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GRB 250916A

GCN Circular 42024

Subject
GRB 250916A: SVOM/VT optical observation
Date
2025-09-29T01:39:36Z (11 days ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, H. L. Li, Y. N. Ma, Z. H. Yao, Y. L. Qiu, C. Wu,  X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), W. K. Zheng (UCT), Z. Q. Wang (GXU) report on behalf of the SVOM/VT team:

SVOM/VT performed a Target of Opportunity observation of GRB 250916A detected by Fermi GBM team (GCN 41839), starting at 2025-09-23T07:32:29 UTC, 6.66 days after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

The optical counterpart (Belkin et al., GCN Circ. 41847; Mohan et al., GCN Circ. 41858; Moskvitin et al., GCN Circ. 41859; Burkhonov et al., GCN Circ. 41860; de Ugarte Postigo et al., GCN Circ. 41863; Moskvitin et al., GCN Circ. 41870; Zheng and Filippenko, GCN Circ. 41873; Pankov et al., GCN Circ. 41892; Hall et al., GCN Circ. 41921 and Moskvitin et al., GCN Circ. 41934, Breeveld et al., GCN 41938) was detected in both channels.

The brightness was VT_R=23.0 +/-0.25 mag, VT_B=23.5+/-0.3 mag, with an effective exposure time of 97*70 seconds. 

Our photometry was in AB magnitude and not corrected for Galactic extinction.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Centre 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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

GCN Circular 41938

Subject
GRB 250916A: Swift/UVOT Detection
Date
2025-09-22T15:32:00Z (17 days ago)
From
Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>
Via
email
A. A. Breeveld (UCL-MSSL) and R. Caputo (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of the Fermi/GBM detected burst GRB 250916A 102120s after the trigger (Fermi GBM team, GCN Circ. 41839).

The optical counterpart detected by Belkin et al., GCN Circ. 41847; Mohan et al., GCN Circ. 41858; Moskvitin et al., GCN Circ. 41859; Burkhonov et al., GCN Circ. 41860; de Ugarte Postigo et al., GCN Circ. 41863; Moskvitin et al., GCN Circ. 41870; Zheng and Filippenko, GCN Circ. 41873; Pankov et al., GCN Circ. 41892; Hall et al., GCN Circ. 41921 and Moskvitin et al., GCN Circ. 41934, is detected in the initial white and u-band UVOT exposures.

Preliminary magnitudes and upper limit using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

u              102120       107275         968            20.5 ± 0.2
white          102446       102767         316            21.1 ± 0.3
v              102774       103051         273            >19.4

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.051 in the direction of the burst (Schlegel et al. 1998).

GCN Circular 41936

Subject
GRB 250916A: CrAO ZTSh R-band optical observations
Date
2025-09-22T11:24:20Z (17 days ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), V. Rumyantsev (CrAO)
report on behalf of IKI-GRB-FuN.

We observed the field of the GRB 250916A (The Fermi GBM team, GCN 41839; Arya et al., GCN 41843; Cheung et al., GCN 41855; Osborne et al., GCN 41862; Waratkar and Grefenstette, GCN 41871; Godwin and Meegan, GCN 41876) with the Shajn 2.65m telescope of CrAO, equipped with CCD-photometer. The observations in the R-filter were carried out on epochs 2025-09-19, 2025-09-20, and 2025-09-21. The OT (Belkin et al., GCN 41847; Mohan et al., GCN 41858; Burkhonov et al., GCN 41860;  de Ugarte Postigo et al., GCN 41863; Moskvitin et al., GCN 41870; Zheng and Filippenko, GCN 41873; Pankov et al., GCN 41892; Hall et al., GCN 41921; Moskvitin et. al, GCN 41934) is clearly detected in the stacked images taken on each epoch.  Preliminary results of our observations are given below:

Date       UT start      t-T0     Exp.    Filter  OT     Err.      UL    
                     (mid, days)  (n*s)                         (3sigma)
2025-09-19 21:40:39    3.42451    120*120   R     21.45  0.02     25.0
2025-09-20 23:47:56    4.48374    78*120    R     21.89  0.03     24.7
2025-09-21 22:21:48    5.44545    109*120   R     22.72  0.05     24.8

Preliminary photometry is based on the nearby objects from PanSTARRS (magnitudes converted via Lupton 2005 transformations) and not corrected for the Galactic extinction.

