GRB 251103B

A. Goyal (IITB), A. Arya (IITB), S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (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 long GRB 251103B which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 42555
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 ), and Swift BAT (Lanava et. al., GCN Circ. 42556
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 ).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-11-03 16:46:36.50 UTC. The measured peak count rate associated with the burst is 87 (+31, -16) counts/s above the background in the combined data of three quadrants (out of four), with a total of 307 (+118, -98) 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 215 (+2, -4) counts/s. Using cumulative rates, we measure a T90 of 5.86 (+0.85, -2.24) s. 

The source was also faintly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-11-03 16:46:41.66 UTC. The measured peak count rate associated with the burst is 148 (+74, -23) counts/s above the background in the combined data of all quadrants, with a total of 677 (+244, -247) counts. The local mean background count rate was 1281 (+6, -7) counts/s. Due to poor statistics, we cannot reliably estimate the T90 from it. 

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

A.A. Breeveld (UCL-MSSL) and S. Lanava (PSU) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 251103B 140 s after the BAT trigger (Lanava et al., GCN Circ. 42556
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 ).
No optical afterglow consistent with the enhanced XRT position (Evans et al., GCN Circ. 42558
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 )is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:

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

u_FC               140          390          246         >20.2
white              420         1363          245         >21.1
v                  470         1413          117         >19.0
b                  395         1339           97         >19.8
w1                 519         1462           97         >18.8
m2                 494         1437          117         >18.8
w2                 445         1389          117         >19.1

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.256 in the direction of the burst (Schlegel et al. 1998).

A. C. Trigg (NPP, NASA MSFC)
report on behalf of the Fermi GBM Team:

"At 16:46:36.44 UT on 03 November 2025, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 251103B (trigger 783881201/251103699),
which was also detected by the Swift/XRT (Evans et al. 2025, GCN 42557
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 )
The Fermi GBM Final Real-time Localization (GCN 42555
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 ) is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 70 degrees.

The GBM light curve shows/consists of rapid multi-peaked emission
with a duration (T90) of about 48 s (50-300 keV).
The time-averaged spectrum from T0-16.1 s to T0+39.8 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.36 +/- 0.02 and
the cutoff energy, parameterized as Epeak, is 82 +/- 2 keV.

The event fluence (10-1000 keV) in this time interval is
(4.67 +/- 0.12)E-6 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.98 s in the 10-1000 keV band
is 5 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

S.Q. Jiang (NAOC), S.Y. Fu (HUST), 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 251103B detected by Swift/BAT (Lanava et al., GCN 42556
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 ) and Fermi/GBM (Fermi GBM team, GCN 42555
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 ) using the HMT-0.5m telescope located at Nanshan, Xinjiang, China. Observations started at 22:31:33 UT on 2025-11-03, i.e., 5.749 hrs after the Swift/BAT trigger, we obtained 25x120 s unfiltered frames.
No new optical source is detected in our stacked image within the enhanced XRT error circle (Evans et al., GCN 42558
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 ), down to the following 3-sigma limiting magnitude R>20.4 at 6.276 hrs post-burst, calibrated with the nearby PanSTARRS field and the magnitude is not corrected for Galactic extinction.

I. Takahashi, H. Hagio, A. Ochi, Y. Kubo, M. Sasada, R. Kato, H. Seki, S. Joshima, Y. Yatsu and N. Kawai (Science Tokyo) report on behalf of the MITSuME collaboration:

We observed the field of GRB 251103B detected by Fermi (Fermi GBM team, GCN 42555
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 ) and Swift (Lanava et al., GCN 42556
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 ) with the optical three-color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50-cm telescope Akeno. 

