GRB 260921A
GCN Circular 45727
Subject
GRB 260921A: NuSTAR detection of prompt emission
Date
2026-09-24T15:38:08Z (2 days ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
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 GRB 260921A in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper.
The NuSTAR SINGS algorithm, triggered at 2026-09-21T09:29:08.42 UTC, shows a detection of GRB 260921A, also detected by Fermi-GBM (GCN 45674), Swift-BAT (GCN 45676), AstroSat-CZTI (GCN 45687), SVOM-GRM (GCN 45725), and Konus-Wind (GCN 45722).
The NuSTAR CsI shield data are recorded at 1 Hz. We detect a single peak lasting for about 10-s. The peak count rate is ~1500-cps with a baseline rate of ~1000-cps during this time period. We do not see any evidence in the signal above 100 keV in the CZT detectors.
The Swift-BAT localization (GCN 45686) at RA = 145.376, Dec = 2.849 implies an offset from the NuSTAR boresight of 101-deg (i.e. from the side) and an offset from the geocenter of 136-deg.
Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260921A
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 45726
Subject
Konus-Wind detection of GRB 260921A (correction to GCN 45722)
Date
2026-09-24T15:34:09Z (2 days ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
Anna Ridnaia reports on behalf of the Konus-Wind Team:
The rest-frame peak energy values of GRB 260921A plotted
in the Amati and Yonetoku diagrams, mentioned in the KW GCN 45722,
were erroneously twice-corrected for the redshift z=1.3
With the correct values of rest-frame Ep, reported in the text
of GCN 45722, GRB 260921A is consistent with 90% prediction bands
of the 'Amati' relation and 68% prediction bands of 'Yonetoku' relation
for the sample of >300 long KW GRBs with known redshifts
(Tsvetkova et al., 2017; Tsvetkova et al., 2021), see the corrected plot
at http://www.ioffe.ru/LEA/GRBs/GRB260921_T34147/GRB260921A_rest_frame_corr.pdf
We apologize for this mistake.
GCN Circular 45725
Subject
GRB 260921A: SVOM/GRM observation
Date
2026-09-24T14:26:37Z (2 days ago)
From
Jin-Ye Luo <jy_luo2002@163.com>
Via
Web form
SVOM/GRM team: Jin-Ye Luo, Chen-Wei Wang, Yue Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Marius Brunet (IRAP)
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by GRB 260921A at 2026-09-21T09:29:10.300 (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #45674), Swift/BAT (Page et al., GCN #45676), AstroSat/CZTI (Goyal et al., GCN #45687), and Konus-Wind (Ridnaia et al., GCN #45722).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of muplti-pulses with a T90 of 8.0 +2.0/-1.5 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here:
https://www.bursthub.cn/gcn/2026/svgrb260921A.png
In addition, the position of this burst, as determined by Swift/BAT (RA = 145.373, Dec = 2.852, Error = 3 arcmin, GCN #45676), is located at about 96 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.
With this localization, the time-averaged spectrum from T0-4.3 to T0+6.7 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.01 +0.13/-0.14 and the cutoff energy, parameterized as Epeak, is 239 +33/-26 keV. The event fluence (10-1000 keV) in this time interval is (9.49 +/−0.57)E-06 erg/cm^2.
The localization of GRB 260921A in the 'Amati' relation diagram is shown at: https://www.bursthub.cn/gcn/2026/svgrb260921A_amati.png'
The localization of GRB 260921A in the 'Yonetoku' relation diagram is shown at:
https://www.bursthub.cn/gcn/2026/svgrb260921A_yonetoku.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/GRM point of contact for this burst is: Jin-Ye Luo (IHEP) (luojy@ihep.ac.cn)
GCN Circular 45722
Subject
Konus-Wind detection of GRB 260921A (possible merger-origin)
Date
2026-09-24T13:39:33Z (2 days ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, V. Panteleeva,
D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 260921A
(Fermi-GBM detection: Fermi GBM Team, GCN 45674;
Sonawane, GCN 45684;
Swift-BAT detection: Page et al., GCN 45676;
Parsotan et al., GCN 45686;
AstroSat-CZTI detection: Goyal et al., GCN 45687)
triggered Konus-Wind at T0=34147.045 s UT (09:29:07.045).
The burst light curve shows a multipeaked structure
which starts at ~T0-2.8 s and has a total duration of ~8.9 s.
The emission is seen up to ~2 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260921_T34147/
As observed by Konus-Wind, the burst
had the total fluence of 7.93(-0.60,+0.68)x10^-6 erg/cm2,
and the 64-ms peak flux, measured from T0+2.880 s,
of 3.71(-0.70,+0.73)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 = -0.92(-0.18,+0.21)
and Ep = 334(-43,+59) keV (chi2 = 78/82 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.3
(chi2 = 79/81 dof).
The spectrum near the maximum count rate
(measured from T0+0.256 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 = -0.90(-0.21,+0.24)
and Ep = 308(-41,+59) keV (chi2 = 80/83 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.3
(chi2 = 80/82 dof).
