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Swift/BAT discovery of SWIFT J0658.6-0330

ATel #11131; D. M. Palmer (LANL) and A. Y. Lien (GSFC/UMBC) report on behalf of the Swift team
on 5 Jan 2018; 20:30 UT
Distributed as an Instant Email Notice Transients
Credential Certification: David M. Palmer (palmer@lanl.gov)

Subjects: X-ray, Binary, Neutron Star

At 05:54:50 UT on 30 Dec 2017, BAT measured a burst in its count rates. Because Swift was slewing at the time, BAT did not produce a corresponding image and trigger on-board. Subsequent ground analysis detected the rate increase and allowed an image reconstruction showing a strong peak. The ground-produced location of this source is RA,Dec = 104.6378, -3.4917 which is
RA(J2000) = 6h 58m 33s
Dec(J2000) = -3d 29' 30"
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single FRED peak with a duration of ~30 seconds. The peak count rate was ~1500 counts/sec (15-350 keV), at the time of the trigger.

The time-averaged spectrum from T-5.5 to T+17.4 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.26 +- 0.16. The fluence in the 15-150 keV band is 7.0 +/- 0.6 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from T+0.17 sec in the 15-150 keV band is 1.2 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level.

A follow-up observation with Swift/XRT has been scheduled to refine the source location, if the source has sufficient residual or quiescent flux to be detected at that time.

Because this source lies on the Galactic plane, and has a lightcurve and spectrum consistent with an X-Ray Burst, we believe that this is a previously-unknown Galactic transient, which we name SWIFT J0658.6-0330 .