Orbital Decay and Evidence of Disk Formation in the X-ray Binary Pulsar OAO 1657-415

TL;DR

This paper analyzes 19 years of observations of the X-ray binary pulsar OAO 1657-415, revealing evidence of orbital decay and two accretion modes, including the formation of a transient accretion disk.

Contribution

It provides the first long-term frequency history of OAO 1657-415, identifying two accretion modes and confirming orbital decay, indicating a transition from wind-fed to disk-mediated accretion.

Findings

Detection of orbital decay with a rate of approximately -3.40e-6 yr^{-1}

Identification of two accretion modes: steady spin-up and flip-flop behavior

Evidence for transient accretion disk formation in the system

Abstract

OAO 1657-415 is an eclipsing X-ray binary wind-fed pulsar that has exhibited smooth spin-up/spin-down episodes and has undergone several torque reversals throughout its long history of observation. We present a frequency history spanning nearly 19 years of observations from the Burst and Transient Source Experiment (CGRO/BATSE) and from the Gamma-Ray Burst Monitor (Fermi/GBM). The analysis suggests two modes of accretion: one resulting in steady spin-up during which we believe a stable accretion disk is present and one that results in what appears to be a random walk in spin frequency where an unstable accretion disk forms alternating in direction ("flip flop"). Orbital elements of the pulsar system are determined at several intervals throughout this history. With these ephemerides, statistically significant orbital decay ($\dot{P}/P =(-3.40 \pm0.15)\times10^{-6} \mathrm{yr^{-1}}$) is established suggesting a transition between wind-fed and disk-mediated accretion.