Observations of The High Mass X-ray Binary A0535+26 in Quiescence

TL;DR

This study analyzes X-ray observations of the high mass X-ray binary A0535+26 during quiescence, revealing continued neutron star accretion and pulsations months after outbursts, with spectral analysis indicating persistent low-level activity.

Contribution

First detailed multi-epoch analysis of A0535+26 in quiescence showing ongoing accretion and pulsations months after outbursts.

Findings

Pulsations detected in the second observation indicating ongoing accretion.

Quiescent flux level decreases over 6.5 years post-outburst.

Continued accretion possible from stellar wind or material at the corotation radius.

Abstract

We have analyzed 3 observations of the High Mass X-ray Binary A0535+26 performed by the Rossi X-ray Timing Explorer (RXTE) 3, 5, and 6 months after the last outburst in 2011 February. We detect pulsations only in the second observation. The 3-20 keV spectra can be fit equally well with either an absorbed power law or absorbed thermal bremsstrahlung model. Re-analysis of 2 earlier RXTE observations made 4 years after the 1994 outburst, original BeppoSAX observations 2 years later, re-analysis of 4 EXOSAT observations made 2 years after the last 1984 outburst, and a recent XMM-Newton observation in 2012 reveal a stacked, quiescent flux level decreasing from ~2 to <1 x 10^{-11} ergs/cm2/s over 6.5 years after outburst. Detection of pulsations during half of the quiescent observations would imply that accretion onto the magnetic poles of the neutron star continues despite the fact that the circumstellar disk may no longer be present. The accretion could come from material built-up at the corotation radius or from an isotropic stellar wind.