GCN Circular 28174
Subject
Fermi-GBM Sub-Threshold candidate for GRB 200729A
Date
2020-07-30T04:09:25Z (4 years ago)
From
Christian Malacaria at NASA-MSFC/USRA <cmalacaria@usra.edu>
C. Malacaria (NASA-MSFC/USRA), E. Burns (NASA-GSFC), C. M. Hui (NASA-MSFC)
and P. Veres (UAH) report on behalf of the Fermi-GBM team:
At 19:38:05 UT on 29 July 2020, the Swift Burst Alert Telescope (BAT)
triggered and located GRB 200729A (Evans et al. 2020, GCN 28165).
There was no Fermi-GBM onboard trigger around the event time of GRB 200729A.
An automated, blind search for short gamma-ray
bursts below the onboard triggering threshold in Fermi-GBM also
identified no counterpart candidates.
The GBM targeted search [1],
the most sensitive, coherent search for GRB-like signals,
was run from +/-30 s around the GRB 200729A time.
The targeted search identified a transient source with an offset
of about 7 s with respect to the Swift-BAT trigger time,
consistent with the Swift-BAT light curve peak.
The candidate most significant timescale is 8.192 s centered at the offset time,
with a log likelihood ratio of 62.
The Swift-XRT location is slightly out of the 3 sigma Fermi-GBM contours.
However, given the reasonable coincidence of trigger time,
light curve peak-time and morphology,
this candidate is likely coincident with GRB 200729A.
The source spectrum at all significant time scales identified by the search
is best fit by a power law function with an exponential
high-energy cutoff.
The fit was performed between the Fermi-GBM trigger time T0 and T0+33 s.
The power law index is 0.70 +/- 0.53,
and the cutoff energy, parameterized as Epeak, is 159 +/- 19 keV.
The spectral analysis results presented above are preliminary.
[1] Goldstein et al. 2019 arXiv:1903.12597