GCN Circular 27894

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL), G. Younes (GWU)
(i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry
downlink,
we report further analysis of SGR-like burst from 1RXS J1708���4009
(trigger #975998) (Bernardini et al., GCN Circ. 27886
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 ).
The BAT ground-calculated position is
RA, Dec = 257.089, -40.134 deg which is
   RA(J2000)  =  17h 08m 21.4s
   Dec(J2000) = -40d 08' 02.9"
with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 89%.

The mask-weighted light curve shows a single-pulse structure that starts
and peaks at ~T0. The main pulse ends at ~T+0.5 s, followed by some tail
emission that lasts till ~T+60 s. T90 (15-350 keV) is 52.0 +- 15.0 sec
(estimated error including systematics).

The time-averaged spectrum from T-0.28 to T+59.72 sec can be fitted
by a simple power-law model (chi squared 57.4 for 57 d.o.f).
The power law index of the time-averaged spectrum is 2.46 +- 0.23.
The fluence in the 15-150 keV band is 5.9 +- 0.8 x 10^-7 erg/cm2.
The 1-s peak spectrum does not fit well with a simple power-law model
(reduced chi squared > 2).

A single blackbody fit to the time-averaged spectrum shows the blackbody
temperature of 7.5 +- 0.8 keV (chi squared 41.3 for 57 d.o.f.).
The fluence in the 15-150 keV band is 4.8 +- 0.8 x 10^-7 erg/cm2.
The 1-sec peak fluence measured from T-0.28 sec in the 15-150 keV
band is 1.73 +- 0.1 x 10^-7 erg/cm2.

A thermal bremsstrahlung model fit shows the temperature of 30.8 +- 7.8 keV
(chi squared 47.6 for 57 d.o.f.). The fluence in the 15-150 keV band
is 5.3 +- 0.9 x 10^-7 erg/cm2. All the quoted errors are at the 90%
confidence level.

The fluence of this burst is about 60 times higher than the previous
burst detected on 2018 August 28 with NuSTAR (Younes et al. 2020,
ApJ, 889, L27). Also, the blackbody temperature is slightly
higher comparing to the 2018 burst (which has a temperature
of ~ 6 keV).

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/975998/BA/