Near-infrared observations of the Be/X-ray binary pulsar A0535+262

We present the results obtained from extensive near-infrared (IR) spectroscopic and photometric observations of the Be/X-ray binary A0535+262/HDE 245770 at different phases of its similar to 111 d orbital period. This observation campaign is part of the monitoring program of selective Be/X-ray binary systems aimed at understanding X-ray and near-IR properties at different orbital phases, especially during the periastron passage of the neutron star. The near-IR observations presented here were carried out using the 1.2 m telescope at the Mt. Abu IR Observatory. Though the source was relatively faint for spectroscopic observations with the 1.2 m telescope, we monitored the source closely during the 2011 February March giant X-ray outburst to primarily investigate whether any drastic changes in the near-IR JHK spectra took 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, which could suggest a mild evacuation/truncation of the circumstellar disk 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 mu m. The presence of all the 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, suggests that the near-IR line emitting regions of the disk are not significantly affected during the X-ray outburst.