X-ray and Infrared Properties of Be/X-ray Binary Pulsars

TL;DR

This paper investigates the X-ray and infrared characteristics of Be/X-ray binary pulsars during Type I outbursts, revealing complex pulse profiles, spectral features, and correlated optical/infrared variations, enhancing understanding of accretion processes and companion star behavior.

Contribution

It provides detailed phase-resolved spectroscopy and multi-wavelength observations, offering new insights into the accretion streams and optical/infrared changes during pulsar outbursts.

Findings

Pulse profiles are complex in soft X-rays with absorption dips.

Broad-band spectra fit a partial covering high energy cutoff power-law model.

Optical/infrared flux decreases during X-ray outbursts, with emission line profile changes.

Abstract

During normal Type I outbursts, the pulse profiles of Be/X-ray binary pulsars are found to be complex in soft X-ray energy ranges. The profiles in soft X-ray energy ranges are characterized by the presence of narrow absorption dips or dip-like features at several pulse phases. However, in hard X-ray energy ranges, the pulse profiles are rather smooth and single-peaked. Pulse phase-averaged spectroscopy of the these pulsars had been carried out during Type I outbursts. The broad-band spectrum of these pulsars were well described by partial covering high energy cutoff power-law model with interstellar absorption and Iron K_alpha emission line at 6.4 keV. Phase-resolved spectroscopy revealed that the presence of additional matter at certain pulse phases that partially obscured the emitted radiation giving rise to dips in the pulse profiles. The additional absorption is understood to be taking place by matter in the accretion streams that are phase locked with the neutron star. Optical/infrared observations of the companion Be star during these Type I outbursts showed that the increase in the X-ray intensity of the pulsar is coupled with the decrease in the optical/infrared flux of the companion star. There are also several changes in the IR/optical emission line profiles during these X-ray outbursts. The X-ray properties of these pulsars during Type I outbursts and corresponding changes in optical/infrared wavebands are discussed in this paper.