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GRB 251126A

GCN Circular 42871

Subject
GRB 251126A: OHP/T193 optical observations and tentative spectroscopic redshift
Date
2025-11-28T14:22:37Z (18 minutes ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
Web form
C. Adami (LAM/Pytheas/AMU), A. Saccardi (CEA/Irfu), B. Schneider (LAM), M. Dennefeld (IAP), E. Le Floc'h (CEA-Saclay) report on behalf of the MISTRAL GRB collaboration:

We carried out observations of the Swift GRB 251126A (Caputo et al., GCN 42843) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 3 exposures of 3 min in the r-band at a midtime of 2025-11-26 22:58 UT corresponding to T-T0 = 3.79  hours.

The afterglow reported by Swain et al. (GCN 42844), Lipunov et al. (GCN 42847), Fu et al. (GCN 42848), Reguitti et al. (GCN 42849), Broens et al. (GCN Circ. 42850), Angulo et al. (GCN 42855), Pulido-Torres et al. (GCN 42856), Gupta et al. (GCN 42857), Cotter et al. (GCN 42858), Seki et al. (GCN 42859), Volnova et al. (GCN 42861), Breeveld and Caputo (GCN 42863), Gupta et al. (GCN 42864), Burkhonov et al. (GCN 42866), Maksut et al. (GCN 42868) and Moskvitin et al. (GCN 42870) is very well detected in r’ with a preliminary magnitude of :

r’ = 20.1 +/- 0.13

We also obtained 1 exposure of 600s in the g-band at a midtime of 2025-11-27 00:38 UT.
The afterglow is only barely visible with an upper magnitude limit of : 

g’ > 21.9 (S/N=3)

The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the STDWeb/STDPipe tools (Karpov 2025), is in the AB system, and is not corrected for Galactic extinction.

In addition, a spectrum covering the wavelength range 4200-8000 AA was secured with the MISTRAL spectroscopic blue setting, starting at 23:10 UT (midtime = T0+4.5h) with a total exposure time of 2x1800 s. In a preliminary reduction of the spectrum, a faint trace is detected down to ~5600 AA (in the flux calibrated spectrum), where a spectral break is observed despite the low S/N. If due to Lyman alpha, this would correspond to a redshift of z~3.6. Our spectroscopic value is in agreement with the photometric redshift reported by Angulo et al. (GCN 42855).
Given the low S/N over the continuum, we caution that this redshift solution should be considered tentative.

We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Stephane Favard, and the SOPHIE observer Felix Savoure.

GCN Circular 42870

Subject
GRB 251126A: further SAO RAS optical observations
Date
2025-11-28T12:46:09Z (2 hours ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), V. Goranskij (SAI MSU, SAO RAS), 
V. S. Shergin (SAO RAS), A. A. Volnova (IKI), A. S. Pozanenko (IKI)
report on behalf of IKI-GRB-FuN and GRB follow-up team.

We observed the field of the Swift GRB 251126A (Caputo et al., 
GCN 42843; Evans, GCN 42845; Goad et al., GCN 42851; Page et al., 
GCN 42860) with the Zeiss-1000 1m telescope of the SAO RAS 
on November 27/28 night. We obtained two series of 300 sec. images
in Rc band under a good weather conditions and seeing (FWHM) ~1".5.

The optical afterglow (Swain, GCN 42844; Fu et al., GCN  42848; 
Reguitti et al., GCN 42849; Broens et al., GCN 42850; Anguloet al., 
GCN 42855; Pulido-Torres et al; GCN 42856; Gupta et al., GCN 42857; 
Cotter et al., GCN 42858; Seki et al., GCN 42859; Volnova et al., 
GCN 42861; Breeveld and Caputo, GCN 42863; Gupta et al., GCN 42864;
Burkhonov et al., GCN 42866; Maksut et al., GCN 42868) is clearly 
detected in the stacked frames. The preliminary results are as 
following.

Date        UT start  t-T0       Exp.    Filter  OT     Err.  UL      
                     (mid,days)  (n*s)                       (3sigma)
                     
2025-11-27  20:20:22  1.07076    11*300  Rc      22.02  0.04  24.1    
2025-11-27  23:31:00  1.19809     8*300  Rc      22.24  0.07  23.8       


The photometry is based on nearby stars from the USNO-B1 catalogue 
(R2 magnitudes)and is not corrected for the Galactic extinction.

