GRO J1008−57: an (almost) predictable transient X-ray binary

Abstract

A study of archival RXTE, Swift, and Suzaku pointed observations of the transient high-mass X-ray binary GRO J1008−57 is presented. A new orbital ephemeris based on pulse arrival-timing shows the times of maximum luminosities during outbursts of GRO J1008−57 to be close to periastron at orbital phase −0.03. This makes the source one of a few for which outburst dates can be predicted with very high precision. Spectra of the source in 2005, 2007, and 2011 can be well described by a simple power law with high-energy cutoff and an additional black body at lower energies. The photon index of the power law and the black-body flux only depend on the 15-50 keV source flux. No apparent hysteresis effects are seen. These correlations allow us to predict the evolution of the pulsar's X-ray spectral shape over all outbursts as a function of just one parameter, the source's flux. If modified by an additional soft component, this prediction even holds during GRO J1008−57's 2012 type II outburst.

High-mass X-ray binaries in the Milky Way

“…An optical counterpart was identified with a B1-B2 Ve star (Coe et al, 2007) at a distance of ∼ 5 kpc (Coe et al, 1994a). Orbital parameters were determined using data from different observatories as follows: P orb = 249.46 ± 0.10 d, ax sin i = 530 ± 60 lt-s, ω = −26 ± 8 deg, e = 0.68 ± 0.02 (Levine and Corbet, 2006a;Coe et al, 2007;Kuehnel et al, 2012). The combined spectrum from the CGRO and ASCA observations can be well approximated by a power law with the high energy cutoff and a 6.4-keV iron emission line (Shrader et al, 1999).…”

Luminosity dependent accretion state change in GRO J1008–57

“…Due to these time constraints detailed analyses of multi-outburst-and -mission-observations are rare. Fortunately, our timing analysis of the BeXRB GRO J1008−57 showed that its outburst times can be predicted with an accuray allowing to schedule observations during particular outburst phases [10].…”

“…By analyzing the arrival times of the 93.7 spulsations as detected during outbursts in 2005 and 2007 by RXTE, Swift, and Suzaku, we were able to improve the time of the periastron passage and the orbital period to 249.48(4) d [10]. Using this ephemeris we calculated the orbital phases of all detected outbursts of GRO J1008−57 in RXTE-ASM and Swift-BAT, which usually last for 14 d. As a result almost all outbursts are consistent with the same orbital phase range close to periastron, which allows us to predict the time of maximum flux during outbursts of this source within 3 d [10].…”

“…In addition to the RXTE pointings during the decays of the 2005 and 2007 outbursts, these data allow to study the spectral evolution of this BeXRB depending on outburst 06003-p.2 phase and luminosity state in great detail. The spectrum of GRO J1008−57 above 10 keV can be well described by a power-law with high exponential cutoff [10,[12][13][14]. Here, we use the form…”

“…Recently, Reig & Nespoli (2013) [14] and ourselves [10] studied the evolution of spectral parameters of GRO J1008−57 depending on its luminosity. In contrast to the first work, which used the 2007 RXTE-data only, we use one simultaneous Suzaku-pointing and one quasi-simultaneous Swiftpointing during that outburst in addition as well as RXTE-monitoring during outbursts in 2005 and 2011.…”