GRB 251027A
GCN Circular 42495
Subject
SVOM trigger sb25102702/GRB 251027A is not a GRB
Date
2025-10-28T00:05:44Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y. H.Cheng (YNU), Y. F. Liang (PMO), M. Brunet (IRAP), D. Turpin (CEA), report on behalf of the SVOM mission team:
At 2025-10-27T09:38:17 SVOM issued an alert with burst-id: sb25102702.
The following circular(s) have been published regarding this event: GCN 42471, 42480, 42481, 42486.
After reprocessing the data with the full telemetry, this trigger is no longer considered as an astrophysical source but is due to coding noise from the Crab in the field of view.
We apologize for any inconvenience this may have caused.
The Burst Advocate (BA) on shift for this alert is Yehao Cheng: yhcheng@mail.ynu.edu.cn.
Please contact the BA by email if you require additional information.
GCN Circular 42486
Subject
GRB 251027A: Swift-XRT observations
Date
2025-10-27T16:36:26Z (3 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected source
GRB 251027A, collecting 1.6 ks of Photon Counting (PC) mode data between T0+4.5 ks
and T0+12 ks after the trigger. We have detected 2 sources. These have been
automatically classified as follows:
* 0 likely counterparts
* 0 candidate counterparts
* 2 uncatalogued X-ray sources
* 0 known X-ray sources
Uncatalogued X-ray sources
--------------------------
Source 1 (SWIFT J062052.1+114644):
==================================
RA (J2000.0): 95.2175 = 06 20 52.20
Dec (J2000.0): +11.7792 = +11 46 45.1
Error: 7.5 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 5.8 arcmin from the SVOM/ECLAIRs position.
Mean rate: (5.4 [+2.7, -2.0])e-3 ct s^-1
Mean flux: (1.90 [+0.95, -0.72])e-10 erg cm^-2 s^-1
Peak rate: (5.4 [+2.7, -2.0])e-3 ct s^-1
Peak flux: (1.90 [+0.95, -0.72])e-10 erg cm^-2 s^-1
ECF: 3.52e-08 erg cm^-2 ct^-1
assuming NH=1.86e+24 cm^-2, gamma=-3.00
determined from a spectral fit.
XMM UL: 2.1e-12 erg cm^-2 s^-1, (0.3-10 keV)
so the source is 2.2-sigma above this 3-sigma upper limit.
There is no evidence for fading.
A SIMBAD object `NVSS J062052+114645' is 0.1" away.
There are 4 2MASS objects within the source's 3-sigma error radius.
Source 3 (SWIFT J062105.2+112959):
==================================
RA (J2000.0): 95.2720 = 06 21 05.28
Dec (J2000.0): +11.4998 = +11 29 59.3
Error: 8.8 (arcsec, radius, 90% confidence).
Detect flag: POOR
Distance: 12.3 arcmin from the SVOM/ECLAIRs position.
Mean rate: (5.7 [+3.5, -2.5])e-3 ct s^-1
Mean flux: (2.4 [+1.5, -1.1])e-13 erg cm^-2 s^-1
Peak rate: (5.7 [+3.5, -2.5])e-3 ct s^-1
Peak flux: (2.4 [+1.5, -1.1])e-13 erg cm^-2 s^-1
XMM UL: 3.3e-12 erg cm^-2 s^-1, (0.3-10 keV)
so the source is not above this 3-sigma upper limit.
There is no evidence for fading.
There are 5 2MASS objects within the source's 3-sigma error radius.
All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
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/SVOM/SVOM_FIELD00046.
This circular is an official product of the Swift-XRT team.
GCN Circular 42481
Subject
GRB 251027A: LCO optical upper limit
Date
2025-10-27T15:13:55Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
A. Saccardi (CEA/Irfu), C. Wu (NAOC), J. T. Palmerio (CEA/Irfu), L. P. Xin (NAOC), D. Turpin (CEA/Irfu), B. Cordier (CEA/Irfu), Y. H. Cheng (YNU), Y. F. Liang (PMO), report on behalf of the SVOM mission team:
We observed the field of the GRB 251027A detected by SVOM/ECLAIRs (Cheng et al., GCN 42471) with the LCO 1m telescope at McDonald Observatory equipped with the Sinistro instrument.
Our observation started on 2025-10-27 at 10:22:43 UT (~45 min after the trigger) and we obtained 3x200 s exposures using the SDSS r filter. In the stacked image, we did not detect any new optical source at the SVOM/ECLAIRs position (Cheng et al., GCN 42471), in agreement with the results reported by Dornic et al. (GCN 42480).
We derived the following upper limit r > 22 AB mag (5-sigma) calibrated against Pan-STARRS1 catalogue and not corrected for Galactic extinction.
This project is funded by the SVOM collaboration.
GCN Circular 42480
Subject
GRB 251027A: SVOM/COLIBRÍ (FM-GFT) optical upper limit
Date
2025-10-27T14:47:37Z (3 days ago)
From
J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr>
Via
Web form
Damien Dornic (CPPM), William H. Lee (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), 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), Y. H.Cheng (YNU) and Y. F. Liang (PMO) :
We imaged the field of the SVOM GRB 251027A (Cheng et al., GCN Circ. 42471) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-10-27T10:08:34 to 2025-10-27T10:47:17 UTC (from 31 to 69 minutes after the trigger) and obtained 32x60 seconds of exposure in the r and z filters.
The data were reduced and coadded with the COLIBRÍ pipeline 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 (Av>2).
In the stacked image, we do not detect any new source at the ECLAIRs source position (Cheng et al., GCN Circ. 42471) down to the following 5-sigma limit:
r > 23.57
z > 22.26
Further analysis is 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 42471
Subject
GRB 251027A: SVOM detection of a faint burst
Date
2025-10-27T10:41:32Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y. H.Cheng (YNU), Y. F. Liang (PMO), O. Godet, S. Guillot (IRAP), S. Schanne (CEA), L. Zhang (IHEP)
At 2025-10-27T09:38:17 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 251027A (SVOM burst-id sb25102702).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was only detected by the Count-Rate Trigger (CRT), which produced a sequence of 1 alert. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 8.33 in the [5-20] keV energy band over a time window of 20.40 seconds starting at 2025-10-27T09:37:56.
The localization of the best alert is R.A., Dec. 95.1860, 11.6869 degrees:
R.A. (J2000) = 6h20m44.64s
Dec. (J2000) = 11d41m12.73s
with a 90% confidence level (C.L.) radius of 9.46 arcmin (including systematic error of 2 arcmin added in quadrature).
The GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb251027A.png
Due to the ECLAIRs detection significance being below the slew threshold, no immediate slew was performed on this burst. But follow-up observations with SVOM/MXT and SVOM/VT have been requested.
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 Yehao Cheng: yhcheng@mail.ynu.edu.cn.
Please contact the BA by email if you require additional information.