A Forest of Bursts from SGR 1935+2154

ATel #13675; David M Palmer (LANL) on behalf of the Swift/BAT Team
on 28 Apr 2020; 03:24 UT
Credential Certification: David M. Palmer (palmer@lanl.gov)

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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 preliminary draft of this ATel was accidentally distributed as GCN #27660. This ATel is the official publication of record.