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

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 ($\dot{P}/P=(-3.40\pm 0.15)\times 10^{-6}\mathrm{y}{\mathrm{r}}^{-1}$) is established suggesting a transition between wind-fed and disk-mediated accretion.

References in corpus (7)

• The Fermi Gamma-Ray Burst Monitor
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• INTEGRAL observations of the variability of OAO 1657-415
• The evolution and masses of the neutron star and donor star in the high mass X-ray binary OAO 1657-415
• Spectral classification of the mass donors in the high-mass X-ray binaries EXO 1722-363 and OAO 1657-415
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