DASCH 100-yr light curves of high-mass X-ray binaries

TL;DR

This study utilizes a century-long dataset of high-mass X-ray binaries' light curves to investigate their variability, revealing long-term changes and potential orbital periods, especially in gamma Cas-like systems, aiding their identification.

Contribution

It provides the first long-term variability analysis of high-mass X-ray binaries using historical photographic data, highlighting characteristic behaviors of gamma Cas-like objects.

Findings

Cyg X-1 shows no significant variability over 100 years.

SAO 49725 exhibits a magnitude variation of about 1 over a decade.

Possible orbital or super-orbital period of 500-600 days identified in SAO 49725.

Abstract

We analyzed the 100-yr light curves of Galactic high-mass X-ray binaries using the Harvard photographic plate collection, made accessible through the DASCH project (Digital Access to a Sky Century at Harvard). As scanning is still in progress, we focus on the four objects that are currently well covered: the supergiant X-ray binary Cyg X-1 (V1357 Cyg), and the Be X-ray binaries 1H 1936+541 (BD+53 2262), RX J1744.7-2713 (HD 161103), and RX J2030.5+4751 (SAO 49725), the latter two objects being similar to gamma Cas. The star associated with Cyg X-1 does not show evidence for variability with an amplitude higher than 0.3 magnitude over a hundred years. We found significant variability of one magnitude with timescales of more than 10 years for SAO 49725, as well as a possible period of 500-600 days and an amplitude of 0.05 magnitude that might be the orbital, or super-orbital period of the system. The data is insufficient to conclude for HD 161103 but suggests a similar long-term variability. We thus observe an additional characteristic of gamma Cas-like objects: their long-term variability. This variability seems to be due to the slow evolution of a decretion disk around the Be star, but may be triggered by the presence of a compact object in the system, possibly a white dwarf. This characteristic could be used to identify further similar objects otherwise difficult to detect.