Gamma-ray Burst 250920A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Sep 25 02:54:36 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      45
TRIGGER_NUM:     780029651
GRB_RA:          178.383d {+11h 53m 32s} (J2000),
                 178.718d {+11h 54m 52s} (current),
                 177.729d {+11h 50m 55s} (1950)
GRB_DEC:         +56.017d {+56d 01' 00"} (J2000),
                 +55.874d {+55d 52' 25"} (current),
                 +56.295d {+56d 17' 42"} (1950)
GRB_ERROR:       3.97 [deg radius, statistical plus systematic]
GRB_INTEN:       1460 [cnts/sec]
DATA_SIGNIF:     46.10 [sigma]
INTEG_TIME:      0.512 [sec]
GRB_DATE:        20938 TJD;   263 DOY;   25/09/20
GRB_TIME:        10446.82 SOD {02:54:06.82} UT
GRB_PHI:          55.00 [deg]
GRB_THETA:        90.00 [deg]
DATA_TIME_SCALE: 0.5120 [sec]
HARD_RATIO:      1.05
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      98%  GRB
2nd_MOST_LIKELY:   1%  Generic SGR
DETECTORS:       0,1,1, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       177.62d {+11h 50m 30s}   +1.03d {+01d 01' 46"}
SUN_DIST:         54.85 [deg]   Sun_angle= -0.1 [hr] (East of Sun)
MOON_POSTN:      159.68d {+10h 38m 42s}   +9.56d {+09d 33' 30"}
MOON_DIST:        48.67 [deg]
MOON_ILLUM:      3 [%]
GAL_COORDS:      138.81, 59.29 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      148.36, 48.98 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_lc_medres34_bn250920121.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 46.93,3.25 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Sep 25 02:54:45 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      58
TRIGGER_NUM:     780029651
GRB_RA:          169.167d {+11h 16m 40s} (J2000),
                 169.536d {+11h 18m 09s} (current),
                 168.444d {+11h 13m 46s} (1950)
GRB_DEC:         +56.633d {+56d 37' 60"} (J2000),
                 +56.493d {+56d 29' 33"} (current),
                 +56.906d {+56d 54' 23"} (1950)
GRB_ERROR:       3.38 [deg radius, statistical plus systematic]
GRB_INTEN:       6330 [cnts/sec]
DATA_SIGNIF:     292.70 [sigma]
INTEG_TIME:      1.024 [sec]
GRB_DATE:        20938 TJD;   263 DOY;   25/09/20
GRB_TIME:        10446.82 SOD {02:54:06.82} UT
GRB_PHI:          56.00 [deg]
GRB_THETA:        85.00 [deg]
DATA_TIME_SCALE: 1.0240 [sec]
HARD_RATIO:      0.68
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      96%  GRB
2nd_MOST_LIKELY:   3%  Generic Transient
DETECTORS:       0,1,1, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       177.62d {+11h 50m 30s}   +1.03d {+01d 01' 46"}
SUN_DIST:         55.84 [deg]   Sun_angle= 0.5 [hr] (West of Sun)
MOON_POSTN:      159.68d {+10h 38m 43s}   +9.56d {+09d 33' 27"}
MOON_DIST:        47.56 [deg]
MOON_ILLUM:      3 [%]
GAL_COORDS:      146.21, 56.00 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      141.67, 46.48 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_lc_medres34_bn250920121.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 46.93,3.25 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Sep 25 02:54:55 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     780029651
GRB_RA:          172.040d {+11h 28m 10s} (J2000),
                 172.393d {+11h 29m 34s} (current),
                 171.350d {+11h 25m 24s} (1950)
GRB_DEC:         +50.820d {+50d 49' 12"} (J2000),
                 +50.678d {+50d 40' 41"} (current),
                 +51.095d {+51d 05' 44"} (1950)
GRB_ERROR:       3.38 [deg radius, statistical only]
DATA_SIGNIF:     412.90 [sigma]
DATA_INTERVAL:   1.024 [sec]
GRB_DATE:        20938 TJD;   263 DOY;   25/09/20
GRB_TIME:        10446.82 SOD {02:54:06.82} UT
GRB_PHI:          50.00 [deg]
GRB_THETA:        86.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:       177.62d {+11h 50m 30s}   +1.03d {+01d 01' 46"}
SUN_DIST:         49.85 [deg]   Sun_angle= 0.3 [hr] (West of Sun)
MOON_POSTN:      159.68d {+10h 38m 43s}   +9.56d {+09d 33' 25"}
MOON_DIST:        42.44 [deg]
MOON_ILLUM:      3 [%]
