Gamma-ray Burst 251023B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #42392
C. Kang (Ehime U.), M. Nakajima (Nihon U.), Y. Kawakubo (AGU), H. Negoro, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, T. Nakamoto (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U) report on behalf of the MAXI team:
The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 08:49:03 UT on October 23, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (72.930 deg, 12.205 deg) = (04 51 43, +12 12 18) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.15 deg and 0.13 deg, respectively. The roll angle of long axis from the north direction is 71.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 341 +- 36 mCrab (4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners:
```
(72.360, 11.893) deg = (04 49 26, +11 53 34) (J2000)
(72.539, 11.711) deg = (04 50 09, +11 42 39) (J2000)
(73.436, 12.551) deg = (04 53 44, +12 33 03) (J2000)
(73.256, 12.734) deg = (04 53 01, +12 44 02) (J2000)
```
There was no significant excess flux in the previous transit at 07:16 UT with an upper limit of 20 mCrab for each.
GCN Circular #42401
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 Einstein Probe/WXT-detected source EP#01709247298, collecting 1.6 ks of Photon Counting (PC) mode data between T0+6.6 ks and T0+8.3 ks after the trigger. A likely counterpart has been found. This source is 43 arcmin from the position of the MAXI-detected transient GRB 251023B / MAXI J0451+122 (Kang et al., GCN Circ. 42392), well outside their error region, thus its association, while likely on temporal grounds, remains unconfirmed. The details of this source are:
Source 1 (SWIFT J045400.4+123929):
==================================
```
    RA (J2000.0):   73.5018  =	04 54 00.43
    Dec (J2000.0):  +12.6583  =  +12 39 29.9
    Error:	    3.7 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    22 arcsec from the Einstein Probe/WXT position.
    Mean rate:	    0.152 +/- 0.011 ct s^-1
    Mean flux:	    (5.94 +/- 0.44)e-12 erg cm^-2 s^-1
    Peak rate:	    0.221 +/- 0.049 ct s^-1
    Peak flux:	    (8.6 +/- 1.9)e-12 erg cm^-2 s^-1
    ECF:	    3.91e-11 erg cm^-2 ct^-1
		      assuming NH=3.15e+21 cm^-2, gamma=1.97
		      determined from a spectral fit.
    XMM UL:	    2.3e-12 erg cm^-2 s^-1, (0.3-10 keV)
		      so the source is 3.2-sigma above this 3-sigma upper limit.
```
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/EP/EP_FIELD00077.
This circular is an official product of the Swift-XRT team.
GCN Circular #42402
S. Nakahira (JAXA), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration:
The long GRB 251023B (MAXI/GSC Detection: Kang et al., GCN Circ 42392) was detected in the ground analysis of the CALET Gamma-ray Burst Monitor (CGBM) data around 08:49:03 on 23 October 2025 (referenced to the MAXI detection: GCN Circ. 42392). (https://cgbm.calet.jp/cgbm_trigger/flight/1445244062/index.html). The burst signal was seen by all CGBM detectors.
The burst light curve shows a multi-peaked structure that starts at T-31.7 sec and ends at T+8.2 sec. The T90 and T50 durations measured by the HXM2 data are 34.9 +/- 4.2 sec and 16.7 +/- 2.2 sec (7-100 keV), respectively.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1445244062/
The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University.
GCN Circular #42404
Nikos Mandarakas (LAM), Ramandeep Gill (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), 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), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of the GRB 251023B / EP#01709247298, that might be associated with MAXI J0451+122 (Kang et al., GCN Circ. 42392) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-23 10:14:07 to 11:41:55 UTC (from 1.42 to 2.88 hours after the trigger) and obtained 32 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed 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 detect an uncatalogued source consistent with the XRT 3.7 arcsec error circle (Evans et al., GCN Circ. 42401) at:
```
RA(J2000) = 04:54:00.63 = 73.50263 degrees
Dec(J2000) = 12:39:28.4 = 12.65789 degrees
```
with an uncertainty of 0.5 arcsec.
