SRG/eROSITA-triggered XMM-Newton observations of three Be/X-ray binaries in the LMC: Discovery of X-ray pulsations

TL;DR

This study reports the discovery of three Be/X-ray binary pulsars in the Large Magellanic Cloud, including two new transients detected by eROSITA, with detailed analysis of their X-ray pulsations, optical spectra, and variability patterns.

Contribution

First detection of two new hard X-ray transients and detailed characterization of three Be/X-ray binary pulsars in the LMC using multi-wavelength observations.

Findings

Detected X-ray pulsations with periods of 40.6 s, 17.3 s, and 784 s.

Measured Halpha emission lines indicating circumstellar discs.

Identified orbital period of approximately 451 days for RX J0501.6-7034.

Abstract

Using data from eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma (SRG), we report the discovery of two new hard transients, eRASSU J050810.4-660653 and eRASSt J044811.1-691318, in the Large Magellanic Cloud. We also report the detection of the Be/X-ray binary RX J0501.6-7034 in a bright state. We initiated follow-up observations to investigate the nature of the new transients and to search for X-ray pulsations coming from RX J0501.6-7034. We analysed the X-ray spectra and light curves from our XMM-Newton observations, obtained optical spectra using the South African Large Telescope to look for Balmer emission lines and utilised the archival data from the Optical Gravitational Lensing Experiment (OGLE) for the long-term monitoring of the optical counterparts. We find X-ray pulsations for eRASSU J050810.4-660653, RX J0501.6-7034, and eRASSt J044811.1-691318 of 40.6 s, 17.3 s, and 784 s, respectively. The Halpha emission lines with equivalent widths of -10.4 A (eRASSU J050810.4-660653) and -43.9 A (eRASSt J044811.1-691318) were measured, characteristic for a circumstellar disc around Be stars. The OGLE I- and V-band light curves of all three systems exhibit strong variability. A regular pattern of deep dips in the light curves of RX J0501.6-7034 suggests an orbital period of ~451 days. We identify the two new hard eROSITA transients eRASSU J050810.4-660653 and eRASSt J044811.1-691318 and the known Be/X-ray binary RX J0501.6-7034 as Be/X-ray binary pulsars.