Near-infrared Observations of Be/X-ray Binary Pulsar A0535+262

TL;DR

This study presents near-infrared spectroscopic and photometric observations of the Be/X-ray binary A0535+262 during different orbital phases, revealing stable emission line regions despite X-ray outbursts and a gradual fading of the circumstellar disc.

Contribution

It provides the first detailed near-IR spectral and photometric monitoring of A0535+262 during an X-ray outburst, highlighting the stability of emission regions and disc evolution.

Findings

Near-IR spectra dominated by hydrogen and HeI emission lines.

No significant change in near-IR continuum during X-ray phases.

Gradual fading of the circumstellar disc observed in JHK light curves.

Abstract

We present results obtained from an extensive near-infrared spectroscopic and photometric observations of the Be/X-ray binary A0535+262/HDE 245770 at different phases of its ~111 day orbital period. This observation campaign is a part of the monitoring programme of selective Be/X-ray binary systems aimed at understanding the X-ray and near-IR properties at different orbital phases, especially during the periastron passage of the neutron star. The near-IR observations were carried out using the 1.2 m telescope at Mt. Abu IR observatory. Though the source was relatively faint for spectroscopic observations with 1.2 m telescope, we monitored the source during the 2011 February--March giant outburst to primarily investigate whether any drastic changes in the near-IR JHK spectra take place at the periastron passage. Changes of such a striking nature were expected to be detectable in our spectra. Photometric observations of the Be star show a gradual and systematic fading in the JHK light curves since the onset of the X-ray outburst that could suggest a mild evacuation/truncation of the circumstellar disc of the Be companion. Near-IR spectroscopy of the object shows that the JHK spectra are dominated by the emission lines of hydrogen Brackett and Paschen series and HeI lines at 1.0830, 1.7002 and 2.0585 micron. The presence of all hydrogen emission lines in the JHK spectra, along with the absence of any significant change in the continuum of the Be companion during X-ray quiescent and X-ray outburst phases suggest that the near-IR line emitting regions of the disc are not significantly affected during the X-ray outburst.