EP261015a
GCN Circular 46016
Subject
EP261015a: VLA non-detection
Date
2026-10-23T19:57:45Z (10 hours ago)
From
Laura Cotter at University College Dublin <laura.cotter@ucdconnect.ie>
Via
Web form
L. Cotter (Cornell), D. Perley (LJMU) and A. Y. Q. Ho (Cornell) report on behalf of a larger collaboration,
We observed the field of EP261015a (Li et al., GCN 45956) with the Karl G. Jansky Very Large Array (VLA) in C configuration under program 25B-363 (PI: Daniel Perley) at a mid time of 2026 October 22 at 10:15 UT, approximately 6.87 days post trigger. The observations lasted 58 minutes at a mean frequency of 10 GHz.
In the preliminary analysis, we do not detect any radio sources at or near the position of the optical counterpart (Corcoran et al., GCN 45955) to a 3-sigma limit of < 17 microJy.
Taking the photometric redshift of 0.57 from the potential underlying host galaxy, this limit corresponds to a rest-frame luminosity of < 1.43e+29 erg/s/Hz. Comparing this to previously detected EP transients, this limit is ~ 2 orders of magnitude below the radio luminosities of GRB 240315C/EP240315A and EP241021A at comparable epochs (e.g. Choza et al. 2026). This limit also lies below the typical luminosities of radio-detected short GRBs at early rest-frame times (e.g., Anderson et al. 2025). However, the limit remains above the radio luminosities measured for some relativistic SNe and low-luminosity GRB-SNe like GRB980425/SN1998bw (e.g. Kulkarni et al 1998), and therefore does not rule out an origin similar to these events.
We thank the operators at the VLA for carrying out our observations.
GCN Circular 46009
Subject
EP261015a: GSP multiband optical follow-up observations
Date
2026-10-22T15:15:22Z (2 days ago)
Edited On
2026-10-22T16:29:24Z (2 days ago)
From
fufu Glow <fufuglow70@gmail.com>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Jinfu Gou at National Astronomial Observatories,CAS <fufuglow70@gmail.com>
Via
Web form
J. F. Gou, W. X. Li, S. J. Xue (NAOC); M. Andrews, D. A. Howell, C. McCully, J. Ridenbaugh and K. Wynn (Las Cumbres Observatory) report:
We observed the field of the Einstein Probe fast X-ray transient EP261015a (Li et al., GCNs 45956 and 45969) using MuSCAT3 on the Las Cumbres Observatory (LCO) 2-m Faulkes Telescope North at Haleakala, Hawaii. Observations were obtained simultaneously in g, r, i, and z_s from 2026 October 20, 14:06:51 to 14:29:01 UTC, with four 300-s exposures in each band, approximately 5.0 days after the EP/WXT event onset.
Following subtraction of same-band archival Legacy Survey DR10 templates with HOTPANTS, we detect residual emission at the position of the optical counterpart reported by Corcoran et al., GCNs 45955 and 45972; Fortin et al., GCN 45991; Pawar et al., GCNs 45998 and 46006; Shivaprasad et al., GCN 46000; Pankov et al., GCN 46003. Preliminary PSF photometry using AutoPhOT (Brennan & Fraser 2022), calibrated against nearby Pan-STARRS DR2 stars, gives:
Magnitude (AB)
g 23.51 ± 0.22
r 22.26 ± 0.06
i > 22.34 (5 sigma)
z > 22.14 (5 sigma)
These observations were taken as part of the Global Supernova Project (GSP).
GCN Circular 46006
Subject
EP261015a: Further optical observations with the 1.3m DFOT
Date
2026-10-21T13:36:45Z (3 days ago)
From
pawarpankaj1492@gmail.com
Via
Web form
Pankaj Pawar, Yash Sharma, Anshika Gupta, Debalina Kar, Dhruv Jain, Kuntal Misra, Amit K Ror and Shashi B. Pandey (ARIES, Nainital, India) report:
We observed the field of EP261015a (Li et al., GCN 45956) and its candidate optical counterpart (Corcoran et al., GCN 45955) with the 1.3m Devasthal Fast Optical Telescope (DFOT), India, on 2026 October 20. Observations started at 19:07:22 UT. We obtained six 300-s exposures in the R filter.
We reduced and stacked the images using standard procedures. After archival Legacy Survey template subtraction, we find a tentative positive residual at the counterpart position. Our preliminary PSF photometry, using an empirical PSF constructed with AutoPhOT (Brennan & Fraser 2022, A&A, 667, A62), is:
Mid-time (UT) Tmid-T0 (d) Filter Exposure (s) Mag (AB)
2026-10-20 19:22:34 5.25484 R 6 x 300 22.30 +/- 0.24
T0 is 2026-10-15 13:15:36 UT (Li et al., GCN 45956). Photometry was calibrated against nearby stars using standardised synthetic Johnson–Cousins R magnitudes derived from Gaia DR3 XP spectra. Magnitudes are reported on the AB system using the zero-point conversion of Bessell & Murphy (2012, PASP, 124, 140), and are not corrected for Galactic extinction.
