TITLE:   GCN CIRCULAR
NUMBER:  27514
SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 200409A
DATE:    20/04/09 18:58:23 GMT
FROM:    Eric Burns at GSFC  <erickayserburns@gmail.com>

E. Burns (GSFC), M. S. Briggs (UAH), C. M. Hui (MSFC), C. Malacaria (MSFC),
and P. Veres (UAH) reports on behalf of the Fermi-GBM Team

Swift-BAT detected GRB 200409A at 03:19:59 UT (GCN 27509). There was no
Fermi-GBM onboard trigger around the event.

An automated, blind search for short gamma-ray bursts below the onboard
triggering threshold in Fermi-GBM identified GRB 200904A with high
reliability [1], with a localization consistent with the Swift-BAT
location, and a discovery timescale of 0.703 s.

The GBM targeted search [2], the most sensitive, coherent search for
GRB-like signals, was run from +/-30 s around BAT trigger time. A transient
source was identified whose most significant timescale according to the
search is 1.024 s, with a log likelihood ratio of 62, with a consistent
location, and is consistent with a soft spectrum for a GRB.

Preliminary spectral analysis statistically prefers a power-law fit over
models that constrain Epeak. Using T0-0.320 to T0+0.320 as the source
interval gives an alpha of -1.84 +/- 0.06, a photon flux of 3.3 +/- 0.4
ph/s/cm^2, and a fluence of (2.0+/-0.2)E-7 erg/s/cm^2.

[1] https://gcn.gsfc.nasa.gov/notices_gbm_sub/608095205.fermi
[2] Goldstein et al. 2019 arXiv:1903.12597