29 research outputs found
Rayleigh - Taylor Gravity Waves and Quasiperiodic Oscillation Phenomenon in X-ray Binaries
Accretion onto compact objects in X-ray binaries [black hole, neutron star
(NS), white dwarf] is characterized by non-uniform flow density profiles. Such
an effect of heterogeneity in presence of gravitational forces and pressure
gradients exhibits Raylegh-Taylor gravity waves (RTGW). They should be seen as
quasioperiodic wave oscillations (QPO). In this paper I show that the main QPO
frequency, which is very close to the Keplerian frequency, is split into
separate frequencies (hybrid and low branch) under the influence of the
gravitational forces in the rotational frame of reference. The observed low and
high QPO frequencies are an intrinsic signature of the RTGW. I elaborate the
conditions for the density profile when the RTGW oscillations are stable. A
comparison of the inferred QPO frequencies with QPO observations is presented.
I find that hectohertz frequencies detected from NS binaries can be identified
as the RTGW low branch frequencies. I also predict that an observer can see the
double NS spin frequency during the NS long (super) burst events when the
pressure gradients and buoyant forces are suppressed. The Coriolis force is the
only force which acts in the rotational frame of reference and its presence
causes perfect coherent pulsations with a frequency twice of the NS spin.Comment: 14 pages and 2 figures, ApJ in pres
Spectral Properties of Accretion Disks Around Galactic and Extragalactic Black Holes
We study the spectral properties of a very general class of accretion disks
which can be decomposed into three distinct components apart from a shock at
: (1) An optically thick Keplerian disk on the equatorial plane
(), (2) A sub-Keplerian optically thin halo above and below this
Keplerian disk and (3) A hot, optically slim, postshock
region where is the Schwarzschild radius. The
postshock region intercepts soft photons from the Keplerian component and
reradiates them as hard X-rays and rays after Comptonization. We solve
two-temperature equations in the postshock region with Coulomb energy exchange
between protons and electrons, and incorporating radiative processes such as
bremsstrahlung and Comptonization. We also present the exact prescription to
compute the reflection of the hard X-rays from the cool disk. We produce
radiated spectra from both the disk components as functions of the accretion
rates and compare them with the spectra of galactic and extragalactic black
hole candidates. We find that the transition from hard state to soft state is
smoothly initiated by a single parameter, namely, the mass accretion rate of
the disk. In the soft state, when the postshock region is very optically thick
and cooled down, bulk motion of the converging flow determines the spectral
index to be about in agreement with observations.Comment: 45 pages of Latex file + 10 figures (Misprints in Description and
Caption of Figure 2 are corrected
The Extended Power Law as Intrinsic Signature For a Black Hole
We analyze the exact general relativistic exact integro-differential equation
of radiative transfer describing the interaction of low energy photons with a
Maxwellian distribution of hot electrons in gravitational field of a
Schwarzschild black hole. We prove that due to Comptonization an initial
arbitrary spectrum of low energy photons unavoidably results in spectra
characterized by an extended power-law feature. We examine the spectral index
by using both analytical and numerical methods for a variety of physical
parameters as such the plasma temperature and the mass accretion rate. The
presence of the event horizon as well as the behaviour of the null geodesics in
its vicinity largely determine the dependence of the spectral index on the flow
parameters. We come to the conclusion that the bulk motion of a converging flow
is more efficient in upscattering photons than thermal Comptonization provided
that the electron temperature in the flow is of order of a few keV or less. In
this case, the spectrum observed at infinity consists of a soft component
produced by those input photons that escape after a few scatterings without any
significant energy change and of hard component (described by a power law)
produced by the photons that underwent significant upscattering. The luminosity
of the power-law component is relatively small compared to that of the soft
component. For accretion into black hole the spectral energy index of the
power-law is always higher than one for plasma temperature of order of a few
keV. This result suggests that the bulk motion Comptonization might be
responsible for the power-law spectra seen in the black-hole X-ray sources.Comment: 31 pages, 3 figures; Astrophysical Journal accepte
X-ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes
We study Compton upscattering of low-frequency photons in a converging flow
of thermal plasma. The photons escape diffusively and electron scattering is
the dominant source of opacity. We solve numerically and approximately
analytically the equation of radiative transfer in the case of spherical,
steady state accretion into black holes. Unlike previous work on this subject,
we consider the inner boundary at a finite radius and this has a significant
effect on the emergent spectrum. It is shown that the bulk motion of the
converging flow is more efficient in upscattering photons than thermal
Comptonization, provided that the electron temperature in the flow is of order
a few keV or less. In this case, the spectrum observed at infinity consists of
a soft component coming from those input photons which escaped after a few
scatterings without any significant energy change and of a power law which
extends to high energies and is made of those photons which underwent
significant upscattering. The luminosity of the power law is relatively small
compared to that of the soft component. The more reflective the inner boundary
is, the flatter the power-law spectrum becomes. The spectral energy power-law
index for black-hole accretion is always higher than 1 and it is approximately
1.5 for high accretion rates. This result tempts us to say that bulk motion
Comptonization might be the mechanism behind the power-law spectra seen in
black-hole X-ray sources.Comment: 37 pages, LaTex, AAS Macros, 8 ps figures, to appear in Ap
The hard X-ray tails in neutron star low mass X-ray binaries: BeppoSAX observations and possible theoretical explanation of the GX 17+2 case
We report results of a new spectral analysis of two BeppoSAX observations of
the Z source GX 17+2. In one of the two observations the source exhibits a
powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work
by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL
of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have
shown that bulk (dynamical) Comptonization of soft photons can be a possible
alternative mechanism for producing hard X-ray tails in such systems. We start
from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX
to better investigate the physical processes at work. We use GX 17+2 as a
representative case. Moreover, we suggest that weakening (or disappearance) of
the hard X-ray tail can be explained by increasing radiation pressure
originated at the surface of the neutron star (NS). As a result the high
radiation pressure stops the bulk inflow and consequently this radiation
feedback of the NS surface leads to quenching the bulk Comptonization.Comment: 6 pages, 3 figures, Accepted for publication in Ap
The converging inflow spectrum is an intrinsic signature for a black hole: Monte-Carlo simulations of Comptonization on free-falling electrons
An accreting black hole is, by definition, characterized by the drain.
