TITLE:   GCN CIRCULAR
NUMBER:  36534
SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 240516B
DATE:    24/05/23 00:17:44 GMT
FROM:    Sarah Dalessi at UAH <sd0104@uah.edu>

S. Dalessi (UAH), S. Bala (USRA), P. Veres (UAH), and C. Meegan (UAH) report on behalf of the Fermi-GBM Team:

AstroSat CZTI detected GRB 240516B at 03:41:01.51 UT (GCN 36527). There was a
Fermi-GBM onboard trigger 259s before the event (GCN 36499) and due to the Fermi-GBM trigger latency, were unable to trigger on this event.

The GBM targeted search (Goldstein et al. 2019 arXiv:1903.12597), 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 1.1e-05 Hz, using the standard search protocol with a S/N of 15. 

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.


The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 145.4, Dec = -9.8 (J2000 degrees, equivalent to
J2000 3h 5m, +47d 6'), with a statistical uncertainty of 4 degrees.
(radius, 1-sigma containment, statistical only. There is additionally a 
systematic error which we have characterized as a core-plus-tail model, 
with 90% of GRBs having a 3.7 deg error and a small tail suffering a 
larger than 10 deg systematic error (Connaughton et al. 2015, ApJS, 216, 32)).

The angle from the Fermi LAT boresight is 41 degrees.

The GBM light curve single emission episode with a duration (T90)
of about 50 s (50-300 keV). The time-averaged spectrum
from T0+259.1 to T0+309.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.44 +/- 0.12 and the cutoff energy,
parameterized as Epeak, is 220 +/- 19 keV.

The event fluence (10-1000 keV) in this time interval is
(6.15 +/- 0.36)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+278.9 s in the 10-1000 keV band is 2.6 +/- 0.2 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

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