EP261015a
GCN Circular 45969
Subject
EP261015a: refined analysis of EP-FXT observations
Date
2026-10-17T05:09:17Z (33 minutes ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO), Z.-X. Li, G.-L. Huang (IHEP), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP261015a was detected by the Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) mission at 2026-10-15T13:15:36 UTC (Li et al., GCN 45956). Follow-up observations with the Follow-up X-ray Telescope (FXT) onboard EP began at 2026-10-16T04:31:12 UTC, approximately 15.26 hours after the WXT trigger, with an exposure time of 3231 s.
We stacked the FXT-A and FXT-B data. Within the EP/WXT error circle (Li et al., GCN 45956), no X-ray source was detected at the position of the optical candidate (Corcoran et al., GCN 45955; Méndez et al., GCN 45960; Ma et al., GCN 45963; Mohan et al., GCN 45967). The derived 0.5–10 keV flux upper limit is approximately 1.33 × 10^-13 erg cm^-2 s^-1 (90% confidence level).
An uncatalogued X-ray source was detected in the stacked data within the EP/WXT error circle (Li et al., GCN 45956). Its position is R.A. = 44.3459 deg, Decl. = −13.3915 deg (J2000), with a positional uncertainty of 10 arcsec in radius (90% confidence level, including statistical and systematic uncertainties). The 0.5-10 keV FXT spectrum of this source can be fitted with an absorbed power-law model, with the Galactic hydrogen column density fixed at 3.5 × 10^20 cm^-2 and a photon index of 1.55 (+1.45, −1.43). The derived unabsorbed 0.5–10 keV flux is 7.01 (+6.27, −3.45) × 10^-14 erg cm^-2 s^-1. All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.
Launched on January 9, 2024, EP is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 45967
Subject
EP261015a: J-band upper limit with WINTER
Date
2026-10-16T23:35:01Z (6 hours ago)
From
tanishk.mohan@iitb.ac.in
Via
Web form
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP261015a (Li et al., GCN 45956) 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 on 2026-10-16 at 05:43:44.591 UTC in the J band (16.47 hr after the WXT observation), consisting of 30*120 sec 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 the optical counterpart reported by Corcoran et al., GCN 45955; Méndez et al., GCN 45960 and Ma et al., GCN 45963 in our stacked image.
We obtain the following 5-sigma upper limit: J = 19 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 45963
Subject
EP261015a: SVOM/VT optical observation
Date
2026-10-16T16:09:27Z (14 hours ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, J. R. Xu, X. H. Han, Y. Xu, J. Wang, 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 a Target of Opportunity observation of EP261015a detected by EP/WXT (Li et al., GCN 45956). SVOM/VT began observing the field at 2026-10-16T02:48:38 UTC, 13.551 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data available, the NOT candidate (Corcoran et al., GCN 45955; Méndez et al., GCN 45960) was detected in both VT_B and VT_R bands. The AB magnitudes are:
mid time (h) | exposure time (s) | band | mag (AB)
-------------|-------------------|------|------------
14.20 | 25*50 | VT_B | 22.3+/-0.2
14.20 | 27*50 | VT_R | 22.3+/-0.2
Our photometry was not corrected for Galactic extinction.
There is a faint extended source at the position of this candidate (Corcoran et al., GCN 45955; Méndez et al., GCN 45960), with magnitudes g = 23.09, r = 22.39, i = 22.30, and z = 21.92 in Legacy Survey. Our photometry is contaminated by the source.
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 45960
Subject
EP261015a: COLIBRÍ optical observations
Date
2026-10-16T08:53:51Z (21 hours ago)
From
damien dornic <damiendornic@gmail.com>
Via
Web form
Enrique Moreno Méndez (UNAM), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), , 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), Camila Angulo (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Marion Guelfand (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM), report:
We imaged the field of the EP 261005a (Li et al, GCN Circ. 45956) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-10-16T04:52:09.644 to 2026-10-16T06:15:23.681 UTC (from 15.61 to 17.00 hours after the trigger) and obtained 62 minutes of exposure in the r filter.
The data were reduced, coadded and analysed with the CPPM COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.
After subtracting template Legacy Survey image, we clearly detected the optical counterpart reported by Corcoran et al., GCN Circ. 45955, at a preliminary magnitude of:
r mag = 23.04 +/- 0.19
Further observations and analysis are ongoing.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
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 45956
Subject
EP261015a: Einstein Probe detection of a fast X-ray transient
Date
2026-10-16T06:21:08Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO), G. -L. Huang, Z.-X. Li (IHEP), M. Nie (YNAO) 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 EP261015a. The transient did not trigger EP-WXT onboard. We sent a GCN Notice manually with trigger ID: 11900947714. The WXT light curve of the transient began at 2026-10-15T13:15:36 UTC, exhibited two peaks, and lasted for 130 s. The WXT position of the source is R.A. = 44.341 deg, DEC = -13.365 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
The 0.5-4 keV WXT spectrum can be fitted by an absorbed powerlaw model with a fixed Galactic column density of 3.5 x 10^20 cm^-2 and a photon index of 1.45 (-0.71,+0.88). The derived 0.5-4 keV unabsorbed flux is 6.25 (-2.91/+3.90) x 10^-10 erg/cm^2/s.
A follow-up observation with the Follow-up X-ray Telescope (FXT) has been scheduled. Further information will be updated when the telemetry data is received.
A possible optical counterpart to the transient was reported by NOT and LCO (Corcoran et al., GCN 45955).
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 45955
Subject
EP261015a (WXT 11900947714): Possible optical counterpart from NOT and LCO
Date
2026-10-16T05:15:19Z (a day ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud), J. Chacon (PUC), and F. E. Bauer (SSI and UTA), O. Stalder (NOT) report on behalf of a larger collaboration:
We observed the field of the Einstein Probe fast X-ray transient EP261015a (WXT trigger 11900947714) using the ALFOSC camera on the Nordic Optical Telescope (NOT). Observations consisted of a sequence of 3 x 300 s exposures in the SDSS r-band, and started on 2026-10-16 at 02:58:58 UT (13.72 hr after the EP/WXT trigger).
After performing image subtraction against Dark Energy Survey stacked images, we identify a potential counterpart at coordinates (J2000):
RA: 02:57:19.83
Dec: -13:21:17.3
We measure a difference magnitude of r = 22.74 +/- 0.21 (AB) at a midtime of 2026-10-16 03:06:55 UT (13.86 hr after the EP/WXT trigger), calibrated using nearby stars in the Pan-STARRS catalog and not corrected for Galactic extinction.
At a similar mid-time (2026-10-16 03:02:53 UT), we performed additional observations using an LCO 1m telescope located at the Cerro Tololo Interamerican Observatory (Chile) equipped with the SINISTRO instrument. Here observations consisted of 9 x 300 s exposures in SDSS r-band. We see the above candidate source in our difference image as well, at r = 22.83 +/- 0.16 (AB).
We caution it is possible that this source is an artefact of the image subtraction due to presence of an underlying galaxy, however the source’s detection from both telescopes and clean residuals in the photometry lend credence to it being real. Given our two observations are at similar times, we cannot make a useful estimate of its temporal evolution.