The uncertainty includes an allowance for tested subtraction systematics; additional uncertainty from residual galaxy contamination and the different science/template passbands may remain. We regard the measurement as tentative.
Our observation follows the optical rebrightening reported by Fortin et al. (GCN 45991). Further observations are encouraged to establish the subsequent evolution of the source.
GCN Circular 46003
Subject
EP261015a: AZT-20 Assy-Turgen optical observations
Date
2026-10-21T06:17:24Z (3 days ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), M. Krugov (FAI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of the Einstein Probe fast X-ray transient EP261015a (Li et al., GCN 45956) with the 1.5-m AZT-20 telescope of the Assy-Turgen Observatory. Observations started on 2026-10-19 at 19:20:07 UT, 4.276 days after the EP/WXT trigger. A series of frames was obtained in the r' filter.
We clearly detect the optical counterpart (Corcoran et al., GCNs 45955 and 45972; Li et al., GCNs 45956 and 45969; Méndez et al., GCN 45960; Ma et al., GCN 45963; Mohan et al., GCN 45967; Fortin et al., GCN 45991; Volnova et al., GCN 45997; Pawar et al., GCN 45998; Shivaprasad et al., GCN 46000) at coordinates (J2000) 02:57:19.83, -13:21:17.3. The residual source is detected after subtraction of a DESI Legacy Survey DR10 r-band template. Preliminary photometry of the residual source is given below.
Date UTstart t-T0 Exp. Filter OT Err. UL
(mid, days) (s) (3sigma)
2026-10-19 19:20:07 4.2764 67*60 r' 22.59 0.06 23.8
The photometry was calibrated against nearby Pan-STARRS1 stars
RA (J2000) Dec (J2000) r
02:57:33.73 -13:21:44.6 16.891
02:57:25.43 -13:24:41.2 19.263
and is not corrected for Galactic extinction.
The residual magnitude is consistent with the Legacy Survey-subtracted measurements of Fortin et al. (GCN 45991; r = 22.58 +/- 0.09 at 3.7 d) and Shivaprasad et al. (GCN 46000; r = 22.52 +/- 0.28 at 4.37 d).
GCN Circular 46000
Subject
EP261015a: Wendelstein 43 cm optical observations
Date
2026-10-20T18:26:49Z (3 days ago)
From
surya.shivaprasad@physik.lmu.de
Via
Web form
Surya Shivaprasad (LMU), Malte Busmann (LMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), Julius Gassert (LMU), Xander J. Hall (CMU), and Antonella Palmese (CMU) report:
We observed the counterpart of EP261015a (Corcoran et al., GCNs 45955 and 45972; Li et al., GCNs 45956 and 45969; Moreno Méndez et al., GCN 45960; Ma et al., GCN 45963; Mohan et al., GCN 45967; Fortin et al., GCN 45991; Volnova et al., GCN 45997; Pawar et al., GCN 45998) with the 43 cm telescope at Wendelstein Observatory. Observations were taken in the r band, with 47 × 300 s exposures starting at 2026-10-19T22:15:03 UT (4.37 days after the trigger). Using a template from the DESI Legacy Surveys for difference imaging, we measure an r-band magnitude of
r = 22.52 ± 0.28 AB mag.
This measurement is consistent with the updated report by Fortin et al. (GCN 45991), which reports r = 22.58 ± 0.09 AB mag at ~3.7 d post-trigger, indicating that the source is evolving slowly at this phase.
Magnitudes are calibrated against the PS1 catalog and are not corrected for Galactic extinction.
GCN Circular 45998
Subject
EP261015a: Follow-up with the 1.3m DFOT
Date
2026-10-20T15:08:00Z (4 days ago)
Edited On
2026-10-20T15:22:03Z (4 days ago)
From
pawarpankaj1492@gmail.com
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of pawarpankaj1492@gmail.com
Via
Web form
Pankaj Pawar, Sarvesh Yadav, Anshika Gupta, Debalina Kar, Dhruv Jain, Yash Sharma, Kuntal Misra and Shashi B. Pandey (ARIES, Nainital, India) report:
We observed the field of the Einstein Probe transient EP261015a
(Li et al., GCN 45956) with the 1.3-m Devasthal Fast Optical Telescope
(DFOT), India, on 2026 October 17. Observations started at 18:29:19 UT,
approximately 53.23 hours after the event onset. We obtained six 300-s
exposures in the R filter.
We detect a source at the position of the candidate optical counterpart
reported by Corcoran et al. (GCN 45955). Our preliminary PSF photometry
of the stacked image, obtained using AutoPhOT (Brennan & Fraser 2022,
A&A, 667, A62), is:
Mid-time (UT) Tmid-T0 (d) Filter Exposure (s) Mag (Vega)
2026-10-17 18:44:31 2.22842 R 6 x 300 22.18 +/- 0.19
T0 is 2026-10-15 13:15:36 UT, the start of the EP-WXT event
(Li et al., GCN 45956). Photometry was calibrated against 26 nearby
Gaia DR3 XP stars using synthetic Johnson-Cousins R magnitudes on the
Vega system. The magnitude is not corrected for Galactic extinction.
