GRB 260518A

D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia,
A. Tsvetkova, M. Ulanov, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 260518A
(AstroSat-CZTI detection: Harsha et al., GCN 44668
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triggered Konus-Wind at T0=27873.529 s UT (07:44:33.529).

The burst light curve shows a multipeaked structure
which starts at ~T0-9 s and has a total duration of ~18 s.
The emission is seen up to ~1 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260518_T27873/

As observed by Konus-Wind, the burst
had the total fluence of 4.59(-0.38,+0.42)x10^-6 erg/cm2,
and the 64-ms peak flux, measured from T0-0.016 s,
of 2.35(-0.47,+0.48)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.11(-0.36,+0.42)
and Ep = 213(-24,+31) keV (chi2 = 66/85 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.9
(chi2 = 66/84 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

Harsha K. H. (IUCAA), S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), A. Goyal (IITB), A. Arya (IITB), G. Waratkar (Caltech/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 GRB 260518A.

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-05-18 07:44:29.50 UTC. The measured peak count rate associated with the burst is 380 (+84, -78) counts/s above the background in the combined data of all quadrants, with a total of 654 (+182, -192) counts. The local mean background count rate was 295 (+5, -6) counts/s. Using cumulative rates, we measure a T90 of 9 (+2, -2) s. 

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2026-05-18 07:44:29.10 UTC. The measured peak count rate associated with the burst is 417 (+67, -72) counts/s above the background in the combined data of all quadrants, with a total of 1270 (+298, -329) counts. The local mean background count rate was 1301 (+5, -5) counts/s. We measure a T90 of 10 (+2, -3) 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 data products like interactive and downloadable light curves for this GRB can be found at: 
https://astrosat.iucaa.in/cift/cift_products/516786266.6/S516786266.6_details.html

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