Investigating the orbital evolution of the eccentric HMXB GX 301$-$2 using long-term X-ray lightcurves

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 years. 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 ${\dot{P}}_{\text{orb}}$ $=-(1.98\pm 0.28)\times 10^{-6}$ s s${}^{-1}$ indicates a rapid evolution timescale of $|P_{\text{orb}}/{\dot{P}}_{\text{orb}}|\sim 0.6\times 10^5$ yr, making it the high mass X-ray binary with the fastest orbital decay. Our estimate of ${\dot{P}}_{\mathrm{orb}}$ is off by a factor of $\sim 2$ from the previously reported value of $-(3.7\pm 0.5)\times 10^{-6}$ 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.

11 pages, 6 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society