GRB 251005C

M. El Kabir (U Rome), O. Guiffreda (UMD),  N. Passaleva  (U Rome),  J. Durbak (UMD),  E. Troja (U Rome), A. S. Kutyrev (NASA/GSFC), S. B. Cenko (NASA/GSFC) 

Following the Swift/BAT detection and Swift/XRT localization of an uncatalogued source (Page et al, GCN 42113
Loading...
 
 ), we observed the transient field in J filter with PRIME ~32.9 hours after the initial BAT trigger. 

Using Two Micron All Sky Survey (2MASS) for preliminary calibration we do not detect any uncatalogued source down to J>20.2 AB (3-sigma) not corrected for galactic extinction at the position of the candidates reported by Swift/XRT (Goad et al. GCN 42116
Loading...
 
 ), and SVOM/VT  (Palmerio et al., GCN 42123
Loading...
 
 ). 

PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024). 

We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations. 

H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.

A second ToO observation from SVOM/VT was performed to the field of GRB 251005C (Page et al., GCN 42113
Loading...
 
 ; Smith et al., GCN 42118
Loading...
 
 ; Arya et al., GCN 42122
Loading...
 
 ; Luo et al., GCN 42135
Loading...
 
 ) which stared at 2025-10-06T17:36:01 UTC, I.e., 25.5 hours post trigger  in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 
 
The optical candidate (Palmerio et al. , GCN 42123
Loading...
 
 ; Yadav et al., GCN 42127
Loading...
 
 ; Watson et al., GCN 42129
Loading...
 
 ) was detected in VT_R stacked image at the mid time of 27.4 hours post trigger with the brightness of VT_R~23.3+/-0.2 mag.  Combining  earlier magnitudes from VT, the source was fading with a power-law temporal slope of about -0.7.

Our photometry was derived 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 Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

SVOM/GRM team: Xing-Hao Luo, Chen-Wei Wang, Zheng-Hang Yu, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Frédéric Daigne (IAP), Stéphane Schanne (CEA)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a burst GRB 251005C(SVOM trigger reference: sb25100502) at 2025-10-05T16:06:54.000 UTC(T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#42118
Loading...
 
 ) and Swift/BAT (K. L. Page et al., GCN#42113
Loading...
 
 )

With the event_by_event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 1.6 +0.6/-0.3 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb251005C.png

In addition, the position of this burst, as determined by Swift/XRT (RA=75.673 , DEC=16.882, GCN#42116
Loading...
 
 ), is located at about 51 degrees form the SVOM optical axis, which is outside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0-0.4 to T0+1.6 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.81 +0.16/-0.14 and the cutoff energy, parameterized as Epeak, is 900 +547/-285 keV. The event fluence (10-1000 keV) in this time interval is (2.17 +0.19/-0.18)E-06 erg/cm^2. 

The localization of GRB 251005C in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb251005C_amati.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Xing-Hao Luo (2952704891@qq.com)

Alan M. Watson (UNAM), Francis Fortin (IRAP), Jean-Luc Atteia (IRAP), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), 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), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM):

We imaged the field of the Swift GRB 251005C (Page et al., GCN Circ. 42113
Loading...
 
 ) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-06 11:23 to 12:30 UTC (from 19.28 to 20.40 hours after the trigger) and obtained 38 minutes of simultaneous exposure in the i and z filters.

The data were reduced and coadded with the tenue software and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

At the position of the XRT source reported by Goad et al. (GCN Circ. 42116
Loading...
 
 ) and at the position of the VT optical source reported by Palmerio et al. (GCN Circ. 42123
Loading...
 
 ), we do not detect any sources above our 3-sigma limits of

i > 23.8
z > 23.3

However, at the position of the VT source there are hints of the presence of a faint resolved object at lower significance. Our results are consistent with the X-shooter photometry reported by Yadav et al. (GCN Circ. 42127
Loading...
 
 ).

We caution that the apparent fading in the i band with respect to the earlier VT_R detection is not conclusive, as the VT_R band above 650 nm includes both i and z, and our limit in z alone is not much deeper than the original detection.

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.

GRB 251005C: VLT/X-shooter Imaging

Muskan Yadav (U Rome), Yu-Han Yang (U Rome), Massine El Kabir (U Rome), Niccolò Passaleva (U Rome),  Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:

We observed the field of GRB 251005C (K. L. Page et al., GCN 42113
Loading...
 
 ) with the Xshooter spectrograph on the ESO VLT UT3 (Melipal). Observations began at T+16.5 hours. We obtained a total exposure of 335 s using the acquisition and guiding (A&G) camera. Observations were carried out in the i’ filter at an average airmass of about 1.4 and seeing 0.75 arcsec.

Within the XRT error region (Goad et al., GCN 42116
Loading...
 
 ) we detect a faint (i~24 AB mag) source at coordinates RA, Dec (J2000) = 05:02:44.03, +16:52:51.632. The object appears extended in our images. 
We also detect the SVOM/VT optical candidate (J. T. Palmeiro et al., GCN 42123
Loading...
 
 ) at a significantly fainter magnitude (i~24 AB mag), confirming its rapid fading. We thus suggest that this is the GRB optical counterpart.

