Trigger 980513: Swift detection of SGR Swift J1818.0-1607

ATel #13850; M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. Sbarrato (INAF-OAB)
on 2 Jul 2020; 08:00 UT
Credential Certification: David M. Palmer (palmer@lanl.gov)

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This is a courtesy copy of GCN #28055 (the reference for citation) for the benefit of the SGR community.

The authors report on behalf of the Neil Gehrels Swift Observatory Team:

At 07:09:38 UT on 2020-07-02, the Swift Burst Alert Telescope (BAT) triggered and located a burst from Soft Gamma Repeater SGR Swift J1818.0-1607 (trigger=980513). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 274.471, -16.167 which is
RA(J2000) = 18h 17m 53s
Dec(J2000) = -16d 09' 59"
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 100 ms. The peak count rate was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 07:11:03.2 UT, 84.4 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 274.5017, -16.1317 which is equivalent to:
RA(J2000) = 18h 18m 00.40s
Dec(J2000) = -16d 07' 54.2"
with an uncertainty of 3.0 arcseconds (radius, 90% containment). This location is 165 arcseconds from the BAT onboard position, within the BAT error circle, and consistent with the catalogued position of SGR Swift J1818.0-1607. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper.

A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density.

UVOT took a finding chart exposure of 150 seconds with the White filter starting 87 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers none of the XRT error circle. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain, extinction expected.

This is the fourth time Swift has triggered on SGR Swift J1818.0-1607; with 5 additional burst detections that did not reach the level needed for a burst response ('trigger'). These detections were:
2020-02-28T22:19:32
2020-03-12T21:16:47 GCN 27373
2020-04-13T19:34:18
2020-05-06T17:36:50 GCN 27696
2020-05-16T15:03:28 GCN 27746
2020-05-17T14:29:54
2020-05-25T20:23:45
2020-06-01T16:52:09