X-ray outbursts of low-mass X-ray binary transients observed in the RXTE era

Zhen Yan, Wenfei Yu

Published 2014-08-21Version 1

We have performed a statistical study of the outburst properties of 110 bright X-ray outbursts in 36 low-mass X-ray binary transients (LMXBTs) seen with the All-Sky Monitor (ASM; 2--12 keV) on board the Rossi X-ray Timing Explorer ({\it RXTE}) in 1996--2011. We have measured a number of outburst properties including peak X-ray luminosity, rate-of-change of luminosity on daily timescale, $e$-folding rise and decay timescales, outburst duration and total radiated energy. We found the average values of some properties such as peak X-ray luminosity, rise and decay timescales, outburst duration and total radiated energy of black hole LMXBTs are at least two times larger than those of neutron star LMXBTs, implying that these properties can be used to infer the nature of the central compact object of a newly discovered LMXBT. We also found the outburst peak X-ray luminosity is correlated with the rate-of-change of X-ray luminosity in both the rise and the decay phases, which is consistent with our previous studies. There are also positive correlations between total radiated energy and peak X-ray luminosity, and between total radiated energy and $e$-folding rise or decay timescale in the outbursts. These correlations suggest that the mass stored in the disc before an outburst is the primary initial condition which sets up the outburst properties seen later on. We also found the outbursts of two transient stellar-mass ULXs in M31 also follow the correlations, which indicate the same outburst mechanism works for the outbursts of these two sources in M31 which reached above the Eddington luminosity.