XMM-Newton follow-up of two eROSITA X-ray binary candidates

TL;DR

This study used XMM-Newton to follow up on two eROSITA X-ray binary candidates, concluding neither are XRBs; one is a chromospherically active star, and the other is a novalike CV with a potential orbital period.

Contribution

First identification of a novalike cataclysmic variable with eROSITA, and detailed follow-up characterization of two X-ray sources.

Findings

1eRASS J061330.8+160440 is an active star, not an XRB.

1eRASS J161201.9-464622 is a novalike CV with a 4.802-hour orbital period.

Neither candidate is likely an X-ray binary.

Abstract

We report on the follow-up observations with XMM-Newton of two X-ray binary candidates identified in the first eROSITA all-sky survey data (eRASS1), 1eRASS J061330.8+160440 and 1eRASS J161201.9-464622. Based on the obtained results, in particular, the observed X-ray spectra and lack of pulsations, as well as properties of the identified optical counterparts, we conclude that both candidates are unlikely to be XRBs. Based on LAMOST optical spectroscopy and SED fit results for 1eRASS J061330.8+160440 we classify it as an M0 chromospherically active subgiant star. ZTF and TESS photometry reveal highly significant period for this object of 7.189 days, which likely attributed to starspot(s). On the other hand, SALT follow-up spectroscopy of 1eRASS J161201.9-464622 solidly classifies this source as a bright novalike cataclysmic variable (CV), the second discovered with eROSITA. A persistent 4.802 h signal is found across all three available TESS observations, and is tentatively identified as the orbital period of the binary. Follow-up high-speed photometry and time-resolved spectroscopy are required to confirm the derived orbital modulation.