EP251222a
GCN Circular 43235
Subject
EP251222a: Optical upper limit with Kinder observations
Date
2025-12-25T09:50:48Z (2 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
M.-H. Lee, A. Aryan, T.-W. Chen, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP251222a (Huang et al., GCN#43200; Guo et al., GCN#43216) using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations started at 11:44 UTC on the 23rd of December 2025 (MJD 61032.489), 20.43 hr after the EP-WXT trigger.
We utilized the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames. Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al., 2019, AJ 157, 168) DR10 image using the 'sfft' (Hu et al., 2022, ApJ, 936, 157) algorithm. In the difference image, we did not detect the optical counterpart candidate (Li et al., GCN#43205; Malesani et al., GCN#43211; Busmann et al., GCN#43232) proposed at a redshift of z = 0.505 (An et al., GCN#43227).
Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | i | 61032.489 | 20.43 | 300 * 24 | >21.4 | 2".8 | 1.16
The non-detection in our observations can be attributed to the relatively shallow limit and late follow-up. Our non-detection is consistent with Kumar et al. (GCN#43210) and Lipunov et al. (GCN#43219).
The presented upper limit is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al., 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_i = 0.23 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al., 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 43232
Subject
EP251222a: FTW optical and NIR observations
Date
2025-12-24T14:24:05Z (3 days ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
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Malte Busmann (LMU), Xander J. Hall (CMU), Mitra Maleki (LMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the counterpart of EP251222a (Huang et al. GCN 43200, Li et al. GCN 43205, Guo et al. GCN 43216) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J-band simultaneously for 10 x 180 s starting at 2025-12-23T21:27:09 UT (1.26 days after the trigger). We detect the counterpart in all bands. Using a template from the DESI Legacy Surveys, we measure an r-band magnitude of
r = 21.64 +/- 0.06 AB mag.
Our measurements are consistent with the reports by Li et al. (GCN 43205), Kumar et al. (GCN 43210), Malesani et al. (GCN 43211), and Lipunov et al. (GCN 43219).
The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Michael Schmidt from the Wendelstein Observatory for obtaining these observations.
GCN Circular 43227
Subject
EP251222a: VLT/FORS2 spectroscopic redshift z = 0.505
Date
2025-12-24T09:47:04Z (3 days ago)
From
Laura Cotter at University College Dublin <laura.cotter@ucdconnect.ie>
Via
Web form
J. An (NAOC), L. Izzo (INAF/OACn & DARK/NBI), A. Saccardi (CEA/Irfu), L. Cotter (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), A. L. Thakur (INAF-IAPS), D. Xu (NAOC), report on behalf of the Stargate collaboration:
We observed the optical counterpart (Li et al., GCN 43205; Malesani et al., GCN 43211) of EP251222a (Huang et al., GCN 43200; Guo et al., GCN 43216) with the ESO VLT UT1 (Antu) equipped with the FORS2 spectrograph. Our observations began on 2025 December 24 at 02:21:56 UT, 1.46 days post trigger.
A spectrum covering the wavelength range 3600-8600 AA was secured using grism 300V (with no order-sorting filter), with a total exposure time of 4x600 s.
In a preliminary reduction of the spectra, we detect a continuum over the entire wavelength range. From the detection of multiple absorption lines, which we interpret as the Mg II doublet (2796, 2804) and Fe II (2587, 2600), we infer a redshift of z = 0.505. At a consistent redshift, we also detect emission lines due to the [O II] 3726,3729 doublet and [O III] 5007 from the host galaxy.
We acknowledge expert support from the observing staff in Paranal, in particular Elyar Sedaghati.
GCN Circular 43219
Subject
EP251222a: Global MASTER-Net observations report
Date
2025-12-23T15:36:29Z (4 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Tunka robotic telescope [1] located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP251222a ( EP Team et al., GCN 43200) errorbox 25213 sec after notice time and 69548 sec after trigger time at 2025-12-23 10:37:14 UT, with upper limit up to 16.4 mag. Observations started at twilight. The observations began at zenith distance = 78 deg. The sun altitude is -13.8 deg.
MASTER-Kislovodsk robotic telescope located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP251222a errorbox 39747 sec after notice time and 84082 sec after trigger time at 2025-12-23 14:39:28 UT, with upper limit up to 16.9 mag. Observations started at twilight. The observations began at zenith distance = 75 deg. The sun altitude is -10.5 deg.
