EP260416a
GCN Circular 44419
Subject
EP260416a: Mondy and Terskol optical upper limits
Date
2026-04-29T08:29:18Z (5 months ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), I. Sokolov (INASAN), E. Klunko (ISTP), A. Pozanenko (IKI), N. Pankov (IKI) report on behalf of IKI GRB-FuN:
We observed the field of the fast X-ray transient EP260416a (Hu et al., GCN 44307, 44324) at z = 1.909 (He et al., GCN 44336; An et al., GCN 44339) in R band with the Zeiss-2000 telescope of the Terskol observatory starting on 2026-04-25 (UT) 20:02:10 and with the AZT-33IK telescope of the Mondy observatory starting on 2026-04-26 (UT) 15:50:09. The optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334; He et al., GCN 44336; An et al., GCN 44339; Jelinek et al., GCN 44341; Guziy et al., GCN 44351; Ho et al., GCN 44354; Pawar et al., GCN 44373; Li et al., GCN 44376; Moskvitin et al., GCN 44395; Volnova et al., GCN 44398) is not detected in the stacked frames of every epoch.
Preliminary photometry and observational details are the following:
Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope
(mid,days) (n*s) (3sigma)
2026-04-25 20:02:10 9.63709 20*180 R n/d n/d 22.5 Terskol/Zeiss-2000
2026-04-26 15:50:09 10.45948 40*120 R n/d n/d 22.9 Mondy/AZT-33IK
The photometry is based on nearby USNO-B1.0 stars (R2-magnitudes) and is not corrected for Galactic extinction.
GCN Circular 44398
Subject
EP260416a: Mondy optical observations
Date
2026-04-23T12:39:12Z (5 months ago)
Edited On
2026-04-23T15:36:00Z (5 months ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), N. Pankov (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of the fast X-ray transient EP260416a (Hu et al., GCN 44307, 44324) at z = 1.909 (He et al., GCN 44336; An et al., GCN 44339) with the AZT-33IK 1.5m telescope of the Mondy observatory starting on 2026-04-21 (UT) 15:28:28 and taking several 120-seconds frames in the R band. The optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334; He et al., GCN 44336; An et al., GCN 44339; Jelinek et al., GCN 44341; Guziy et al., GCN 44351; Ho et al., GCN 44354; Pawar et al., GCN 44373; Li et al., GCN 44376; Moskvitin et al., GCN 44395) is clearly detected in the stacked frame.
Preliminary photometry and observational details are the following:
Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope
(mid,days) (n*s) (3sigma)
2026-04-21 15:28:28 5.43677 27*120 R 22.72 0.09 23.3 Mondy/AZT-33IK
The photometry is based on nearby USNO-B1.0 stars (R2-magnitudes) and is not corrected for Galactic extinction.
GCN Circular 44395
Subject
EP260416a: ZTSh R band observations
Date
2026-04-22T23:58:50Z (6 months ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), V. Rumyantsev (CrAO), A. Volnova, N. Pankov,
A. Pozanenko (IKI), Tao An, Yuanqi Liu (SHAO), Ankur Ghosh,
Soebur Razzaque (U. of Johannesburg, RSA) report on behalf
of GRB follow-up collaboration.
We observed the field of EP260416a (Hu et al., GCN 44307; GCN 44324)
with 2.6-m ZTSh of CrAO equipped with BVRI photometer and FLI PL4240
camera. We obtained a series of images in R band on 2026-04-22,
21:11:50--22:14:42 UT (t_mid - T0 = 6.6762 days).
The optical transient discovered by COLIBRÍ (Ducoin et al., GCN 44310;
and also observed by Li et al., GCN 44315; Hua et al., GCN 44318;
Lee et al., GCN 44319; Martin-Carrillo et al., GCN 44328; Li et al.,
GCN 44334; He et al., GCN 44336; An et al., GCN 44339; Jelinek et al.,
GCN 44341; Guziy et al., GCN 44351; Ho et al., GCN 44354) is clearly
detected in the stacked image with the following brightness.
