131 research outputs found
Doughnut strikes sandwich: the geometry of hot medium in accreting black hole X-ray binaries
We study the effects of the mutual interaction of hot plasma and cold medium
in black hole binaries in their hard spectral state on the value of the
truncation radii of accretion discs. We consider a number of different
geometries. In contrast to previous theoretical studies, we use a modern
energy-conserving code for reflection and reprocessing from cold media. We show
that a static corona above a disc extending to the innermost stable circular
orbit produces spectra not compatible with those observed. They are either too
soft or require a much higher disc ionization than that observed. This
conclusion confirms a number of previous findings, but disproves a recent study
claiming an agreement of that model with observations. We show that the cold
disc has to be truncated in order to agree with the observed spectral hardness.
However, a cold disc truncated at a large radius and replaced by a hot flow
produces spectra which are too hard if the only source of seed photons for
Comptonization is the accretion disc. Our favourable geometry is a truncated
disc coexisting with a hot plasma either overlapping with the disc or
containing some cold matter within it, also including seed photons arising from
cyclo-synchrotron emission of hybrid electrons, i.e. containing both thermal
and non-thermal parts.Comment: 12 pages, 8 figures; A&A, in pres
The origin of seed photons for Comptonization in the black hole binary Swift J1753.5-0127
Aims. The black hole binary SWIFT J1753.5-0127 is providing a unique data set
to study accretion flows. Various investigations of this system and of other
black holes have not, however, led to an agreement on the accretion flow
geometry or on the seed photon source for Comptonization during different
stages of X-ray outbursts. We place constraints on these accretion flow
properties by studying long-term spectral variations of this source. Methods.
We performed phenomenological and self-consistent broad band spectral modeling
of Swift J1753.5-0127 using quasi-simultaneous archived data from
INTEGRAL/ISGRI, Swift/UVOT/XRT/BAT, RXTE/PCA/HEXTE and MAXI/GSC instruments.
Results. We identify a critical flux limit, F \sim 1.5 \times 10^{-8}
erg/cm^2/s, and show that the spectral properties of SWIFT J1753.5-0127 are
markedly different above and below this value. Above the limit, during the
outburst peak, the hot medium seems to intercept roughly 50 percent of the disk
emission. Below it, in the outburst tail, the contribution of the disk photons
reduces significantly and the entire spectrum from the optical to X-rays can be
produced by a synchrotron-self-Compton mechanism. The long-term variations in
the hard X-ray spectra are caused by erratic changes of the electron
temperatures in the hot medium. Thermal Comptonization models indicate
unreasonably low hot medium optical depths during the short incursions into the
soft state after 2010, suggesting that non-thermal electrons produce the
Comptonized tail in this state. The soft X-ray excess, likely produced by the
accretion disk, shows peculiarly stable temperatures for over an order of
magnitude changes in flux. Conclusions. The long-term spectral trends of SWIFT
J1753.5-0127 are likely set by variations of the truncation radius and a
formation of a hot, quasi-spherical inner flow in the vicinity of the black
hole. (abridged)Comment: 16 pages, 8 figures, published in A&
Interference as an Origin of the Peaked Noise in Accreting X-Ray Binaries
We propose a physical model for the peaked noise in the X-ray power density spectra of accreting X-ray binaries. We interpret its appearance as an interference of two Comptonization continua: one coming from the upscattering of seed photons from the cold thin disk and the other fed by the synchrotron emission of the hot flow. Variations of both X-ray components are caused by fluctuations in mass accretion rate, but there is a delay between them corresponding to the propagation timescale from the disk Comptonization radius to the region of synchrotron Comptonization. If the disk and synchrotron Comptonization are correlated, the humps in the power spectra are harmonically related and the dips between them appear at frequencies related as odd numbers 1:3:5. If they are anti-correlated, the humps are related as 1:3:5, but the dips are harmonically related. Similar structures are expected to be observed in accreting neutron star binaries and supermassive black holes. The delay can be easily recovered from the frequency of peaked noise and further used to constrain the combination of the viscosity parameter and disk height-to-radius ratio α(H/R)2 of the accretion flow. We model multi-peak power spectra of black hole X-ray binaries GX 339-4 and XTE J1748-288 to constrain these parameters.</p