GCN Circular 42868

Subject
GRB 251126A NUTTelA-TAO Early Measurements
Date
2025-11-28T10:49:00Z (4 hours ago)
From
Zhanat Maksut at Nazarbayev University <zhanat.maksut@nu.edu.kz>
Via
Web form
Z. Maksut (NU), B. Grossan (UCB, NU), T. Komesh (NU), E. Abdikamalov (NU), M. Krugov (FAI) and D. Berdikhan (NU)
 
The Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO) pointed at GRB 251126A on receipt of an automated GCN / BAT position alert, observing in Sloan g' and i' bands with the Burst Simultaneous Three-Channel Imager (BSTI; Grossan, Kumar & Smoot 2019, JHEA, 32, 14). Observations started at UT 2025 November 26, 19:11:32, 52 s after the SWIFT/BAT trigger (Caputo et al., GCN 42843). Weather conditions were good within the first 1 hour of observation, but gradually deteriorated to cloudy over the course of the night. A new optical counterpart reported by Swain et al., GCN 42844; Fu et al., GCN 42848; Reguitti et al., GCN 42849; Broens et al., GCN 42850;  Angulo et al., GCN 42855; Pulido-Torres et al., GCN 42856; Gupta et al., GCN 42857; Cotter et al., GCN 42858; Seki et al., GCN 42859, was detected. We report the following preliminary, uncorrected, selected photometric values for the optical transient:
 
tc-t0(s)  t_exp  i'(mag)  err(mag) g’(mag) err(mag) 
-------- ------ -------  --------
76    	48.0  18.12 	0.08	19.49  	0.06
142  	30.0  17.38 	0.07	18.96  	0.06
322  	30.0  17.62 	0.08	19.03  	0.09
502  	30.0  17.88 	0.09	19.31  	0.08
 
tc-t0 = trigger time minus image center time. Calibration was done with 5 bright Pan-STARRS catalog stars on our images (see Komesh, T. et al. 2023, MNRAS 520, 6104).
 
We caution the reader that these are preliminary results, without color or other corrections, and will likely change. Please also note that times are approximate. Systematic errors are estimated from previous measurements. We welcome requests for additional data.
 
----------------------------------
NU = Nazarbayev University, Astana, Kazakhstan
UCB = University of California, Berkeley, USA
FAI = Fesenkov Astrophysical Institute, Kazakhstan
 
This research has been funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP26103591). The NUTTelA-TAO Team acknowledges the support of the staff of the Assy-Turgen Astrophysical Observatory, Almaty, Kazakhstan, and the Fesenkov Astrophysical Institute, Almaty, Kazkhstan.

GCN Circular 42866

Subject
GRB251126A: MAO/AZT-22 optical observations
Date
2025-11-28T05:08:59Z (10 hours ago)
From
Yodgor Rajabov at UBAI <rajabov@astrin.uz>
Via
Web form
O. Burkhonov, Y. Rajabov, B.Abidkhanov, S. Ehgamberdiev, Y. Tillayev (UBAI), T. Boyqobilov, A. Shaymanov (Maidanak Observatory/UBAI) report on behalf of UBAI team.

We observed the field of GRB 251126A detected by Swift (Caputo et al. 2025, GCN 42843) ), with the AZT-22 1.5m telescope of the Maidanak Observatory (MAO) starting on 2025-11-27 at 22:17:50 UT, i.e, ~1.13 days after the Swift trigger. In total we obtained 6x300 s exposures in the R-band using 4kx4k CCD SNUCAM camera (Im et al., 2010). We stacked the images after the alignment. We detect the optical counterpart in our stacked image within the error box of GIT (Swain et al. 2025, GCN 42844). 

Our detection is consistent with Swain et al. 2025 (GCN 42844); Lipunov et al. 2025 (GCN 42847); Fu et al. 2025 (GCN 42848); Reguitti et al. 2025 (GCN 42849); Broens et al. 2025 (GCN 42850); Angulo et al. 2025 (GCN 42855); and Torres et al. 2025 (GCN 42856); Gupta et al. 2025 (GCN 42857); Cotter et al. 2025 (GCN 42858); Seki et al. 2025 (GCN 42859); Volnova et al. 2025 (GCN 42861); Breeveld et al. 2025 (GCN 42863); Gupta et al. 2025 (GCN 42864). 

