GRB 250620C

The Fermi GBM team reports the detection of a likely LONG GRB

At 16:07:09 UT on 20 Jun 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250620C (trigger 772128434.916854 / 250620672).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 142.6, Dec = 55.0 (J2000 degrees, equivalent to J2000 09h 30m, 55d 00'), with a statistical uncertainty of 1.0 degrees.

The angle from the Fermi LAT boresight is 124.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250620672/quicklook/glg_skymap_all_bn250620672.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250620672/quicklook/glg_healpix_all_bn250620672.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250620672/quicklook/glg_lc_medres34_bn250620672.gif

O. Mukherjee (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 16:07:09.92 UT on 20 June 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB250620C (trigger 772128434/250620672).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 142.61, Dec = 54.96 (J2000 degrees, equivalent to
J2000 9h 30m, +54d 57'), with a statistical uncertainty of 1.00 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a
systematic error which we have characterized as a mixture of two Gaussians,
one with a radius of 1.8 degrees (52% contribution) and one with a radius
of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).

The angle from the Fermi LAT boresight is 124 degrees.

The GBM light curve consists of multiple emission episodes with a duration (T90)
of about 32 s (50-300 keV). The time-averaged spectrum
from T0 to T0+35.7 s is best fit by
a Band function with Epeak= 371 +/- 15 keV, alpha = -0.93 +/- 0.02 and beta = -2.6 +/- 0.2.

The event fluence (10-1000 keV) in this time interval is
(6.64 +/- 0.07)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.96 s in the 10-1000 keV band is 50.1 +/- 0.7 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/"

SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Ulysse Jacob (LUPM)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a bright burst GRB 250620C (SVOM trigger reference: sb25062006) at 2025-06-20T16:07:10.100 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#40794
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 ).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a narrow spike with a T90 of 30.1 +/-1.5 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250620C.png

In addition, the position of this burst, as determined by Fermi/GBM (RA= 142.6, DEC= 55.0, GCN#40794
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 ), is located at about 161 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0-1 to T0+40 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.22 +0.13/-0.12 and the cutoff energy, parameterized as Epeak, is 502 +74/-55 keV. The event fluence (10-1000 keV) in this time interval is (9.15 +0.26/-0.28)E-07 erg/cm^2. 

The localization of GRB 250620C in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/grb250620C_amati.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)

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

The bright,long-duration GRB 250620C
(Fermi-GBM detection: The Fermi GBM team, GCN 40794
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 ;
Mukherjee, GCN 40799
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 ;
SVOM-GRM detection: Wang et al., GCN 40807
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 ;
IPN triangulation: Svinkin et al., GCN 40809
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 )
triggered Konus-Wind at T0=58026.172 s UT (16:07:06.172).

The burst light curve shows a multipeaked structure
which starts at ~T0-0.2 s and has a total duration of ~39 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250620_T58026/

As observed by Konus-Wind, the burst
had a fluence of 7.87(-0.47,+0.43)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+1.344 s,
of 4.05(-0.28,+0.28)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+39.424 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.00(-0.03,+0.03),
the high energy photon index beta = -2.83(-0.42,+0.21),
the peak energy Ep = 363(-17,+19) keV
(chi2 = 116/97 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+1.536 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.71(-0.05,+0.05),
the high energy photon index beta = -2.44(-0.14,+0.12),
the peak energy Ep = 618(-45,+47) keV
(chi2 = 85/59 dof).

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

S.Salunke (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 and bright GRB 250620C which was also detected by Fermi/GBM (Fermi GBM team, GCN Circ. 40794
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 ), SVOM/GRM (Chen-Wei Wang et al, GCN Circ. 40807
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 ), and Konus-Wind (Svinkin et al, GCN Circ. 40810
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 ).

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 2025-06-20 16:07:11.50 UTC. The measured peak count rate associated with the burst is 1004 (+55, -46) counts/s above the background in the combined data of two quadrants (out of four), with a total of 8579 (+269, -224) counts. The local mean background count rate was 150 (+2, -3) counts/s. Using cumulative rates, we measure a T90 of 31.3 (+0.3, -0.4) s. In the preliminary analysis, we find 1018 Compton events associated with this event.

The source was also clearly 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 2025-06-20 16:07:10.63 UTC. The measured peak count rate associated with the burst is 4717 (+120, -137) counts/s above the background in the combined data of all quadrants, with a total of 17904 (+519, -667) counts. The local mean background count rate was 1222 (+6, -5) counts/s. We measure a T90 of 31 (+1, -2) 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