Spectral Properties of Accretion Disks Around Black Holes II -- Sub-Keplerian Flows With and Without Shocks

Close to a black hole, the density of the sub-Keplerian accreting matter becomes higher compared to a spherical flow due to the presence of a centrifugal barrier independent of whether or not a standing shock actually forms. This hot dense flow intercepts soft photons from a cold Keplerian disk and reprocesses them to form high energy X-rays and gamma rays. We study the spectral properties of various models of accretion disks where a Keplerian disk on the equatorial plane may or may not be flanked by a sub-Keplerian disk and the sub-Keplerian flow may or may not possess standing shocks. From comparison with the spectra, we believe that the observed properties could be explained better when both the components (Keplerian and sub-Keplerian) are simultaneously present close to a black hole, even though the sub-Keplerian halo component may have been produced out of the Keplerian disk itself at larger radii. We are able to understand soft and hard states of black hole candidates, properties of X-ray novae outbursts, and quasi-periodic oscillations of black hole candidates using these two component models. We fit spectra of X-ray novae GS1124-68 and GS2000+25 and satisfactorily reproduce the light curves of these objects.Comment: 15 Latex pages plus 12 figures. Macros included. Astrophysical Journal (In press

Where is the Inner Edge of an Accretion Disk Around a Black Hole?

What is meant by the "inner edge" of an accretion disk around a black hole depends on the property that defines the edge. We discuss four such definitions using data from recent high-resolution numerical simulations. These are: the "turbulence edge", where flux-freezing becomes more important than turbulence in determining the magnetic field structure; the "stress edge", where plunging matter loses dynamical contact with the outer accretion flow; the "reflection edge", the smallest radius capable of producing significant X-ray reflection features; and the "radiation edge", the innermost place from which significant luminosity emerges. All these edges are dependent on the accretion rate and are non-axisymmetric and time-variable. Although all are generally located in the vicinity of the marginally stable orbit, significant displacements can occur, and data interpretations placing the disk edge precisely at this point can be misleading. If observations are to be used successfully as diagnostics of accretion in strong gravity, the models used to interpret them must take careful account of these distinctions.Comment: accepted by Ap.J., 26 p

On the light-bending model of X-ray variability of MCG-6-30-15

We apply the light bending model of X-ray variability to Suzaku data of the Seyfert 1 galaxy MCG-6-30-15. We analyze the energy dependence of the root mean square (rms) variability, and discuss conditions necessary for the model to explain the characteristic decrease of the source variability around 5-8 keV. A model, where the X-ray source moves radially rather than vertically close to the disk surface, can indeed reproduce the reduced variability near the energy of the Fe Kalpha line, although the formal fit quality is poor. The model then predicts the energy spectra, which can be compared to observational data. The spectra are strongly reflection dominated, and do not provide a good fit to Suzaku spectral data of the source. The inconsistency of this result with some previous claims can be traced to our using data in a broader energy band, where effects of warm absorber in the spectrum cannot be neglected.Comment: 6 pages, PASJ, accepte

Timing and Spectral Properties of X-ray Emission from the Converging Flows onto Black hole: Monte-Carlo Simulations

We demonstrate that a X-ray spectrum of a converging inflow (CI) onto a black hole is the sum of a thermal (disk) component and the convolution of some fraction of this component with the Comptonization spread (Green's) function. The latter component is seen as an extended power law at energies much higher than the characteristic energy of the soft photons. We show that the high energy photon production (source function) in the CI atmosphere is distributed with the characteristic maximum at about the photon bending radius, 1.5r_S, independently of the seed (soft) photon distribution. We show that high frequency oscillations of the soft photon source in this region lead to the oscillations of the high energy part of the spectrum but not of the thermal component. The high frequency oscillations of the inner region are not significant in the thermal component of the spectrum. We further demonstrate that Doppler and recoil effects (which are responsible for the formation of the CI spectrum) are related to the hard (positive) and soft (negative) time lags between the soft and hard photon energy channels respectively.Comment: 9 pages and 4 figures, to be published in the Astrophysical Journal Letter

A Disk--Jet interaction model for the X--Ray Variability in Microquasars

We propose a simple dynamical model that may account for the observed spectral and temporal properties of GRS 1915+105 and XTE J1550-5634. The model is based on the assumption that a fraction of the radiation emitted by a hot spot lying on the accreting disk is dynamically Comptonized by the relativistic jet that typically accompanies the microquasar phenomenon. We show that scattering by the jet produces a detectable modulation of the observed flux. In particular, we found that the phase lag between hard and soft photons depends on the radial position of the hot spot and, if the angle between the jet and the line of sight is sufficiently large, the lags of the fundamental and its harmonics may be either positive or negative.Comment: 14 pages, 4 figures, accepted for publication in ApJ Part

