State Transitions in Bright Galactic X-Ray Binaries: Luminosities Span by Two Orders of Magnitude

Abstract

Using X-ray monitoring observations with the All-Sky Monitor on board the Rossi X-Ray Timing Explorer and the Burst Alert Telescope on board the Swift, we are able to study the spectral state transitions occurred in about 20 bright persistent and transient black hole and neutron star binaries. We have confirmed that there is a correlation between the X-ray luminosity corresponding to the hard-to-soft transition and the X-ray luminosity of the following soft state. This correlation holds over a luminosity range spanning by 2 orders of magnitude, with no indication of a flux saturation or cutoff. We have also found that the transition luminosity correlates with the rate of increase in the X-ray luminosity during the rising phase of an outburst or flare, implying that the origin of the variation of the transition luminosity is associated with non-stationary accretion in both transient sources and persistent sources. The correlation between the luminosity corresponding to the end of the soft-to-hard transition and the peak luminosity of the preceding soft state is found insignificant. The results suggest that the hysteresis effect of spectral state transitions is primarily driven by non-stationary accretion when the mass accretion rate increases rather than the mass accretion rate decreases. Our results also imply that Galactic X-ray binaries can reach more luminous hard states during outbursts of higher luminosities and of similar rise timescales as those observed. Based on the correlations, we speculate that bright hard state beyond the Eddington luminosity will be observed in Galactic binaries in the next century. We also suggest that some ultra-luminous X-ray sources in nearby galaxies, which stay in the hard states during bright, short flares, harbor stellar-mass compact stars.

Discovery of the correlation between peak episodic jet power and X-ray peak luminosity of the soft state in black hole transients

“…This characteristic suggests that more luminous or dimmer episodic jet emission can be produced during more brighter or dimmer outbursts respectively, which hints that episodic jet production is somehow related to the occurrence of outbursts or specifically the rate-of-change in the mass accretion rate. This is similar to the conclusion obtained from the empirical relation between the hard-to-soft transition luminosity and the peak luminosity of the following soft state (Yu & Yan 2009). Both empirical correlations probably extend to much higher luminosities such as those of the ultra-luminous X-ray sources (ULXs) in nearby galaxies (Yu & Yan 2009).…”

“…The solid arrows mark the flux peaks of the soft states. There was only one exceptionwe excluded the isolated ASM flux peak in the soft state for XTE J1752-223 in order to avoid false measurement of the peak flux, as discussed in (Yu & Yan 2009).…”

“…On the other hand, as for the outbursts in the low mass X-ray binary (LMXB) transients, no saturation of the peak luminosity, up to the Eddington luminosity, is seen in the relation between the X-ray outburst peak luminosity and the orbital period in the Galactic transients (Wu et al 2010). These empirical relations suggested that some stellar mass compact objects can turn into ultra-luminous X-ray sources during brighter outbursts of similar duration or shorter outbursts (Yu & Yan 2009), which has been indeed observed (Middleton et al 2013). The correlation between the peak power of episodic jet and the corresponding peak luminosity of the soft state in Galactic sources suggests that the investigation of the relation between the episodic jet power and the peak luminosity of the soft state in transient ULXs would tell us about the nature of the ULXs as compared with the Galactic black hole transients; whether ULXs contains intermediate mass black holes or just extreme versions of the Galactic microquasars.…”

“…Since there is a correlation between the hard X-ray peak luminosity (i.e., the luminosity of the hard-to-soft transition in outbursts with state transitions) and the peak luminosity of the following soft state (Yu et al 2004(Yu et al , 2007aYu & Yan 2009), the correlation between the peak episodic jet power and the peak luminosity of the soft state in this study implies a correlation between the peak luminosity of the hard state (or the luminosity of the hard-to-soft state transition) and the peak power of the episodic jet. This seems to be supported by the apparent correlation between the power of the episodic jet and the accretion power measured before episodic jet ejection seen in GRS 1915+105.…”

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).…”