EP250630b: A new Be white dwarf binary in outburst in the LMC
ATel #17428; C. Maitra (IUCAA, MPE), F. Haberl (MPE), A. Udalski (Univ. of Warsaw), C. Jin (CAS)
on 2 Oct 2025; 17:02 UT
Credential Certification: Chandreyee Maitra (cmaitra@mpe.mpg.de)
Subjects: X-ray, Binary, Transient
EP250630b was discovered as a new long-term X-ray transient by EP-WXT on 2025-06-30T09:23:01 (ATEL#17306). A follow-up observation with the Follow-up X-ray Telescope (FXT) indicated
two possible counterparts, a nova candidate AT 2025ggn and a Be star both located in the LMC.
We followed up EP250630b with a 60 ks XMM-Newton ToO observation on 2025-08-30. The source was detected at the following position (after astrometrical corrections):
RA (J2000) = 5:03:37.63 , DEC (J2000) = -69:24:09.64
with a 0.5" statistical uncertainty (including 1 sigma statistical and systematics). This favours the Be star candidate 2MASS J05033739-6924111 (V=15.7 mag) as the likely
counterpart of the X-ray source as opposed to AT 2025ggn which falls outside the 3 sigma error circle.
A 2 ks Chandra HRC-I DDT observation was performed on 2025-09-11 in which
the source was detected at the following position:
RA (J2000) = 5:03:37.28 , DEC (J2000) = -69:24:10.48
with a 0.21" statistical uncertainty ( 1 sigma). This further
favours the Be star as the possible optical counterpart.
The X-ray spectrum is typical of a Supersoft X-ray source with an absorbed luminosity (0.2-5 keV) of ~ 1e37 erg/s assuming a distance of 50 kpc to the LMC.
OGLE-IV I-band light curve of the Be star (OGLE J05033753-6924110) revealed high variability and a 2.9 mag outburst observed just before the X-ray brightening, and a possible orbital period of ~13 or ~40 days.
All of the above indicates that EP250630b is a candidate Be white dwarf binary in outburst in the LMC. This discovery makes it only the fifth known in this rare class of systems in the LMC (and 5 in the SMC). Detailed analysis of the EP, XMM, Chandra and OGLE observations will be published elsewhere.
We thank the XMM-Newton and Chandra SOC teams for the quick scheduling of the observations.
