EP251102a
GCN Circular 42597
Subject
EP251102a: Xinglong 2.16-m clear-band observations of the optical counterpart
Date
2025-11-06T06:42:49Z (7 hours ago)
From
Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
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Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Junjun-Jia(NAOC), Jie-Zheng(NAOC), Yinan-Zhu(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration:
We observed the field of the X-ray transient EP251102a, detected by EP/WXT, using the Xinglong 2.16-m telescope at Xinglong, Hebei, China. We obtained three 300 s clear-band frames with a median time of 2025-11-03T20:56:36, i.e. 9.2 hr after the EP trigger. The stacked clear-band image has a 5σ limiting magnitude of ≈23.0 mag, calibrated against Pan-STARRS g-band field stars. We detect the optical counterpart in the stacked image at
RA (J2000) = 07:32:39.024 = 113.1626 deg
Dec (J2000) = +07:19:18.88 = +7.32191 deg.
This position is consistent with the candidate counterpart identified by Schneider et al. (GCN 42544). The preliminary magnitude of this source is 18.738 ± 0.025 (AB), which is brighter than reported in other follow-up observations (GCN 42544, 42546).
We note the presence of a bright source ≈3 arcsec from the candidate counterpart, whose measured magnitude is consistent with its Pan-STARRS g-band magnitude, indicating that our flux calibration and PSF point-source decomposition are reliable. Therefore, the brightness of EP251102a in the clear band is likely intrinsically higher, or may be attributed to an unusual spectral energy distribution (SED) of the source. Further observations are encouraged.
GCN Circular 42578
Subject
EP251102a: FTW optical and NIR observations
Date
2025-11-04T13:49:28Z (2 days ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
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Malte Busmann, Rasika Deshpande (LMU), Xander Hall, Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:
We observed the counterpart of EP251102a (Schneider et al., GCN 42544; Eyles-Ferris et al., GCN 42546; Liang et al., GCN 42548; Mo et al., GCN 42563; Adami et al., GCN 42575) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands simultaneously for 7 x 180 s starting at 2025-11-04T04:50:46 UT (0.94 days after the trigger). We detect the counterpart in all bands and report the following magnitude:
r = (21.02 +/- 0.06) AB mag
The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Christoph Ries for obtaining these observations.
GCN Circular 42576
Subject
EP251102a: update of EP-FXT follow-up observation
Date
2025-11-04T13:08:10Z (2 days ago)
From
Hao Zhou at Purple Mountain Observatory, CAS <haozhou@pmo.ac.cn>
Via
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H. Zhou, Y.-F. Liang (PMO, CAS) and H.-W. Pan (NAO, CAS) report on behalf of the Einstein Probe team:
The Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) performed one ToO observation for EP251102a, starting at 2025-11-03T11:21:51 (UTC) with an exposure time of about 2.1 ks. The ground-processed position of EP251102a is R.A. = 113.1622 deg, DEC = 7.3217 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The updated position is consistent with the counterpart reported before (Schneider et al. GCN 42544, Eyles-Ferris et al. GCN 42546, Liang et al. GCN 42548, Adami et al. GCN 42575).
The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 9.1 x 10^20 cm^-2 and a photon index of 1.93+/-0.36. The derived average unabsorbed 0.5-10 keV flux is (4.8+/-1.3) x 10^(-13) erg/s/cm^2.
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 42575
Subject
EP251102a: OHP/T193 optical observations
Date
2025-11-04T11:32:43Z (2 days ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
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C. Adami (LAM/Pytheas/AMU), N.A. Rakotondrainibe (LAM), B. Schneider (LAM), S. Basa (UAR Pytheas), E. Le Floc'h (CEA/Irfu) report on behalf of the MISTRAL GRB collaboration:
We carried out observations of the EP251102a (Schneider et al., GCN 42544; Eyles-Ferris et al., GCN 42546; Liang et al., GCN 42548; Mo et al., GCN 42563) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 75 min (15 exposures of 5 min) in the r-band at a midtime of 2025-11-04 03:34:45 UT corresponding to T-T0 = 30.2064 hours.