GCN Circular 41934

Subject
GRB 250916A: SAO RAS Zeiss-1000 optical observations
Date
2025-09-22T09:39:39Z (17 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova (SAO), N. Pankov (HSE), A. Pozanenko 
(IKI), A. Ghosh, S. Razzaque (CAPP, University of Johannesburg), 
Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) 
report on behalf of GRB follow-up collaboration and IKI-GRB-FuN.

We observed the field of the GRB 250916A (Fermi GBM team, GCN 41839; 
Arya et al., GCN 41843; Cheung et al., GCN 41855; Osborne et al., 
GCN 41862; Waratkar and Grefenstette, GCN 41871; Godwin and Meegan, 
GCN 41876) with the 1-m SAO RAS telescope Zeiss-1000 equipped 
with the CCD-photometer. We obtained 9 x 300 sec. images Rc band 
on September 21, 23:36:57 -- September 22, 00:47:22 UT 
(t_mid - T0 = 5.44639 days).

The optical transient (Belkin et al., GCN 41847; Mohan et al., 
GCN 41858; Moskvitin et al., GCN 41859; Burkhonov et al., GCN 41860;
de Ugarte Postigo et al., GCN 41863; Moskvitin et al., GCN 41870;
Zheng and Filippenko, GCN 41873; Pankov et al., GCN 41892; 
Hall et al., GCN 41921) is clearly detected in the stacked frame 
with the brightness of R = 22.6 +/- 0.1.

Date        UT start  t-T0     Exp.  Filter  OT     Err. UL (3sigma)
                      (mid, d) (s)       
2025.09.21  23:36:57  5.44639  9 x 300 Rc  22.6 +/- 0.1 23.3
                      
Preliminary photometry is based on the nearby objects from PanSTARRS 
(magnitudes converted with Lupton 2005 equations) 
and has not corrected for the Galactic extinction.

GCN Circular 41921

Subject
GRB 250916A: FTW optical and NIR observations of the counterpart
Date
2025-09-20T22:03:31Z (19 days ago)
From
Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>
Via
Web form
Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the counterpart of GRB 250916A (Fermi GBM Team, GCN 41839; Belkin et al., GCN 41847) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 5 x 180 s starting at 2025-09-19 23:23:41 UT (3.4 days after the trigger). We detect the counterpart clearly in all filters and report the following magnitude:

r = (21.75 +/- 0.06) mag.

The magnitude is calibrated against the PS1 catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.

We thank Christoph Ries from the Wendelstein Observatory for obtaining these observations.

GCN Circular 41892

Subject
GRB 250916A: Mondy R-band optical observations
Date
2025-09-19T20:44:29Z (20 days ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) A. Moskvitin (SAO RAS) report on behalf of IKI-GRB-FuN:

We observed the field of a long GRB 250916A at the redshift z = 2.015 (de Ugarte Postigo et al., GCN 41863) detected by Fermi/GBM (The Fermi GBM team, GCN 41839), AstroSat/CZTI (Arya et al., GCN 41843), Glowbug (Cheung et al., GCN 41855), Swift/XRT (Osborne et. al, GCN 41862), and NuSTAR (Waratkar et. al, GCN 41871). The observations were carried with the AZT-33IK 1.5m reflector of Sayan Solar Observatory (Mondy) equipped with the CMOS-photometer. The series of 25x120 s images was obtained in the R-band starting on 2025-09-18 at 18:36 UT, i.e. ~2.23 days since GRB. In the stacked image we clearly detect the OT found initially by GOTO (Belkin et al., GCN 41847) and followed-up by (Mohan et al., GCN 41858; Moskvitin et. al, GCN 41859; Burkhonov et al., GCN 41860; Moskvitin et. al, GCN 41870; Zheng & Filippenko, GCN 41873). Preliminary observation results are presented below:


Date       UT start      t-T0     Exp.    Filter  OT     Err.      UL    
                     (mid, days)  (n*s)                         (3sigma)
2025-09-18 18:36:27   2.23063    25x120    R     20.52   0.02     21.5  

The photometry is based on nearby stars of PanSTARRS-DR1 (R-magnitudes were obtained via the Lupton 2005 transformations; see Moskvitin et. al, GCN 41859) and not corrected for the Galactic extinction.