The preliminary magnitudes derived for that source is:
```
r = 21.86 +/- 0.04
z = 20.45 +/- 0.04
```
Further observations are planned.
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 #42407
C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 251023B, which was also detected by MAXI (as MAXI J0451+122; GCN 42392) and CALET (GCN 42402), and possibly associated with the Einstein Probe/WXT-detected source EP#01709247298 (GCN 42401).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-10-23 08:48:53.664 with a duration of 7.9 s and a total significance of about 7.0 sigma. The onset measured by Glowbug is 9.4 s prior to the MAXI-reported T0. The Glowbug light curve is dominated by a single peak corresponding to the maximum in the CALET light curve (GCN 42402).
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS, and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12.
```
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
```
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular #42415
Y. Wu (NJU), D. F. Hu (PMO, CAS), H. N. Yang, T. Zhao, W. D. Zhang (NAO, CAS), 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 EP251023b. The transient triggered EP-WXT (ID: 01709247298) at 2025-10-23T08:48:47 (UTC). The refined analysis of the WXT data shows that the event started at T0=2025-10-23T08:46:06 (UTC) and lasted for ~160 s. The WXT position of the source is R.A. = 73.502 deg, Dec. = 12.648 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic). The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.69 x 10^(21) cm^-2 and a photon index of 1.66（-/+0.25）. The derived average unabsorbed 0.5-4 keV flux is 3.9 (-/+0.5) x 10^(-9) erg/s/cm^2.
We performed a Target-of-Opportunity observation with the Follow-up X-ray Telescope on board EP. The observation began at 2025-10-23T14:40:06 (UTC).An uncatalogued X-ray source was detected within the WXT error circle at R.A. = 73.5021 deg, Dec. = 12.6574 deg (J2000), with a positional uncertainty of 10 arcsec in radius (90% confidence level, including both statistical and systematic uncertainties). The FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with the same fixed Galactic hydrogen column density value and a photon index of 2.16 (-/+0.21). The derived average unabsorbed 0.5-10 keV flux is 2.03 (-0.29/+0.34) x 10^(-12) erg/s/cm^2. The source is spatially consistent with the counterpart reported (Kang et al. GCN 42392, Evans et al. GCN 42401, Nakahira et al. GCN 42402, Mandarakas et al. GCN 42404, Cheung et al. GCN 42407).
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 #42416
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-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP251023b ( EP Team et al., GCN 42415) errorbox 40434 sec after trigger time at 2025-10-23 20:02:41 UT, with upper limit up to 20.1 mag. The observations began at zenith distance = 38 deg. The sun altitude is -34.7 deg.
The galactic latitude b = -18 deg., longitude l = 187 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3022366
We obtain a following upper limits.

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   40494 | 2025-10-23 20:02:41 |        MASTER-Tunka | (04h 53m 57.12s , +12d 53m 42.1s) |   C |   120 | 16.4 |        
