The Three Spectral Regimes Found in the Stellar Black Hole XTE J1550−564 in Its High/Soft State

Abstract

The present paper describes the analysis of multiple RXTE/PCA data of the black hole binary with superluminal jet, XTE J1550 − 564, acquired during its 1999-2000 outburst. The X-ray spectra show features typical of the high/soft spectral state, and can approximately be described by an optically thick disk spectrum plus a power-law tail. Three distinct spectral regimes, named standard regime, anomalous regime, and apparently standard regime, have been found from the entire set of the observed spectra. When the X-ray luminosity is well below ∼ 6 × 10 38 erg s −1 (assuming a distance of 5 kpc), XTE J1550 − 564 resides in the standard regime, where the soft spectral component dominates the power-law component and the observed disk inner radius is kept constant. When the luminosity exceeds the critical luminosity, the apparently standard regime is realized, where luminosity of the optically thick disk rises less steeply with the temperature, and the spectral shape is moderately distorted from that of the standard accretion disk. In this regime, radial temperature gradient of the disk has been found to be flatter than that of the standard accretion disk. The results of the apparently standard regime are suggestive of a slim disk (e.g., Abramowicz et al. 1988, Watarai et al. 2000 which is a solution predicted under high mass accretion rate. In the intermediate anomalous regime, the spectrum becomes much harder, and the disk inner radius derived using a simple disk model spectrum apparently varies significantly with time. These properties can

An Optically Thick Disk Wind in Gro J1655–40?

“…Similar very soft X-ray spectra with no or only a weak nonthermal power-law component are sometimes observed in other BH low-mass X-ray binaries, such as GRS 1915+105, XTE J1550−564, and GX 339−4 (e.g., Kubota & Makishima 2004;Zdziarski & Gierliński 2004), in their brightest phase. They appear in the luminosity range from~L 0.4 Edd to~L 1 Edd , which is consistent with the luminosity of GRO J1655−40 at the Chandra epoch that we estimated in the SED modeling.…”

“…Some previous studies suggest that the disk is truncated at similar luminosities: in the bright low/hard state above~L 0.01 Edd (e.g., Done et al 2007;Shidatsu et al 2011b;Yamada et al 2013;Kolehmainen et al 2014;Shidatsu et al 2014;Plant et al 2015) and in the very high state (Tamura. et al 2012;Hori et al 2014) over~L 0.2 Edd (Kubota & Makishima 2004). However, the properties of the hypersoft state are nothing like these states; the X-ray spectra in the hypersoft state are much softer and the variability power on timescales of ∼0.01-10 s is more than one order of magnitude lower (see, e.g., Remillard 2006 andDone et al 2007 for spectral and timing properties in each state).…”

Two Eclipsing Ultraluminous X-Ray Sources in M51

“…They fare relatively better (c » n 1.6 2 and c » n 1.5 2 , respectively) than a power-law model, but they are still not good fits. They also require a surprisingly low peak color temperature, » kT 0.6 in -0.7 keV; this is inconsistent with the disk temperatures expected near or just above the Eddington limit (»1.0-1.3 keV: e.g., Kubota & Makishima 2004;Remillard & McClintock 2006), and would require a heavy stellar-mass BH (as we shall discuss later).…”

MAXI observations of long-term variations of Cygnus X-1 in the low/hard and the high/soft states

“…In this modeling, the disk photons are partially fed into nthComp, so that the parameter r in in the diskbb model is smaller than the true inner radius which is denoted R in hereafter. This R in was calculated from the sum of 0.01-100 keV photon flux of the diskbb component, F disk ph , and that of nthComp component, F nth ph , utilizing the innermost temperature T in of diskbb and the equation by Kubota & Makishima (2004) as…”