GAL_COORDS:      151.03, 61.52 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      148.16, 42.56 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_lc_medres34_bn250920121.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_780029651.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        Bright hard burst in the GBM.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Sep 25 02:55:21 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     780029651
GRB_RA:          173.830d {+11h 35m 19s} (J2000),
                 174.177d {+11h 36m 43s} (current),
                 173.152d {+11h 32m 36s} (1950)
GRB_DEC:         +49.920d {+49d 55' 12"} (J2000),
                 +49.778d {+49d 46' 39"} (current),
                 +50.197d {+50d 11' 48"} (1950)
GRB_ERROR:       1.71 [deg radius, statistical only]
DATA_SIGNIF:     36.60 [sigma]
DATA_INTERVAL:   2.048 [sec]
GRB_DATE:        20938 TJD;   263 DOY;   25/09/20
GRB_TIME:        10446.82 SOD {02:54:06.82} UT
GRB_PHI:          49.00 [deg]
GRB_THETA:        87.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       177.62d {+11h 50m 30s}   +1.03d {+01d 01' 46"}
SUN_DIST:         48.84 [deg]   Sun_angle= 0.2 [hr] (West of Sun)
MOON_POSTN:      159.68d {+10h 38m 44s}   +9.56d {+09d 33' 19"}
MOON_DIST:        41.99 [deg]
MOON_ILLUM:      3 [%]
GAL_COORDS:      150.42, 62.95 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      150.12, 42.41 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_lc_medres34_bn250920121.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_780029651.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Sep 25 02:55:31 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      2
TRIGGER_NUM:     780029651
GRB_RA:          173.780d {+11h 35m 07s} (J2000),
                 174.127d {+11h 36m 31s} (current),
                 173.102d {+11h 32m 24s} (1950)
GRB_DEC:         +49.910d {+49d 54' 36"} (J2000),
                 +49.768d {+49d 46' 03"} (current),
                 +50.187d {+50d 11' 12"} (1950)
GRB_ERROR:       1.00 [deg radius, statistical only]
DATA_SIGNIF:     309.30 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        20938 TJD;   263 DOY;   25/09/20
GRB_TIME:        10446.82 SOD {02:54:06.82} UT
GRB_PHI:          49.00 [deg]
GRB_THETA:        87.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       177.62d {+11h 50m 30s}   +1.03d {+01d 01' 46"}
SUN_DIST:         48.83 [deg]   Sun_angle= 0.2 [hr] (West of Sun)
MOON_POSTN:      159.68d {+10h 38m 44s}   +9.55d {+09d 33' 16"}
MOON_DIST:        41.97 [deg]
MOON_ILLUM:      3 [%]
GAL_COORDS:      150.49, 62.94 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      150.09, 42.39 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_lc_medres34_bn250920121.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_780029651.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        Bright hard burst in the GBM.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Sep 25 03:48:41 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     780029651
GRB_RA:          175.270d {+11h 41m 05s} (J2000),
                 175.613d {+11h 42m 27s} (current),
                 174.601d {+11h 38m 24s} (1950)
GRB_DEC:         +48.960d {+48d 57' 36"} (J2000),
                 +48.817d {+48d 49' 02"} (current),
                 +49.237d {+49d 14' 14"} (1950)
GRB_ERROR:       1.00 [deg radius, statistical only]
GRB_DATE:        20938 TJD;   263 DOY;   25/09/20
GRB_TIME:        10446.82 SOD {02:54:06.82} UT
GRB_PHI:          47.99 [deg]
GRB_THETA:        87.87 [deg]
E_RANGE:         50.000 - 300.000 [keV]
LOC_ALGORITHM:   415 (Gnd S/W Version number)
SUN_POSTN:       177.66d {+11h 50m 38s}   +1.01d {+01d 00' 54"}
SUN_DIST:         47.83 [deg]   Sun_angle= 0.1 [hr] (West of Sun)
MOON_POSTN:      160.11d {+10h 40m 26s}   +9.34d {+09d 20' 22"}
MOON_DIST:        41.56 [deg]
MOON_ILLUM:      3 [%]
GAL_COORDS:      150.14, 64.28 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      151.88, 42.10 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_lc_medres34_bn250920121.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_locplot_all_bn250920121.png
COMMENTS:        Fermi-GBM Final Position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file should be available by the time this FINAL notice is produced.  