   40494 | 2025-10-23 20:02:41 |             MASTER- | (04h 53m 27.18s , +12d 59m 22.7s) |   C |   120 | 18.8 |        
   40627 | 2025-10-23 20:04:54 |             MASTER- | (04h 53m 23.22s , +13d 00m 50.4s) |   C |   120 | 19.8 |        
   40628 | 2025-10-23 20:04:54 |        MASTER-Tunka | (04h 53m 53.26s , +12d 55m 10.3s) |   C |   120 | 16.4 |        
   40761 | 2025-10-23 20:07:07 |        MASTER-Tunka | (04h 53m 53.45s , +12d 53m 31.3s) |   C |   120 | 16.9 |        
   40761 | 2025-10-23 20:07:07 |             MASTER- | (04h 53m 23.36s , +12d 59m 11.7s) |   C |   120 | 19.9 |        
   41086 | 2025-10-23 20:13:02 |        MASTER-Tunka | (04h 53m 57.11s , +12d 54m 43.4s) |   C |    60 | 19.7 |        
   41176 | 2025-10-23 20:13:02 |        MASTER-Tunka | (04h 53m 57.11s , +12d 54m 43.3s) |   C |   240 | 20.0 |  Coadd 
   41086 | 2025-10-23 20:13:02 |             MASTER- | (04h 53m 26.63s , +13d 00m 28.7s) |   C |    60 | 19.7 |        
   41159 | 2025-10-23 20:14:15 |        MASTER-Tunka | (04h 53m 50.19s , +12d 54m 07.5s) |   C |    60 | 19.7 |        
   41159 | 2025-10-23 20:14:15 |             MASTER- | (04h 53m 19.72s , +12d 59m 52.6s) |   C |    60 | 19.7 |        
   41263 | 2025-10-23 20:15:30 |        MASTER-Tunka | (04h 53m 51.71s , +12d 55m 04.5s) |   C |   120 | 19.8 |        
   41263 | 2025-10-23 20:15:30 |             MASTER- | (04h 53m 21.21s , +13d 00m 49.4s) |   C |   120 | 19.8 |        
   41398 | 2025-10-23 20:17:44 |        MASTER-Tunka | (04h 53m 57.01s , +12d 54m 00.3s) |   C |   120 | 19.8 |        
   41518 | 2025-10-23 20:17:44 |        MASTER-Tunka | (04h 53m 57.02s , +12d 54m 00.4s) |   C |   360 | 20.0 |  Coadd 
   41398 | 2025-10-23 20:17:44 |             MASTER- | (04h 53m 26.46s , +12d 59m 45.2s) |   C |   120 | 19.8 |        
   41530 | 2025-10-23 20:19:57 |        MASTER-Tunka | (04h 53m 50.71s , +12d 52m 55.2s) |   C |   120 | 19.7 |        
   41530 | 2025-10-23 20:19:57 |             MASTER- | (04h 53m 20.15s , +12d 58m 40.0s) |   C |   120 | 19.7 |        
   41663 | 2025-10-23 20:22:10 |        MASTER-Tunka | (04h 53m 57.82s , +12d 53m 08.0s) |   C |   120 | 19.7 |        
   41663 | 2025-10-23 20:22:10 |             MASTER- | (04h 53m 27.32s , +12d 58m 52.6s) |   C |   120 | 19.7 |        
   41796 | 2025-10-23 20:24:22 |        MASTER-Tunka | (04h 53m 54.06s , +12d 54m 19.5s) |   C |   120 | 19.7 |        
   41916 | 2025-10-23 20:24:22 |        MASTER-Tunka | (04h 53m 54.06s , +12d 54m 19.5s) |   C |   360 | 20.1 |  Coadd 
   41796 | 2025-10-23 20:24:22 |             MASTER- | (04h 53m 23.57s , +13d 00m 03.9s) |   C |   120 | 19.6 |        
   41929 | 2025-10-23 20:26:35 |        MASTER-Tunka | (04h 53m 54.16s , +12d 52m 58.5s) |   C |   120 | 19.8 |        
   41929 | 2025-10-23 20:26:35 |             MASTER- | (04h 53m 23.66s , +12d 58m 43.0s) |   C |   120 | 19.8 |        
   42062 | 2025-10-23 20:28:48 |        MASTER-Tunka | (04h 53m 57.79s , +12d 54m 17.4s) |   C |   120 | 19.9 |        
   42062 | 2025-10-23 20:28:48 |             MASTER- | (04h 53m 27.25s , +13d 00m 01.6s) |   C |   120 | 19.9 |        
