EP250812a
GCN Circular 41349
Subject
EP250812a: Nickel optical upper limit
Date
2025-08-13T18:18:02Z (a year ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
Andreas Betz, Riley Patlak, WeiKang Zheng and Alex Filippenko
(UC Berkeley) report on behalf of the KAIT GRB team:
We observed the field of EP250812a (Zhang et al., GCN 41327)
with the 1-m Nickel telescope located at Lick observatory,
California. Observations were performed in the R band with
600s x 5 exposures. In our coadd image, we do not detect the
optical counterpart (Xin et al., GCN 41340; Ducoin et al.,
GCN 41342) with an upper limit of R > 20.7 (Vega) at a
mid-time of 23.8 hours after the trigger.
GCN Circular 41345
Subject
EP250812a: NOT, NTT, and GS observations
Date
2025-08-13T15:23:05Z (a year ago)
From
Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>
Via
Web form
A. P. C. van Hoof (Radboud), J. A. Quirola-Vasquez (Radboud), J. N. D. van Dalen (Radboud), D. B. Malesani (DAWN/NBI and Radboud), L. Cotter (UCD), M. Fraser (UCD), G. Corcoran (UCD), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. J. Levan (Radboud and Warwick), F. E. Bauer (SSI and UTA), report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP250812a (Zhang et al., GCN 41327, GCN 41339) using the Nordic Optical Telescope (NOT, equipped with the ALFOSC camera), the New Technology Telescope (NTT, with the EFOSC2 instrument), and the Gemini South Telescope (GS, with the FLAMINGOS2, F2, instrument).
Observations with NTT were also carried out in the r filter (3x600 s) starting on 2025-08-13 05:22:07.8 UT, i.e., ~16.2 hr after the trigger. Moreover, NOT and NTT observations were carried out in the z filters (15x80 s and 6x300 s) starting on 2025-08-13 05:31:57.8 and 2025-08-13 05:40:17.2 UT, i.e, ~16.4 and 16.5 hr after the X-ray trigger, respectively. The GS-F2 visited the field using the H filter (20x10 s) starting on 2025-08-13 07:35:11.0 UT, i.e.,~18.4 hr after the trigger.
At the location of the transient reported by Zhang et al. (GCN 41327) and the optical counterpart from SVOM/VT reported by Xin et al. (GCN 41340) and from COLIBRI (Ducoin et al., GCN 41342), also consistent with the EP/FXT (Zhang et al., GCN 41339) and Swift/XRT positions (Evans et al., GCN 41330), no source is detected, down to 3-sigma limiting magnitudes:
H > 22.0 (GS)
z > 22.5 (NOT)
z > 22.3 (NTT)
r > 22.6 (NTT)
These upper limits are in AB magnitudes and were calibrated using nearby stars from the Pan-STARRS and 2MASS catalogues and not corrected by galactic extinction.
We thank the excellent support from the observing staff at Gemini (especially Cinthya Rodrigez), NOT (in particular Emil Knudstrup), and NTT (in particular Pablo Arias).
GCN Circular 41342
Subject
EP250812a: COLIBRÍ detection of the VT candidate and evidence of flattening
Date
2025-08-13T13:44:03Z (a year ago)
Edited On
2025-08-13T15:02:33Z (a year ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Jean-Grégoire Ducoin (CPPM), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (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), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of the EP250812a (Zhang et al., GCN Circ. 41327, 41339) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-13 08:00
to 10:21 UTC (from 18.8 to 21.2 hours after the trigger) and obtained 96 minutes of exposure in the i 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.
In the stacked image, we marginally detect the VT candidate (Xin et al., GCN Circ. 41330), which is also consistent with the XRT Source 1 position (Evans et al., GCN Circ. 41340), at:
i = 22.5 +/- 0.3
This value is similar to the second observation reported by VT (Xin et al., GCN Circ. 41330) at T + 14.34 h, but brighter than in the Legacy Survey DR10 catalog (Dey et al. 2019):
i = 23.5 +/- 0.1
Therefore, we suggest that the candidate is indeed the afterglow of EP250812a and that it showed evidence of a flattening in its evolution at the time of our observations.
Further observations and analysis are ongoing.
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.
GCN Circular 41341
Subject
EP250812a: Liverpool Telescope upper limits
Date
2025-08-13T10:28:50Z (a year ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek and D. A. Perley (LJMU) report:
We observed the field of the fast X-ray transient EP250812a (Zhang et al., GCN 41327) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 8x100s exposures in the SDSS r’ and z’ filters starting at 2025-08-13 03:13:06 UT, approximately 14 hours after the trigger.
