Gamma-ray Burst 251026A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #42454
J.X Cao (GXU), X. Tian (GXU), J-L. Atteia (IRAP), W.J Xie (NAOC), F. Lacreu (IAP), M. Moita (CEA), report on behalf of the SVOM mission team:
At 2025-10-26T08:27:22 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 251026A (SVOM burst-id sb25102605).
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. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 11.73 in the [8-120] keV energy band over a time window of 81.92 seconds starting at 2025-10-26T08:26:46.
The localization of the best alert is R.A., Dec. 72.5819, 51.8058 degrees (J2000) with a 90% confidence level (C.L.) radius of 6.86 arcmin (including systematic error of 2 arcmin added in quadrature).
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2025-10-26T08:31:19 UTC, 237 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 72.707, 51.776 degrees:
```
R.A. (J2000) = 04h50m49s
Dec. (J2000) = 51d46m32s
```
with a 90% C.L. radius of 65 arcseconds.
This location is 5.17 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.
VT began observing the field after the slew. The analysis of the data will be published in a future circular.
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 Jiaxin Cao: cjx@st.gxu.edu.cn. Please contact the BA by email if you require additional information.
GCN Circular #42456
Antonio de Ugarte Postigo (LAM), Margarita Pereyra (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), 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), Damien Dornic (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), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), J.X Cao (GXU), X. Tian (GXU):
We imaged the field of the SVOM GRB 251026A (Cao et al., GCN Circ. 42454) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-10-26 08:49:51 to 09:08:59 UTC (from 22.5 to 41.6 minutes after the trigger) and obtained 16x60 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/SkyMapper catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source within the MXT source position (Cao et al., GCN Circ. 42454) down to the following 5-sigma limit:
```
r > 23.0 mag
z > 21.6 mag
```
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 #42458
Chao WU (NAOC), Zhe Kang (CHO), Liping Xin(NAOC), Xuhui Han(NAOC), Pinpin Zhang (NAOC), Xiaomeng Lu (NAOC), Zhenwei Li (CHO), You Lv (CHO), Ruosong Zhang (NAOC), Yujie Xiao(NAOC), Yulei Qiu(NAOC), Jing Wang (NAOC), Jinsong Deng(NAOC), Lei Huang(NAOC), Jianyan Wei (NAOC), J.X Cao (GXU), X. Tian (GXU) report on behalf of the SVOM/C-GFT team:
We observed the field of GRB 251026A (sb25102605) detected by SVOM/ECLAIRs (Cao et al., GCN 42454) with LATIOS on SVOM/C-GFT. Observations started at 2025-10-26T10:31:43 UTC, ~2.07 hr after the trig-ger.
A series of g, r, and i band images were obtained. No credible candidate was detected within the error box provided by SVOM/MXT (Cao et al., GCN 42454) in our stacked images. The corresponding three sigma upper limit is, mag_i=19.65 at the midpoint of the observations, T₀ + 2.15 hr.
The photometry was calibrated against nearby Pan-STARRS1 stars and no correction for Galactic dust extinction was applied. This result is consistent with Postigo et al. (GCN 42456).
We thank the observation assistants Bowen Li and Hongxu Xuan at Jilin observatory for their excellent support.
The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcsec x 21 arcsec FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching.

GCN Circular #42461
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 251026A (SVOM burst-id sb25102605), collecting 1.7 ks of Photon Counting (PC) mode data between T0+5.8 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

We note that neither source is consistent with the SVOM/MXT position given in GCN Circ. 42454.
Uncatalogued X-ray sources
--------------------------
Source 1 (SWIFT J044944.9+514605):
==================================

    RA (J2000.0):   72.4374  =	04 49 44.98
    Dec (J2000.0):  +51.7683  =  +51 46 05.9
    Error:	    5.1 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    5.8 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    0.0123 +/- 0.0033 ct s^-1
    Mean flux:	    (4.6 +/- 1.2)e-13 erg cm^-2 s^-1
    Peak rate:	    0.0123 +/- 0.0033 ct s^-1
    Peak flux:	    (4.6 +/- 1.2)e-13 erg cm^-2 s^-1
    ECF:	    3.75e-11 erg cm^-2 ct^-1
		      assuming NH=1.57e+22 cm^-2, gamma=2.98
		      determined from a spectral fit.
    XMM UL:	    3.5e-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.
    A SIMBAD object `NVSS J044945+514603' is 7.5" away.
    There are 2 2MASS objects within the source's 3-sigma error radius.

Source 2 (SWIFT J045052.5+515630):
==================================

    RA (J2000.0):   72.7189  =	04 50 52.54
    Dec (J2000.0):  +51.9418  =  +51 56 30.5
    Error:	    9.7 (arcsec, radius, 90% confidence).
    Detect flag:    REASONABLE
    Distance:	    9.6 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    (4.0 [+2.6, -1.8])e-3 ct s^-1
    Mean flux:	    (1.74 [+1.10, -0.78])e-13 erg cm^-2 s^-1
    Peak rate:	    (4.0 [+2.6, -1.8])e-3 ct s^-1
    Peak flux:	    (1.74 [+1.10, -0.78])e-13 erg cm^-2 s^-1
    XMM UL:	    2.3e-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.
    A SIMBAD object `NVSS J045052+515630' is 3.9" away.
    There are 6 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_FIELD00045.
This circular is an official product of the Swift-XRT team.

