TITLE: GCN CIRCULAR NUMBER: 30728 SUBJECT: POLAR detection of a SGR-like burst DATE: 21/08/27 13:34:35 GMT FROM: Hancheng Li at IHEP/POLAR lihc@ihep.ac.cn

G. Koziol (UniGE), H. C. Li (UniGE), M. Kole (UniGE), Y. H. Wang (IHEP),

N. Produit (UniGE), J. C. Sun (IHEP) and Y. Zhao (IHEP/BNU)

report on behalf of the POLAR collaboration:

During on-ground blind search of archive POLAR data, we found a SGR-like

burst whick took place on 2017-01-18 16:16:25.5 UT (T0). The burst has a

duration (T90) of 0.32 +/- 0.02 seconds, the light curve and data files

(both in ROOT and FITS formats) can be found at:

https://www.astro.unige.ch/polar/content/sgr-candidate-170118

Using the localization method of POLAR (Y. H. Wang et al, 2021),

the location of this event is best fitted as (in J2000):

RA : 33.5 [deg] (l = 136.04)

Dec: +50.7 [deg] (b = -10.06)

Err: 8.1 [deg] (radius, 1 sigma containment, statistical only)

The incident angle in the POLAR local coordinate at T0 is:

Theta: 60.0 [deg]

Phi: 50.0 [deg]

The spectra is better fitted with "bbody+powerlaw" model than Band function

or single powerlaw. The temperature (kT) is 13.85 +/- 0.79 keV with a

normalization of 99.34 +/- 7.93, and the power law index is 1.73 +/- 0.43

with a normalization of 74.21 +/- 182.29. The fluence (50-500 keV) from

T0 to T0 + 1.0 s is (5.09 +/- 1.33) E-06 erg/cm^2, and the average photon

flux (50-500 keV) over this time period is 40 +/- 1 ph/s/cm^2.

The results presented above are preliminary. Refined analyses are ongoing.

Additionally, more burst events are expected to be found in the blind

search of data which was recently started, and to be circulated by GCN.

POLAR was a dedicated Gamma-Ray Burst polarimeter which was launched

on-board the Chinese space laboratory Tiangong-2 (TG-2) on Sep 15, 2016,

and ended operation in April 2017. More information about POLAR can be

found at https://www.astro.unige.ch/polar/ , http://polar.ihep.ac.cn/en/ ,

and http://polar.psi.ch/html/ .