Preliminary photometry is as follows:

Date       UTstart  Exptime     t-T0     Filter  OT    Err.     UL    Site/Telescope
                    (nxs)     (mid, days)                 
2025-11-27 22:37:36 6x300      1.144       R   22.14   0.11    22.93   MAO/AZT-22

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousins filters were calibrated using the Gaia DR3 Synphot 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 42864

Subject
GRB251126A: Continued optical observation from 1.3m DFOT
Date
2025-11-28T01:22:33Z (13 hours ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Dhruv Jain, Debalina Kar, Pankaj Pawar, and Kuntal Misra (ARIES) report:

We observed the field of GRB 251126A detected by Swift (Caputo et al. 2025, GCN 42843) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on 2025-11-27 at 18:10:19 UT, i.e., ~22.99 hours after the Swift trigger. We have taken multiple frames with an exposure time of 300 s in the R filter. We stacked the images after the alignment. We detect the optical counterpart in our stacked image within the error box of GIT (Swain et al. 2025, GCN 42844). We obtain the following preliminary magnitude in the stacked image:


Date Start_UT T_start-T0 (hour) Filter  Exp time (s)  Magnitude
======================================================================
2025-11-27  18:10:19    ~22.99    R     300*12       21.62+/- 0.02


Our detection is consistent with Swain et al. 2025 (GCN 42844); Lipunov et al. 2025 (GCN 42847); Fu et al. 2025 (GCN 42848); Reguitti et al. 2025 (GCN 42849); Broens et al. 2025 (GCN 42850); Angulo et al. 2025 (GCN 42855); and Torres et al. 2025 (GCN 42856); Gupta et al. 2025 (GCN 42857); Cotter et al. 2025 (GCN 42858); Seki et al. 2025 (GCN 42859); Volnova et al. 2025 (GCN 42861); Breeveld et al. 2025 (GCN 42863). 

The magnitude is not corrected for the Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the USNO-B1 catalog.

GCN Circular 42863

Subject
GRB 251126A: Swift/UVOT Detection
Date
2025-11-27T14:51:12Z (a day 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 GRB 251126A 86s after the BAT trigger (Caputo et al., GCN Circ. 42843).
The fading optical afterglow (Swain et al., GCN 42844; Fu et al., GCN 42848; Reguitti et al., GCN 42849; Broens et al., GCN 42850; Angulo et al., GCN 42855; Pulido-Torres et al., GCN 42856; Gupta et al., GCN 42857; Cotter et al., GCN 42858; Seki et al., GCN 42859 and Volnova et al., GCN 42861) is clearly detected in the initial UVOT exposures.

The non detection in the U and UV filters is consistent with the redshift of z=3.5 found by Angulo et al., (GCN Circ. 42855).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposures and subsequent exposures are:

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

white_FC            86          236          147          20.0 ± 0.1
white              579          772           39          20.3 ± 0.3
white              880         1377          186          20.5 ± 0.2
u_FC               299          548          246         >20.4
v                  629         1080           58          18.7 ± 0.3
b                 1159         1352           39          19.4 ± 0.3
uvw1               853         1467           35         >18.32
uvm2               653          847           39         >18.00
uvw2               778         1402           58         >18.70

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.063 in the direction of the burst (Schlegel et al. 1998).

GCN Circular 42861

Subject
GRB 251126A: Mondy and SAO RAS optical observations
Date
2025-11-27T13:38:15Z (a day ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), A. S. Moskvitin (SAO RAS), V. P. Goranskij (SAI MSU, SAO RAS), E. Klunko (ISTP), V. S. Shergin (SAO RAS), N. Pankov (HSE, IKI), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of the Swift GRB 251126A (Caputo et al., GCN 42843; Evans, GCN 42845; Goad et al., GCN 42851) with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) in R-band and the Zeiss-1000 1m telescope of the SAO RAS in Rc-band on November 26/27 night. Observations with the AZT-33IK started at 20:35:11 UT (1.4097 hours after the trigger), observations with the Zeiss-1000 started at 21:31:39 UT (2.3508 hours after the trigger).