COMMENTS:        This notice has human-in-the-loop processing.

GCN Circular #41895
The Fermi GBM team reports the detection of a likely LONG GRB
"At 02:54:06.82 UT on 20 September 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250920 (trigger 780029651/250920121).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 175.27, Dec = 48.96 (J2000 degrees, equivalent to J2000 11h 41m, +48d 57'), with a statistical uncertainty of 1.00 degrees.
The angle from the Fermi LAT boresight is 88 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_skymap_all_bn250920121.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/bn250920121/quicklook/glg_healpix_all_bn250920121.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920121/quicklook/glg_lc_medres34_bn250920121.gif"
GCN Circular #41896
T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:
The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger 780029651 at 02:54:06 on 20 Sept. 2025 were automatically fitted for spectrum and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427; Berlato et al. 2019, ApJ 873, 60).
The best-fit position is:
```
RA(2000.0) = 174.6 deg
Decl.(2000.0) = 52.8 deg
```
The 1 sigma statistical error radius is 1.9 deg. We estimate an additional systematic error of 2 deg.
Further details are available at: https://grb.mpe.mpg.de/grb/GRB250920121/
The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB250920121/healpix
The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB250920121/json
GCN Circular #41899
Harsha K. H. (IUCAA), S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), A. Goyal (IITB), A. Arya (IITB), U. Pathak(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 bright, long GRB 250920A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 41895).
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 2025-09-20 02:54:08.95 UTC. The measured peak count rate associated with the burst is 636 (+45, -50) counts/s above the background in the combined data of two quadrants (out of four), with a total of 1944 (+129, -140) counts. The local mean background count rate was 149 (+2, -3) counts/s. Using cumulative rates, we measure a T90 of 9.3 (+0.5, -0.4) s.
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 2025-09-20 02:54:08.30 UTC. The measured peak count rate associated with the burst is 2813 (+106, -113) counts/s above the background in the combined data of all quadrants, with a total of 4822 (+303, -320) counts. The local mean background count rate was 1366 (+7, -7) counts/s. We measure a T90 of 8.7 (+0.7, -0.4) 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
GCN Circular #41912
SVOM/GRM team: Chen-Wei Wang. Wen-Jun Tan, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Jean-Luc Atteia (IRAP)
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by a bright burst GRB 250920A (SVOM trigger reference: sb25092001) at 2025-09-20T02:54:09.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#41895) and AstroSat (Harsha K. H., GCN#41899).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 9.6 +0.3/-0.1 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250920A.png
In addition, the position of this burst, as determined by Fermi/GBM (RA= 175.27, DEC= 48.96, GCN#41895), is located at about 144 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view, but this burst still clearly detected by ECLAIRs through the shield.
With this localization, the time-averaged spectrum from T0-3 to T0+11 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.58 +0.06/-0.06 and the cutoff energy, parameterized as Epeak, is 202 +17/-13 keV. The event fluence (10-1000 keV) in this time interval is (6.24 +0.14/-0.13)E-05 erg/cm^2.
The localization of GRB 250919A in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb250920A_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)
GCN Circular #41918
R. Sonawane (IISER, TVM) and J.R. Smith (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 02:54:06.82 UT on 20 September 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250920A (trigger 780029651/250920121). which was also detected by AstroSat-CZTI( Harsha K. H. et al. 2025, GCN 41899), and SVOM-GRM (Chen-Wei Wang et al. 2025, GCN 41912).
The Fermi GBM Final Localization is presented in GCN 41895.
The angle from the Fermi LAT boresight is 88 degrees.
The GBM light curve consists of two emission episodes with a duration (T90) of about 9.4 s (50-300 keV). The time-averaged spectrum from T0+0.003 to T0+13.312 s is best fit by a Band function with Epeak = 151 +/- 3 keV, alpha = -0.81 +/- 0.02, and beta = -2.39 +/- 0.04.