We do not detect any new objects within the EP/FXT error region of the transient (Zhang et al., GCN 41327; Zhang et al., GCN 41339). The 3-sigma limiting magnitudes on the stacked images are r > 21.71 mag and z > 21.53 mag. This is in agreement with the detection of an optical counterpart candidate by SVOM/VT at VT_R = 22.74 ± 0.15 at 14.3 hours (Xin et al., GCN 41340).
GCN Circular 41340
Subject
EP250812a: SVOM/VT optical counterpart
Date
2025-08-13T09:40:18Z (a year ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, H. L. Li, Y. N. Ma, Y. L. Qiu, C. Wu, Z. H. Yao, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM performed two Target of Opportunity observations of EP250812a detected by EP/WXT (Zhang et al., GCN 41327). The first epoch is from 2025-08-12T20:32:53 UTC, 7.38 hours after the EP trigger time. The second epoch is from 2025-08-13T01:47:59, 12.64 hours post the trigger.
No any uncatalogued sources, compared to Lagecy survey, was found using VT X-band data, within the errorbox of EP/FXT (Zhang et al., GCN 41327, GCN 41339) or Swift/XRT (Evans et al., GCN 41330).
However, there is a cataloged source in Legacy survey within the errorbox of Swift/XRT source1 with a distance of 3.4 arcseconds at R.A.=30.993037 Dec=-9.318883 degrees:
RA (J2000) = 02:03:58.329
Dec (J2000) = -09:19:07.98
with an uncertainty of 0.5 arcsec.
The cataloged source was detected in both VT_B and VT_R images. The magnitudes are derived as below:
DeltaT(Mid time) | exposure time (s) | band | mag (AB) | mag err
---------------------- |-------------------|------|----------|--------
7.62 hours | 26*70 | VT_B | 22.52 | 0.15
7.62 hours | 26*70 | VT_R | 22.04 | 0.12
14.34 hours | 75*70 | VT_B | 23.10 | 0.16
14.34 hours | 67*70 | VT_R | 22.74 | 0.15
Our photometry is not corrected for Galactic extinction.
Given the fading of the source in our observations, we proposed that it is the counterpart of the transient.
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 41339
Subject
EP250812a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-08-13T08:57:14Z (a year ago)
Edited On
2025-08-13T15:03:00Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
M.H. Zhang (NAO, CAS), H. Zhou (PMO, CAS), W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250812a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 41327). Refined analysis of the WXT data shows that the event started at T0=2025-08-12 13:09:03 (UTC) and lasted for about 37 s before the interruption of the autonomous follow-up. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.5 x 10^20 cm^-2 and a photon index of 1.0 (-0.3/+0.3). The derived average unabsorbed 0.5-4 keV flux is 6.7(-0.8/+0.8) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 186 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 30.9931, DEC = -9.3191 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the WXT transient. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.5 x 10^20 cm^-2 and a photon index of 2.21 (-0.18/+0.20). The absorption from the host galaxy cannot be constrained. The derived average unabsorbed 0.5-10 keV flux is 1.97 (-0.13/+0.21) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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 41332
Subject
EP 250812a: GOTO optical upper limit
Date
2025-08-12T17:59:49Z (a year ago)
From
kendall.ackley@warwick.ac.uk
Via
Web form
K. Ackley, A. Kumar, D. O'Neill, G. Ramsay, B.Godson, B. P. Gompertz, R. Starling, M. J. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al. 2024) in response to EP250812a (Zhang et al. GCN 41327).
Three targeted observations were performed by GOTO-South beginning at 14:58:30UT and continuing up to 17:23:59UT on 2025-08-12, corresponding to 1.84 hours, 3.03 hours, and 4.18 hours after trigger. Each observation consisted of 4x90 s exposures in the GOTO L-band (400-700 nm).
We serendipitously observed the field by GOTO-South at 9.5 hours prior to the trigger time at 03:39:10UT on 2025-08-12, where each observation consisted of 4x45s exposures in the GOTO L-band (400-700 nm).
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues.
We do not identify any candidate optical counterparts at the location of the Swift-XRT afterglow candidate (Evans et al. GCN 41330) or within the EP/WXT localisation uncertainty region after the trigger time, down to a 3-sigma limiting magnitude for the three sets of observations of L >18.9 (AB), L >19.4 (AB), L >19.8 (AB). The observation at 9.5 hours prior to the trigger shows no source to a 3-sigma limit of L >18.1 mag (AB).
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.