The observation started at 2025-11-03 16:47:11 UT (35 seconds after the trigger). We stacked the images taken under good conditions. We did not detect any obvious point sources within the enhanced Swift/XRT error region (Evans et al., GCN 42558
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 ​​). We obtained the 5-sigma limits of the stacked images as follows:

T0+[sec] | MID-UT | T-EXP[sec] | 5-sigma limits
------------------------------------------------------------------------------------------------
124 | 2025-11-03 16:48:40 | 80 | Rc>16.6, Ic>16.3
821 | 2025-11-03 17:00:17 | 1020 | g'>17.8, Rc>18.4, Ic>18.1
7041 | 2025-11-03 18:43:57 | 9180 | g'>19.4, Rc>19.5, Ic>19.1
------------------------------------------------------------------------------------------------
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).

R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), S.
Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 251103B, from 62 s to 23.6
ks after the   trigger. The data comprise 143 s in Windowed Timing (WT)
mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+10.3 ks) can be modelled with a
power-law decay with a decay index of alpha=1.0 (+/-0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.21 (+/-0.08). The
best-fitting absorption column is  9.0 (+/-0.7) x 10^21 cm^-2, in
excess of the Galactic value of 3.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.09 (+0.22, -0.21)
and a best-fitting absorption column of 4.9 (+1.2, -1.1) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.0 x 10^-11 (6.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.9 (+1.2, -1.1) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^21 cm^-2
Excess significance: 2.5 sigma
Photon index:	     2.09 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
1.0, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.4 x
10^-13 (9.2 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/01409824.

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

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-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 251103B ( S. Lanava et al., GCN 42556
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 ) errorbox  16520 sec after notice time and 16537 sec after trigger time at 2025-11-03 21:22:14 UT, with upper limit up to  18.3 mag. The observations began at zenith distance = 70 deg. The sun  altitude  is -40.6 deg. 

The galactic latitude b = -8 deg., longitude l = 237 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3031294

We obtain a following upper limits.  

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

   16628 |         MASTER-SAAO |   C |   180 | 18.3 |        
   16628 |         MASTER-SAAO |   C |   180 | 17.4 |        
   16840 |         MASTER-SAAO |   C |   180 | 17.4 |        
   16840 |         MASTER-SAAO |   C |   180 | 18.3 |        
Filter C is a clear (unfiltred) band. 


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

P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1251 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 251103B, 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 = 105.87339, -24.87217 which is equivalent
to:

RA (J2000): 07h 03m 29.61s
Dec (J2000): -24d 52' 19.8"

with an uncertainty of 2.3 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.

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using 1.3 ks of promptly downlinked XRT event data for GRB 251103B, we
find an enhanced XRT position of the afterglow: RA, Dec: 105.87365,
-24.87162 which is equivalent to:
   RA (J2000)  = 07 03 29.68
   Dec (J2000) = -24 52 17.8
with an uncertainty of 2.0 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1409824.

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.

S. Lanava (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC),
M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and
B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 16:46:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251103B (trigger=1409824).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 105.864, -24.843 which is 
   RA(J2000) = 07h 03m 27s
   Dec(J2000) = -24d 50' 35"
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 200 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~2 sec after the trigger.

The XRT began observing the field at 16:47:49.2 UT, 72.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 105.87352, -24.87083 which
is equivalent to:
   RA(J2000)  = 07h 03m 29.64s
   Dec(J2000) = -24d 52' 15.0"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 104 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (3.33 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.5
(+2.76/-2.40) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 2.78e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
139 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.255.

Burst Advocate for this burst is S. Lanava (sml7284 AT psu.edu).
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 16:46:36 UT on 3 Nov 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251103B (trigger 783881201.440798 / 251103699).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 107.5, Dec = -22.0 (J2000 degrees, equivalent to J2000 07h 10m, -22d 00'), with a statistical uncertainty of 4.3 degrees.

The angle from the Fermi LAT boresight is 67.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251103699/quicklook/glg_skymap_all_bn251103699.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/bn251103699/quicklook/glg_healpix_all_bn251103699.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251103699/quicklook/glg_lc_medres34_bn251103699.gif