EP251021b, GRB 251021A
GCN Circular 42393
Subject
EP251021b/GRB 251021A: SVOM/VT optical candidate
Date
2025-10-23T10:18:34Z (21 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H.L. Li , L.P. Xin, Y. L. Qiu, C. Wu, Y.N. Ma, Z.H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observation to the field of EP251021b/GRB 251021A (Dai et al., GCN 42377; Ravasio et al., GCN 42379). The observation stared at 2025-10-22T20:09:33 UTC, I.e., 29.69 hours post trigger in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
Within the error box of EP-FXT (Dai et al., GCN 42377), an uncatalogued source was detected, compared to the Legacy survey DR9 catalog. The position is at R.A., Dec. = 306.745094, -38.3034732 degrees, equivalent to:
R.A. (J2000) = 20:26:58.82
Dec. (J2000) = -38:18:12.50
with an uncertainty of 0.5 arcsec.
The candidate's magnitudes are given below:
Mid_time Band Exposure Time Magnitude (AB)
30.75 hour VT_B 45*70 sec 23.62+/-0.25 mag
30.75 hour VT_R 54*70 sec 22.80+/-0.20 mag
Our photometry was derived in AB magnitude and not corrected for Galactic extinction.
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 42379
Subject
GRB 251021A/EP251021b: Fermi-GBM Sub-Threshold Detection
Date
2025-10-22T13:56:22Z (22 days ago)
From
mariaedvige.ravasio@ru.nl
Via
Web form
M. E. Ravasio (Radboud Univ.), E. Burns (LSU), and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:
Fermi-GBM had full spatial and temporal coverage of the transient EP251021b detected by EP-WXT (Dai et al., GCN 42377). There was no Fermi-GBM onboard trigger around the EP starting time at T0=2025-10-21T14:28:05 UTC.
The GBM Targeted Search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [-50;+500] s from the EP T0. A transient was found most significantly at ~T0+7 s on a 4.096 s timescale, with a false alarm rate of 4.9e-05 Hz. The localization is consistent with the EP one, with a spatial association probability of 99.2%. Among the three spectral templates tested, the transient was best-fit with a "soft" spectrum (i.e., a Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 42378
Subject
EP251021b: BOOTES-7 optical upper limit
Date
2025-10-22T09:05:45Z (22 days ago)
From
I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>
Via
Web form
I. Perez-Garcia, S.-Y. Wu, E. Fernandez-Garcia, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar, A. Castellon, A. Reina (Univ. de Malaga),Y.-D. Hu (GXU), L. Hernandez-Garcia (Univ. de Valparaiso), M. Gritsevich (Univ. of Helsinki), D. R. Xiong (Yunnan Observatories of CAS), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP251021b by the Einstein Probe (Dai et al. GCN 42377), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) observed the fast X-ray transient location starting on Oct 22, 03:57 UT (~13.08 h after trigger) in different optical bands. No new optical source is detected on the first co-added clear-filter images (mid exposure time 2025-10-22 04:07:57) within the EP/FXT 20 arcsec radius error circle down to 20.0 mag.
We thank the staff at San Pedro de Atacama Celestial Explorations observatory for their excellent support.
GCN Circular 42377
Subject
EP251021b: Einstein Probe detection of a fast X-ray transient
Date
2025-10-22T08:30:35Z (22 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
C. Y. Dai (NJU), Q. J. Huang(PMO), W. F. Wen (SZTU), J. H. Wu (GZHU), H. W. Pan (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 EP251021b. The transient triggered EP-WXT (ID: 01709247263) at 2025-10-21T14:28:28 (UTC). The ground analysis of the WXT data shows that the event started at T0=2025-10-21T14:28:05 (UTC) and lasted for around 30 s. The WXT position of the source is R.A. = 306.742 deg, Dec. = -38.286 deg (J2000) with an uncertainty of 2.3 arcmin in radius (90% C.L. statistical and systematic).
We performed a Target-of-Opportunity observation with the Follow-up X-ray Telescope on board EP. The observation began at 2025-10-22T04:41:25 (UTC), around 14 hours after the trigger. An uncatalogued X-ray source was detected within the WXT error circle at R.A. = 306.7451 deg, Dec. = –38.3048 deg (J2000), with a positional uncertainty of 20 arcsec in radius (90% confidence level, including both statistical and systematic uncertainties).
Further information will be updated when the telemetry data is received. 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).