Our photometry was performed on unsubtracted images and is contaminated
by the underlying galaxy noted by Ma et al. (GCN 45963). The reported
measurement includes galaxy light and is not a galaxy-subtracted
transient magnitude.
Our observation predates the optical rebrightening reported by Fortin
et al. (GCN 45991). The galaxy contribution and different passbands prevent
a direct comparison with their template-subtracted r-band photometry.
GCN Circular 45997
Subject
EP261015a: Terskol Zeiss-2000 optical upper limit
Date
2026-10-20T15:07:34Z (4 days ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), D. Andreev (INASAN), A. Pozanenko (IKI), A. Moskvitin (SAO), N. Pankov (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of Einstein Probe fast X-ray transient EP261015a (Li et al., GCN 45956) with the Zeiss-2000 2-meter telescope of the Terskol Observatory on Oct. 17, 2026, starting at 21:30 UT and taking several exposures in R-band.
The optical counterpart reported previously (Corcoran et al., GCNs 45955, 45972; Fortin et al., GCN 45991) is not detected in the stacked frame.
Preliminary photometry and observational details are the following:
Date UT start t-T0 Exp, Filter Obj Err UL Site/Telescope
(mid,days) (n*s) (3sigma)
2026-10-17 21:30:24 2.32810 20*180 R n/d n/d 22.8 Terskol/Z2000
The photometry is based on several nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction.
GCN Circular 45991
Subject
EP261015a: COLIBRÍ optical observations of a rebrightening
Date
2026-10-19T15:08:36Z (5 days ago)
Edited On
2026-10-20T15:21:20Z (4 days ago)
From
F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Via
Web form
Francis Fortin (IRAP), Leonardo García-García (UNAM), Noémie Globus (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (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), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), 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 EP261015a (Li et al, GCN Circ. 45956) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-10-19T05:20:43 to 08:18:14 UTC (from 88.09 to 91.06 hours after the trigger) and obtained 2.17 hours of exposure in the r 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.
After subtracting template Legacy Survey image, we detect a rebrightening of the optical counterpart reported by Corcoran et al., GCN Circ. 45955, GCN Circ. 45972, at a preliminary magnitude of:
r = 22.58 +/- 0.09
Further observations are ongoing. We recommend following-up EP261015a in case this is the rising of a supernova associated to the event.
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 45972
Subject
EP261015a: Tentative fading of the candidate optical counterpart from the NOT
Date
2026-10-17T16:03:10Z (7 days 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), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud) A. L. Bouquin (NOT), A. Caballero Almagro (NOT), E. Georgiou (NOT and Turku Univ.) report on behalf of a larger collaboration
We observed the field of the Einstein Probe fast X-ray transient EP261015a (Li et al., GCN 45956) using the ALFOSC camera on the Nordic Optical Telescope (NOT). Observations consisted of a sequence of 5 x 300 s exposures in SDSS-r band and started on 2026-10-17 at 02:12:26 UT (1.54 days after event start). Due to the telescope pointing and dither pattern, there are some artifacts caused by a bright star to the north of the target that affect part of our images.
After dropping two images affected by the bright star and performing image subtraction against a Dark Energy Survey stacked image, we clearly detect the candidate discussed by Corcoran et al. (GCN 45955), Mendez et al (GCN 45960), and Ma et al. (GCN 45963). We measure a difference magnitude of r = 23.18 +/- 0.10 (AB) calibrated against nearby stars in the Pan-STARRS catalog and not corrected for Galactic extinction at a midtime of 2026-10-17 02:20:22 UT (1.54 days after event start).
We also refined our photometry on the previous epoch presented in GCN 45955 to r = 22.64 +/- 0.12. This shows fading between epochs with a decay of t^{-0.51+/-0.15}. We note that the uncertainties on our photometry neglect the difficult to estimate systematic uncertainties inherent to image subtraction between instruments, therefore the statistical significance of the fading cannot be reliably determined.
The nature of this object is still not fully clear. An X-ray source has been reported by EP/FXT within the EP/WXT error circle (Li et al., GCN 45969), positionally inconsistent with the above candidate. This source is however the likely counterpart of a spectroscopically confirmed QSO (https://www.legacysurvey.org/viewer/desi-spectrum/dr1/targetid39627462111402716), thus likely unrelated to the EP/WXT event (no significant variability is detected in the QSO flux between our two epochs). The optical fading would then favor an association of the candidate with EP261015a, though an unrelated transient (e.g. a SN) cannot be fully discarded.
Further monitoring is planned and encouraged to clarify the nature of this object.
GCN Circular 45969
Subject
EP261015a: refined analysis of EP-FXT observations
Date
2026-10-17T05:09:17Z (7 days 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 (7 days 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 (8 days 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 (8 days 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 (8 days 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 (8 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
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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.