   42195 | 2025-10-23 20:31:02 |        MASTER-Tunka | (04h 53m 52.11s , +12d 53m 34.1s) |   C |   120 | 19.9 |        
   42315 | 2025-10-23 20:31:02 |        MASTER-Tunka | (04h 53m 52.11s , +12d 53m 34.1s) |   C |   360 | 20.1 |  Coadd 
   42195 | 2025-10-23 20:31:02 |             MASTER- | (04h 53m 21.54s , +12d 59m 18.5s) |   C |   120 | 19.9 |        
   42327 | 2025-10-23 20:33:13 |        MASTER-Tunka | (04h 53m 50.56s , +12d 54m 30.9s) |   C |   120 | 20.0 |        
   42327 | 2025-10-23 20:33:13 |             MASTER- | (04h 53m 19.95s , +13d 00m 15.1s) |   C |   120 | 19.9 |        
   42460 | 2025-10-23 20:35:27 |        MASTER-Tunka | (04h 53m 57.26s , +12d 53m 27.1s) |   C |   120 | 19.9 |        
   42460 | 2025-10-23 20:35:27 |             MASTER- | (04h 53m 26.58s , +12d 59m 11.0s) |   C |   120 | 19.9 |        
   42595 | 2025-10-23 20:37:41 |        MASTER-Tunka | (04h 53m 52.25s , +12d 52m 22.4s) |   C |   120 | 19.9 |        
   42685 | 2025-10-23 20:37:41 |        MASTER-Tunka | (04h 53m 52.25s , +12d 52m 22.4s) |   C |   300 | 20.1 |  Coadd 
   42595 | 2025-10-23 20:37:41 |             MASTER- | (04h 53m 21.54s , +12d 58m 06.1s) |   C |   120 | 19.9 |        
   42730 | 2025-10-23 20:39:56 |        MASTER-Tunka | (04h 53m 57.37s , +12d 52m 45.8s) |   C |   120 | 20.0 |        
   42730 | 2025-10-23 20:39:56 |             MASTER- | (04h 53m 26.63s , +12d 58m 29.3s) |   C |   120 | 20.0 |        
   42833 | 2025-10-23 20:42:09 |        MASTER-Tunka | (04h 53m 54.47s , +12d 53m 48.3s) |   C |    60 | 19.9 |        
   42833 | 2025-10-23 20:42:09 |             MASTER- | (04h 53m 23.67s , +12d 59m 31.7s) |   C |    60 | 19.9 |        
   42907 | 2025-10-23 20:43:24 |        MASTER-Tunka | (04h 53m 54.53s , +12d 52m 27.5s) |   C |    60 | 19.8 |        
   42967 | 2025-10-23 20:43:24 |        MASTER-Tunka | (04h 53m 54.54s , +12d 52m 27.5s) |   C |   180 | 20.0 |  Coadd 
   42907 | 2025-10-23 20:43:24 |             MASTER- | (04h 53m 23.70s , +12d 58m 10.7s) |   C |    60 | 19.8 |        
   42980 | 2025-10-23 20:44:36 |        MASTER-Tunka | (04h 53m 56.90s , +12d 54m 01.3s) |   C |    60 | 19.7 |        
   42980 | 2025-10-23 20:44:36 |             MASTER- | (04h 53m 26.04s , +12d 59m 44.2s) |   C |    60 | 19.7 |        
   43052 | 2025-10-23 20:45:49 |        MASTER-Tunka | (04h 53m 52.30s , +12d 53m 09.6s) |   C |    60 | 19.5 |        
   43052 | 2025-10-23 20:45:49 |             MASTER- | (04h 53m 21.42s , +12d 58m 52.8s) |   C |    60 | 19.5 |        
   43134 | 2025-10-23 20:47:11 |        MASTER-Tunka | (04h 53m 51.00s , +12d 54m 07.5s) |   C |    60 | 19.2 |        
   43194 | 2025-10-23 20:47:11 |        MASTER-Tunka | (04h 53m 50.99s , +12d 54m 07.5s) |   C |   180 | 19.7 |  Coadd 
   43135 | 2025-10-23 20:47:11 |             MASTER- | (04h 53m 20.10s , +12d 59m 50.4s) |   C |    60 | 19.1 |        
   43208 | 2025-10-23 20:48:25 |        MASTER-Tunka | (04h 53m 58.03s , +12d 53m 05.4s) |   C |    60 | 19.2 |        
   43208 | 2025-10-23 20:48:25 |             MASTER- | (04h 53m 27.12s , +12d 58m 48.7s) |   C |    60 | 19.1 |        
