GCN Circular 27665

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.