EP260110a
GCN Circular 43615
J. Sanchez-Sierras (Radboud), A. van Hoof (Radboud), P. G. Jonker (Radboud), D. Mata-Sanchez (IAC and ULL), A. J. Levan (Radboud), D. B. Malesani (DAWN/NBI and Radboud Univ.) report on behalf of a larger collaboration:
We observed the field of fast X-ray transient EP260110a (Zhang et al., GCNs 43363, 43367) with the Espectrógrafo Multiobjeto Infra-Rojo (EMIR) attached to the Gran Telescopio Canarias (GTC). The observations were performed in imaging mode and consisted of 260x5 s exposures, with a total exposure time of 1280 s in the H band, starting on 2026-02-01 at 06:08:10 UT (i.e. 21.76 days after the EP-WXT trigger).
We used this observation as a template to perform image subtraction from the GTC/EMIR image reported by Corcoran et al. (observed 19.02 hr after the trigger; GCN 43391), which had the same exposure time and instrument configuration. The subtraction was made using the ZOGY algorithm (Zackay et al. 2016), as implemented in PyZOGY (Guevel et al. 2021). No new sources are detected inside the EP-FXT error circle in the subtraction, down to the limit of the least deep image, i.e. upper limiting AB magnitude of H > 23.3 (calibrated using nearby stars from the 2MASS catalog).
GCN Circular 43391
G. Corcoran (UCD), P. G. Jonker (Radboud), J. Sanchez-Sierras (Radboud), A. Martin-Carrillo (UCD), J. A. Quirola-Vasquez (Radboud), A. van Hoof (Radboud), D. Mata Sanchez (IAC), D. Slumstrup (GTC), N. Castro Rodríguez (GTC), V. Vuolteenaho (NOT and Oulu Univ.) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260110a (Zhang et al., GCNs 43363, 43367) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera as well as in the near infrared with the Gran Telescopio Canarias (GTC) equipped with the EMIR instrument. Our observations consisted of two epochs of 3 x 300 s in SDSS-r band and 5 x 200 s in SDSS-z, taken from the NOT, and a single epoch in H with a total exposure time of 1280 s taken from the GTC.
Performing image subtraction with PyZOGY on our r-band stacked images and visually comparing the other bands to a stack of archival r-band CFHT images we do not detect any new sources within the EP/FXT uncertainty region in any of our images. We obtain the following upper limits:
| Mid time (UT) | Time since trigger (hr) | Band | 3 sigma UL (AB) |
|---|---|---|---|
| 2026-01-11 01:46:32 | 13.80 | r | 23.5 |
| 2026-01-11 02:04:27 | 14.10 | z | 22.3 |
| 2026-01-11 06:35:44 | 19.02 | H | 23.3 |
| 2026-01-12 03:03:04 | 39.08 | r | 24.2 |
| 2026-01-12 03:20:59 | 39.38 | z | 22.7 |
The r and z band upper limits were calibrated using nearby stars from the Pan-STARRS catalog while the 2MASS catalog was used for the H-band observations and the photometry is not corrected for Galactic extinction. Our upper limits are consistent with Perez-Garcia et al. (GCN 43364), Kang et al. (GCN 43365), Antier et al. (GCN 43366), Zheng et al. (GCN 43368), Lipunov et al. (GCN 43372), Li et al. (GCN 43375), and Zhu et al. (GCN 43390).
We note the presence of two faint sources within the EP/FXT uncertainty region in both our r- and H-band images, which are also seen in the archival CFHT imagery. While no residual is visible at their position in the r-band subtraction, due to the lack of sufficiently deep H-band templates we cannot test for NIR variability at this time.
GCN Circular 43390
Z.P. Zhu (NAOC), S. Tinyanont, K. Noysena, K. Chanchaiworawit(NARIT), J. An, X. Liu, S.Q. Jiang, L.B. He, N.C. Sun, W.X. Li, D. Xu (NAOC), S.Y. Fu (HUST) report on behalf of a large collaboration:
We observed the field of EP260110a detected by the Einstein Probe (EP) (Zhang et al., GCN 43363), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Sierra Remote Observatories in California. We obtained several 360 s frames in the R filter. Observations started at 07:38:55 UTC on 2026-01-11, i.e., 0.44 day after the EP/WXT trigger time.
