGLE-1999-BUL-32

We present an upper limit on the 3--20 keV X-ray flux from the black hole - microlens OGLE-1999-BUL-32, based on RXTE/PCA scans over the Galactic Center region in 1999-2000. It is shown that the X-ray luminosity of the black hole did not exceed L(3-20 keV)<3e33(d/1kpc)^2 ergs/s (where d is the distance to the black hole). Near the maximum of the background star amplification by the microlens (July 6, 1999), the upper limit on the X-ray flux corresponds to an X-ray luminosity L(3-20 keV)<7e33(d/1kpc)^2 ergs/s.

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arXiv2001

Possible 38 day X-ray period of KS1731-260

We report the detection of a 38 day period in the X-ray flux of the transient burster KS1731-260. The narrow peak of periodicity was detected during ~TJD 10150--11050 when the source had a high and relatively stable X-ray flux. After TJD 11100 the source became strongly variable on a time scale of months that contaminates the search for the 38 day periodicity. The detected period can not be a binary period. The binary with Roche lobe overflow has in this case large radii of the secondary and of the accretion disk. Disk and secondary star illumination by X-ray flux from luminous neutron star would lead to high infrared brightness of the binary. That clearly contradicts the infrared data even for the brightest infrared sources within CHANDRA error box of KS1731-260. Remaining possibility is that observed periodicity is connected with the accretion disk precession, similar to that was observed for SS 433, Her X-1, Cyg X-1 etc.

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arXiv2001

Chandra localization of KS1731--260

We present the analysis of the Chandra image of KS 1731-260. The improvement of the source localization (down to ~0.6'') allowed us to rule out 12 of 13 infrared companion candidates proposed by Barret et al. (1998). The remaining possible infrared counterpart of KS 1731-260 (angular distance between sources ~1.45'', or ~2σ) has the brightness m_{J}~16 in the J band. If this star is really the counterpart of KS 1731-260, the lower limit on its luminosity is L_{tot}>L_{J,H}~10L_{\odot}. The strong drop of the X-ray flux from the source allow us to propose the additional check of whether the decribed star is the real counterpart of KS1731-260. If the optical and infrared brightness of the binary system is partially caused by the reprocessing of X-rays, as it is usual for the low mass binary systems, the infrared brightness of real counterpart should strongly reduce in 2001, when the X-ray flux from KS1731-260 turned off (Wijnands et al. 2001)

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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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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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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

RXTE observations of XTE J2012+381 during its 1998 outburst

We present an analysis of the RXTE observations of X-ray transient source XTE J2012+381 during its 1998 outburst. The spectral and timing properties of the source emission and their evolution during the outburst are very similar to what of the X-ray Novae.

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arXiv2000

Discovery of 0.08 Hz QPO in the power spectrum of black hole candidate XTE J1118+480

We found a strong QPO feature at 0.085+/-0.002 Hz in the power spectrum of X-ray transient XTE J1118+480. The QPO was detected in PCA/RXTE data with an amplitude close to 10% rms, and the width 0.034+/-0.006 Hz. The shape of the power spectrum is typical for black hole candidates: almost flat at frequencies lower than 0.03 Hz, roughly power law with slope ~1.2 from 0.03 to 1 Hz, with a following steepening to ~1.6 at higher frequencies. The hard energy spectrum detected up to ~150 keV and the absence of significant X-ray variability at the high frequencies above 100 Hz strongly support the identification of XTE J1118+480 as black hole transient.

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arXiv2000

Fourier power spectra at high frequencies: a way to distinguish a neutron star from a black hole

We analyzed the power density spectra of a sample of 9 neutron star and 9 black hole binaries in the low/hard spectral state. In the power density spectra of accreting neutron stars with a weak magnetic field a significant power is contained at frequencies close to one kHz. At the same time, most Galactic accreting black holes demonstrate a strong decline in the power spectra at the frequencies higher than 10--50 Hz. We propose to use this empirical fact as a method to distinguish the accreting neutron stars from black holes in X-ray transients. The X-ray transients that demonstrate significant noise in their X-ray flux at frequencies above ~500 Hz should be considered neutron stars. We propose to explain the observed difference as a result of the existence of a radiation dominated spreading layer on the neutron star surface (Inogamov & Sunyaev 1999). The possible very high frequency variabilities of this layer are discussed.

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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

The study of the X-ray emission from the accreting black holes and neutron stars

The thesis studies the X-ray emission from the Galactic compact objects (accreting neutron stars and black holes), using mainly the RXTE data. In particular following results have been included: spectral evolution of X-ray transients GRS 1739-278, XTE J1755-324, GS 1354-644, XTE J1748-288; the outburst and detailed spectroscopy of the anomalous X-ray Novae XTE J0421+560; study of the outburst of the millisecond pulsar SAX J1808.4-3658 and stability of its spectrum; study of the SAX J1808.4-3658 pulse profile and relativistic distortions of the profile, the linear velocity of the emitting area on the surface of the neutron star SAX J1808.4--3658, inferred from our simplest model is about ~0.1c and the neutron star radius is about ~13 km; frequency resolved spectroscopy and its application to the hard state of Cyg X-1 and GX 339-4, correlations of the spectral and timing properties for Cyg X-1, GX 339-4 and possible connection to the disk-spheroid model of the accreting flow in the vicinity of the 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

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

X-ray outburst of CI Cam/XTE J0421+560: RXTE observations

We present the results of observations of XTE J0421+560 X-ray transient with Rossi X-ray Timing Explorer in the beginning of April 1998. Lightcurve, obtained by ASM all-sky monitor, shows unusually fast decrease in the object's luminosity. Spectra from series of observations by PCA and HEXTE experiments were studied in detail. Two emission line regions with energies around 6.5 keV and around 8 keV have been mentioned in the spectra. The possibility of generation of the observed emission in relativistic plasma jet has been discussed. Some analogy is found with known Galactic jet source SS433.

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arXiv1999

Timing analysis of the X-ray transient source XTE J1806--246 (2S1803--245)

An outburst of the X-ray transient source XTE J1806--246 (2S1803--245) has been observed by Rossi X-ray Timing Explorer in April-July 1998. Strong quasi-periodical oscillations (QPO) with a central peak frequency around 9 Hz was detected in one observation of the series performed by PCA/RXTE experiment. X-ray flux from the source during the observation with QPO was maximal. The energy spectrum of XTE J1806--246 at this time was softer than for other observations. A sufficient variability of the QPO parameters has been detected, in short-term correlation with the flux variability. Fractional amplitudes of Very Low frequency Noise and QPO component of power density spectrum demonstrate strong energy dependence, while other parameters do not change significantly.

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