Phase Lags and Coherence of X-Ray Variability in Black Hole Candidates

The ``low'' (hard or ``non-thermal'') state of black hole candidates is sometimes modelled via an optically thick, hot Compton cloud that obscures a softer input source such as an accretion disk. In these models the observed output spectra consist entirely of photons reprocessed by the cloud, making it difficult to extract information about the input spectra. Recently Miller (1995) has argued that the Fourier phase (or time) lag between hard and soft X-ray photons in actuality represents the phase lags intrinsic to the input source, modulo a multiplicative factor. The phase lags thus would be a probe of the input photon source. In this paper we examine this claim and find that, although true for the limited parameter space considered by Miller, the intrinsic phase lag disappears whenever the output photon energy is much greater than the input photon energy. The remaining time lags represent a ``shelf'' due to differences between mean diffusion times across the cloud. As pointed out by Miller, the amplitude of this shelf -- which is present even when the intrinsic time lags remain -- is indicative of the size and temperature of the Compton cloud and is a function of the two energies being compared. However, we find that with previous instruments such as Ginga the shelf, if present, was likely obscured by counting noise. A more sensitive measure of Compton cloud parameters may be obtainable from the coherence function, which is derived from the amplitude of the Fourier cross power spectral density. This function has been seen to exponentially decrease at high Fourier frequencies in Cygnus X-1. Coherence loss is characteristic of Compton clouds that undergo large variations of size and/or temperature on time scales longer than about 10 seconds. We argue that observing phase lags and coherenceComment: 14 pages, uuencoded postscript, accepted for publication in Monthly Notice

Coronal--Temporal Correlations in GX339-4: Hysteresis, Possible Reflection Changes, and Implications for ADAFs

We present spectral fits and timing analysis of Rossi X-ray Timing Explorer observations of GX339-4. These observations were carried out over a span of more than two years and encompassed both the soft/high and hard/low states. Hysteresis in the soft state/hard state transition is observed. The hard state exhibits a possible anti-correlation between coronal compactness (i.e., spectral hardness) and the covering fraction of cold, reflecting material. The correlation between `reflection fraction' and soft X-ray flux, however, appears to be more universal. Furthermore, low flux, hard state observations - taken over a decline into quiescence- show that the Fe line, independent of `reflection fraction', remains broad and at a roughly constant equivalent width, counter to expectations from ADAF models. All power spectral densities (PSD) of the hard state X-ray lightcurves are describable as the sum of just a few broad, quasi-periodic features with frequencies that roughly scale as coronal compactness to the -3/2 power. Similar to observations of Cyg X-1, time lags between soft and hard variability anti-correlate with coronal compactness. A stronger correlation is seen between the time lags and the `reflection fraction'.Comment: 29 Pages, 17 Figures, 6 Tables. Accepted for Publication in MNRAS. (Abstract Abridged

The Future of X-ray Spectroscopy of Galactic Black Hole Binaries

There are four major X-ray satellites currently in operation, with two more shortly to follow, and several very ambitious observatories in various stages of planning. This very rich period of X-ray observation is leading to great advances in our understanding of the accretion flow onto the black hole, although we are quickly learning (or perhaps better put, remembering) exactly how complicated this flow can be. This review was meant to assess future prospects for X-ray spectroscopy of black hole binaries; however, I first look backward to the observations and theories that helped us arrive at our current `paradigm'. I then discuss current and near-future spectroscopic studies, which increasingly (and very fruitfully) treat X-ray spectroscopy as part of a larger, intimately connected picture along with radio, optical, and gamma-ray spectroscopy. Equally importantly, and in large part thanks to the success of RXTE, there is now a strong realization that spectral-temporal correlations, even across wavelength bands, are crucial to our understanding of the physics of these systems. Going forward, we are well-poised to continue to advance our knowledge via X-ray spectroscopy, both with existing satellites that have a long lifetime ahead of them, and with the next generation of instruments. If there is any `hole' in this bright future, it is the potential loss of RXTE, with no designated follow-up mission. Studies of multi-wavelength spectral-temporal correlations will become more difficult due to the loss of two important attributes of RXTE: its fast timing capabilities and its extremely flexible scheduling which has made many of these studies possible.Comment: To appear in Proceedings of "X-ray Timing 2003: Rossi and Beyond", ed. P. Kaaret, F.K. Lamb, & J.H. Swan

X-Ray Variability Coherence: How to Compute it, What it Means, and How it Constrains Models of Cyg X-1 and GX 339-4

We describe how the coherence function, a Fourier frequency-dependent measure of the linear correlation between time series measured simultaneously in two energy channels, can be used in conjunction with energy spectra, power spectra, and time delays between energy channels to constrain models of the spectrum and variability of x-ray binaries. Here we present a procedure for estimating the coherence function in the presence of counting noise. We apply this method to the black hole candidates Cyg X--1 and GX 339--4, and find that the near perfect coherence between low and high energy x-ray photons rules out a wide range of models that postulate: spatially extended fluctuating emission, thermal flares, and overlapping shot-noise.Comment: Latex file (emulateapj macro included, see comments at beginning of file), 1 eps figure. To be published in ApJ Letters, Jan. 1, 199