EP260105a
GCN Circular 43488
Subject
EP260105a: GTC/HiPERCAM optical observations
Date
2026-01-21T16:05:28Z (7 months ago)
From
javisansie@gmail.com
Via
Web form
J. Sanchez-Sierras (Radboud), G. Corcoran (UCD), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), D. Mata Sanchez (IAC), A. L. Cabrera Lavers (GTC), report on behalf of a larger collaboration:
We observed the field of EP260105a (Zhou et al., GCN 43327) with the HiPERCAM instrument attached to the Gran Telescopio Canarias (GTC; La Palma, Spain). The observation simultaneously covered the u, g, r, i and z bands with a total exposure time of 1612 s (150 x 10.75 s), starting on 2026-01-15 at 05:49:40 UT (i.e. 10 days after the trigger).
Several sources are detected within the EP/FXT error circle, but comparison with archival i-band imaging from the Hyper Suprime-Cam Legacy Archive (Tanaka et al. 2021, doi:10.1093/pasj/psab034) shows all of them to be pre-existing. We carried out subtraction in the i-band with PyZOGY (Guevel et al. 2021), using the HSC image as a template. The subtraction does not show any new source inside the EP-FXT error region (Zhou et al., GCN 43333) down to a limiting AB magnitude of i > 25.5, not corrected by Galactic extinction.
This non-detection is consistent with the others reported by Lipunov et al. (GCN 43328), Corcoran et al. (GCN 43332), Li et al. (GCN 43337) and Busmann et al. (GCN 43345).
GCN Circular 43345
Subject
EP260105a: FTW optical observations
Date
2026-01-06T15:03:15Z (7 months ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann, Jule Augustin, Mitra Maleki (LMU), Xander J. Hall, Brendan O'Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the 90% FXT localization of EP260105a (Zhou et al., GCNs 43327 and 43333) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r and i-band simultaneously for 10 x 180 s starting at 2026-01-06T02:04:38 UT (0.84 days after the trigger). We performed difference imaging with templates from the DESI Legacy Surveys and did not detect any uncatalogued sources to a 3-sigma upper limit of
r > 22.7 AB mag.
The non-detection is consistent with the reports by Lipunov et al. (GCN 43328), Corcoran et al. (GCN 43332), and Li et al (GCN 43337).
The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Christoph Ries from the Wendelstein Observatory for obtaining these observations.
GCN Circular 43337
Subject
EP260105a: SVOM/VT optical upperlimit
Date
2026-01-06T03:03:52Z (7 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, Y. Xu, X. H. Han, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observation on the fast X-ray transient EP260105a (Zhou et al., GCN 43327). The observation started on 2026-01-05T07:51:04 UT (i.e. 2.06 hour post trigger) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
Inside the error box of the EP-FXT (Zhou et al., GCN 43333) we did not detect any new uncatalogued source, which was consistent with the non detections reported by MASTER-Net (Lipunov et al., GCN 43328) and LCO (Corcoran et al., GCN 43332). The 3 sigma limit magnitudes are derived as below:
Mid time | Band | Exposure Time | 3 sigma limit magnitude
4.68 hour VT_B 30*100 sec > 23.5 mag
4.68 hour VT_R 30*100 sec > 23.5 mag
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 43333
Subject
EP260105a: refined analysis of the EP-WXT and EP-FXT observations
Date
2026-01-05T14:30:43Z (7 months ago)
Edited On
2026-01-09T19:57:15Z (7 months 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
H. Zhou (PMO, CAS), Z. C. Zou (NJU), D. Y. Li, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260105a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhou et al., GCN 43327). The refined analysis of the WXT data shows that the event started at T0=2026-01-05T05:46:19 (UTC) and lasted for 100 seconds. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.4 x 10^20 cm^-2 and a photon index of 0.89(-0.65/+0.67). The derived average unabsorbed 0.5-4 keV flux is (1.1+/-0.5) x 10^(-9) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-01-05T05:52:07 (UTC), about 4.3 minutes after T0. The exposure time of this observation is 2698s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 181.0148, DEC = 22.7092 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. A significant decay was detected in the light curve during the initial phase of FXT observation. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.4 x 10^20 cm^-2 and a photon index of 2.07 (-0.56/+0.58). The derived average unabsorbed 0.5-10 keV flux is 4.2 (-1.7/+2.1) x 10^(-13) erg/s/cm^2.
No optical counterpart of EP260105a has been detected so far (Lipunov et al., GCN 43328; Corcoran et al., GCN 43332) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260105a.
Another ToO observation with EP-FXT is being performed start from 2026-01-05T12:31:46 (UTC). 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).