GCN Circular 41876

Subject
GRB 250916A: Fermi GBM Observation
Date
2025-09-19T02:43:19Z (21 days ago)
From
Matt Godwin <msg0028@uah.edu>
Via
Web form
Matt Godwin (UAH) and C. Meegan (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 13:29:21.01 UT on 16 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250916A (trigger 779722166/250916562).
which was also detected by AstroSat ( Arya et al. 2025, GCN 41843) with an optical counterpart by GOTO (Belkin et al. 2025, GCN 41847).

The angle from the Fermi LAT boresight is 66 degrees.

GBM triggered on what appears to be precursor and the main emission episode begins about 150 seconds after the precursor. The lightcurve has a duration (T90)
of about 80 s (50-300 keV). The time-averaged spectrum of the main emission
from T0+230 to T0+290 s is best fit by
a Band function with Epeak = 144 +/- 9 keV,
alpha = -1.08 +/- 0.03, and beta = -2.00 +/- 0.03.

The event fluence (10-1000 keV) within this time interval is
(4.61 +/- 0.07)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+270 s in the 10-1000 keV band is 5.65 +/- 0.4 ph/s/cm^2.

The time-averaged spectrum of the percursor
from T0-12 to T0+05 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.92 +/- 0.3 and the cutoff energy,
parameterized as Epeak, is 110 +/- 22 keV.

The event fluence (10-1000 keV) within this time interval is
(1.31 +/- 0.15)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+270 s in the 10-1000 keV band is 0.67 +/- 0.05 ph/s/cm^2. The localization for the preliminary pulse is uncertain,
but consistent with the secondary pulse.

The event fluence (10-1000 keV) within the whole time interval is
(4.73 +/- 0.16)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+270 s in the 10-1000 keV band is 4.57 +/- 0.3 ph/s/cm^2. The localization for the preliminary pulse is uncertain,
but consistent with the secondary pulse.

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 41873

Subject
GRB 250916A: KAIT optical observations
Date
2025-09-18T22:13:06Z (21 days ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on

behalf of the KAIT GRB team:


The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, observed the field of GRB 250916A (Fermi GBM

team, GCN 41839; Arya et al., GCN 41843; Cheung et al., GCN 41855;

Waratkar et al., GCN 41871) starting at ~0.783d and again at ~1.738d

after the burst. A set of clear (roughly R) filter images were

obtained. We clearly detected the optical afterglow (Belkin et al.,

GCN 41847; Mohan et al., GCN 41858; Moskvitin et al., GCN 41859;

Burkhonov et al., GCN 41860; de Ugarte Postigo et al., GCN 41863;

Moskvitin et al., GCN 41870) in our coadd images. We estimate the

afterglow to be 19.35 +/- 0.1 and 20.10 +/- 0.1 mag (Vega) at a mid

time of ~0.824d and ~1.807d respectively.

GCN Circular 41871

Subject
GRB 250916A: NuSTAR detection of the prompt emission
Date
2025-09-18T19:45:02Z (21 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 the long-duration GRB 250916A 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 2025-09-16 13:33:27.000 (with a resolution ~5-seconds). This is consistent with the detections by Fermi-GBM (Fermi GBM Team, GCN Circ., 41839), Astrosat-CZTI (Arya et al., GCN Circ. 41843), Glowbug (Cheung et al., GCN Circ. 41855), and CALET-GBM (Trig ID. 1442064382).

The NuSTAR CsI shield data are recorded at 1 Hz. The burst appears to be composed of two significantly-detected peaks. We do not detect the initial faint burst detected by Fermi-GBM which occurred ~250-s before our detection. The total duration for the event is at roughly 50-s. The peak count rate is ~500-cps over a baseline rate of ~1,000-cps during this time period. We do not see clear evidence in the signal above 100 keV in the CdZnTe detectors.

The localization from the optical counterpart candidate (Belkin et al., GCN Circ. 41847) at RA = 26.57, Dec = 36.16 implies an offset from the NuSTAR boresight of 69-deg (e.g., through the side of the instrument) and the offset from the geocenter of 142-deg.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250916A/ 

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 41870

Subject
GRB 250916A: further SAO RAS optical observations
Date
2025-09-18T15:39:58Z (21 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), A. Pozanenko (IKI), N. Pankov (HSE, IKI)
report on behalf of GRB follow-up collaboration and IKI-GRB-FuN.