Assuming the redshift z=1.300 (Segura et al., GCN 45678)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 3.65(-0.27,+0.31)x10^52 erg,
the peak luminosity L_iso is 3.93(-0.74,+0.77)x10^52 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 768(-99,+136) keV and the spectrum near the maximum count rate
Ep,p,z is 708(-95,+135) keV.
With the obtained estimates, GRB 260921A is a clear outlier
in the 'Amati' relation and nearly an outlier in the 'Yonetoku'
relation derived for the sample of >300 long KW GRBs with known redshifts
(Tsvetkova et al., 2017; Tsvetkova et al., 2021), but is generally consistent
with the Type I (merger-origin) population,
see http://www.ioffe.ru/LEA/GRBs/GRB260921_T34147/GRB260921A_rest_frame.pdf
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
GCN Circular 45687
Subject
GRB 260921A: AstroSat CZTI detection
Date
2026-09-22T05:23:16Z (5 days ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
A. Goyal (IITB), A. Arya (IITB), Harsha K. H. (IUCAA), S. Salunke(IUCAA), 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 GRB 260921A which was also detected by Fermi (Fermi GBM team, GCN Circ. 45674), Swift BAT (Parsotan et. al, GCN Circ. 45686).
The source was 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 2026-09-21 09:29:13.18 UTC. The measured peak count rate associated with the burst is 123 (+38, -16) counts/s above the background in the combined data of all quadrants, with a total of 551 (+257, -252) counts. The local mean background count rate was 307 (+2, -4) counts/s. Due to poor statistics, we cannot reliably estimate the T90 from it.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2026-09-21 09:29:10.55 UTC .The measured peak count rate associated with the burst is 563 (+55, -61) counts/s above the background in the combined data of all quadrants, with a total of 1892 (+187, -211) counts. The local mean background count rate was 682 (+4, -4) counts/s. We measure a T90 of 7 (+1, -2) s from the cumulative Veto light curve.
CZTI data products like interactive and downloadable light curves for this GRB can be found at: https://astrosat.iucaa.in/cift/cift_products/ET20260921T092910/ET20260921T092910_details.html
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 45686
Subject
GRB 260921A: Swift-BAT refined analysis
Date
2026-09-22T01:58:07Z (5 days ago)
From
Amy <yarleen@gmail.com>
Via
email
T. Parsotan (GSFC), S. D. Barthelmy (GSFC), R. Gupta (GSFC), H. A. Krimm
(NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), D.
Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):
Using the data set from T-61 to T+243 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 260921A (trigger #1512356)
(Page et al., GCN Circ. 45676). The BAT ground-calculated position is
RA, Dec = 145.376, 2.849 deg which is
RA(J2000) = 09h 41m 30.2s
Dec(J2000) = +02d 50' 55.6"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 27%.
The mask-weighted light curve shows a single-pulse structure that starts at
~T-7 s, peaks at ~T0, and ends at ~T+7 s. T90 (15-350 keV) is 8.62 +- 1.44
sec (estimated error including systematics).
The time-averaged spectrum from T-6.88 to T+6.93 sec is best fit by a
simple power-law model. The power law index of the time-averaged spectrum
is 1.29 +- 0.07. The fluence in the 15-150 keV band is 3.4 +- 0.1 x 10^-06
erg/cm2. The 1-sec peak photon flux measured from T+0.13 sec in the 15-150
keV band is 7.5 +- 0.6 ph/cm2/sec. All the quoted errors are at the 90%
confidence level.
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1512356
GCN Circular 45684
Subject
GRB 260921A: Fermi GBM Observation
Date
2026-09-21T18:45:14Z (5 days ago)
From
Rushikesh Sonawane at IISER TVM <rushikesh23@iisertvm.ac.in>
Via
Web form
R. Sonawane (IISER, TVM) reports on behalf of the
Fermi Gamma-ray Burst Monitor Team:
"At 09:29:08.42 UT on 21 September 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260921A (trigger 811675753/260921395),
which was also detected by Swift BAT (Page et al. 2026, GCN 45676).
MASTER reported a possible optical counterpart with a candidate
host galaxy at z=1.3 (Lipunov et al. 2026, GCN 45678).
The Fermi GBM real-time localization (GCN 45674) is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 95 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 17.3 s (50-300 keV). The time-averaged spectrum
from T0-6.3 to T0+13.9 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.97 +/- 0.02 and the cutoff energy,
parameterized as Epeak, is 280 +/- 3 keV.
The event fluence (10-1000 keV) in this time interval is
(9.3 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.0 s in the 10-1000 keV band is 12.3 +/- 0.3 ph/s/cm^2.
A Band function fits the spectrum equally well
with Epeak = 261 +/- 3 keV, alpha = -0.95 +/- 0.01 and beta = -2.58 +/- 0.05.
