GRB 240516B

J. Joshi (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 240516B. While this is separated by only ~ 266 seconds from GRB 240516A, the relative intensities in different quadrants of CZT and Veto detectors indicates a different source direction. Inspection of Fermi-GBM lightcurve shows the presence of a peak temporally coincident with this. The relative counts in various GBM detectors also indicate that the direction of this burst is different from GRB 240516A. There is no solar activity at this time in GOES data. This leads us to conclude that the two events are indeed separate GRBs.

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2024-05-16 03:41:01.50 UTC. The measured peak count rate associated with the burst is 115 (+38, -14) counts/s above the background in the combined data of all quadrants, with a total of 1333 (+280, -277) counts. The local mean background count rate was 338 (+2, -2) counts/s. Using cumulative rates, we measure a T90 of 29 (+1, -9) s.

These total counts are not enough to localise the source with CZTI, but rough count ratios in various quadrants yield a direction consistent with the direction in which corresponding Fermi detectors were pointing at the instant of GRB 240516B.

The source was also detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2024-05-16 03:41:03.52 UTC. The measured peak count rate associated with the burst is 163 (+74, -18) counts/s above the background in the combined data of all quadrants, with a total of 2002 (+608, -714) counts. The local mean background count rate was 1444 (+5, -5) counts/s. We measure a T90 of 33 (+7, -15) s from the cumulative Veto light curve.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

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
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 ). There was a
Fermi-GBM onboard trigger 259s before the event (GCN 36499
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 ) 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
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 ), 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/"