   43284 | 2025-10-23 20:49:41 |        MASTER-Tunka | (04h 53m 50.96s , +12d 52m 02.9s) |   C |    60 | 19.0 |        
   43284 | 2025-10-23 20:49:41 |             MASTER- | (04h 53m 20.05s , +12d 57m 45.8s) |   C |    60 | 19.0 |        
   43358 | 2025-10-23 20:50:54 |        MASTER-Tunka | (04h 53m 58.58s , +12d 52m 08.7s) |   C |    60 | 18.8 |        
   43418 | 2025-10-23 20:50:54 |        MASTER-Tunka | (04h 53m 58.58s , +12d 52m 08.7s) |   C |   180 | 19.3 |  Coadd 
   43358 | 2025-10-23 20:50:54 |             MASTER- | (04h 53m 27.65s , +12d 57m 51.3s) |   C |    60 | 18.7 |        
   43435 | 2025-10-23 20:52:12 |        MASTER-Tunka | (04h 53m 54.73s , +12d 53m 32.9s) |   C |    60 | 18.7 |        
   43435 | 2025-10-23 20:52:12 |             MASTER- | (04h 53m 23.76s , +12d 59m 15.7s) |   C |    60 | 18.7 |        
   43507 | 2025-10-23 20:53:24 |        MASTER-Tunka | (04h 53m 54.77s , +12d 52m 13.3s) |   C |    60 | 18.3 |        
   43507 | 2025-10-23 20:53:24 |             MASTER- | (04h 53m 23.79s , +12d 57m 56.2s) |   C |    60 | 18.2 |        
   43579 | 2025-10-23 20:54:36 |        MASTER-Tunka | (04h 53m 57.61s , +12d 53m 34.0s) |   C |    60 | 18.7 |        
   43639 | 2025-10-23 20:54:36 |        MASTER-Tunka | (04h 53m 57.62s , +12d 53m 34.0s) |   C |   180 | 19.9 |  Coadd 
   43579 | 2025-10-23 20:54:36 |             MASTER- | (04h 53m 26.61s , +12d 59m 16.6s) |   C |    60 | 18.7 |        
   43654 | 2025-10-23 20:55:50 |        MASTER-Tunka | (04h 53m 52.49s , +12d 52m 52.2s) |   C |    60 | 19.6 |        
   43654 | 2025-10-23 20:55:50 |             MASTER- | (04h 53m 21.48s , +12d 58m 34.6s) |   C |    60 | 19.5 |        
   43727 | 2025-10-23 20:57:04 |        MASTER-Tunka | (04h 53m 51.37s , +12d 53m 50.1s) |   C |    60 | 19.5 |        
   43727 | 2025-10-23 20:57:04 |             MASTER- | (04h 53m 20.37s , +12d 59m 32.4s) |   C |    60 | 19.4 |        
   43800 | 2025-10-23 20:58:16 |        MASTER-Tunka | (04h 53m 57.11s , +12d 52m 48.1s) |   C |    60 | 19.6 |        
   43890 | 2025-10-23 20:58:16 |        MASTER-Tunka | (04h 53m 57.11s , +12d 52m 48.2s) |   C |   240 | 20.0 |  Coadd 
   43800 | 2025-10-23 20:58:16 |             MASTER- | (04h 53m 26.13s , +12d 58m 30.3s) |   C |    60 | 19.6 |        
   43873 | 2025-10-23 20:59:30 |        MASTER-Tunka | (04h 53m 52.86s , +12d 51m 46.0s) |   C |    60 | 19.6 |        
   43873 | 2025-10-23 20:59:30 |             MASTER- | (04h 53m 21.87s , +12d 57m 27.9s) |   C |    60 | 19.5 |        
   43984 | 2025-10-23 21:00:51 |        MASTER-Tunka | (04h 53m 58.73s , +12d 51m 48.7s) |   C |   120 | 19.2 |        
   43984 | 2025-10-23 21:00:51 |             MASTER- | (04h 53m 27.73s , +12d 57m 30.8s) |   C |   120 | 19.2 |

Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.

GCN Circular #42418
L.B. He, J. An, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei, H.Z. Wu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration:
We observed the field of EP251023b/GRB 251023B (Kang et al., GCN 42392; Nakahira et al., GCN 42402; Wu et al., GCN 42415) using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Observations started at 2025-10-23 16:01:41 UT, i. e. 7.3 hours after the trigger.