GCN Circular #42476
A. Volnova (IKI), A. Pozanenko (IKI) , N. Pankov (HSE, IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN:
We observed the field of GRB 251026A detected by SVOM/ECLAIRs (Cao et al., GCN 42454) in the R-filter with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) starting Oct. 26, 2025, 20:15:58 (UT) taking several frames with 120 s exposures. Inside the error box of the SVOM/MXT (Cao et al., GCN 42454) we did not detect any new uncatalogued source. Photometry and observational details are the following:
```
Date        UT start  t-T0       Exp.    Filter  OT     Err.  UL
                     (mid,days)  (n*s)                       (3sigma)
2025-10-26  20:15:58  0.51293    30*120  R       n/d    n/d   22.4
```
The photometry is based on several nearby PS1 stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction.
GCN Circular #42482
A. Li (BNU), J. H. Wu (GZHU), H. Q. Cheng and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251026a. The ground analysis of the WXT data shows that the event started at T0=2025-10-26T08:26:33 (UTC). The WXT position of the source is R.A. = 72.663 deg, DEC = 51.775 deg (J2000) with an uncertainty of 2.1 arcmin in radius (90% C.L. statistical and systematic). This transient is temporally and spatially coincident with GRB 251026A (GCN 42454).
The analysis of the WXT data shows that the event lasted for about 500 seconds. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.77 x 10^21 cm^-2 and a photon index of 1.00 (-0.37/+0.38). The derived average unabsorbed 0.5-4 keV flux is 1.35 (-0.6/+0.7) x 10^-9 erg/s/cm^2. The peak flux is around 2.1 x 10^-9 erg/s/cm^2 with a fixed Galactic hydrogen column density of 3.77 x 10^21 cm^-2 and a photon index of 1.07 (-0.53/+0.55).
We note that the late release of this transient is mostly due to the delay in the data transmission. Follow-up observations with EP-FXT has been scheduled and more information will be reported when the telemetry data arrives.
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).
GCN Circular #42511
A. Li (BNU), J. H. Wu (GZHU), H. Q. Cheng and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:
EP-FXT performed two follow-up observations of GRB 251026A/EP251026a (GCN 42454) at 2025-10-27T09:41:08 and 2025-10-28T01:39:45 (UTC), respectively, about 25 and 41 hours after the ECLAIRs' trigger, with an exposure time of 2.7 and 6.0 ks. No source was detected in the first observation. The second observation detected a weak uncatalogued source within the WXT error circle at R.A. = 72.6573, DEC = 51.7653 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). Based on the spectral fitting result, we got a powerlaw spectrum with the absorption fixed at the Galactic value of 3.77x10^21 cm^-2 and the photon index of 2.5, the flux in 0.5-10 keV is derived to be approximately 5.4x10^-14 erg/s/cm2 from the FXT count rate.
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).
GCN Circular #42512
H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, Z. H. Yao, Y. N. Ma, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu., J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), J.X Cao (GXU), X. Tian (GXU) report on behalf of the SVOM/VT team.
SVOM/VT performed an automatic slew on the long burst triggered by SVOM/ECLAIRs (Cao et al., GCN 42454) and EP-WXT (Li et al.., GCN 42482). SVOM/VT began observing the field automatically on 2025-10-26T08:34:26, i.e., 424 sec with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X band downlinked data, an uncatalogued and fading optical counterpart within EP-FXT's error box (Li et al., GCN 42511) was detected in both channels, compared to the PanSTARRS catalogue. The position is at R.A., Dec. = 72.653080, 51.763733 degrees, equivalent to:
```
R.A. (J2000) = 04:50:36.74 
Dec. (J2000) = +51:45:49.44
```
with an uncertainty of 0.5 arcsec.
This location is 3.6 arcminutes from the ECLAIRs onboard position and 2.1 arcminutes from the MXTs position (Cao et al., GCN 42454).
The afterglow was fading with a temporal slope of -0.7 from VT_R~19.51+/-0.05 mag, VT_B~21.35+/-0.18 mag at 445 sec, followed by a bump with the peak of VT_R~20.8+/-0.1 mag at the mid time of 3.3 hours post trigger.
With the check of VT VHF data, the optical counterpart was also detected in source list and 1 bit images of both channels in Sequence 1.
Our photometry was estimated in AB magnitude and not corrected for Galactic extinction.
Given the heavy extinction of the Galactic plane (Av = 3.7), the burst was intrinsically optically bright.
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.
GCN Circular #42541
Antonio de Ugarte Postigo (LAM), Margarita Pereyra (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (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), 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), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Alvarez (UNAM), Benjamin Schneider (LAM), J.X Cao (GXU), X. Tian (GXU):
We reanalysed our earlier observation (de Ugarte Postigo et al., GCN Circ. 42456) of the field of the SVOM GRB 251026A (Cao et al., GCN Circ. 42454) using the DDRAGO wide-field, two-channel imager on the SVOM/COLIBRÍ (FM-GFT) telescope, following the report of the optical counterpart by Li et al. (GCN Circ. 42512). We observed from 2025-10-26 08:49:51 to 11:18:57 UTC (from 22 to 172 minutes after the trigger) and obtained 48 minutes of simultaneous exposure in the r and z filters at three epochs.
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.
We detected the optical counterpart reported by Li et al. (GCN Circ. 42512) at preliminary magnitudes of:
```
r = 22.15 +/- 0.07
z = 20.62 +/- 0.06
```
We did not previously report this source as it is significantly outside the 90% confidence region for SVOM/MXT (Cao et al., GCN Circ. 42454).
Our light curve is consistent with the report of Li. et al. (GCN Circ. 42512) of an initially fading source followed by a rebrightening at about 3 hours.
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.

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