No new source is detected within or at the border of the EP/FXT error region (Zhang et al., GCN 43367), being consistent with the non-detection reported at other telescopes (Perez-Garcia et al., GCN 43364; Kang et al., GCN 43365; Antier et al., GCN 43366; Zheng et al., GCN 43368; Lipunov et al., GCN 43372; Li et al., GCN 43375). From the stacked images we obtain the following 5-sigma upper limits in Vega magnitude:
| Mid-time(d) | filter | upper limit | site |
|---|---|---|---|
| 0.46 | R | > 19.6 | SRO |
| 1.45 | R | > 20.9 | SRO |
| 2.44 | R | > 21.0 | SRO |
calibrated using nearby stars from the Pan-STARRS DR1 catalog and are not corrected for Galactic extinction.
GCN Circular 43375
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, P. P. Zhang, Y. Xu, 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 EP260110a (Zhang et al., GCN 43363; 43367). The observation started on 2026-01-10T14:29:11 UT (i.e. 2.52 hour post trigger) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
Inside the error box of the EP-FXT (Zhang et al., GCN 43367) we did not detect any new uncatalogued source, which was consistent with the non detections reported by other groups (Perez-Garcia et al., GCN 43364; Kang et al., GCN 43365; Antier et al., GCN 43366; Zheng et al., GCN 43368; Lipunov et al., GCN 43372). The 3 sigma limits in AB magnitude are derived as below:
Mid time | Band | Exposure Time | 3 sigma limit magnitude
4.11 hour VT_B 52*70 sec > 23.5
4.64 hour VT_R 50*70 sec > 23.5
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 43372
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-Kislovodsk robotic telescope [1] located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP260110a ( EP Team et al., GCN 43363) errorbox 1 days 21876 sec after notice time and 1 days 25310 sec after trigger time at 2026-01-11 18:58:48 UT, with upper limit up to 17.6 mag. The observations began at zenith distance = 62 deg. The sun altitude is -54.1 deg.
The galactic latitude b = 51 deg., longitude l = 118 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3099928
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
111800 | 2026-01-11 18:58:48 | MASTER-Kislovodsk | (13h 17m 27.89s , +65d 50m 39.1s) | C | 180 | 16.4 |
111800 | 2026-01-11 18:58:48 | MASTER-Kislovodsk | (13h 17m 45.69s , +65d 27m 35.9s) | C | 180 | 17.6 |
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 43368
WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and
Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of EP260110a (Zhang et al.,
GCN 43363; 43367) starting at 12:19, Jan 10 UT, about 21 minutes
after the burst. A set of 20s, 40s and 60s clear (roughly R) filter
images were obtained. In our coadd image, we did not detect any new
source within the EP-WXT error circle (Zhang et al., GCN 43367) down
to a limiting mag of 21.0 mag (Vega) at a mid-time of 1.0 hours after
the burst. Our result is consistent with the upper limit reported
by other groups (Perez-Garcia et al., GCN 43364; Kang et al., GCN
43365; Antier et al., GCN43366).
GCN Circular 43367
B. Zhang, J. P. Feng (USTC), Y. Wu (NJU), F. F. Song(YNAO, CAS), A. Li (BNU), C. Y. Wang (THU), G. Y. Zhao (SYSU), Y. Liu (NAOC, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260110a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 43363). The refined analysis of the WXT data shows that the event started at T0=2026-01-10T 11:56:58(UTC), and lasted for approximately 85 seconds, after which the WXT light curve was interrupted due to the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.59×10^20 cm^-2 and a photon index of 0.99(-0.47/+0.50). The derived average unabsorbed 0.5-4 keV flux is 1.02(-0.30/+0.39)×10^(-9) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-01-10T12:00:52(UTC, T0+234 s). The exposure time of this observation is 4576s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 199.4321, DEC = 65.8490 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.59×10^20 cm^-2 and a photon index of 2.02 (-0.14/+0.14). The derived average unabsorbed 0.5-10 keV flux is 1.52 (-0.19/+0.22)×10^(-12) erg/s/cm^2.
