GRB 260921A

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
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 ), Swift BAT (Parsotan et. al, GCN Circ. 45686
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 ).

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

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
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 ).  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

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
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 ).
MASTER reported a possible optical counterpart with a candidate 
host galaxy at z=1.3 (Lipunov et al. 2026, GCN 45678
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 ).

The Fermi GBM real-time localization (GCN 45674
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 ) 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/"

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

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
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 ) 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

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/)

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