We observed the field of the GRB 250916A (Fermi GBM team, GCN 41839; 
Arya et al., GCN 41843; Cheung et al., GCN 41855) with the BTA, 
6-m telescope of SAO RAS equipped with SCORPIO-1 focal reducer.
We obtained 3 x 60 sec. images in Rc band  under mediocre weather 
conditions on September 18, 00:07:09--00:15:29 UT 
(t_mid - T0 = 1.4458 days = 34.6994 hours).

The OT (Belkin et al., GCN 41847; Mohan et al., GCN 41858; 
Burkhonov et al., GCN 41860) at the redshift z = 2.015 
(de Ugarte Postigo et al., GCN 41863) is clearly detected 
in the stacked frame with a brightness of R = 19.72 +/- 0.05.

The preliminary photometry is based on PanSTARRS data on nearby
objects (magnitudes calculated using the Lupton 2005 transformations)
and does not take into account Galactic extinction.

GCN Circular 41863

Subject
GRB 250916A: OSIRIS+/GTC redshift z = 2.015
Date
2025-09-18T10:56:55Z (21 days ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. F. Agui Fernandez (CAHA), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), F. Pérez Toledo (GTC)  and D. González Gonzalez (GTC) report:

We observed the optical afterglow (Belkin et al., GCN 41847) of GRB 250916A (Fermi GBM team, GCN 41839) with OSIRIS+, mounted on the 10.4 m GTC telescope, at Roque de los Muchachos Observatory, on the island of La Palma (Spain). The observation consisted of an r-band acquisition image followed by 3x900 s of spectroscopy using grism R1000B, with a spectral coverage between 3600 and 7800 AA and a resolving power of ~600. The observation started at 2025-09-18 04:15:12.011 UT (1.61 days after the burst onset).

In the acquisition image we detect the afterglow at r = 20.0 +/- 0.1 mag (AB), as compared to Pan-STARRS field stars and not corrected by Galactic extinction. 

The spectrum shows a clear continuum over the entire spectral range. The spectrum continuum shows a moderate curvature, which could be indicative of host galaxy extinction. We detect multiple spectral features that correspond to Lyman alpha, S II, Si II, Si II*, O I, C II, Ni II, Ni II*, Si IV, C IV, Fe II, Fe II*, Al II, Al III, Cr II, Zn II, and Mn II, all at a common redshift of 2.015, which we identify as the redshift of the GRB.

We also note the presence of at least one intervening system with C IV features at z = 1.853.

GCN Circular 41862

Subject
GRB 250916A: Swift-XRT observations
Date
2025-09-18T07:58:29Z (21 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), C. Salvaggio
(INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows
(PSU), S. Lanava (PSU), S. Dichiara (PSU) and P.A. Evans (U. Leicester)
reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/GBM-detected burst GRB 250916A, collecting  1.6 ks of Photon
Counting (PC) mode data between T0+101.9 ks and T0+107.2 ks. 

One uncatalogued X-ray source has been detected within the estimated
3-sigma GOTO error region (0.8 arcsec) given in GCN Circ. 41847, it is
below the RASS limit and shows no definitive signs of fading. However,
given its coincident location with the fading optical source, it is
indeed likely to be the afterglow. Details of this source are given
below:

Source 1:
  RA (J2000.0):  26.5708  =  01h 46m 16.99s
  Dec (J2000.0): +36.1652  =  +36d 09' 54.6"
  Error: 3.7 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: 0.0274 +/- 0.0049 ct s^-1   
  Distance: 2 arcsec from GOTO position.
  Flux: (9.2 +/- 1.6)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

An uncatalogued source was also detected, however this was too far from
the GRB position to be the afterglow.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021859.

This circular is an official product of the Swift-XRT team.

GCN Circular 41860

Subject
GRB 250916A: MAO/AZT-22 optical observations
Date
2025-09-18T05:09:34Z (22 days ago)
Edited On
2025-09-18T15:55:05Z (21 days ago)
From
Yodgor Rajabov at UBAI <rajabov@astrin.uz>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Yodgor Rajabov at UBAI <rajabov@astrin.uz>
Via
Web form
O. Burkhonov, Y. Rajabov, B.Abidkhanov, S. Ehgamberdiev, Y. Tillayev (UBAI), A. Shaymanov (Maidanak Observatory/UBAI) report on behalf of UBAI team.

We observed the field of the GRB 250916A (Fermi GBM team, GCN 41839; Arya et al., GCN 41843; Cheung et al., GCN 41855) with the 1.5-m at Maidanak Observatory telescope AZT-22 equipped with the 4kx4k CCD SNUCAM camera(Im et al., 2010).
We obtained series of 300 sec. images in Bessel R filter on September 17, 18:35:18-19:02:20 UTC.

