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

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.

💡 Research Summary

This paper presents a coordinated near‑infrared (NIR) photometric and spectroscopic monitoring campaign of the Be/X‑ray binary pulsar A0535+262 and its optical companion HDE 245770. Observations were carried out with the 1.2 m telescope at the Mt. Abu Infrared Observatory between March 2010 and April 2011, covering three orbital cycles (≈111 days) and, crucially, the giant Type II X‑ray outburst that occurred in February–March 2011.

Photometric monitoring in the J, H, and K bands shows that during quiescent intervals the magnitudes remain essentially constant (≈7.5–8.0 mag). At the onset of the X‑ray outburst, all three bands exhibit a systematic fading of about 0.12 mag, corresponding to a ∼12 % decrease in flux. The J–H and H–K colour indices, however, stay unchanged within the measurement uncertainties, indicating that the flux reduction is roughly equal across the NIR spectrum. The authors interpret this behaviour as a mild evacuation or truncation of the Be star’s circumstellar disc, which supplies the bulk of the free‑free and bound‑free emission that dominates the NIR output.

Spectroscopically, the NIR spectra are dominated by hydrogen recombination lines: Paschen series (e.g., Pa β, Pa γ) in the J band, Brackett series (Br γ, Br 10–14) in the H and K bands, and He I lines at 1.083 µm, 1.7002 µm, and 2.0585 µm. The line centres match laboratory wavelengths, and the equivalent widths and line profiles show no significant variation between the quiescent and outburst phases. Notably, the dramatic disc‑loss event reported in 1998 (where Br γ switched from emission to absorption) is absent; the disc remains present throughout the 2011 outburst. The continuum shape also remains stable, reinforcing the conclusion that the line‑forming region of the disc is not strongly perturbed by the neutron star’s periastron passage.

These findings support a picture in which the neutron star’s interaction with the Be disc during periastron primarily affects the outer, low‑density parts of the disc that dominate the NIR free‑free emission, while the denser inner disc that produces the recombination lines remains largely intact. The modest NIR dimming, together with unchanged colours, suggests a relatively uniform loss of disc material rather than a temperature or density change confined to a specific region.

Methodologically, the study demonstrates that even modest‑aperture facilities can provide valuable constraints on disc dynamics in Be/X‑ray binaries, provided that observations are timed to capture key orbital phases. However, the limited spectral resolution (R ≈ 1000) and signal‑to‑noise (S/N ≈ 20–40) restrict the ability to detect subtle line‑profile changes or weak diagnostic lines. The authors recommend follow‑up observations with larger telescopes and higher‑resolution NIR spectrographs, ideally coordinated with simultaneous X‑ray, optical, and radio monitoring, to map the full three‑dimensional response of the disc to neutron‑star accretion events. Such multi‑wavelength campaigns will be essential for building quantitative models of mass transfer, disc truncation, and outburst triggering in Be/X‑ray binaries.