S. Mereghetti (INAF, IASF-Milano), V. Savchenko (ISDC, Versoix), D. Gotz
(CEA, Saclay), L. Ducci, C. Ferrigno, E. Bozzo (ISDC, Versoix), J.Borkowski
(CAMK, Torun) and A. Bazzano (INAF, IAPS-Roma) report
A bright short burst has been detected by IBAS in the IBIS/ISGRI data at
14:34:24 UT of April 28 (Alert Packet n. 8611), and promptly identified as
originating from the currently active SGR 1935+2154 (GCN #27667, #27666,
#27665, #27664, #27663, #27662, #27661, #27659, #27657, #27631, ATel
#13682, #13679, #13678, #13675). Note that the above time is not corrected
for the light-travel but it is consistent with the one of the radio burst
from the same source reported in ATel #13681, #13684.
The burst was at an off-axis angle of 8.1 degrees, in the field of view of
the IBIS and SPI instruments, but outside those of JEM-X and OMC.
A preliminary analysis indicates that the burst had a duration of about 0.3
s, a fluence of 3.9+/-0.2e-7 erg/cm2 in the 25-80 keV range and a peak flux of
1.2+/-0.1 e-6 erg/cm2/s on 10 ms timescale.
The burst was also marginally detected in the SPI-ACS with S/N=4.65 on a
0.65s time scale. The ACS observation independently gives a FAP at the
level of 0.0019 (2.9 sigma).
We note that another weaker burst from SGR 1935+2154, lasting about 0.1 s
was detected by IBAS at 09:51:05 UT of April 28 (Weak Alert Packet n.
ISGRI and SPI-ACS lightcurves of the bright event are available at:
[GCN OPS NOTE(29apr20): Per author's request, the fluence in the 3rd paragraph
was changed from "3.9+/-0.2e-5" to "3.9+/-0.2e-7 erg/cm2" (ie the value
went down by 100x and units were added. A thank-you to V.Palshin
for pointing out the typo.]