The OT (Belkin et al., GCN 41847; Mohan et al., GCN 41858, Moskvitin et al. GCN 41859) is clearly detected in the individual and stacked frames with the brightness of R = 19.76 +/- 0.03 (t_mid - T0 = 1.2229 days = 29.4917 hours).

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). 
Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog.
For image template subrtaction we used PanSTARRS DR2 catalog. 
The data has not corrected for the Galactic extinction.

Maidanak astronomical observatory (MAO) is an observational facility of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences (http://maidanak.uz/).

GCN Circular 41859

Subject
GRB 250916A: SAO RAS optical observations
Date
2025-09-17T23:24:23Z (22 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova (SAO RAS), A. Pozanenko (IKI), 
A. Ghosh, S. Razzaque (CAPP, University of Johannesburg),
Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) 
report on behalf of GRB follow-up collaboration and IKI-GRB-FuN.

We observed the field of the GRB 250916A (Fermi GBM team, GCN 41839; 
Arya et al., GCN 41843; Cheung et al., GCN 41855) with the 1-m SAO RAS 
telescope Zeiss-1000 equipped with CCD-photometer.
We obtained series of 300 sec. images in BVRcIc bands on September 17, 
20:21:44--21:42:59 UT.

The OT (Belkin et al., GCN 41847; Mohan et al., GCN 41858) is clearly 
detected in the individual and stacked frames with the brightness 
of R = 19.60 +/- 0.02 (t_mid - T0 = 1.3059 days = 31.3419 hours).

Preliminary photometry is based on the nearby objects from PanSTARRS 
(magnitudes converted with Lupton 2005 equations) 
and has not corrected for the Galactic extinction.

GCN Circular 41858

Subject
GRB 250916A : GIT optical observations
Date
2025-09-17T21:43:01Z (22 days ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form

T. Mohan, V. Swain, A.P. Saikia, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and R. Norbu (IAO) report on behalf of the GIT team:

We observed the field of Fermi GRB 250916A (Fermi GBM team, GCN 41839

Loading...
 
 
) with 0.7m GROWTH-India Telescope (GIT) in r' filter. We started the observation at 2025-09-17 17:34:08 UT, i.e., 28.08 hours after the Fermi GBM trigger. The source is detected at the position reported by GOTO (Belkin et al., GCN 41847) and photometry result follows as:

MJD (mid)tmid-t0 (hours)FilterExposure (s)Mag (AB)
60935.7341128.13r'36019.73 +- 0.06

The source is fading and photometric result is consistent with Belkin et al., GCN 41847

Loading...
 
 
.

More broadband observations are under way. The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

GCN Circular 41855

Subject
GRB 250916A: Glowbug gamma-ray detection
Date
2025-09-17T19:22:38Z (22 days ago)
From
C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil>
Via
Web form
C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
 
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 250916A, which was also detected by Fermi/GBM (GCN 41839), AstroSat/CZTI (GCN 41843), and CALET (Trig ID. 1442064382).
 
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-09-16 13:33:04.840 with a duration of 64.5 s and a total significance of about 110 sigma.  The light curve comprises two primary peaks at ~T0+22s and ~T0+44s. A search of the Glowbug data for the earlier, fainter emission reported in GCN 41839 by Fermi/GBM (trigger time 13:29:21) was inconclusive.
 
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
 
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC.  It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS, and operated until 2024 April when it was put in safe storage on orbit. Glowbug was recently removed from storage and resumed operation on 2025 September 12.
 
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
 
Distribution Statement A: Approved for public release.  Distribution is unlimited.

GCN Circular 41853

Subject
GRB 250916A: Swift ToO observations
Date
2025-09-17T19:02:32Z (22 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the Fermi/GBM-detected event
GRB 250916A. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021859
Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the Fermi/GBM event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

This circular is an official product of the Swift-XRT team.

GCN Circular 41847

Subject
GRB 250916A: GOTO optical counterpart candidate
Date
2025-09-17T06:44:08Z (22 days ago)
From
Sergey Belkin at Monash University <sergey.belkin@monash.edu>
Via
Web form
S. Belkin, D. O'Neill, B. P. Gompertz, R. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, M. Pursiainen, report on behalf of GOTO collaboration:

We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 250916A (Fermi GBM team, GCN 41839; Arya et al., GCN 41843).