GOTO (https://goto-observatory.org/) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular 41330
Subject
EP250812a: Swift-XRT counterpart detection
Date
2025-08-12T17:22:44Z (a year ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250812a, collecting 1.7 ks of Photon Counting (PC) mode data between T0+1.5
ks and T0+3.4 ks after the trigger. A likely counterpart has been found. The
position is consistent with the uncatalogued X-ray source detected by EP-FXT (Zhang
et al., GCN 41327). The details of this source are:
Source 1 (SWIFT J020358.1-091907):
==================================
RA (J2000.0): 30.9921 = 02h 03m 58.10s
Dec (J2000.0): -9.3187 = -09d 19' 07.3"
Error: 3.7 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 67 arcsec from the Einstein Probe/WXT position.
Mean rate: 0.1069 +/- 0.0091 ct s^-1
Mean flux: (4.00 +/- 0.34)e-12 erg cm^-2 s^-1
Peak rate: 0.203 +/- 0.046 ct s^-1
Peak flux: (7.6 +/- 1.7)e-12 erg cm^-2 s^-1
ECF: 3.75e-11 erg cm^-2 ct^-1
assuming NH=4.71e+20 cm^-2, gamma=1.83
determined from a spectral fit.
LSXPS UL: 1.5e-03 ct/sec, (0.3-10 keV)
so the source is 4.4-sigma above this 3-sigma upper limit.
The source may be fading, at the 2.6-sigma level.
We have detected a total of 3 sources. These have been automatically classified as
follows:
* 1 likely counterpart
* 0 candidate counterparts
* 0 uncatalogued X-ray sources
* 2 known X-ray sources
Known X-ray sources
-------------------
Source 2 (SWIFT J020340.7-092134):
==================================
RA (J2000.0): 30.9196 = 02h 03m 40.70s
Dec (J2000.0): -9.3596 = -09d 21' 34.6"
Error: 5.8 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 4.8 arcmin from the Einstein Probe/WXT position.
Mean rate: (8.5 [+3.0, -2.5])e-3 ct s^-1
Mean flux: (2.42 [+0.87, -0.71])e-13 erg cm^-2 s^-1
Peak rate: (8.5 [+3.0, -2.5])e-3 ct s^-1
Peak flux: (2.42 [+0.87, -0.71])e-13 erg cm^-2 s^-1
ECF: 2.86e-11 erg cm^-2 ct^-1
assuming NH=4.16e+20 cm^-2, gamma=2.23
determined from a spectral fit.
This matches a catalogued X-ray source LSXPS J020340.7-092134
in the LSXPS catalogue. Details:
Separation: 1.0" from the XRT source
Cat Rate: 4.6e-03 +/- 8.6e-04 ct s^-1
Cat Flux: 1.3e-13 +/- 2.5e-14 erg cm^-2 s^-1 (0.3-10 keV)
so the source is 1.3-sigma above the catalogued flux.
There is no evidence for fading.
A SIMBAD object `[VV2006] J020341.0-092134' is 3.4" away.
Source 4 (SWIFT J020404.7-091504):
==================================
RA (J2000.0): 31.0199 = 02h 04m 04.78s
Dec (J2000.0): -9.2513 = -09d 15' 04.7"
Error: 5.8 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 4.0 arcmin from the Einstein Probe/WXT position.
Mean rate: (6.6 [+2.8, -2.2])e-3 ct s^-1
Mean flux: (2.14 [+0.92, -0.72])e-13 erg cm^-2 s^-1
Peak rate: (6.6 [+2.8, -2.2])e-3 ct s^-1
Peak flux: (2.14 [+0.92, -0.72])e-13 erg cm^-2 s^-1
ECF: 3.25e-11 erg cm^-2 ct^-1
assuming NH=2.48e+20 cm^-2, gamma=1.94
determined from a spectral fit.
This matches a catalogued X-ray source LSXPS J020405.1-091509
in the LSXPS catalogue. Details:
Separation: 7.3" from the XRT source
Cat Rate: 5.6e-03 +/- 1.5e-03 ct s^-1
Cat Flux: 1.8e-13 +/- 4.9e-14 erg cm^-2 s^-1 (0.3-10 keV)
so the source is 0.3-sigma above the catalogued flux.
There is no evidence for fading.
All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP/EP_FIELD00058.
This circular is an official product of the Swift-XRT team.
GCN Circular 41327
Subject
EP250812a: Einstein Probe detection of an X-ray transient
Date
2025-08-12T13:59:12Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
M.-H. Zhang (NAO,CAS), H. Zhou (PMO,CAS) and W.-D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250812a. The transient triggered EP-WXT (ID: 01709198788) at 2025-08-12T13:09:44 (UTC). The WXT position of the source is R.A. = 30.979 deg, DEC = -9.305 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 30.9964 deg, DEC = -9.3206 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
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).