GCN Circular 43332
Subject
EP260105a: LCO Optical Upper limit
Date
2026-01-05T11:05:43Z (7 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
G. Corcoran (UCD), J. Quirola-Vasquez (Radboud), F. E. Bauer (SSI and UTA), A. Levan (Radboud), A. Martin-Carrillo (UCD), P.G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260105a (Zhou et al. GCN 43327) using the Las Cumbres Observatory (LCO) network at the Cerro Tololo Inter-American Observatory node using the Sinistro instrument on the 1m telescope. A series of 6x300s exposures were taken in the SDSS-r filter starting at 2026-01-05T07:10:04.556 UTC (1.326 hours after the WXT trigger).
We do not detect any sources within the EP/FXT uncertainty region. From the stacked image we obtain a 3 sigma upper limit of:
r > 21.5.
This upper limit is in AB magnitudes, was calibrated using nearby stars from the Pan-STARRS catalog and is not corrected for Galactic extinction. Our observations are consistent with the upper limits reported by Lipunov et al. GCN 43328.
GCN Circular 43328
Subject
EP260105a: Global MASTER-Net observations report
Date
2026-01-05T06:36:38Z (7 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-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260105a ( EP Team et al., GCN 43327) errorbox 302 sec after trigger time at 2026-01-05 05:55:32 UT, with upper limit up to 17.7 mag. The observations began at zenith distance = 76 deg. The sun altitude is -33.4 deg.
The galactic latitude b = 79 deg., longitude l = 235 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3096319
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
333 | 2026-01-05 05:55:32 | MASTER-OAFA | (12h 04m 39.81s , +22d 45m 13.6s) | C | 60 | 17.0 |
404 | 2026-01-05 05:56:38 | MASTER-OAFA | (12h 04m 39.97s , +22d 45m 19.5s) | C | 70 | 17.2 |
490 | 2026-01-05 05:57:55 | MASTER-OAFA | (12h 04m 40.18s , +22d 45m 26.3s) | C | 90 | 17.3 |
597 | 2026-01-05 05:59:31 | MASTER-OAFA | (12h 04m 40.39s , +22d 45m 34.0s) | C | 110 | 17.3 |
723 | 2026-01-05 06:01:28 | MASTER-OAFA | (12h 04m 40.60s , +22d 45m 42.6s) | C | 130 | 17.4 |
874 | 2026-01-05 06:03:44 | MASTER-OAFA | (12h 04m 40.84s , +22d 45m 52.0s) | C | 160 | 17.5 |
991 | 2026-01-05 06:06:30 | MASTER-OAFA | (12h 04m 41.03s , +22d 45m 59.4s) | C | 60 | 17.5 |
1056 | 2026-01-05 06:07:36 | MASTER-OAFA | (12h 04m 41.10s , +22d 46m 03.1s) | C | 60 | 17.5 |
1121 | 2026-01-05 06:08:41 | MASTER-OAFA | (12h 04m 41.18s , +22d 46m 07.0s) | C | 60 | 17.5 |
1188 | 2026-01-05 06:09:47 | MASTER-OAFA | (12h 04m 41.28s , +22d 46m 10.9s) | C | 60 | 17.5 |
1253 | 2026-01-05 06:10:53 | MASTER-OAFA | (12h 04m 41.36s , +22d 46m 14.7s) | C | 60 | 17.6 |
1319 | 2026-01-05 06:11:58 | MASTER-OAFA | (12h 04m 41.43s , +22d 46m 18.6s) | C | 60 | 17.7 |
1385 | 2026-01-05 06:13:05 | MASTER-OAFA | (12h 04m 41.51s , +22d 46m 22.1s) | C | 60 | 17.7 |
1451 | 2026-01-05 06:14:10 | MASTER-OAFA | (12h 04m 41.60s , +22d 46m 25.6s) | C | 60 | 17.6 |
1516 | 2026-01-05 06:15:16 | MASTER-OAFA | (12h 04m 41.63s , +22d 46m 29.0s) | C | 60 | 17.7 |
1582 | 2026-01-05 06:16:22 | MASTER-OAFA | (12h 04m 41.67s , +22d 46m 32.3s) | C | 60 | 17.7 |
1648 | 2026-01-05 06:17:27 | MASTER-OAFA | (12h 04m 41.73s , +22d 46m 35.5s) | C | 60 | 17.7 |
1714 | 2026-01-05 06:18:33 | MASTER-OAFA | (12h 04m 41.76s , +22d 46m 38.8s) | C | 60 | 17.7 |
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 43327
Subject
EP260105a: Einstein Probe detection of an X-ray transient
Date
2026-01-05T06:17:04Z (7 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Zhou (PMO, CAS), Z. C. Zou (NJU), D. Y. Li, 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 EP260105a. The transient triggered EP-WXT (ID: 01709250995) at 2026-01-05T05:50:30 (UTC). The WXT position of the source is R.A. = 181.036 deg, DEC = 22.723 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. = 181.0158 deg, DEC = 22.7089 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).