TITLE:   GCN CIRCULAR
NUMBER:  35776
SUBJECT: Fermi GBM Sub-Threshold Detection of EPW20240219aa as a likely GRB
DATE:    24/02/22 22:31:42 GMT
FROM:    Cori Fletcher at USRA <cfletcher@usra.edu>

C. Fletcher (USRA), S. Lesage (UAH) and P. Jenke (UAH) report on behalf of the Fermi-GBM Team:

Einstein Probe detected a bright X-ray flare (EPW20240219aa; ATel 16463; ATel 16472) at 06:21:42 UT on February 19, 2024. There was no Fermi-GBM onboard trigger around the event. Zhang et al. 2024 (GCN 35773) found a possible GRB event in Fermi-GBM data at that time.

An automated, blind search developed for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified EPW20240219aa with a low chance of being a short gamma-ray burst. The rate of GBM signals that meet the low reliability threshold (including onboard triggers) is 200/year. An initial estimate of the chance coincidence, including only temporal information, is ~4.1 sigma.

The GBM targeted search [1], the most sensitive, coherent search for
GRB-like signals identified a transient most significantly on the 8.192 s
timescale, with a false alarm rate of 7.7e-05 Hz and a location consistent with
the Einstein Probe event, using the standard search protocol with a S/N of 9.2. 
The GBM targeted search event was found with the highest
significance with a "normal" spectrum (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3) for a GRB.


[1] Goldstein et al. 2019 arXiv:1903.12597