The optical afterglow (Swain, GCN 42844; Fu et al., GCN  42848; Reguitti et al., GCN 42849; Broens et al., GCN 42850; Anguloet al., GCN 42855; Pulido-Torres et al; GCN 42856; Gupta et al., GCN 42857; Cotter et al., GCN 42858; Seki et al, GCN 42859) is clearly detected in individual frames. 
Preliminary photometry is given below.

Date        UT start  t-T0       Exp.    Filter  OT     Err.  UL      Seeing" Site/Instrum
                     (mid,days)  (n*s)                       (3sigma)
2025-11-26  20:35:11  0.06221    5*120   R       19.99  0.08  21.1    5.0     Mondy/AZT-33IK
2025-11-26  20:45:12  0.07125    8*120   R       19.84  0.07  21.4    5.0     Mondy/AZT-33IK
2025-11-26  21:01:12  0.08584   13*120   R       19.89  0.06  21.7    5.0     Mondy/AZT-33IK
2025-11-26  21:31:39  0.10164    2*300   Rc      19.81  0.08  21.2    2.2     SAO/Zeiss-1000
2025-11-26  21:58:22  0.12230    3*300   Rc      19.95  0.03  22.7    2.0     SAO/Zeiss-1000
2025-11-27  01:14:02  0.25960    2*600   Rc      20.63  0.04  23.2    1.6     SAO/Zeiss-1000
2025-11-27  01:57:56  0.29372    3*600   Rc      20.62  0.03  23.4    1.6     SAO/Zeiss-1000

The photometry is based on several nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction.

GCN Circular 42860

Subject
GRB 251126A: Swift-XRT refined Analysis
Date
2025-11-27T13:02:50Z (a day ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
K.L. Page (U. Leicester), M. Capalbi (INAF-OAR), M. Perri (SSDC &
INAF-OAR), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), J.A. Kennea
(PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 251126A, from 66 s to 56.7
ks after the   trigger. The data comprise 9 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.5 (+0.6, -0.5). At T+210 s  the decay
flattens to an alpha of -0.50 (+0.00, -0.17) before breaking again at
T+1954 s to a final decay with index alpha=1.13 (+0.21, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.11 (+0.25, -0.24). The
best-fitting absorption column is  1.7 (+0.8, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 9.5 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.3 x 10^-11 (4.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 9.5 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     2.11 (+0.25, -0.24)

If the light curve continues to decay with a power-law decay index of
1.13, the count rate at T+24 hours will be 8.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-13 (3.9 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01417793.

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

GCN Circular 42859

Subject
GRB 251126A : MITSuME Akeno optical afterglow candidate detection
Date
2025-11-27T12:08:00Z (a day ago)
From
seki@hp.phys.titech.ac.jp
Via
Web form
H. Seki, I. Takahashi, M. Sasada, Y. Kubo, H. Hagio, A. Ochi, R. Kato, S. Joshima, Y. Yatsu and N. Kawai (Science Tokyo) report on behalf of the MITSuME collaboration:

We observed the field of GRB 251126A detected by Swift (Caputo et al., GCN 42843) with the optical three-color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50-cm telescope Akeno. 

The observation started at 2025-11-26 19:11:15 UT (39 seconds after the trigger). We stacked the images taken under good conditions. We detected a point source in the g'-, Rc- and Ic-band images at a position of the reported optical candidate (Swain et al., GCN 42844; Fu et al., GCN 42848; Reguitti et al., GCN 42849; Broens et al., GCN 42850;  Angulo et al., GCN 42855; Pulido-Torres et al., GCN 42856; Gupta et al., GCN 42857; Cotter et al., GCN 42858).  Here we report the magnitudes of the source as follows.

T0+[sec] | MID-UT | T-EXP[sec] | magnitudes
---------------------------------------------------------------------------------------------------------------
1310 | 2025-11-26 19:32:26 | 1560 | g'=20.5+/-0.3, Rc=18.7+/-0.1, Ic=18.2+/-0.1
---------------------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the trigger
T-EXP: Total exposure time

We used the PS1 catalog for flux calibration. The catalog magnitudes in PS1 g, r and i bands were converted to our g', Rc and Ic band magnitudes following Tonry et al. (2012), Table 6. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ; https://github.com/MNiwano/Eclaire).