Targeted observations were performed beginning at 2025-09-16 23:39:53 UT (+10.18h post trigger) and continued through to 2025-09-17 04:55:45 UT (+15.44h post trigger). 211 images were taken, across 10 unique pointings, covering 422.1 sq deg within the 90% localisation contour. ~68.1% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.0 mag.

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.

A new optical source, GOTO25hed, is identified within the GBM 90% localisation region with coordinates (J2000) RA = 26.570161, DEC = 36.16543. The source was initially detected with magnitude L = 18.90 ± 0.11 AB mag (+10.43h post trigger), before fading to L = 19.28 ± 0.12 AB mag (+14.47h post trigger). In total, the source was detected 3 times in our observations. The observed decay rate across the 4 hours our observations span is ~t^(-1.05±0.03). ATLAS forced photometry (Shingles et al. 2021) reveals a non-detection down to a 5-sigma upper limit of o > 19.6 AB mag at 2025-09-16 11:50:38 UT (-1.65h before trigger). We find no evidence of the source prior to the GRB trigger time in previous GOTO or ATLAS observations. A summary of our observations is presented below.
+-----------------------------------------------------------------------------+
|  Observation time (UT) |    t-t0   | filter |     AB mag   |
+-----------------------------------------------------------------------------+
|   2025-09-15 05:57:16  |  -31.53h  |    L   |    >19.00    |
|   2025-09-16 23:55:24  |  +10.43h  |    L   | 18.90 ± 0.11 |
|   2025-09-17 02:49:13  |  +13.33h  |    L   | 19.17 ± 0.07 |
|   2025-09-17 03:57:43  |  +14.47h  |    L   | 19.28 ± 0.12 |
+-----------------------------------------------------------------------------+

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

GCN Circular 41843

Subject
GRB 250916A: AstroSat CZTI detection of a bright long burst
Date
2025-09-16T18:23:31Z (23 days ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
A. Arya (IITB), A. Goyal (IITB), M. Tembhurnikar (IUCAA), 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 bright long-duration GRB 250916A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 41839), and CALET (Trig ID. 1442064382). Note that we detected the bright burst roughly 250 s after the initial fainter spike as seen in the GBM lightcurve. 

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 2025-09-16 13:33:28.50 UTC. The measured peak count rate associated with the burst is 79 (+29, -4) counts/s above the background in the combined data of two quadrants (out of four), with a total of 1695 (+232, -241) counts. We caution that there is a 0.3 s readout dead time in CZT data during the burst which affects the calculated total counts. The local mean background count rate was 135 (+1, -2) counts/s. Using cumulative rates, we measure a T90 of 48 (+2, -6) 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 2025-09-16 13:33:26.00 UTC. The measured peak count rate associated with the burst is 556 (+80, -24) counts/s above the background in the combined data of all quadrants, with a total of 13048 (+742, -896) counts. The local mean background count rate was 1180 (+5, -4) counts/s. We measure a T90 of 50 (+2, -5) 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 41840

Subject
Fermi GRB 250916A: Global MASTER-Net observations report
Date
2025-09-16T15:30:58Z (23 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
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, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-Tunka robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Applied Physics Institute, Irkutsk State University) started inspect of the Fermi GRB 250916A ( Fermi GBM team, GCN 41839) errorbox  2134 sec after notice time and 2165 sec after trigger time at 2025-09-16 14:05:26 UT, with upper limit up to  20.8 mag. The observations began at zenith distance = 44 deg. The sun  altitude  is -24.7 deg. 

The galactic latitude b = -20 deg., longitude l = 137 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2992979

