Swift J1728.9-3613: Swift/XRT localization

ATel #12445; Jamie A. Kennea (Penn State)
on 29 Jan 2019; 01:23 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 16:50UT on January 28th, 2019, Swift began a 1ks target of opportunity observation of the newly discovered transient Swift J1728.9-3613 (ATEL #12436, ATEL #12437, GCN #23800). In the XRT Photon Counting mode data we find a bright point source at the following location: RA/Dec(J2000) = 262.24435, -36.24380, which is equivalent to:

RA(J2000) =17h 28m 58.64s,
Dec(J2000) = -36d 14m 37.7s,

with an estimated error radius of 1.7 arc-seconds (90% confidence). We note that this position lies 0.7 arc-minutes from the Swift/BAT position reported in ATEL #12436/GCN #23800, and is the only X-ray source in the XRT field of view consistent with being the transient. In addition the X-ray position is 11.6 arc-seconds from the cataloged X-ray source 1RXS J172859.6-361438 in the ROSAT All-Sky Survey Faint Source Catalogue (Voges et al., 2000), suggesting that this may be a detection of Swift J1728.9-3613 in quiescence.

This X-ray source lies 2.8 arc-minutes from the optical transient reported in ATEL #12443, and therefore that source is not related to Swift J1728.9-3613.

A spectral analysis of pile-up corrected data show that the source has a high absorption, N_H = 4.6 +/- 0.6 x 10^22 cm^-2, and is well fit by a power-law model with photon index = 1.86 +/- 0.22. The observed flux is 4.4 x 10^-8 erg/s/cm^2 (0.5 - 10 keV), 10^-9 erg/s/cm^2 correcting for absorption. Temporal analysis of the short 1ks exposure shows a steady count rate of ~50 c/s, after correcting for pile-up, with no evidence of variability.

Further observations in order to determine the nature of this source are encouraged.