GCN Circular 42858

Subject
GRB 251126A: NOT optical observations
Date
2025-11-27T11:11:11Z (a day ago)
From
Laura Cotter at University College Dublin <laura.cotter@ucdconnect.ie>
Via
Web form
L. Cotter (UCD), A. Martin-Carrillo (UCD), S.Y. Fu (HUST), J. An (NAOC), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), T. Pursimo (NOT) report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 251126A (Caputo et al., GCN 42843) with the Nordic Telescope (NOT) equipped with the ALFOSC Camera. We obtained 3x300s r-band images with observations starting on 2025-11-26 at 23:31:36 UTC, under good seeing (~1”). However, unfortunately, observations had to be interrupted due to adverse weather conditions.

The optical counterpart reported by Swain et al. (GCN 42844), Lipunov et al. (GCN 42847), Fu et al. (GCN 42848), Reguitti et al. (GCN 42849), Broens et al. (GCN 42850), Angulo et al. (GCN 42855), Torres et al. (GCN 42856) and Gupta et al. (GCN 42857) is well detected in our stacked image with an AB magnitude of r = 20.72 ± 0.05 at a mid epoch of 2025-11-26 23:34:06.554 (4.4 hr after the Swift/BAT trigger). This magnitude was calibrated against nearby objects from the Pan-STARRS catalog and not corrected for Galactic extinction.

GCN Circular 42857

Subject
GRB 251126A: 1.3m DFOT optical afterglow detection
Date
2025-11-27T08:14:19Z (a day ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Dhruv Jain, Pankaj Pawar, Debalina kar and Kuntal Misra (ARIES) report:


We observed the field of GRB 251126A detected by Swift (Caputo et al. 2025, GCN 42843) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on   2025-11-26 at 22:17:19 UT, i.e., ~3.11 hour after the Swift trigger. We have taken multiple frames with an exposure time of 300s in the I filter. We stacked the images after the alignment. We detect the optical counterpart in our stacked image within the error box of GIT (Swain et al. 2025, GCN 42844). We obtain the following preliminary magnitude in the stacked image:


Date Start_UT T_start-T0 (hour) Filter  Exp time (s)  Magnitude
======================================================================
2025-11-26  22:17:19    ~3.11    I     300*12      19.41+/- 0.01


Our detection is consistent with Swain et al. 2025 (GCN 42844); Lipunov et al. 2025 (GCN 42847); Fu et al. 2025 (GCN 42848); Reguitti et al. 2025 (GCN 42849); Broens et al. 2025 (GCN 42850); Angulo et al. 2025 (GCN 42855); and Torres et al. 2025 (GCN 42856). 

The magnitude is not corrected for the Galactic extinction in the direction of the burst. 
Photometric calibration is performed using the standard stars from the USNO-B1 catalog.

GCN Circular 42856

Subject
GRB 251126A: ULL-ASTRO-MASTER detection of the optical afterglow with LCO 1-m telescope at McDonald Observatory
Date
2025-11-27T06:40:21Z (a day ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
M. Pulido-Torres, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, V. Ghiraldo, J. Hernández Fung, L. Juliá-Maroto,  E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo,  M. Quintana-Ansaldo, A. Schenone-Zanuzzi, A. Selezneva, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL).

Following the detection of the Swift GRB 251126A (Caputo et al., GCN Circ. 42843; Evans et al.,  GCN Circ. 42845; and Goad et al., GCN Circ. 42851), we observed the field with one of the two Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at McDonald Observatory, Texas. The observation, a single exposure of 300 sec in the SDSS r' filter, started on 2025-11-27 at 04:41:19 UT, about 9.51 hours after the Swift trigger. The optical counterpart first detected by the GROWTH-India Telescope (GIT) (Swain et al., GCN Circ. 42844) is clearly detected in our image with a magnitude of r' = 20.86 +/- 0.13 (AB), calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction. Our result is consistent with other optical observations: Swain et al. (GCN Circ. 42844), Lipunov et al. (GCN Circ. 42847), Fu et al. (GCN Circ. 42848), Reguitti et al. (GCN Circ. 42849), Broens et al. (GCN Circ. 42850), and Angulo et al. (GCN Circ. 42855).
 
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2025B-010). These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (La Laguna, Tenerife, Canary Islands, Spain).

This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).