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 45678
Subject
Swift GRB 260921A: MASTER OT J094129.58+025052.5 possible optical counterpart detection with host galaxy (z=1.3)
Date
2026-09-21T10:29:29Z (6 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
J. Segura,F.Podesta, C.Francile, R.Podesta, E. Gonzalez (OAFA, SJNU),
V.Lipunov, I.Panchenko, P.Balanutsa, K.Zhirkov,.Gorbovskoy,
G.Antipov, N.Tiurina, Ya.Kechin, V.Senik, A.Chasovnikov, V.Topolev,D.Vlasenko,
A.Sankovich, I.Gorbunov, V.Fedorova, A.Telnova (Lomonosov MSU),
N.Budnev, O.Gress (ISU),
D.Buckley (SAAO),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich (Blagoveschensk Educational StateUniversity)
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez,A.R.Corella,
J.Tanori, L. Villalobos, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics
Observatory)
MASTER-OAFA robotic telescope of MASTER Global Robotic Net[1-4]
discovery MASTER OT J094129.58+025052.5 at 09 30 32 UT with error +-4"., with ~14 mag unfiltered. There is galaxy at z=1.3 (LEGACY catalog}.
[1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L
[2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271
[3] Lipunov et a. 2019, ARep, vol.63, 293
[4] Lipunov et al. 2023, Astronomical Robotic Networks and Operative Multichanel
Astrophysics,Lomonosov MSU PRESS, 591pp.
http://www.pereplet.ru/lipunov/625.html#625
GCN Circular 45677
Subject
Swift GRB 260921A: Global MASTER-Net observations report
Date
2026-09-21T09:59:36Z (6 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
M. Segura, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA)
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya..Kechin, V.Senik, A.Chasovnikov, K.Labsina (Lomonosov MSU)
D.Buckley (SAAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
A.Sosnovskij (Lomonosov MSU)
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory)
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
R.Rebolo (The Instituto de Astrofisica de Canarias)
O.Gress, N.Budnev(ISU)
MASTER-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 260921A ( K. L. Page et al., GCN 45676) errorbox 58 sec after notice time and 80 sec after trigger time at 2026-09-21 09:30:32 UT, with upper limit up to 15.5 mag. Observations started at twilight. The observations began at zenith distance = 76 deg. The sun altitude is -12.8 deg.
The galactic latitude b = 39 deg., longitude l = 233 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3508447
We obtain a following upper limits.
Tmid-T0 | Site |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________
91 | MASTER-OAFA | C | 20 | 15.2 |
117 | MASTER-OAFA | C | 20 | 15.3 |
148 | MASTER-OAFA | C | 30 | 15.3 |
183 | MASTER-OAFA | C | 30 | 15.2 |
224 | MASTER-OAFA | C | 40 | 15.5 |
275 | MASTER-OAFA | C | 50 | 15.1 |
337 | MASTER-OAFA | C | 60 | 15.3 |
407 | MASTER-OAFA | C | 70 | 14.9 |
494 | MASTER-OAFA | C | 90 | 14.6 |
600 | MASTER-OAFA | C | 110 | 15.3 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 45676
Subject
GRB 260921A: Swift detection of a burst
Date
2026-09-21T09:52:28Z (6 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
K. L. Page (U Leicester), V. D'Elia (SSDC & INAF-OAR),
S. Dichiara (PSU), R. A. J. Eyles-Ferris (U Leicester),
N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL), T. M. Parsotan (GSFC)
and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels
Swift Observatory Team:
At 09:29:12 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 260921A (trigger=1512356). Swift did not automatically slew
to the burst. The BAT on-board calculated location is
RA, Dec (145.373, +2.852), which is
RA(J2000) = 9h 41m 29s
Dec(J2000) = +2d 51' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The available BAT light curve (cut off at
T+8 s) showed a complex structure with a duration of about 10 sec.
The peak count rate was ~4500 counts/sec (15-350 keV), at ~1 sec
after the trigger.
Swift's automatic slews are currently disabled, and the burst is Sun
constrained until 8th October, so there will be no XRT or UVOT data for
this trigger.
This is the first BAT-triggered GRB since GRB 260127A. Since then,
Swift has been hibernating in preparation for the LINK reboost attempt.
With the rescope of that mission, Swift has been returned to
operation and we will continue to make science observations for
as long as possible until re-entry.
Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
GCN Circular 45674
Subject
GRB 260921A: Fermi GBM Final Real-time Localization
Date
2026-09-21T09:39:40Z (6 days ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 09:29:08 UT on 21 Sep 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260921A (trigger 811675753.423246 / 260921395).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 148.9, Dec = 0.6 (J2000 degrees, equivalent to J2000 09h 55m, 0d 36'), with a statistical uncertainty of 1.4 degrees.
The angle from the Fermi LAT boresight is 91.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260921395/quicklook/glg_skymap_all_bn260921395.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260921395/quicklook/glg_healpix_all_bn260921395.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260921395/quicklook/glg_lc_medres34_bn260921395.gif