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S-CUBED confirms turn on of CXOU J005245.0-722844, a possible WD hosting BeXRB?

ATel #16633; J. A. Kennea (PSU), M. J. Coe (Southampton), T. Gaudin (PSU) and P. A. Evans (Leicester)
on 28 May 2024; 13:46 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 16634, 16636, 16638

On May 7th, 2024 at 08:41UT Einstein Probe detected a transient source in the SMC that they named EP J0052.9-7230, noting itâs closeness to CXOU J005245.0-722844 (ATEL #16631). The S-CUBED SMC survey, performed by Swift, scans the Small Magellanic Cloud weekly (Kennea et al, 2018), and observed this region on May 7th at 22:29UT with a 55s X-ray Telescope photon counting mode exposure. In this observation we detect a bright point source at the following location: RA/Dec(J2000) = 13.18825, -72.47907, which is equivalent to:

RA(J2000) = 00h 52m 45.18s,
Dec(J2000) = 72d 28m 44.6s,

with an estimated uncertainty of 2.4 arc-seconds radius (90% confidence). This position is 0.94 arc-seconds from the location of CXOU J005245.0-722844, confirming that the EP J0052.9-7230 is indeed that source. No other X-ray sources are detected near the EP localization.

In the S-CUBED observation the count rate is 5.7 +/- 1.0 count/s. In previous observations taken on May 21st, the source was not detected, therefore we confirm that CXOU J005245.0-722844 has entered a transient outburst.

The XRT spectrum is soft, with a blackbody fit give kT = 0.094 +/- 0.028 keV, and a flux of 1.22 +- 0.26 x 10^-10 erg/s/cm^2 (0.3-10 keV). Correcting for absorption, this flux is 1.6x10^-9 erg/s/cm^2. Therefore the spectral parameters are consistent with the measured Einstein Probe values.

Given the soft spectrum, if this source is a Be X-ray Binary system, the compact object is likely a White Dwarf, as Neutron Star hosting BeXRBs are characterized by having a hard (photon index ~1) power-law spectra. Be/WD binaries are a rare class of BeXRB system, of which 2 have previously been discovered by S-CUBED in the SMC (Kennea et al. 2021, MNRAS, 508, 781, Coe et al., 2020, MNRAS, 497, L50).