Investigating the orbital evolution of the eccentric HMXB GX 301–2 using long-term X-ray light curves

Abstract

ABSTRACT We report the orbital decay rate of the high-mass X-ray binary GX 301–2 from an analysis of its long-term X-ray light curves and pulsed flux histories from CGRO/BATSE, RXTE/ASM, Swift/BAT, Fermi/GBM, and MAXI by timing the pre-periastron flares over a span of almost 30 yr. The time of arrival of the pre-periastron flares exhibits an energy dependence (hard lag) and the orbital period decay was estimated after correcting for it. This method of orbital decay estimation is unaffected by the fluctuations in the spin rate of the X-ray pulsar associated with variations in the mass accretion rate. The resulting $\dot{P}_\textrm {orb}$ = −(1.98 ± 0.28) × 10−6 s s−1 indicates a rapid evolution time-scale of $|P_\textrm {orb}/\dot{P}_\textrm {orb}|\sim 0.6\times 10^{5}$ yr, making it the high mass X-ray binary with the fastest orbital decay. Our estimate of $\dot{P}$orb is off by a factor of ∼2 from the previously reported value of −(3.7 ± 0.5) × 10−6 s s−1 estimated from pulsar TOA analysis. We discuss various possible mechanisms that could drive this rapid orbital decay and also suggest that GX 301–2 is a prospective Thorne–Żytkow candidate.