The galactic latitude b = -35 deg., longitude l = 194 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3081644
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
66002 | 2025-12-23 09:36:37 | MASTER- | (04h 15m 01.67s , -00d 43m 08.5s) | C | 180 | 16.3 |
69578 | 2025-12-23 10:37:14 | MASTER-Tunka | (04h 09m 44.57s , -01d 55m 17.1s) | C | 60 | 16.4 |
69578 | 2025-12-23 10:37:14 | MASTER-Tunka | (04h 09m 58.83s , -02d 01m 41.0s) | C | 60 | 16.1 |
69649 | 2025-12-23 10:38:24 | MASTER-Tunka | (04h 09m 58.86s , -02d 01m 41.4s) | C | 60 | 15.7 |
69649 | 2025-12-23 10:38:25 | MASTER-Tunka | (04h 09m 44.41s , -01d 55m 16.5s) | C | 60 | 16.1 |
84112 | 2025-12-23 14:39:28 | MASTER-Kislovodsk | (04h 07m 57.86s , -01d 42m 13.8s) | C | 60 | 16.9 |
84112 | 2025-12-23 14:39:28 | MASTER-Kislovodsk | (04h 07m 58.58s , -01d 19m 40.7s) | C | 60 | 15.8 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 43216
Subject
EP251222a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-12-23T14:23:50Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
C.-L. Guo (NAO, CAS), R. Shi, Q.-J. Huang (PMO,CAS), X. Mao, and Y. Liu (NAO,
CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP251222a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 43200), and followed by several telescopes (Li et al., GCN 43205, Kumar et al., GCN 43210, Malesani et al., GCN 43211).
The refined analysis of the WXT data shows that this observation of the event started at T0=2025-12-22T14:59:14 (UTC). During the observation period of WXT (1100s), there was no significant short-timescale variability in the light curve. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.23 x 10^20 cm^-2 and a photon index of 1.58 (-0.4/+0.4). The derived average unabsorbed 0.5-4 keV flux is 6.35 (-4.9/+8.2) x 10^(-11) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2025-12-22T15:21:49 (UTC), about 22 minutes after T0. The exposure time of this observation is 4081s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 62.5014, DEC = -1.4795 (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 7.23 x 10^20 cm^-2 and a photon index of 2.34 (-0.05/+0.05). The derived average unabsorbed 0.5-10 keV flux is 7.54 (-5.2/+16.7) x 10^(-12) 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 43211
Subject
EP251222a: NTT/ULTRACAM optical observations
Date
2025-12-23T08:11:19Z (4 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), M. Stepney (CATA), D. Sahman (Sheffield), L. Cotter (UCD), G. Corcoran (UCD), P. G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP251222a (Huang et al., GCN 43200) using the ULTRACAM instrument on the ESO NTT telescope in La Silla (Chile). Observations consisted of 120 exposures by 15 s each, taken at a mean epoch 2025 December 23, 03:07:14 UT (11.81 hr after the EP trigger).
The optical counterpart identified by Li et al. (GCN 43205) is well detected in our coadded image, for which we measure a magnitude r = 21.56 +- 0.08 (AB), calibrated against nearby stars from the Pan-STARRS catalog and not corrected for Galactic extinction. This confirms the rebrightening reported by Li et al. (GCN 43205). The consistency with the EP/FXT X-ray position (Huang et al., GCN 43200) makes this source the likely counterpart of EP251222a.
Further follow-up is encouraged.
GCN Circular 43210
Subject
EP251222a: GOTO optical upper limit
Date
2025-12-23T08:03:47Z (4 days ago)
From
Amit Kumar at The Open University, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar, G. Ramsay, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, S. Belkin, D. O'Neill, B. Godson, 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 EP251222a (Huang et al., GCN 43200). Two sets of targeted observations were performed by GOTO-North at 2025-12-22 22:19:38 UT and 2025-12-23 01:14:12 UT, corresponding to 7.0 and 9.9 hours post-trigger, respectively. Each observation consisted of 4x90 s 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 new optical source within the EP/WXT localisation uncertainty region after the trigger time, down to a 5-sigma limiting magnitude for the two sets of observations of L >20.4 (AB) and L >20.5 (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, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular 43205
Subject
EP251222a: GSP Optical Counterpart Detection
Date
2025-12-23T05:56:12Z (4 days ago)
From
Wenxiong Li at NAOC <liwenxiong1992@gmail.com>
Via
Web form
W. X. Li, S. J. Xue (NAOC); M. Andrews, J. Farah, D. A. Howell, C. McCully, and Kathryn Wynn (Las Cumbres Observatory), on behalf of a larger collaboration, report optical follow-up observations of EP251222a (Huang et al. GCN 43200) obtained with a Las Cumbres Observatory (LCO) 1-m telescope at at the Cerro Tololo Inter-American Observatory in Chile, ~12.5 hours after the EP-WXT trigger.
We acquired 2 × 300 s exposures using the broad optical w band. Within the EP-FXT error region, we found that the source at R.A. = 62.5025, Dec. = –1.4792 brightened, increasing in brightness from r = 22.9 mag (DESI Legacy Survey) to r = 21.6 mag, with no other optical counterpart detected.
These observations were taken as part of the Global Supernova Project (GSP).
GCN Circular 43200
Subject
EP251222a: Einstein Probe detection of an X-ray transient
Date
2025-12-23T03:34:27Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q. J. Huang, R. Shi (PMO, CAS), C. L. Guo (NAO, CAS), X. Mao (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 EP251222a. The transient triggered EP-WXT (ID: 01709250453) at 2025-12-22 15:18:06 (UTC). The WXT position of the source is R.A. = 62.519 deg, DEC = -1.506 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. = 62.5014 deg, DEC = -1.4795 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 are 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).