EP251229a
GCN Circular 43278
Subject
EP251229a: Optical upper limit with Kinder observations
Date
2025-12-31T10:27:16Z (2 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
A. Sankar.K, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), Y.-H. Lee, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, W.-J. Hou, H.-Y. Hsiao, 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 EP251229a (Wang et al., GCN#43253) 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 20:18 UTC on the 29th of December 2025 (MJD 61038.846), 0.32 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. We did not detect any new and uncataloged source within the EP-FXT localization circle with a radius of 20", either in the stacked or difference image.
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 | r | 61038.846 | 0.32 | 300 * 12 | >22.4 | 2".01 | 1.35
Our non-detection is deeper than Lipunov et al. (GCN#43254), consistent with Martin-Carrillo et al. (GCN#43266) and Perez-Garcia et al. (GCN#43268). The absence of any associated optical counterpart down to 22.4 mag with the optical observations starting at 0.32 hours of its X-ray discovery aligns well with the 'Dark' nature of FXTs (Aryan et al., 2025, ApJS, 281, 20).
The presented upper limit is calibrated using the field stars from the PanSTARRs DR2 and is not corrected for the expected Galactic foreground extinction of A_r = 0.14 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 43268
Subject
EP251229a: BOOTES-2 early optical upper limit
Date
2025-12-30T12:07:44Z (2 months ago)
From
I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>
Via
Web form
I. Perez-Garcia, M.D. Caballero-Garcia, E. J. Fernandez-Garcia, R. Sanchez-Ramirez, S.-Y. Wu, S. Guziy, and A. J. Castro-Tirado (IAA-CSIC, Granada), Y.-D. Hu (GXU), C. Perez del Pulgar and A. Reina (Univ. of Malaga), M. Gritsevich (Univ. of Helsinki), R. Fernandez-Munoz (IHSM/UMA-CSIC), D. R. Xiong (Yunnan Observatories of CAS), and M. Jelinek (ASU-CAS, Ondrejov Obs.), on behalf of a larger collaboration, report:
Following the detection of the X-ray transient EP251229a by Einstein Probe (Wang et al. GCN 43253), the 0.6m BOOTES-2/TELMA robotic telescope at IHSM UMA “La Mayora” (Malaga, Spain) automatically responded to this high-energy event starting on Dec 29, 20:11:10 UT (i.e. 12 min after detection). On a coadd image (14 x 60sec) at 20:50:40 UT (mid exposure time; i.e. 51 min after detection), nothing is detected down to 19.3 mag (clear filter) within the FXT error region (Wang et al. GCN 43264), in agreement with the deeper observations taken later on (Martin-Carrillo et al. GCN 43266).
We would like to thank the staff at IHSM-/UMA-CSIC “La Mayora” for their excellent support.
GCN Circular 43266
Subject
EP251229a: NOT optical upper limits
Date
2025-12-30T10:31:06Z (2 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
A. Martin-Carrillo (UCD), J. Chacón (PUC), Z.P. Zhu, D. Xu (NAOC), G. Corcoran (UCD), F. E. Bauer (SSI and UTA), P.G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP251229a (Wang et al., GCN 43253) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were obtained in the SDSS i and z bands consisting of 5x200 s exposures, starting on 2025-12-29 at 23:36:08 UTC (3.622 hours after the WXT trigger).
No new source is detected within or at the border of the EP/FXT uncertainty region (Wang et al., GCN 43253). From the stacked images we obtain the following 3-sigma upper limits:
i > 24.0
z > 23.1
These upper limits are in AB magnitudes and were calibrated using nearby stars from the Pan-STARRS catalog and are not corrected for Galactic extinction.
GCN Circular 43264
Subject
EP251229a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-12-30T09:05:29Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B.T. Wang (YNAO, CAS), J.P. Chen (SYSU), D. Zhu, K.J. Zhang (YNU), and Z.X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP251229a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Wang et al., GCN 43253) and followed by an optical telescope (Lipunov et al., GCN 43254). The refined analysis of the WXT data shows that the event started at T0=2025-12-29T19:54:09 (UTC) and lasted for 173 s with a single pulse. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 5.3 (-2.2/+2.6) x 10^21 cm^-2 and a photon index of 2.3 (-0.68/+0.77). The derived average unabsorbed 0.5-4 keV flux is 1.62 (-0.41/+0.87) x 10^(-9) erg/s/cm^2.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2025-12-29T20:01:28 (UTC), about 7 minutes after T0. The exposure time of this observation is 855 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 124.1027, 37.2068 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 6.0 x 10^20 cm^-2 and a photon index of 1.89 (-0.13/+0.13). The derived average unabsorbed 0.5-10 keV flux is 3.46 (-0.37/+0.40) x 10^(-11) 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 43254
Subject
EP251229a: Global MASTER-Net observations report
Date
2025-12-30T01:54:26Z (2 months 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 EP251229a ( EP Team et al., GCN 43253) errorbox 635 sec after trigger time at 2025-12-29 20:09:24 UT, with upper limit up to 15.9 mag. The observations began at zenith distance = 19 deg. The sun altitude is -45.6 deg.
