4XMM J182531.5–144036: A new persistent Be/X-ray binary found within the XMM-Newton serendipitous survey

Mason, A.B.; Norton, A.J.; Clark, J. S.; Farrell, S.A and Gosling, A.J. (2024). 4XMM J182531.5–144036: A new persistent Be/X-ray binary found within the XMM-Newton serendipitous survey. Publications of the Astronomical Society of Australia (Early access).

DOI: https://doi.org/10.1017/pasa.2024.6

Abstract

We aim to investigate the nature of time-variable X-ray sources detected in the XMM-Newton serendipitous survey. The X-ray light curves of objects in the XMM-Newton serendipitous survey were searched for variability and coincident serendipitous sources observed by Chandra were also investigated. Subsequent infrared spectroscopy of the counterparts to the X-ray objects that were identified using UKIDSS was carried out using ISAAC on the VLT. We found that the object 4XMM J182531.5–144036 detected in the XMM-Newton serendipitous survey in April 2008 was also detected by Chandra as CXOU J182531.4–144036 in July 2004. Both observations reveal a hard X-ray source displaying a coherent X-ray pulsation at a period of 781 s. The source position is coincident with a K = 14 mag infrared object whose spectrum exhibits strong HeI and Brγ emission lines and an infrared excess above that of early B-type dwarf or giant stars. We conclude that 4XMM J182531.5–144036 is a Be/X-ray binary pulsar exhibiting persistent X-ray emission and is likely in a long period, low eccentricity orbit, similar to X Per.

Plain Language Summary

We have discovered a new binary star system comprising a neutron star accreting from the circumstellar disc of material expelled by its partner star. The neutron star spins on its axis every 781 seconds emitting X-rays whose intensity varies with the period. The companion star is identified as exhibiting an excess of infrared emission and strong emission lines in its spectrum. The system has similarities to other previously discovered systems, notably that known as X Per.

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