Colors and patterns of black hole X-ray binary GX 339-4
Black hole X-ray binaries show signs of non-thermal emission in the
optical/near-infrared range. We analyze the optical/near-infrared SMARTS data
on GX3394 over the 2002--2011 period. Using the soft state data, we estimate
the interstellar extinction towards the source and characteristic color
temperatures of the accretion disk. We show that various spectral states of
regular outbursts occupy similar regions on the color-magnitude diagrams, and
that transitions between the states proceed along the same tracks despite
substantial differences in the observed light curves morphology. We determine
the typical duration of the hard-to-soft and soft-to-hard state transitions and
the hard state at the decaying stage of the outburst to be one, two and four
weeks, respectively. We find that the failed outbursts cannot be easily
distinguished from the regular ones at their early stages, but if the source
reaches 16 mag in -band, it will transit to the soft state. By subtracting
the contribution of the accretion disk, we obtain the spectra of the
non-thermal component, which have constant, nearly flat shape during the
transitions between the hard and soft states. In contrast to the slowly
evolving non-thermal component seen at optical and near-infrared wavelengths,
the mid-infrared spectrum is strongly variable on short timescales and
sometimes shows a prominent excess with a cutoff below Hz. We show
that the radio to optical spectrum can be modeled using three components
corresponding to the jet, hot flow and irradiated accretion disk.Comment: Accepted for publication in Astronomy & Astrophysics, 19 pages, 6
tables, 18 figure
Expanding hot flow in the black hole binary SWIFT J1753.5-0127: evidence from optical timing
We describe the evolution of optical and X-ray temporal characteristics
during the outburst decline of the black hole X-ray binary SWIFT J1753.5-0127.
The optical/X-ray cross-correlation function demonstrates a single positive
correlation at the outburst peak, then it has multiple dips and peaks during
the decline stage, which are then replaced by the precognition dip plus peak
structure in the outburst tail. Power spectral densities and phase lags show a
complex evolution, revealing the presence of intrinsically connected optical
and X-ray quasi-periodic oscillations. For the first time, we quantitatively
explain the evolution of these timing properties during the entire outburst
within one model, the essence of which is the expansion of the hot accretion
flow towards the tail of the outburst. The pivoting of the spectrum produced by
synchrotron Comptonization in the hot flow is responsible for the appearance of
the anti-correlation with the X-rays and for the optical quasi-periodic
oscillations. Our model reproduces well the cross-correlation and phase lag
spectrum during the decline stage, which could not be understood with any model
proposed before.Comment: 13 pages, 11 figures, MNRAS submitte
Colours of black holes: infrared flares from the hot accretion disc in XTE J1550-564
Outbursts of the black hole (BH) X-ray binaries are dramatic events occurring in our Galaxy approximately once a year. They are detected by the X-ray telescopes and often monitored at longer wavelengths. We analyse the X-ray and optical/infrared (OIR) light curves of the BH binary XTE J1550–564 during the 2000 outburst. By using the observed extreme colours as well as the characteristic decay time-scales of the OIR and X-ray light curves, we put strong constraints on the extinction towards the source. We accurately separate the contributions to the OIR flux of the irradiated accretion disc and a non-thermal component. We show that the OIR non-thermal component appears during the X-ray state transitions both during the rising and the decaying part of the outburst at nearly the same X-ray hardness but at luminosities differing by a factor of 3. The line marking the quenching/recovery of the non-thermal component at the X-ray hardness–flux diagram seems to coincide with the ‘jet line’ that marks the presence of the compact radio jet. The inferred spectral shape and the evolution of the non-thermal component during the outburst, however, are not consistent with the jet origin, but are naturally explained in terms of the hybrid hot flow scenario, where non-thermal electrons emit synchrotron radiation in the OIR band. This implies a close, possibly causal connection between the presence of the hot flow and the compact jet. We find that the non-thermal component is hardening during the hard state at the decaying stage of the outburst, which indicates that the acceleration efficiency is a steep function of radius at low accretion rate. </p