The event fluence (10-1000 keV) in this time interval is (4.22 +/- 0.03)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1.7 s in the 10-1000 keV band is 102.5 +/- 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/"
GCN Circular #41926
G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:
The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 250920A in both the NuSTAR CsI anti-coincidence shields. This GRB was identified through a blind search using the CsI shield rates. Details of the search algorithm will be described in a future paper.
The NuSTAR SINGS algorithm triggered at 2025-09-20T02:54:07.000 (with a resolution ~5-seconds). This is consistent with the detections of GRB 250920A by Fermi/GBM (Fermi GBM Team, GCN Circ. 41895, Sonawane et al., GCN Circ. 41918), AstroSat/CZTI (Harsha et al., GCN Circ. 41898), and SVOM/GRM (Wang et al., GCN Circ. 41912).
The NuSTAR CsI shield data are recorded at 1 Hz. The burst appears to be composed of one bright peak lasting for ~4-s, followed by a relatively fainter peak lasting for 3-s. The peak count rate is ~5000-cps over a baseline rate of ~1,000-cps during this time period. We also see clear evidence in the signal above 100 keV in the CdZnTe detectors for the first episode.
The Fermi/GBM localization (Fermi GBM Team, GCN Circ. 41895) at RA = 175.27, Dec = 48.96 implies an offset from the NuSTAR boresight of 104-deg (i.e., through the side of the instrument) and an offset from the geocenter of 94-deg.
Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250920A
Information on NuSTAR SINGS can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/
NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.
GCN Circular #41940
A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team,
D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team,
A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi GBM team,
and
W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report:
The bright, long-duration GRB 250920A (Ferm-GBM detection: The Fermi GBM team, GCN 41895; Sonawane and Smith, GCN 41918; BALROG localization: Preis and Greiner, GCN 41896 AstroSat-CZTI detection: Harsha et al., GCN 41899; SVOM-GRM detection: Wang et al., GCN 41912; NuSTAR-detection: Waratkar et al., GCN 41926) was detected by Fermi (GBM trigger 780029651), SVOM (GRM), AstroSat (CZTI), NuSTAR, Konus-Wind, and Mars-Odyssey (HEND), at about 10447 s UT (02:54:07).
We have triangulated it to a preliminary, 3 sigma error box whose coordinates are:
```
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  176.909 (11h 47m 38s) +54.466 (+54d 27' 56")
 Corners:
  177.868 (11h 51m 28s) +54.769 (+54d 46' 07")
  177.751 (11h 51m 00s) +54.775 (+54d 46' 28")
  175.963 (11h 43m 51s) +54.152 (+54d 09' 06")
  176.079 (11h 44m 19s) +54.148 (+54d 08' 54")
 ---------------------------------------------
```
The error box area is 168 sq. arcmin, and its maximum dimension is 1.27 deg (the minimum one is 2.3 arcmin). The Sun distance was 53 deg.
This localization may be improved.
A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250920_T10444/IPN/ The HEALPix triangulation map is the multi-order HEALPix in units of probability density.
The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular.
GCN Circular #41941
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 250920A (Ferm-GBM detection: The Fermi GBM team, GCN 41895; Sonawane and Smith, GCN 41918; BALROG localization: Preis and Greiner, GCN 41896 AstroSat-CZTI detection: Harsha et al., GCN 41899; SVOM-GRM detection: Wang et al., GCN 41912; NuSTAR-detection: Waratkar et al., GCN 41926; IPN triangulation: Kozyrev et al., GCN 41940) triggered Konus-Wind at T0=10444.538 s UT (02:54:04.538).
The burst light curve shows a multipeaked structure which starts at ~T0-1.1 s and has a total duration of ~12.5 s. The emission is seen up to ~15 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250920_T10444/ There are no data for the ~18-70 keV band in the triggered mode.
As observed by Konus-Wind, the burst had the total fluence of 4.52(-0.15,+0.15)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+1.984 s, of 4.06(-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+18.432 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.84(-0.05,+0.05), the high energy photon index beta = -2.61(-0.09,+0.08), the peak energy Ep = 162(-6,+6) keV (chi2 = 106/97 dof).
The spectrum near the maximum count rate (measured from T0+1.792 to T0+2.048 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.58(-0.09,+0.09), the high energy photon index beta = -2.46(-0.14,+0.11), the peak energy Ep = 332(-27,+31) keV (chi2 = 44/46 dof).
All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.

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Jochen Greiner, last update: 22-Sep-2025 [Disclaimer]