Optical Spectroscopy and Photometry of the Likely Counterpart to EP J115415.8-501810

ATel #16549; Antonio Rodriguez (Caltech), S. R. Kulkarni (Caltech)
on 21 Mar 2024; 17:59 UT
Credential Certification: Shri Kulkarni (srk@astro.caltech.edu)

Tweet

EP J115415.8-501810 was recently discovered as a highly variable X-ray source by the Einstein Probe (EP) mission (ATel # 16546), and found to be consistent with sources detected previously by XMM-Newton and the eROSITA telescope aboard the SRG observatory. Gaia DR3 5370642890382757888 was found to be a likely optical counterpart, consistent with all X-ray positions. No other Gaia sources are consistent with the EP X-ray position. In part due to its brightness (average Gaia G = 16.2), DR3 5370642890382757888 has an XP spectrum available as of Gaia Data Release 3 (DR3; Gaia Collaboration 2022). The spectrum shows prominent H alpha and H beta, as well as a likely detection of He II 4686 and He I 7065, all in emission. There is an archival light curve from the ASAS-SN survey, and indeed this object is also present in the catalog of ASAS-SN variable stellar sources as ASASSN-V J115415.90-501802.1 (also in the Variable Star Index, VSX). It shows erratic variability between V=17.2 mag and 14.4 mag, which likely led to its VSX classification as a non-magnetic cataclysmic variable (CV) of U Geminorum type. However, the detection of high X-ray variability and He II 4686 (with an equivalent width similar to that of H beta) suggests that this object is instead a magnetic CV. The erratic optical behavior would then correspond to accretion state changes commonly seen in magnetic CVs, not dwarf nova outbursts. A period search of the light curve, where minimal erratic variability is present, was conducted using a Lomb-Scargle periodogram, but no significant period (> 5 sigma) aside from the sidereal day was found. Further optical observations are encouraged to determine the orbital period and confirm the nature of this system. Link to Gaia XP spectrum: https://www.dropbox.com/scl/fi/gusyywzoigud5wsm6xqsa/ATel_CV.png?rlkey=9hrehgwjqab1saompxi3xohdn&dl=0 Link to ASAS-SN Light curve: http://asas-sn.ifa.hawaii.edu/skypatrol/objects/412317207926