The Correlation between Hard X-Ray Peak Flux and Soft X-Ray Peak Flux in the Outburst Rise of Low-Mass X-Ray Binaries

Abstract

We have analyzed Rossi X-ray timing explorer (RXTE) pointed observations of the outbursts of black hole and neutron star soft X-ray transients in which an initial low/hard state or 'island' state, followed by a transition to a softer state, was observed. In three sources, the black hole transient XTE J1550-564, the neutron star transient Aquila X-1 and a quasi-persistent neutron star low mass X-ray binary (LMXB) 4U 1705-44, two such outbursts were found. We find that the flux of the soft X-ray peak, which lags the hard X-ray peak by a few days to several weeks, scales with the flux of the hard X-ray peak. We conclude that we are able to predict the soft X-ray outburst peak flux based on the 'preceding' hard X-ray peak flux, implying an early set up of the outbursts. We also find that the X-ray luminosity corresponding to the peak of the hard X-ray flux, which corresponds to the X-ray luminosity of the start of the hard-tosoft state transition, varies by a factor of about 2. If the accretion geometry early in the outburst rise is composed of two flows (e.g. a hot sub-Keplerian halo flow and a Keplerian disk flow, or an outflow and a Keplerian disk flow), the correlation indicates that the two flows are initially related, probably due to processes in the outer part of the accretion disk. We discuss constraints on a single flow model and a disk-jet model from these observations.

On the relation of hard X-ray peak flux and outburst waiting time in the black hole transient GX 339-4

“…Independent of accretion geometry considerations, Yu & Yan (2009) performed a comprehensive study of spectral state transitions in bright Galactic X-ray binaries. Their results confirmed the correlation between LH-to-HS transition luminosity and the peak luminosity of the following soft state shown found by previous studies (Yu et al 2004Yu & Dolence 2007), and provided strong evidence that: a) non-stationary accretion plays a dominant role in generating a bright LH state and b) the rate-of-increase of the mass accretion rate may be the dominant parameter determining spectral state transitions. The empirical relation between the LH-to-HS state transition luminosity and the peak luminosity of the following HS state and the empirical relation studied in this paper connect both the mass in the accretion disk (its cause and initial condition) and the peak luminosity of the hard state (its effect) with the rate-ofincrease in the mass accretion rate.…”

“…However, this is not the case. The correlation between the hard X-ray peak flux and the peak flux of the corresponding HS state is found to hold for individual black hole binaries and neutron-star low-mass X-ray binaries (Yu et al 2004;Yu & Dolence 2007;Yu & Yan 2009). Given that the neutron star has a hard surface, the observed X-ray flux from the neutron star system should in general reflect the instantaneous mass accretion rate.…”

States and Transitions in Black Hole Binaries

“…6. The similarities between X-ray outbursts of black-hole and neutron-star transients were explored by [51,102,103] and clearly suggest that the overall phenomenon of outburst evolution is similar. A similar evolution was recently found in the Xray emission of a persistent neutron-star X-ray binary, 4U 1636-53 [12].…”

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

“…The luminosity corresponding to the hard-to-soft (hereafter H-S) transition is found usually higher than that of the soft-to-hard (hereafter S-H) transition in transient sources (Miyamoto et al 1995;Nowak 1995;Maccarone & Coppi 2003;Gladstone et al 2007), suggesting that the transitions between the two states do not occur at the same luminosity when transition directions are opposite. On the other hand, additional studies show evidence that the H-S transition luminosity is not fixed (Homan et al 2001;Zdziarski et al 2004), which could vary by a factor of 10 (Yu et al 2004;Yu & Dolence 2007). Many different parameters have been suggested to determine spectral state transitions, including the size of the corona (Homan et al 2001), the history of the mass accretion rate (Homan & Belloni 2005), the history of the location of the inner disk radius (Zdziarski et al 2004), two different flows in the accretion geometry which contribute 1 Also Graduate School of the Chinese Academy of Sciences.…”