Two Different Long‐Term Behaviors in Black Hole Candidates: Evidence for Two Accretion Flows?

Abstract

We discuss the results of long-term hard X-ray monitoring of Galactic black hole candidates 1E 1740.7À2942, GRS 1758À258, Cyg X-1, GX 339À4, and Cyg X-3 with the Rossi X-Ray Timing Explorer. The objects divide into two classes. In the first class, exemplified by Cyg X-1, luminosity and spectral hardness evolve simultaneously. In the second class, the relation is more complicated: the softest spectra occur while the count rate is dropping. Most models of accretion, tailored to Cyg X-1, do not predict the second sort of behavior. One interpretation is a simple model with two simultaneous, independent accretion flows: a thin disk and a hot halo. A drop in the accretion rate affecting both flows would propagate through the halo immediately but might take up to several weeks to propagate through the disk. While the inner halo is thus temporarily depleted compared to the disk, a temporary soft state is expected. This picture is supported by the observation that those sources that show delays (1E 1740.7À2942, GRS 1758À258, and GX 339À4) are expected to have low-mass companions, and those that do not (Cyg X-1, Cyg X-3) are known or thought to have high-mass companions. Low-mass companions imply accretion by Roche lobe overflow, with a high specific angular momentum in the accreting material, and therefore a large disk with a long viscous timescale. Wind accretion from massive companions is expected to result in a much smaller disk and thus little viscous delay.

1E 1740.7–2942: Temporal and spectral evolution from INTEGRAL and RXTE observations

“…Although the opposite behaviour, with the flux in the soft state larger than that in the hard state, is most typical of black-hole binaries, the behaviour observed by us is also seen in other low-mass X-ray binaries, e.g., in GX 339-4. Smith et al (2002a) often observed GRS 1758-258 at low luminosities in the typical soft state. This is due to the existence of two accretion solutions at a given accretion rate, which allows for such a hysteretic behaviour (e.g., ).…”

“…During RXTE observations of 1E 1740.7-2942, an interesting correlation between the time derivative of the flux and the hardness was discovered; no such correlation was found in Cyg X-1 (Smith et al 2002a). A periodic modulation with amplitude 3-4% at 12.73 ± 0.05 days has been measured and interpreted as the orbital period, suggesting that the object could have a red-giant companion (Smith et al 2002b).…”

Black hole spin of Cygnus X-1 determined from the softest state ever observed

“…The transients show large scale hysteresis, where the hard-to-soft transition during the rapid outburst rise can occur at a much higher (up to at least a factor of 10) luminosity than the reverse transition on the decline, and where the luminosity of the hardto-soft transition itself is variable between different outbursts of the same source (e.g. Smith et al 2002;Maccarone & Coppi 2003;Gierliński & Newton 2006;Gladstone et al 2007;Yu & Yan 2009;Dunn et al 2010). By contrast, in Cyg X-1, the hysteresis is only small scale, less than a factor of 2-3, and the hard-to-soft transition luminosity is fairly stable (Smith et al 2002;Zdziarski et al 2002;Meyer et al 2007).…”

Role of Disk Models in Indentifying Astrophysical Black Holes

“…Is the evaporation because intrinsic 'boiling' of the disk, or due to irradiation of X-rays emitted further close to the hole, or both? The observations seem to indicate that the corona is dynamic, it is changing its character very fast, in a free-fall time (Smith et al 2001(Smith et al , 2002. Thus, the only possibility is that it is another flow which engulfs the Keplerian disk on the equatorial plane.…”