TITLE:   GCN CIRCULAR
NUMBER:  28603
SUBJECT: SGR 1830-0645: pre-discovery detection in Swift/BAT
DATE:    20/10/11 22:39:41 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

SGR 1830-0645 was discovered by Swift/BAT trigger #999571 on
2020-10-10T14:29:24 UTC (GCN 28594, ATEL 14083). The hypothesis of
magnetar origin was strengthened by measurement of a spin-period in
the Swift/XRT data (Gogus et al. ATEL 14085).

We performed a sub-threshold search in the Swift/BAT data, and find
that BAT detected a soft (<50 keV), short (~20 ms), flare from this
source at 2020-10-05T18:20:35.566  UTC, 5 days before the discovery
trigger.

The origin of this flare is confirmed to come from SGR 1830-0645 from
time-tagged event data recorded by Swift/BAT. This TTE data was saved
by BAT triggering on this flare in the rates domain in real-time, with
failed trigger #998860. This was not promoted to a successful trigger
as BAT failed to find a source location in the associated image
produced on-board. This was due to a suboptimal foreground interval
selection by the real time analyses. The close proximity to the SAA,
and elevated background, likely also contributed.

We cleaned the TTE data of hot/noisy detectors. We detect the flare
from SGR 1830-0645 with a rates significance of >10 sigma, and confirm
the source location in the image domain at >7 sigma.