The afterglow is detected in r’ after image subtraction performed relative to r-band images from PanSTARRS. We measured the following preliminary magnitude:
r’ = 20.94 +/- 0.07
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the STDWeb/STDPipe tools (Karpov 2025), is in the AB system, and is not corrected for Galactic extinction.
We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Yoann Degot-Longhi.
GCN Circular 42563
Subject
EP251102a: NIR observations with WINTER
Date
2025-11-03T22:02:24Z (3 days ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
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Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (Caltech), Viraj Karambelkar (Columbia/CCA), Benjamin Schneider (LAM), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP251102a (Liang et al., GCN 42548) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began at 2025-11-03T10:34:56 UTC in the J band (13.2 hours after the EP trigger), consisting of 15 x 120 s exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We do not detect a source at the optical counterpart location (Schneider et al., GCN 42544; Eyles-Ferris et al., GCN 42546) after image subtraction performed relative to J-band images from the UKIRT Hemisphere survey (Dye et al., 2017). We set a 3-sigma upper limit of J ~ 19.6 mag (AB) and a 5-sigma upper limit of J ~ 19.1 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 42548
Subject
EP251102a: Einstein Probe detection of a fast X-ray transient
Date
2025-11-03T12:39:07Z (3 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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Y. F. Liang, H. Zhou (PMOC) and H. W. Pan (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 EP251102a. The source did not trigger the WXT on-board trigger unit. A GCN Notice was sent manually earlier (tigger ID: 11900459650). The ground analysis of the WXT data shows that the event started at T0=2025-11-02T21:22:22 (UTC) and lasted for around 400 seconds. The WXT position of the source is R.A. = 113.140 deg, DEC = 7.322 deg (J2000) with an uncertainty of 2.4 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 9.1 x 10^20 cm^-2 and a photon index of 1.6(-/+0.1). The derived average unabsorbed 0.5-4 keV flux is 7.2(-1.6/+1.5) x 10^(-10) erg/s/cm^2. The peak flux is around 1.7 x 10^(-9) erg/s/cm^2.
We performed a target-of-opportunity follow-up observation with the Follow-up X-ray Telescope (FXT). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 113.1642 deg, DEC = 7.3199 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the FXT telemetry data are received.
We note that an optical counterpart has been reported (GCN 42544, 42546), which is positionally consistent with the FXT source.
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 42546
Subject
EP251102a (WXT 11900459650): LCO observations confirm COLIBRÍ candidate
Date
2025-11-03T11:45:32Z (3 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
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R. A. J. Eyles-Ferris (U. Leicester), J. A. Quirola-Vasquez (Radboud), J. A. Chacón (PUC), G. Corcoran (UCD), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), F. E. Bauer (SSI and UTA), and A. J. Levan (Radboud and Warwick) report on behalf of a larger collaboration:
We observed the field of the new transient event EP251102a (Trigger ID: 11900459650) detected by the Einstein Probe Wide-field X-ray Telescope (EP-WXT) with the Las Cumbres Observatory (LCO) network in the r-band on 2025-11-03 08:33:04. The total exposure time was 6x300s. We performed image subtraction on the stacked image using the ZOGY algorithm (Zackay et al. 2016), as implemented in PyZOGY (Guevel et al. 2021), with archival reference r-band images from Pan-STARRS.
The candidate counterpart identified by Schneider et al. (GCN 42544) is faintly detected in our subtraction, and we measure r = 20.18 +/- 0.14, consistent with the measurement of Schneider et al. at a similar time. We note the proximity of a bright source ~3 arcsec from the counterpart.
We do not identify any further candidates within the WXT error region down to a limiting magnitude of r >~ 21.2. Observations are ongoing.