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

    2196 | 2025-09-16 14:05:26 |        MASTER-Tunka | (01h 01m 35.41s , +42d 32m 50.5s) |   C |    60 | 20.7 |        
    2196 | 2025-09-16 14:05:26 |             MASTER- | (01h 02m 07.93s , +42d 25m 57.0s) |   C |    60 | 20.5 |        
    2271 | 2025-09-16 14:06:42 |        MASTER-Tunka | (01h 31m 07.38s , +48d 15m 08.4s) |   C |    60 | 19.6 |        
    2271 | 2025-09-16 14:06:42 |             MASTER- | (01h 31m 43.29s , +48d 08m 17.2s) |   C |    60 | 19.5 |        
    2825 | 2025-09-16 14:15:56 |        MASTER-Tunka | (01h 01m 37.13s , +42d 32m 44.7s) |   C |    60 | 20.6 |        
    2825 | 2025-09-16 14:15:56 |             MASTER- | (01h 02m 10.56s , +42d 25m 34.6s) |   C |    60 | 20.6 |        
    2900 | 2025-09-16 14:17:11 |        MASTER-Tunka | (01h 31m 06.83s , +48d 15m 06.1s) |   C |    60 | 20.2 |        
    2900 | 2025-09-16 14:17:11 |             MASTER- | (01h 31m 43.65s , +48d 07m 58.6s) |   C |    60 | 19.8 |        
    4244 | 2025-09-16 14:39:34 |        MASTER-Tunka | (01h 17m 38.67s , +38d 43m 55.2s) |   C |    60 | 19.8 |        
    4244 | 2025-09-16 14:39:34 |             MASTER- | (01h 18m 09.42s , +38d 37m 35.4s) |   C |    60 | 19.7 |        
    4985 | 2025-09-16 14:51:56 |        MASTER-Tunka | (01h 01m 35.37s , +42d 30m 26.2s) |   C |    60 | 20.7 |        
    4985 | 2025-09-16 14:51:56 |             MASTER- | (01h 02m 11.14s , +42d 22m 49.2s) |   C |    60 | 20.6 |        
    5059 | 2025-09-16 14:53:10 |        MASTER-Tunka | (01h 31m 07.69s , +48d 15m 07.8s) |   C |    60 | 20.2 |        
    5059 | 2025-09-16 14:53:10 |             MASTER- | (01h 31m 47.54s , +48d 07m 33.5s) |   C |    60 | 19.9 |        
    5553 | 2025-09-16 15:01:23 |        MASTER-Tunka | (01h 17m 43.01s , +38d 42m 15.8s) |   C |    60 | 19.8 |        
    5553 | 2025-09-16 15:01:23 |             MASTER- | (01h 18m 15.41s , +38d 35m 49.9s) |   C |    60 | 19.7 |        
    5775 | 2025-09-16 15:05:05 |        MASTER-Tunka | (01h 01m 31.47s , +42d 30m 13.1s) |   C |    60 | 20.8 |        
    5775 | 2025-09-16 15:05:06 |             MASTER- | (01h 02m 08.39s , +42d 23m 51.2s) |   C |    60 | 20.6 |        
    5852 | 2025-09-16 15:06:22 |        MASTER-Tunka | (01h 31m 05.98s , +48d 12m 03.1s) |   C |    60 | 20.3 |        
    5852 | 2025-09-16 15:06:22 |             MASTER- | (01h 31m 46.96s , +48d 05m 44.0s) |   C |    60 | 19.9 |        
    6285 | 2025-09-16 15:13:36 |        MASTER-Tunka | (01h 17m 33.49s , +38d 39m 54.6s) |   C |    60 | 19.9 |        
    6285 | 2025-09-16 15:13:36 |             MASTER- | (01h 18m 04.22s , +38d 34m 21.2s) |   C |    60 | 19.7 |        
    6508 | 2025-09-16 15:17:18 |        MASTER-Tunka | (01h 01m 28.89s , +42d 27m 40.0s) |   C |    60 | 20.7 |        
    6508 | 2025-09-16 15:17:18 |             MASTER- | (01h 02m 05.60s , +42d 21m 48.9s) |   C |    60 | 20.7 |        
    6582 | 2025-09-16 15:18:33 |        MASTER-Tunka | (01h 31m 05.87s , +48d 12m 44.6s) |   C |    60 | 19.7 |        
    6582 | 2025-09-16 15:18:33 |             MASTER- | (01h 31m 46.70s , +48d 06m 57.1s) |   C |    60 | 19.6 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

GCN Circular 41839

Subject
GRB 250916A: Fermi GBM Final Real-time Localization
Date
2025-09-16T13:40:16Z (23 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 13:29:21 UT on 16 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250916A (trigger 779722166.007904 / 250916562).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 29.3, Dec = 40.3 (J2000 degrees, equivalent to J2000 01h 57m, 40d 17'), with a statistical uncertainty of 9.8 degrees.

The angle from the Fermi LAT boresight is 66.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250916562/quicklook/glg_skymap_all_bn250916562.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/bn250916562/quicklook/glg_healpix_all_bn250916562.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250916562/quicklook/glg_lc_medres34_bn250916562.gif
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