GCN Circular 27665
Subject
A Forest of Bursts from SGR 1935+2154
Date
2020-04-28T14:28:46Z (5 years ago)
From
David Palmer at LANL <dmopalmer@gmail.com>
A Forest of Bursts from SGR 1935+2154
David M. Palmer (LANL) reports on behalf of the BAT Team:
At 18:26:20 of 2020-04-27 UT, the Swift Burst Alert Telescope (BAT)
triggered and located a burst from the Soft Gamma Repeater SGR 1935+2154
(Trigger #968211) (GCN #27657; Barthelmy et al.). This burst, and many
subsequent bursts described below, continuing to at least T+7 hours (the
time of this writing) were also seen by Fermi/GBM (GCN #27659; Fletcher
et al.)
This initial burst was followed by an intense sequence of bursts
starting at ~T+300s after the first trigger time. This includes two
separate time segments, 3 seconds and 15 seconds long, made up of rapid
sequences of multiple bursts during which the count rate never returns
to baseline on the 64 ms timescale (the highest time-resolution data
that has been downlinked so far).
During those time intervals, the peak count rate reaches up to 130k
counts/s on a 64 ms timescale over the 15-350 keV band, and 350k
counts/s on a 1 second timescale over the full detector sensitivity
range. (The majority of these additional counts would be below the 15 keV
calibrated energy bin but above the Low-Level-Discriminator level.
This LLD level varies from detector-to-detector in BAT's 32k-element
array, but is typically 12-14 keV. This indicates that the emission
spectrum is very steep around those energies.)
During the first 24 minutes of the episode, there were at least 35
clearly-distinguishable bursts outside of the piled-up time intervals.
This is similar to the forests of bursts seen 2006-03-29 from
SGR 1900+14 (Israel et al, 2008, ApJ 685:1114) and 2008-05-28 from
SGR 1627-41. (GCN #7777; Palmer et al.).
SGR 1935+2154's recent activation was first detected with a burst 5 days
earlier, which was seen by multiple spacecraft, providing timing
information that identified the location to be this source (GCN #27625;
Hurley et al.). The previous BAT detection was 9 bursts in ~24 hours in
November 2019.
Note: A draft copy of this report was accidentally distributed to the
GCN (as #27660) before the final version was submitted to ATel,
and then as this courtesy copy to GCN. The ATel #13675 submission
is the citable publication of record.