TITLE: GCN CIRCULAR NUMBER: 30753 SUBJECT: Fermi GBM observation of a burst from SGR J1555.2-5402 DATE: 21/09/03 14:54:10 GMT FROM: Oliver J Roberts at USRA/NASA oliver.roberts@nasa.gov

O.J. Roberts (USRA/NASA-MSFC), R. Dunwoody (UCD), J. Mangan (UCD) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 05:20:49.405 UT on 27 August 2021, the Fermi Gamma-Ray Burst Monitor triggered on an SGR-like burst from the direction of SGR J1555.2-5402 (trigger 651734454/bn210827223). The SGR burst was also detected by Swift-BAT (Palmer, 2021, ATel 14887).

The on-ground calculated location, using the Fermi GBM trigger data, is consistent with the Swift position and known position of the SGR.

The angle from the Fermi LAT boresight is 21 degrees.

The burst has a duration of about 0.024 s (8-200 keV).

The time-averaged spectrum from T+0.384s to T0+0.408s is best fit by a by a power law function with an exponential high-energy cutoff. The power law index is -0.41 +/- 0.65 and the cutoff energy, parameterized as Epeak, is 32 +/- 4 keV.

The event fluence (8-200 keV) from T0+0.384s to T0+0.408s is (3.123 +/- 0.336)E-08 erg/cm^2. The average photon flux in the 8-200 keV band during this period is 31 +/- 3 ph/s/cm^2.

The spectral analysis results presented above are preliminary.

For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"