The previously reported optical counterpart (Mandarakas et al., GCN 42404) was detected in our stacked z-band frame with a magnitude z~21.4 at a mid-time 9.75 hrs post-burst. The magnitude was callibrated with nearby PanSTARRS1 dr2 catalog without the Galactic extinction correction.
We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.
GCN Circular #42421
C. Adami (LAM/Pytheas/AMU), N.A. Rakotondrainibe (LAM), E. Le Floc'h (CEA/Irfu), report on behalf of the MISTRAL GRB collaboration:
We carried out observations of GRB 251023B (Cheung et al., GCN 42407; Mandarakas et al., GCN 42404; Nakahira et al., GCN 42402; Evans et al., GCN 42401; Kang et al., GCN 42392) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 6 exposures of 15 min in the r-band at a midtime of 2025 10 24 01:51:23 UT, corresponding to T-T0 = 17.04 hours.
We do not significantly detect the optical source identified by Mandarakas et al. (GCN 42404) at the 3-sigma upper limit of:
r’ < 21.78 mag (AB)
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction. We used the STDWeb/STDPipe tools (Karpov 2025).
We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen.
GCN Circular #42424
Enrique Moreno Méndez (UNAM), Nikos Mandarakas (LAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), 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), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We reimaged the field of the GRB 251023B / EP251022b (Kang et al., GCN Circ. 42392; Nakahira et al., GCN Circ. 42402; Wu et al., GCN Circ. 42415; Cheung et al., GCN Circ. 42407), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-24 07:06 to 08:12 UTC (from 22.3 to 23.4 hours after the trigger) and obtained 16 minutes of exposure in each of the r and i filters and 32 minutes of exposure in the z filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed 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.
In our second epoch, we do not detect the optical candidate reported by Mandarakas et al. (GCN Circ. 42404) down to the following 3-sigma limits:
r > 22.8
i > 22.4
z > 22.0
These values are consistent with the non-detection reported by Adami et al. (GCN Circ. 42421) and confirm the fading nature of the source, supporting its association with GRB 251023B / EP251022b.
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 #42478
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 the GRB 251023B / EP251023b (Kang et al., GCN Circ. 42392; Nakahira et al., GCN Circ. 42402; Wu et al., GCN Circ. 42415; Cheung et al., GCN Circ. 42407) in the R-filter with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) starting Oct. 25, 2025, 17:50:36 UT, and taking several frames with 120 s exposures. The optical candidate reported by Mandarakas et al. (GCN Circ. 42404) is not detected in the stacked frame. Photometry and observational details are the following:
```
Date        UT start  t-T0       Exp.    Filter  OT     Err.  UL
                     (mid,days)  (n*s)                       (3sigma)
2025-10-25  17:50:36  2.40386    40*120  R       n/d    n/d   22.7
```
The photometry is based on several nearby SDSS stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction.
GCN Circular #42488
Swift-UVOT has performed follow-up observations of the EP-detected burst GRB 251023B (Wu et al. GCN 42415), collecting 1.7 ks of observations between T0+6.6 ks and T0+26.9 ks. No source was detected in the UVOT data at the position of the optical afterglow (Mandarakas et al., 42404; He at al., GCN 42418; Adami et al., 42421; Méndez et al. 42424).
3-sigma upper limits (AB) using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are:
```
Filter		T_start(s)	T_stop(s)	Exp(s)		Mag
U		6573		26943		1737		>21.92
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.416 in the direction of the burst (Schlegel et al. 1998).
We note that in the first snapshot (1531.80 s worth of data), the optical afterglow's position landed on a UVOT detector patch of small scale sensitivity inhomogeneity. In the U-band this can cause a source's flux to appear 5-16% lower than it actually is. The above limits are not corrected for this. See https://swift.gsfc.nasa.gov/analysis/uvot_digest/sss_check.html for more information.

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Jochen Greiner, last update: 27-Oct-2025 [Disclaimer]