October 31, 1996
With the advent of RXTE, which is capable of broad spectral coverage and fast timing, as well as other instruments which are increasingly being used in multi-wavelength campaigns (via both space-based and ground-based observations), we must demand more of our theoretical models. No current model mimics all facets of a system as complex as an x-ray binary. However, a modern theory should qualitatively reproduce -- or at the very least not fundamentally disagree with - all of Cygnus X-1's most basic average properties: energy spectrum (viewed within a broader framework of black hole candidate spectral behavior), power spectrum (PSD), and time delays and coherence between variability in different energy bands. Below we discuss each of these basic properties in turn, and we assess the health of one of the currently popular theories: Comptonization of photons from a cold disk. We find that the data pose substantial challenges for this theory, as well as all other currently discussed models.
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October 8, 1998
We present results from a 20 ksec RXTE observation of the black hole candidate Cyg X-1. We apply self-consistent accretion disk corona models to these hard state data and show that Comptonization in a spherical corona irradiated by soft photons from an exterior cold disk is able to successfully model the spectrum. We also present the power spectrum, the coherence function, and the time lags for lightcurves from four energy bands. By modeling the high-resolution lightcurves wi...
December 7, 1997
There have been a number of recent spectral models that have been successful in reproducing the observed X-ray spectra of galactic black hole candidates (GBHC). However, there still exists controversy over such issues as: what are the sources of hard radiation, what is the system's geometry, is the accretion efficient or inefficient, etc. A potentially powerful tool for distinguishing among these possibilities, made possible by the Rossi X-ray Timing Explorer (RXTE), is the v...
February 7, 1997
We report the results from our timing analysis of 15 RXTE observations of Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be divided into 3 distinct phases: (1) transition from the hard to soft state, (2) soft state, and (3) transition from the soft state back to the hard state. The observed X-ray properties in phases 1 and 3 are remarkably similar, suggesting that the same physical processes are likely involved in triggering such transitions. The p...
March 20, 2000
Recently, there appeared many papers devoted to the modeling of X-ray properties of Cygnus X-1 and other black hole accretion disk candidates: e.g. J.Poutanen, J.Krolik, F.Ryde, MNRAS 292 (1997) L21; E.Agol, J.Krolik, ApJ 507 (1998) 304; A.Beloborodov, in ``High Energy Processes in Accreting Black Holes'', ASP Conf. Series, 161, (1999), p.295. The goal of this electronic publication is to draw attention to our old papers where many ideas of recent discussions were anticipated...
December 8, 1999
We show that waves propagating in a transition disk can explain the short term temporal behavior of Cygnus X-1. In the transition disk model the spectrum is produced by saturated Comptonization within the inner region of the accretion disk where the temperature varies rapidly with radius. Recently, the spectrum from such a disk has been shown to fit the average broad band spectrum of this source better than that predicted by the soft-photon Comptonization model. Here, we cons...
October 6, 2005
(abbreviated abstract) We present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15GHz using ~200 pointed observations taken between early 1999 and late 2004 with RXTE and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., compTT or eqpair, can reproduce this simplicity. Broken power l...
November 19, 1998
We present a model for the short timescale spectral variability of accreting black holes. It describes well the time-averaged spectra as well as temporal characteristics such as power-density spectrum, time/phase lags, and coherence function of Cygnus X-1. We assume that X/gamma-rays are produced in compact magnetic flares at radii < 100GM/c^2 from the central black hole. The tendency for magnetic loops to inflate and detach from the underlying accretion disc causes the spect...
April 26, 2004
The peculiar X-ray binary Cygnus X-3 has been observed on several occasions with the X/gamma-ray instruments on board INTEGRAL. We have collected data from available public and Galactic Plane Scan observations between December 2002 and December 2003 and summed them together into two broad-band spectra, representing different physical spectral states of the source. We have fitted the two spectra with models including Comptonization and Compton reflection, similar to those foun...
March 13, 2018
We develop new techniques to deconvolve the radial structure of the X-ray emission region in the bright low/hard state of the black hole Cygnus X-1 using both spectral and timing data in the 3-35~keV range. The spectrum at these energies is dominated by Comptonisation rather than the disc, but there is a complex pattern in the time lags between different energy bands and differences in the normalisation and shape in the power spectra of these bands, which clearly shows that t...
August 14, 2017
We present a timing analysis of threeRossi X-ray Timing Explorer observations of the black hole binary Cygnus X-1 with the propagating mass accretion rate fluctuations model PROPFLUC. The model simultaneously predicts power spectra, time lags, and coherence of the variability as a function of energy. The observations cover the soft and the hard state of the source, and the transition between the two. We find good agreement between model pre-dictions and data in the hard and i...