The galactic latitude b = 33 deg., longitude l = 185 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3090301
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
665 | 2025-12-29 20:09:24 | MASTER-Tunka | (08h 15m 31.75s , +37d 38m 46.9s) | P- | 60 | 15.1 |
665 | 2025-12-29 20:09:24 | MASTER-Tunka | (08h 14m 38.46s , +37d 47m 55.5s) | P| | 60 | 14.4 |
729 | 2025-12-29 20:10:27 | MASTER-Tunka | (08h 15m 31.53s , +37d 38m 43.3s) | P- | 60 | 15.7 |
729 | 2025-12-29 20:10:27 | MASTER-Tunka | (08h 14m 38.47s , +37d 47m 47.0s) | P| | 60 | 13.9 |
792 | 2025-12-29 20:11:31 | MASTER-Tunka | (08h 15m 31.64s , +37d 38m 42.0s) | P- | 60 | 15.7 |
792 | 2025-12-29 20:11:31 | MASTER-Tunka | (08h 14m 38.68s , +37d 47m 50.3s) | P| | 60 | 14.4 |
856 | 2025-12-29 20:12:34 | MASTER-Tunka | (08h 15m 31.62s , +37d 38m 39.6s) | P- | 60 | 15.7 |
856 | 2025-12-29 20:12:34 | MASTER-Tunka | (08h 14m 42.54s , +37d 48m 11.2s) | P| | 60 | 11.9 |
919 | 2025-12-29 20:13:38 | MASTER-Tunka | (08h 14m 38.71s , +37d 47m 49.4s) | P| | 60 | 14.4 |
919 | 2025-12-29 20:13:38 | MASTER-Tunka | (08h 15m 31.64s , +37d 38m 37.7s) | P- | 60 | 15.7 |
983 | 2025-12-29 20:14:41 | MASTER-Tunka | (08h 15m 31.63s , +37d 38m 36.0s) | P- | 60 | 15.7 |
983 | 2025-12-29 20:14:41 | MASTER-Tunka | (08h 14m 38.72s , +37d 47m 38.4s) | P| | 60 | 14.0 |
1050 | 2025-12-29 20:15:49 | MASTER-Tunka | (08h 15m 31.60s , +37d 38m 34.3s) | P- | 60 | 15.7 |
1050 | 2025-12-29 20:15:49 | MASTER-Tunka | (08h 14m 38.88s , +37d 47m 44.8s) | P| | 60 | 14.4 |
1114 | 2025-12-29 20:16:52 | MASTER-Tunka | (08h 15m 31.84s , +37d 38m 36.2s) | P- | 60 | 15.0 |
1114 | 2025-12-29 20:16:52 | MASTER-Tunka | (08h 14m 38.84s , +37d 47m 29.7s) | P| | 60 | 14.0 |
1177 | 2025-12-29 20:17:56 | MASTER-Tunka | (08h 15m 31.72s , +37d 38m 31.1s) | P- | 60 | 15.9 |
1177 | 2025-12-29 20:17:56 | MASTER-Tunka | (08h 14m 38.85s , +37d 47m 27.3s) | P| | 60 | 14.0 |
1240 | 2025-12-29 20:18:59 | MASTER-Tunka | (08h 14m 39.62s , +37d 47m 47.2s) | P| | 60 | 14.3 |
1240 | 2025-12-29 20:18:59 | MASTER-Tunka | (08h 15m 32.02s , +37d 38m 31.4s) | P- | 60 | 15.2 |
1304 | 2025-12-29 20:20:02 | MASTER-Tunka | (08h 15m 31.81s , +37d 38m 27.8s) | P- | 60 | 15.9 |
1367 | 2025-12-29 20:21:06 | MASTER-Tunka | (08h 15m 32.10s , +37d 38m 28.0s) | P- | 60 | 15.4 |
1367 | 2025-12-29 20:21:06 | MASTER-Tunka | (08h 14m 39.08s , +37d 47m 30.2s) | P| | 60 | 14.6 |
1431 | 2025-12-29 20:22:09 | MASTER-Tunka | (08h 14m 38.88s , +37d 47m 07.2s) | P| | 60 | 14.4 |
1431 | 2025-12-29 20:22:09 | MASTER-Tunka | (08h 15m 31.88s , +37d 38m 24.7s) | P- | 60 | 15.7 |
1494 | 2025-12-29 20:23:13 | MASTER-Tunka | (08h 15m 32.16s , +37d 38m 24.7s) | P- | 60 | 15.1 |
1494 | 2025-12-29 20:23:13 | MASTER-Tunka | (08h 14m 39.04s , +37d 47m 20.0s) | P| | 60 | 13.9 |
1558 | 2025-12-29 20:24:16 | MASTER-Tunka | (08h 15m 32.26s , +37d 38m 21.8s) | P- | 60 | 15.3 |
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 43253
Subject
EP251229a: Einstein Probe detection of an X-ray transient
Date
2025-12-30T01:49:26Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B. T. Wang (YNAO, CAS), J. P. Chen (SYSU), D. Zhu, K. J. Zhang (YNU), Y. L. Wang (NAO, CAS; ICE, CSIC-IEEC) and Z. X. Ling (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 EP251229a. The transient triggered EP-WXT (ID: 01709250690) at 2025-12-29T19:58:49 (UTC). The WXT position of the source is R.A. =124.103 deg, DEC = 37.224 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. = 124.1057 deg, DEC = 37.2073 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).