Swift/XRT confirms outburst of Swift J0243.6+6124

ATel #15984; J. A. Kennea (PSU), A. Bahramian (Curtin) and H. Negoro (Nihon U)
on 10 Apr 2023; 01:48 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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Following on a report of a possible outburst of a transient consistent with either Swift J0243.6+6124, LS I +61 303 or a new transient by MAXI (ATEL #15983), we performed a Swift Target of Opportunity observation of the MAXI error region, utilizing 14 pointings with 100s exposure each to cover it. We detected both Swift J0243.6+6124 and LS I +61 303, however Swift J0243.6+6124 is ~7x brighter than LS I +303, and we therefore suggest this is likely the source of the MAXI trigger. No other bright X-ray source is detected inside the MAXI error region.

The position we find for Swift J0243.6+6124 is RA/Dec(J2000) = 40.9197, 61.4349, which is equivalent to:

RA(J2000) = 02h 43m 40.72s,
Dec(J2000) = +61d 26m 05.8s,

with an error radius 3.5 arc-seconds (90% confidence). This position lies 2.9 arc-seconds from the known position of Swift J0243.6+6124. The spectrum is well fit by an absorbed power-law model, with a photon index of 0.2 +/- 0.4 and an observed flux of 8 +/- 2 x 10^-10 erg/s/cm^2 (0.3 - 10 keV).

We note that the most recent observation of this source by Swift/XRT was in 2021 April 27, at which time the source was undetected in a 2.6ks exposure (obsid: 00010645052). Therefore we suggest that this detection represents a new outburst of this source.

Monitoring of this outburst is recommended to understand if this is the start of a new major Type-II outburst of this BeXRB, or simply a smaller Type-I outburst.

This analysis was supported by the Swift Guest Investigator program in Cycle 19.