We thank the staff at the VLT, for the rapid execution of these observations.

J. T. Palmerio (CEA), 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) report on behalf of the SVOM/VT team:
    
SVOM/VT performed a Target of Opportunity observation of GRB 251005C detected by Swift (Page et al., GCN 42113
Loading...
 
 ) and Fermi GBM (Smith et al., GCN 42118
Loading...
 
 ), starting at 2025-10-06T02:00:33 UTC, 9.90 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 
    
An uncatalogued optical counterpart, compared to the PanSTARRS catalogue, is detected in VT_R at R.A., Dec. = 75.68455, 16.88019 degrees, equivalent to:
R.A. (J2000) = 05h02m44.3s 
Dec. (J2000) = +16d52m48.7s
with an uncertainty of 0.5 arcsec.

This location is 4.6 arcsec from the Swift-XRT position (Goad et al. GCN 42116
Loading...
 
 ), outside of the 90% confidence region of 2.3 arcsec.
The candidate's magnitudes are given below:

  Mid_time     |   Band     |    Exposure Time  |     Magnitude (AB)
11.18  hour        VT_R            36*50 sec             22.64+/-0.25  mag                      
12.79  hour        VT_R            36*50 sec             22.81+/-0.25  mag
10.71  hour        VT_B            60*50 sec             >23.7 mag
12.86  hour        VT_B            50*50 sec             >23.6 mag

The fading of this candidate cannot be confirmed given the uncertainties.

Our photometry is 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.

A. Arya (IITB), U. Pathak (IITB),  A. Goyal (IITB), Harsha K. H. (IUCAA), M. Tembhurnikar (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 GRB251005C which was also detected by Fermi/GBM (GCN Circ. 42118
Loading...
 
 ), Swift BAT (GCN Circ. 42113
Loading...
 
 ). 

The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-10-05 16:06:52.04 UTC. The measured peak count rate associated with the burst is 218 (+69, -55) counts/s above the background in the combined data of all quadrants, with a total of 801 (+399,  -369)  counts. The local mean background count rate was 1373 (+5,  -9) counts/s. Using cumulative rates, we measure a T90 of 9.6 (+5.3,  -5.8) s.

The source was also faintly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-10-05 16:06:54.50 UTC. The measured peak count rate associated with the burst is 90 (+28,  -31) counts/s above the background in the combined data of three quadrants (out of four), with a total of 133 (+86,  -91) counts. The local mean background count rate was 225 (+3, -3) 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

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-OAR), M.
Perri (SSDC & INAF-OAR), S. Lanava (PSU), S. Dichiara (PSU), J.A.
Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.6 ks of XRT data for GRB 251005C, from 129 s to 29.0
ks after the   trigger. The data are entirely in Photon Counting (PC)
mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.34 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.41 (+0.27, -0.25). The
best-fitting absorption column is  7.1 (+2.8, -2.2) x 10^21 cm^-2, in
excess of the Galactic value of 3.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.4 x 10^-11 (8.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.1 (+2.8, -2.2) x 10^21 cm^-2
Galactic foreground: 3.2 x 10^21 cm^-2
Excess significance: 2.9 sigma
Photon index:	     1.41 (+0.27, -0.25)

If the light curve continues to decay with a power-law decay index of
1.34, the count rate at T+24 hours will be 4.2 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x
10^-14 (3.5 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01401726.

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

Jacob Smith (UAH), B. Mailyan (Florida Tech) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 16:06:54.00 UT on 05 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251005C (trigger 781373219/251005671),
which was also detected by Swift BAT (K. L. Page et al. 2025, GCN 42113
Loading...
 
 ).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 98 degrees.

The GBM light curve consists of a double emission episode with a duration (T90)
of about 6 s (50-300 keV). The time-averaged spectrum
from T0+0.003 to T0+5.248 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.9 +/- 0.1 and the cutoff energy,
parameterized as Epeak, is 2136 +/- 866 keV.

The event fluence (10-1000 keV) in this time interval is
(3.2 +/- 0.2)E-06 erg/cm^2. The 64-millisec peak photon flux measured
starting from T0+0.58 s in the 10-1000 keV band is 6.1 +/- 1.2 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/"

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

Using 1541 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 251005C, 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 = 75.68325, +16.88054 which is equivalent
to:

RA (J2000): 05h 02m 43.98s
Dec (J2000): +16d 52' 49.9"

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 http://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.

K. L. Page (U Leicester), S. Dichiara (PSU), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL) and M. J. Moss (GSFC)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 16:06:53 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251005C (trigger=1401726).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 75.673, +16.882 which is 
   RA(J2000) = 05h 02m 42s
   Dec(J2000) = +16d 52' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 2 sec.  The peak count rate
was ~4,500 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 16:08:52.2 UT, 118.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 75.68331, 16.88007
which is equivalent to:
   RA(J2000)  = 05h 02m 44.00s
   Dec(J2000) = +16d 52' 48.3"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 36 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 (3.19 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 5
(+3.76/-3.16) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 122 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 19.6 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 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.461.

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)