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

TL;DR

This study measures the rapid orbital decay of the high mass X-ray binary GX 301-2 over 30 years using long-term X-ray lightcurves, revealing the fastest decay among similar systems and suggesting potential underlying mechanisms.

Contribution

It introduces a new method for estimating orbital decay unaffected by pulsar spin fluctuations and provides the most rapid decay rate measured for this class of binaries.

Findings

Orbital decay rate of a0a0=-(1.98a0a0a0a0) imes10^{-6} s s^{-1}

Orbital decay timescale of a0a0a0a0a0a0a0 yr, fastest among HMXBs

Potential candidate for Thorne-Żytkow object

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 $\dot P_\textrm{orb}$ $=-(1.98\pm0.28)\times10^{-6}$ s s$^{-1}$ indicates a rapid evolution timescale of $|P_\textrm{orb}/\dot P_\textrm{orb}|\sim 0.6\times10^{5}$ 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 $\sim2$ from the previously reported value of $-(3.7\pm0.5)\times10^{-6}$ 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-\.{Z}ytkow candidate.