Namely, the matter falls into a black hole much the same way as water
disappears down a drain - matter goes in and nothing comes out. As this can
only happen in a black hole, it provides an unique way to see it. The accretion
proceeds almost in free fall close to the black hole horizon. In this paper we
calculate (by using Monte -Carlo simulations) the specific features of X-ray
spectra formed as a result of upscattering of the soft (disk) photons in the
converging inflow (CI) within about 3 Schwarzschild radii of the black hole.
The full relativistic treatment has been implemented to reproduce these
spectra. We show that spectra in the soft state of black hole systems can be
described as the sum of a thermal (disk) component and the convolution of some
fraction of this component with the CI upscattering spread function. The latter
boosted photon component is seen as an extended power-law at energies much
higher than the characteristic soft photons energy. We demonstrate the
stability of the power spectral index (alpha= 1.8) over a wide range of the
plasma temperature 0-10 keV and mass accretion rates (higher than 2 in
Eddington units). We also demonstrate that the sharp high energy cutoff occurs
at energies of 200-400 keV which are related to the average rest energy of
electrons impinging upon the horizon. The spectrum is practically identical to
the standard thermal Comptonization spectrum when the CI plasma temperature is
getting of order of 50 keV (hard state of BHS). Also, the change of spectral
shapes from the soft to the hard X-ray state is clearly to be related with the
temperature of the bulk flow. These Monte-Carlo simulated CI spectra are then a
inevitable stamp of the BHS.Comment: 30 pages TeX format, 6 PS figures, accepted for ApJ Main Journa
Broad redshifted line as a signature of outflow
We formulate and solve the diffusion problem of line photon propagation in a
bulk outflow from a compact object (black hole or neutron star) using a generic
assumption regarding the distribution of line photons within the outflow.
Thomson scattering of the line photons within the expanding flow leads to a
decrease of their energy which is of first order in v/c, where v is the outflow
velocity and c is the speed of light. We demonstrate that the emergent line
profile is closely related to the time distribution of photons diffusing
through the flow (the light curve) and consists of a broad redshifted feature.
We analyzed the line profiles for the general case of outflow density
distribution. We emphasize that the redshifted lines are intrinsic properties
of the powerful outflow that are supposed to be in many compact objects.Comment: 16 pages, 1 black-white figure and 2 color figures; accepted for
publication in the Astrophysical Journa
Correlations between X-ray Spectral and Timing Characteristics in Cyg X-2
Correlations between the quasi-periodic oscillations (QPOs) and the spectral
power-law index have been reported for a number of black hole candidate sources
and for four neutron star (NS) sources, 4U 0614+09, 4U 1608-52, 4U 1728-34 and
Sco X-1. An examination of QPO frequencies and index relationship in Cyg X-2 is
reported herein. The RXTE spectrum of Cyg X-2 can be adequately represented by
a simple two-component model of Compton up-scattering with a soft photon
electron temperature of about 0.7 keV and an iron K-line. Inferred spectral
power-law index shows correlation with the low QPO frequencies. We find that
the Thomson optical depth of the Compton cloud (CC) tau, in framework of
spherical geometry, is in the range of ~4-6, which is consistent with the
neutron star's surface being obscured. The NS high frequency pulsations are
presumably suppressed as a result of photon scattering off CC electrons because
of such high values of tau. We also point out a number of similarities in terms
timing (presence of low and high frequency QPOs) and spectral (high CC optical
depth and low CC plasma temperature) appearances between Cyg X-2 and Sco X-1.Comment: 7 pages, 4 figures, accepted for publication in ApJ (October 1, 2007,
v667n2 issue
Power-law Tails from Dynamical Comptonization in Converging Flows
The effects of bulk motion comptonization on the spectral formation in a
converging flow onto a black hole are investigated. The problem is tackled by
means of both a fully relativistic, angle-dependent transfer code and a
semi-analytical, diffusion-approximation method. We find that a power-law
high-energy tail is a ubiquitous feature in converging flows and that the two
approaches produce consistent results at large enough accretion rates, when
photon diffusion holds. Our semi-analytical approach is based on an expansion
in eigenfunctions of the diffusion equation. Contrary to previous
investigations based on the same method we find that, although the power-law
tail at really large energies is always dominated by the flatter spectral mode,
the slope of the hard X-ray portion of the spectrum is dictated by the second
mode and it approaches Gamma=3 at large accretion rate, irrespective of the
model parameters. The photon index in the tail is found to be largely
independent on the spatial distribution of soft seed photons when the accretion
rate is either quite low ( 10).
On the other hand, the spatial distribution of source photons controls the
photon index at intermediate accretion rates, when Gamma switches from the
first to the second mode. Our analysis confirms that a hard tail with photon
index Gamma <3 is produced by the up-scattering of primary photons onto
infalling electrons if the central object is a black hole.Comment: to be published in the Astrophysical Journal, 16 pages and 9 figure