GRB 251203C

N. Dagoneau (CEA), M. Brunet, N. A. Webb (IRAP)

Using the event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 251203C (SVOM burst-id sb25120305). 

The burst that triggered ECLAIRs onboard (GCN 42976
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 ) consists of a multiple peaks lightcurve with a duration of T90 = 13.0 -4.7/+1.6 s in the 4-120 keV energy band. 

The time-averaged spectrum (from T0-3.59s to T0+9.49s) in the energy range 5-120 keV is best fit by a simple power-law model with a photon index -1.98 -0.17/+0.18. The total fluence in the 4-120 keV band is (3.63 -0.59/+0.27)e-7 erg/cm^2.

All the quoted errors are at the 68% confidence level.

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe.

Nikos Mandarakas (LAM), Alan M. Watson (UNAM), Fredd Sanchez Alvarez (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), 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), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), M. Brunet (IRAP), N. A. Webb  (IRAP):

We imaged the field of the SVOM GRB 251203C (Brunet et al., GCN 42976
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 ) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-12-05T04:55:27 to 2025-12-05T06:22:35 UTC (from 34.36 to 35.81 hours after the trigger) and obtained 3840 seconds of exposure in the r and z filters.
The data were reduced, coadded, and analyzed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the Pan-STARRS-DR1 catalog, is in the AB system, and was not corrected for Galactic extinction.
In the stacked image, we no longer detect the optical counterpart reported by H.L. Li (GCN Circ. 42987
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 ) and D. Turpin et al. (CGN Circ. 42988), down to the following 3σ limit:

r > 22.91
z > 22.47

We do detect a nearby extended source with coordinates RA = 51.782, Dec = -7.31, which is also seen in the Pan-STARRS-DR1 catalog, and could potentially correspond to the host of the GRB. The magnitudes we detect are:

r = 22.34 +/- 0.14
z = 21.96 +/- 0.14

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.

D. Turpin, A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), J. T. Palmerio (CEA/Irfu), L. P. Xin, C. Wu (NAOC), B. Cordier (CEA/Irfu), M. Brunet, N. A. Webb (IRAP), report on behalf of the SVOM mission team:

Following the report of an optical counterpart by SVOM/VT (Li et al., GCN 42987
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 ) of the SVOM GRB 251203C (Brunet et al., GCN 42976
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 ), we inspected again our stacked image from LCO (Turpin et al., GCN 42980
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 ).

The VT candidate afterglow is detected in our r-band image. At a mean epoch of 37 min after trigger, we measure an SDSS r-band magnitude of r = 19.91 +/- 0.14 (AB, which is significantly brighter than the SVOM/VT magnitudes (despite the filter system difference) measured at a later epoch (> 3.3 hr after trigger).

This further confirms that this source is the optical afterglow of GRB 251203C.

The z-band upper limit reported by Turpin et al. (GCN 42980
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 ) is still confirmed.

This project is funded by the SVOM collaboration.

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

SVOM/VT performed ToO observation to the field of GRB 251203C triggered by SVOM (Brunet et al., GCN 42976
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 ). The observation started at 2025-12-03T21:46:05 UTC, i.e., 3.2 hours post trigger in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

An uncatalogued source was detected within the ECLAIRs's error box (Brunet et al., GCN 42976
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 ), compared to the Legacy survey catalog. The position is at R.A., Dec. = 51.701538,  -7.382688 degrees, equivalent to:
    R.A. (J2000) =   03:26:48.36
    Dec. (J2000) =  -07:22:57.67
with an uncertainty of 0.5 arcsec.

The measurements in AB magnitudes are given below:

    Mid_time       Band        Exposure Time     Magnitude (AB)
    3.3 hour       VT_B           9*70 sec         22.1+/-0.1 mag
    3.3 hour       VT_R           9*70 sec         21.7+/-0.1 mag
    9.7 hour       VT_B          44*30 sec         23.0+/-0.3 mag 
    9.7 hour       VT_R          44*30 sec         22.4+/-0.2 mag 

Our photometry was not corrected for Galactic extinction.
    
As this source has significantly faded during our observations, we suggest it is the optical afterglow of GRB 251203C.

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.

H.Z. Wu (HUST), Y. Wu (NJU), H. Y. Ren, Tian-Yu Liu, G. J. Yang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 251203C (SVOM/sb25120305, Brunet et al., GCN #42976
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 ) at 2025-12-03 20:12:47 (UTC), about 52 minutes after the SVOM/ECLAIRs trigger, with an exposure time of 4569 s. Four uncataloged sources are detected within the ECLAIRs error circle.
Preliminary analysis on these sources is automatically conducted, and the details are listed as follows. 

EPF_J032718.1-071802
RA (J2000): 51.8268
Dec (J2000):  -7.3024
Flux: 9.24 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 2.78 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J032703.9-072334
RA (J2000): 51.7662
Dec (J2000): -7.3929
Flux: 1.07 x 10^-13 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 3.23 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J032718.1-071802
RA (J2000): 51.8255
Dec (J2000): -7.3005
Flux: 5.85 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 2.12 x 10^-14 erg/s/cm2 (1 sigma)

EPF_J032658.4-072128
RA (J2000): 51.7434
Dec (J2000):  -7.3577
Flux: 1.12 x 10^-13 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 2.77x 10^-14 erg/s/cm2 (1 sigma)

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

D. Turpin, A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), J. T. Palmerio (CEA/Irfu), L. P. Xin, C. Wu (NAOC), B. Cordier (CEA/Irfu), M. Brunet, N. A. Webb (IRAP), report on behalf of the SVOM mission team:

We observed the field of the SVOM GRB 251203C (Brunet et al., GCN 42976
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 ) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument. 

Our observation started on 2025-12-03 at 19:05:17 UT (about 31 min after the trigger) and we obtained 3x200 s exposures in the SDSS r and 2x200 s exposures in the Pan-STARRS z filters. In the stacked image, we do not detect any new source within the ECLAIRs error circle.

We measure the following upper limits calibrated against the Pan-STARRS catalog, and not corrected for Galactic extinction:

r > 20.2 AB (mid-time 37 min after the trigger);
z > 19.0 AB (mid-time 58 min after the trigger).

This project is funded by the SVOM collaboration.

M. Brunet, N. A. Webb  (IRAP), N. Dagoneau, M. Moita, D. Turpin (CEA) report on behalf of the SVOM mission team

At 2025-12-03T18:33:50 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 251203C (SVOM burst-id sb25120305).

The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 11 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 11.74 in the [8-50] keV energy band over a time window of 10.20 seconds starting at 2025-12-03T18:33:45.

The localization of the best alert is R.A., Dec. 51.7804, -7.3100 degrees:
R.A. (J2000) = 3h27m07.31s
Dec. (J2000) = -7d18m36.09s
with a 90% confidence level (C.L.) radius of 6.85 arcmin (including systematic error of 2 arcmin added in quadrature).

No immediate slew was performed on this burst.
A SVOM ToO has been programmed for follow-up.

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this alert is Marius Brunet: mbrunet@irap.omp.eu.
Please contact the BA by email if you require additional information.