Evidence For Advective Flow From Multi-Wavelength Observations Of Nova Muscae

We model the UV/optical spectrum of the black hole binary Nova Muscae as a sum of black body emissions from the outer region of an accretion disk. We show for self-consistency that scattering effects in this region are not important. The black hole mass ($M \approx 6 M_\odot$), the inclination angle ($\mu \approx 0.5$) and the distance to the source ($D \approx 5$ kpc) have been constrained by optical observations during quiescence (Orosz et al. 1996). Using these values we find that the accretion rate during the peak was ${\dot M} \approx 8 \times 10^{19}$ g sec$^{-1}$ and subsequently decayed exponentially. We define a radiative fraction ($f$) to be the ratio of the X-ray energy luminosity to the total gravitational power dissipated for a keplerian accretion disk. We find that $f \approx 0.1$ and remains nearly constant during the Ultra-soft and Soft spectral states. Thus for these states, the inner region of the accretion disk is advection dominated. $f$ probably increased to $\approx 0.5$ during the Hard state and finally decreased to $\approx 0.03$ as the source returned to quiescence.Comment: 5 figures. uses aasms4.sty, accepted by Ap

Mass Determination of Black Holes in LMC X-1 and Nova Muscae 1991 from their High-Energy Spectra

We offer a brief description of the bulk-motion Comptonization (BMC) model for accretion onto black holes, illustrated by its application to observational data for LMC X-1, and Nova Muscae 1991. We then extract some physical parameters of these systems from observables (within the context of the BMC model}, drawing from results on GRO J1655-40, for which we presented extensive analysis previously. We derive estimates of the mass, (16 +/- 1) solar masses and mass accretion rate in the disk in Eddington units around 2 for LMC X-1, and (24 +/- 1)d_{5.5} and the disk mass acretion rate around 3 for Nova Muscae 1991 [where d_{5.5} stands for the distance in 5.5 kpc units]. Differences between these estimates and previous estimates based on dynamical studies are discussed. It is further shown that the disk inner radius increases with the high-to-low state transition in Nova Muscae 1991. Specifically, our analysis suggests that the inner-disk radius increases to 17 Scwarzschild radii as the transition to the low-hard state occurs.Comment: 14 pages, 3 figures, The paper is accepted for publication in the Astrophysical Journal Letter

Detection of anti-correlated hard X-ray time lag in Cygnus X-3

The wide-band X-ray spectra of the high mass X-ray binary Cygnus X-3 exhibits a pivoting behavior in the `low' (as well as `hard') state, correlated to the radio emission. The time scale of the soft and hard X-rays' anti-correlation, which gave rise to the pivoting feature, was found to be less than a day from the monitoring observations by RXTE--ASM and CGRO--BATSE. In this Letter we report the detection of a lag of $\lesssim$ 1000s in the anti-correlation of the hard X-ray emission (20--50 keV) to that of the soft X-ray emission (2--7 keV), which may be attributed to the viscous time scale of flow of matter in the accretion disk. This suggests the geometrical picture of a truncated accretion disc with a Compton cloud inside the disc, the relative sizes of which determine the spectral shape. Any change in the disc structure will take place in a viscous time scale, with corresponding anti-correlated change in the Compton cloud. We also report the pivoting in the spectra in one span of a pointed observation when an episode of the rearranging of the accretion system is serendipitously observed. This is the first such observation of hard X-ray delay seen in the persistent Galactic microquasars, within the precincts of the hard state.Comment: Accepted in The Astrophysical Journal (Letters): in pres

On the spectral slopes of hard X-ray emission from black hole candidates

Most black hole candidates exhibit characteristic power-law like hard X-ray emission above \sim 10 keV. In the {\em high state}, in which 2 -- 10 keV luminosity is relatively high, the energy index of the hard X-ray emission is usually greater than 1 --- typically \sim 1.5. On the other hand, in the {\em low state}, the hard X-ray energy index is 0.3 -- 0.9. In this paper, we suggest that this difference of the hard X-ray spectral slopes may be due to two different Comptonization mechanisms. We propose that, in the high state, the hard component is governed by the Comptonization due to the bulk motion of the almost freely falling (convergent accretion) flow close to the black hole, rather than thermal Comptonization. The spectral slope of the hard component is insensitive to the disk accretion rate governing the soft component, hence is nearly invariant in spite of the soft component variations. The power-law component due to the bulk motion Comptonization has a sharp cut-off at around the electron rest mass energy, which is consistent with high energy observations of the high state. In the low state, the spectrum is formed due to thermal Comptonization of the low-frequency disk radiation by a sub-Keplerian component (possibly undergoing a centrifugally-supported shock) which is originated from the Keplerian disk. In the limit of low disk accretion rate, the power law index is uniquely determined by the mass accretion rate of the sub-Keplerian component