Pulsar Wind-heated Accretion Disk and the Origin of Modes in Transitional Millisecond Pulsar PSR J1023+0038
Transitional millisecond pulsars provide a unique set of observational data for understanding accretion at low rates onto magnetized neutron stars. In particular, PSR.J1023+0038 exhibits a remarkable bimodality of the X-ray luminosity (low and high modes), pulsations extending from the X-ray to the optical band, GeV emission, and occasional X-ray flares. We discuss a scenario for the pulsar interaction with the accretion disk capable of explaining the observed behavior. We suggest that during the high mode the disk is truncated outside the light cylinder, allowing the pulsar wind to develop near the equatorial plane and strike the disk. The dissipative wind-disk collision energizes the disk particles and generates synchrotron emission, which peaks in the X-ray band and extends down to the optical band. The emission is modulated by the pulsar wind rotation, resulting in a pulse profile with two peaks 180 degrees apart. This picture explains the high mode luminosity, spectrum, and pulse profile (X-ray and optical) of PSR.J1023+0038. It may also explain the X-ray flares as events of sudden increase in the effective disk cross section intercepting the wind. In contrast to previously proposed models, we suggest that the disk penetrates the light cylinder only during the low X-ray mode. This penetration suppresses the dissipation caused by the pulsar wind-disk collision, and the system enters the propeller regime. The small duty cycle of the propeller explains the low spin-down rate of the pulsar
Doughnut strikes sandwich: the geometry of hot medium in accreting black hole X-ray binaries
We study the effects of the mutual interaction of hot plasma and cold medium in black hole binaries in their hard spectral state. We consider a number of different geometries. In contrast to previous theoretical studies, we use a modern energy-conserving code for reflection and reprocessing from cold media. We show that a static corona above an accretion disc extending to the innermost stable circular orbit produces spectra not compatible with those observed. They are either too soft or require a much higher disc ionization than that observed. This conclusion confirms a number of previous findings, but disproves a recent study claiming an agreement of that model with observations. We show that the cold disc has to be truncated in order to agree with the observed spectral hardness. However, a cold disc truncated at a large radius and replaced by a hot flow produces spectra which are too hard if the only source of seed photons for Comptonization is the accretion disc. Our favourable geometry is a truncated disc coexisting with a hot plasma either overlapping with the disc or containing some cold matter within it, also including seed photons arising from cyclo-synchrotron emission of hybrid electrons, i.e. containing both thermal and non-thermal parts
Evidence for a black hole spin--orbit misalignment in the X-ray binary Cyg X-1
Recently, the accretion geometry of the black-hole X-ray binary Cyg X-1 was
probed with the X-ray polarization. The position angle of the X-ray emitting
flow was found to be aligned with the position angle of the radio jet in the
plane of the sky. At the same time, the observed high polarization degree could
be obtained only for a high inclination of the X-ray emitting flow, indicating
a misalignment between the binary axis and the black hole spin. The jet, in
turn, is believed to be directed by the spin axis, hence similar misalignment
is expected between the jet and binary axes. We test this hypothesis using very
long (up to about 26 years) multi-band radio observations. We find the
misalignment of --. However, on the contrary to the earlier
expectations, the jet and binary viewing angles are found to be similar, while
the misalignment is seen between position angles of the jet and the binary axis
on the plane of the sky. Furthermore, the presence of the misalignment
questions our understanding of the evolution of this binary system.Comment: ApJL, in pres
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