GRB 251023B

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
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 ; Mandarakas et al., GCN 42404
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 ; Nakahira et al., GCN 42402
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 ; Evans et al., GCN 42401
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 ; Kang et al., GCN 42392
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 ) 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
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 ) 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.

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
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 ) and CALET (GCN 42402
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 ), and possibly associated with the Einstein Probe/WXT-detected source EP#01709247298 (GCN 42401
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 ).
 
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
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 ). 
 
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
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[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.

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
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 ) 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
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 ) 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.

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
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 ) 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
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 ).
(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.

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.