We performed one target-of-opportunity (ToO) follow-up observations with FXT at 2026-01-10T16:23:31 (UTC, T0+15,993 s) with an exposure time of 3,032s. The average FXT 0.5-10 keV spectrum of the ToO observation can be fitted with an absorbed power law, with a fixed Galactic hydrogen column density of 1.59×10^20 cm^-2, and a photon index of 2.11 (-0.84/+1.00) . The derived average unabsorbed 0.5-10 keV flux is 1.15 (-0.57/+1.33)×10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
No optical counterpart of EP260110a has been detected so far (Perez-Garcia et al., GCN 43364; Kang et al., GCN 43365; Antier et al., GCN43366) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260110a.
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 43366
Sarah Antier (IJCLAB), Dalya Akl (NYUAD), Edilberto Aguilar-Ruiz (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata(UNAM), Nikos Mandarakas (LAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of the EP 260110a using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-10 12:17:40 to 13:21 UTC (from 18 min after the trigger) and obtained about 50 minutes of simultaneous exposure in the r and z filters.
The data were reduced, coadded, and analyzed with the two COLIBRÍ pipelines, and were also analyzed 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 do not detect any new source against Legacy at the WXT and FXT localization error box (Zhang et al., GCN Circ. 43363) down to the following 5-sigma limit:
r > 22.8
z > 21.8
This upper limit is consistent with the ones reported by Perez-Garcia et al. GCN Circ. 43364 and Kang at al. Circ. 43365.
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 43365
Z. Kang (CHO), C. WU, L.P. Xin, X.H. Han, P.P. Zhang, X.M. Lu (NAOC), Z.W. Li (CHO), Y. Lv (CHO), R.S. Zhang, Y.J. Xiao, Y.L. Qiu, J. Wang, J.S. Deng, L. Huang, J.Y. Wei (NAOC) report on behalf of the SVOM/C-GFT team:
We observed the field of EP260110a, detected by EP/WXT (Zhang et al., GCN 43363), using the LATIOS instrument on the SVOM/C-GFT. Observations started at 2026-01-10T12:20:27 UTC, ~22.07 min after the trigger.
A series of i-band images were obtained. After preliminary processing, no credible candidate was detected within the error box provided by EP/FXT (Zhang et al., GCN 43363) in our images. The three-sigma upper limit is:
| Date-Obs (mid-time) | Mid_t - T0 (min) | Exposure Time (s) | Band | Upper Limit (AB) |
|---|---|---|---|---|
| 2026-01-10T12:20:57 | 22.57 | 2×30 | i | 18.6 |
The photometry was calibrated against nearby Pan-STARRS1 stars and no correction for Galactic dust extinction was applied.
This result is consistent with Perez-Garcia et al., GCN 43364.
We thank the observation assistants Yin-Huai Hao and Chun-Lei Guo at Jilin observatory for their excellent support.
The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcsec x 21 arcsec FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching.
GCN Circular 43364
I. Perez-Garcia, E. J. Fernandez-Garcia, M. D. Caballero-Garcia, S.-Y. Wu, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), I. M. Carrasco (SMA), M. Gritsevich (Univ. of Helsinki), C. Perez del Pulgar (Univ. of Malaga), Y.-D. Hu (GXU), S. Jeong (ADD, Daejeon), G. Garcia-Segura and D. Hiriart (IA-UNAM, Ensenada), D. R. Xiong (Yunnan Observatories of CAS, Kunming), and W. H. Lee (UNAM, Mexico DF), on behalf of a larger collaboration, report:
Following the detection of EP260110a by EP (Zhang et al., GCN 43363), the 0.6m BOOTES-5/JGU robotic telescope at San Pedro Martir Observatory (Mexico) automatically responded to this high-energy event starting on Jan 10, 12:18:31 UT (i.e. 20 min after detection). Nothing is detected down to 17.3 in this first image. On a coadd image (12 x 60sec) at 12:41:13 UT (mid exposure time), nothing is detected down to 20.1 mag (clear filter) within the FXT error region. All photometric calibration was done using Gaia Gmag band.
We would like to thank the staff at San Pedro Martir Observatory for their excellent support.
GCN Circular 43363
B.Zhang, J. P. Feng (USTC), Y. Wu (NJU), F. F. Song(YNAO, CAS), A. Li (BNU), C. Y. Wang (THU), G. Y. Zhao (SYSU). Y. Liu (NAOC, 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 EP260110a. The transient triggered EP-WXT (ID: 01709251242) at 2026-01-10T11:58:23 (UTC). The WXT position of the source is R.A. = 199.406 deg, DEC = 65.845 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. = 199.4371 deg, DEC = 65.8476 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).