GCN Circular 42855

Subject
GRB 251126A: COLIBRÍ photometric redshift z = 3.5
Date
2025-11-27T04:57:59Z (a day ago)
From
Camila Angulo Valdez at UNAM <camiangulo@astro.unam.mx>
Via
Web form
Camila Angulo (UNAM), Ny Avo Rakotondrainibe (LAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of the Swift GRB 251126A (Caputo et al., GCN Circ. 42843) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-11-27 02:49 to 04:14 UTC (from 7.6 to 9.1 hours after the trigger) and obtained, respectively, 10, 27, 11, 23, and 23 minutes of exposure in the g, r, i, z, and y filters.

The data were reduced, coadded, and analyzed with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

We detected the optical counterpart reported by Swain et al. (GCN Circ. 42844), Lipunov et al. (GCN Circ. 42847), Fu et al. (GCN Circ. 42848), Reguitti et al. (GCN Circ. 42849), and Broens et al. (GCN Circ. 42850) at preliminary magnitudes of:

r = 20.62 +/- 0.05
z = 19.86 +/- 0.07

In our data, we see a clear break in the spectrum in g. After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.063 mag (Schlafly & Finkbeiner 2011) and fitting a power-law model to the grizy-bands with SMC extinction, we derive a photometric redshift of z = 3.52 (+0.18, -0.20, 1-sigma confidence interval).

Further observations are ongoing.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

GCN Circular 42851

Subject
GRB 251126A: Enhanced Swift-XRT position
Date
2025-11-27T01:31:57Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 633 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 251126A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 99.94120, +54.98588 which is equivalent
to:

RA (J2000): 06h 39m 45.89s
Dec (J2000): +54d 59' 09.2"

with an uncertainty of 4.5 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at https://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

GCN Circular 42850

Subject
GRB 251126A: Kilonova-Catcher optical afterglow detection
Date
2025-11-26T21:24:46Z (2 days ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
E. Broens (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (OCA/IJCLab), M. Coughlin (UMN), S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 251126A detected by Swift (Caputo et al., GCN 42843) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a private telescope T-BRO operated by E. Broens. Our observations started at TGRB+35 min and were taken unfiltered.

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we marginally detect the optical counterpart reported by GIT (Swain, GCN 42844). Despite the pretty low SNR (while the source is visible by eye in our image), we performed a forced photometry at the position we found to be at RA, Dec (J2000): 99.9440, +54.9864.

We report our follow-up results in the table below:

+---------------+-----------+---------+----------------+-------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter  | Magnitude      | Instrument  |
+===============+===========+=========+================+=============+
| 0.83          | 10 x 180s | r (AB)  | 19.43 +/- 0.42 | T-BRO       | 
+---------------+-----------+---------+----------------+-------------+

Compared to the magnitude reported by GIT (Swain, GCN 42844) the source has significantly faded even when considering our marginal detection which induced large error bars on the source photometry. Therefore, we confirm that this source is likely the optical afterglow of GRB 251126A. This is in agreement with the detections also reported by the Nanshan/HMT team (Fu et al., GCN 42848) and the Asiago team (Reguitti et al., GCN 42849).

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Clear filters were calibrated using the PanSTARRS DR1 catalog using the PS1-r filter.

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). 

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

GCN Circular 42849

Subject
GRB 251126A: Asiago i-band detection
Date
2025-11-26T21:14:37Z (2 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
A. Reguitti (INAF/OAPD), L. Tomasella (INAF/OAPD), D. B. Malesani (DAWN/NIB and Radboud), report on behalf of a larger collaboration:

We observed the optical afterglow (Swain et al., GCN 42844; Fu et al., GCN 42848) of GRB 251126A (Caputo et al., GCN 42843) using the 67/92 Schmidt Telescope located at the Asiago observatory (Italy). Observations were carried out using the Sloan i filter.

In a single exposure with start time 2025 Nov 26.8252 UT (37.8 min after the Swift/BAT trigger), the afterglow is detected with a magnitude i = 18.27 +- 0.15 (AB), calibrated against nearby stars fron the Pan-STARRS catalog, and not corrected for Galactic extinction.

Compared to the earlier optical measurements (Swain et al., GCN 42844; Fu et al., GCN 42848), our images seem to indicate a red r-i color for the optical afterglow.

GCN Circular 42848

Subject
GRB 251126A: Nanshan/HMT optical afterglow detection
Date
2025-11-26T21:01:26Z (2 days ago)
From
syfu@nao.cas.cn
Via
Web form
S.Y. Fu (HUST), S.Q. Jiang, J. An, X. Liu, L.B. He, Z.P. Zhu, D. Xu (NAOC), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:

We observed the field of GRB 251126A detected by Swift/BAT (Caputo et al., GCN 42843) using the HMT-0.5m telescope located at Nanshan, Xinjiang, China. Observations started at 19:11:52 UT on 2025-11-26, i.e., 76 seconds after the Swift/BAT trigger, and a series of unfiltered frames with different exposures were obtained.

An uncatalogued and varying optical source is detected within the XRT error circle (Caputo et al., GCN 42843) at coordinates:

R.A.(J2000) = 06:39:46.56
Dec.(J2000) = +54:59:12.27

with an uncertainty of ~0.5 arcsec. This position is consistent with the previous report by GIT (Swain, GCN 42844). The source has ~ 19.0 mag in the first image, calibrated with the nearby Pan-STARRS field and not corrected for Galactic extinction. The early light curve initially shows a slow rise, reaching a peak at ~150s, and then decays with power law index of ~0.5. Thus, we conclude that the source is the afterglow of GRB 251126A.

Observations are still ongoing.

GCN Circular 42847

Subject
Swift GRB 251126A: Global MASTER-Net observations report
Date
2025-11-26T20:34:46Z (2 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-Kislovodsk robotic telescope  [1]  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB 251126A ( R. Caputo et al., GCN 42843) errorbox  4521 sec after notice time and 4538 sec after trigger time at 2025-11-26 20:26:15 UT, with upper limit up to  18.4 mag. The observations began at zenith distance = 31 deg. The sun  altitude  is -67.1 deg. 

The galactic latitude b = 21 deg., longitude l = 161 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3052530

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

    4629 |   MASTER-Kislovodsk |   C |   180 | 15.7 |        
    4629 |   MASTER-Kislovodsk |   C |   180 | 18.4 |        
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 42845

Subject
GRB 251126A: Prompt enhanced Swift-XRT position
Date
2025-11-26T19:53:46Z (2 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-XRT team:

Using 1.4 ks of promptly downlinked XRT event data for GRB 251126A, we
find an enhanced XRT position of the afterglow: RA, Dec: 99.94311,
54.98585 which is equivalent to:
   RA (J2000)  = 06 39 46.35
   Dec (J2000) = +54 59 09.1
with an uncertainty of 2.1 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1417793.

Position enhancement is described by Goad et al. (2007, A&A, 476, 1401)
and Evans et al. (2009, MNRAS, 397, 1177).

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

GCN Circular 42844

Subject
GRB 251126A: GIT discovery of the likely optical afterglow
Date
2025-11-26T19:41:15Z (2 days ago)
Edited On
2025-11-26T21:57:00Z (2 days ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form

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

We observed the field of GRB 251126A detected by Swift (Caputo et al., GCN 42843

Loading...
 
 
) with 0.7m GROWTH-India Telescope (GIT). We started observations at 2025-11-26T19:14:32 UT, i.e., 3.93 mins after the Swift/BAT trigger. We obtained multiple frames in the r' band of 360 sec each. We detected an uncatalogued source at RA 06:39:46.42, Dec: +54:59:11.78 with an uncertainty of 0.4 arcsec. There is no minor planet present at this position. The photometric results follow as:

JD (mid)tmid-t0 (mins)FilterExposure (s)Mag (AB)
2461006.30175925933.93r'36018.26 +- 0.06

We encourage photometric for further confirmation and spectroscopic follow-up for redshift measurement. The magnitudes are 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 42843

Subject
GRB 251126A: Swift detection of a burst
Date
2025-11-26T19:26:44Z (2 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
R. Caputo (GSFC), M. J. Moss (GSFC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 19:10:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251126A (trigger=1417793).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 99.910, +54.987 which is 
   RA(J2000) = 06h 39m 39s
   Dec(J2000) = +54d 59' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 20 sec.  The peak count rate
was ~2600 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 19:11:58.3 UT, 81.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 99.94124, 54.98591 which is equivalent
to:
   RA(J2000)  = 06h 39m 45.90s
   Dec(J2000) = +54d 59' 09.3"
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This
location is 64 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper. No
spectrum from the promptly downlinked event data is yet available to
determine the column density.

There are no UVOT data available at this time. Analysis will be reported at a later time.

Burst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov).
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/)
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