Date, UT_start, t_mid - T0, filter, exp, mag +/- err UL
days s
2026-04-22T21:11:50 6.67620 R 30*120 23.12 +/- 0.17 24.1
This preliminary photometry is based on the nearby USNO-B1 stars
and not corrected for the Galactic extinction.
GCN Circular 44376
Subject
EP260416a: J band upper limit by SYSU 80cm infrared telescope
Date
2026-04-21T11:00:51Z (6 months ago)
From
lijj328@mail2.sysu.edu.cn
Via
Web form
Jin-Ji Li, Hao-Nan Yang, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Rui-Chen Gao, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Yun Shi, Yan Yu, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of the fast X-ray transient EP260416a (Hu et al., GCN 44307) using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our observations were carried out on 2026 Apr 19 18:39 UT, 85.1 hours after the trigger, using stacked images in the J band.
We do not detect any counterpart at the position of the optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334, He et al. GCN 44336, An et al. GCN 44339, Jelinek et al. GCN 44341; Guziy et al., GCN 44351; Ho et al., GCN 44354), down to a 5-sigma depth of J ~ 18.1 Vega magnitudes.
The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 44373
Subject
EP260416a: 1.3m DFOT optical observation
Date
2026-04-21T10:02:49Z (6 months ago)
From
pawarpankaj1492@gmail.com
Via
Web form
Pankaj Pawar, Anshika Gupta, Debalina Kar, Dhruv Jain and Kuntal Misra (ARIES) report:
We observed the field of EP260416a (Hu et al., GCN 44307) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of observational sciencES (ARIES), India.
The observations began on 2026-04-20 at 15:58:17 UT, approximately ~106 hours after the trigger. We obtained a series of 300s exposures in the R filter. The frames were aligned and stacked to improve the signal-to-noise ratio.
The optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334; He et al. GCN 44336; An et al. GCN 44339; Jelinek et al. GCN 44341; Guziy et al., GCN 44351; and Ho et al., GCN 44354) was not detected in the stacked image.
The details of observations and measured upper limit are as follows:
Date (UT) Start Time T_start-T0 (days) Filter Exposure (s) Magnitude (AB)
=======================================================================================
2026-04-20 15:58:17 ~4.44 R 300s*13 >22.6
The magnitude is calibrated using Pan-STARRS field stars.
GCN Circular 44354
Subject
EP260416a: Optical follow-up observations with Kinder
Date
2026-04-20T08:42:49Z (6 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
K. N.-T. Ho, A. Aryan, T.-W. Chen, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), A. Sankar.K, M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP260416a (Hu et al., GCN 44307) using the 40cm SLT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 11:48 UTC on the 16th of April 2026 (MJD 61146.4918), 6.32 hr after the EP-WXT detection.
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. Owing to the faint optical counterpart candidate (Ducoin et al., GCN 44310), the transient was not detected in our stacked image.
After the reported optical rebrightening (Hua et al., GCN 44318), we performed another SLT epoch of observations starting at UTC 12:33 on April 18th, 2026 (61148.5228), 55.05 hr post the WXT discovery. The optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334, He et al. GCN 44336, An et al. GCN 44339, Jelinek et al. GCN 44341; and Guziy et al., GCN 44351) was detected in the stacked images.
We observed the transient for one more epoch, starting at UTC 11:30 on April 19th, 2026 (61149.4790), about 78.0 hr since the WXT discovery. The optical counterpart candidate was not detected in the stacked image.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and measured photometry/the 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 | 61146.4918 | 6.31 | 300 * 24 | >21.3 | 2".13 | 1.26
SLT | r | 61148.5228 | 55.05 | 300 * 19 | 21.16 +/- 0.08 | 2".05 | 1.13
SLT | r | 61149.4790 | 78.00 | 300 * 18 | >20.9 | 2".66 | 1.35
The presented magnitudes are calibrated using the field stars from ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitudes/upper-limits are not corrected for an expected galactic extinction of A_r = 0.04 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 44351
Subject
EP260416a: 1.5m OSN and 2.2m CAHA optical monitoring
Date
2026-04-20T04:03:59Z (6 months ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
S. Guziy, F. J. Aceituno, A. J. Castro-Tirado, M.D. Caballero-García, M. Gritsevich, I. Pérez-García, R. Sánchez-Ramírez, S.-Y. Wu (IAA-CSIC) and A. Fernández-Martin and M. M. Sánchez-Andújar (CAHA), on behalf of a larger collaboration,report:
We observed EP260416a (Hu et al., GCN 44307) with both the 1.5m telescope at Observatorio de Sierra Nevada (OSN) and the 2.2m telescope at Calar Alto Observatory (CAHA), in Southern Spain. A series of 300 s BVRI exposures were obtained at the 1.5m OSN starting on Apr 17, 21:04 UT, complemented by one 100s i-band image at the 2.2m CAHA, on Apr 18, 01:04 UT., i.e. 39.6 h and 43.6 h respectively, after the initial trigger.
The optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334, He et al. GCN 44336, An et al. GCN 44339, Jelinek et al. GCN 44341) is well detected in our images. We measure B = 21.08 +/- 0.14 and R = 20.99 +/- 0.07 on Apr 17, 21:15 UT and I = 19.82 +/-0.18 on Apr 18, 01:04 UT (none corrected for Galactic extinction).
The night after, a second epoch BVRI observation was conducted at the 1.5m OSN, for which measure R = 21.59 +/- 0.10 on Apr 18, 20:14 UT. Our measurement is consistent with the continued fading of the afterglow.
Further optical monitoring is being carried out.
GCN Circular 44341
Subject
EP260416a: Ondrejov D50 optical detection
Date
2026-04-18T19:52:07Z (6 months ago)
From
Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>
Via
email
M. Jelinek, J. Strobl, F. Novotny, A. Malenakova and R. Hudec
(ASU CAS Ondrejov, CZ) report:
We observed the field of EP260416a (Hu et al., GCN 44307) with
the 50 cm robotic telescope (D50) of the Astronomical
Institute Ondrejov, near Prague, Czech Republic. We obtained
a series of 120 s Sloan-r band exposures starting at 20:09 UT
on 2026 April 17, i.e. 38.66 h after the initial trigger, and
combined 68 frames with a mean exposure time of 40.03 h after
the trigger.
The optical counterpart (Ducoin et al., GCN 44310; Li et al.,
GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng
et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et
al., GCN 44334) is detected in the combined image.
The preliminary AB magnitude of the object is r = 21.08 +/-
0.23, calibrated against the ATLAS catalog and not corrected
for Galactic extinction.
Our measurement is consistent with the continued fading of the
afterglow.
GCN Circular 44339
Subject
EP260416a: VLT/FORS2 spectroscopic redshift confirmation
Date
2026-04-18T12:32:28Z (6 months ago)
From
Laura Cotter at University College Dublin <laura.cotter@ucdconnect.ie>
Via
Web form
J. An (NAOC), A. Saccardi (CEA/Irfu), L. Cotter (UCD), D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), A. L. Thakur (INAF-IAPS) report on behalf of the Stargate collaboration:
We observed the optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al,. GCN 44334) of EP260416a (Hu et al., GCNs 44307, 44324) using the ESO VLT UT1 (Antu) equipped with the FORS2 spectrograph.
Our observation started on 2026 April 18 at 03:04:13 UT (1.90 days after the EP trigger). A series of 3 spectra of 900 s each were obtained with the 300V grism, covering the wavelength range 3500-8600 AA.
From a 60 s acquisition image (1.89 days after trigger), we measure a magnitude R = 20.99 +/- 0.06 (Vega), calibrated against nearby stars from the Pan-STARRS catalog using the Lupton (2005) transformations, and not corrected for Galactic extinction.
In a preliminary reduction of the spectra, we detect a continuum over the entire wavelength range. At the blue edge of the spectrum, even though the S/N is low, we identify a broad trough at around 3540 AA, likely due to H I. We also detect multiple absorption features including O I, C II, Si IV, C IV, Si II, Fe II, Al II, Al III, at a common redshift of z = 1.910, which we thus suggest to be the redshift of EP260416a. The lack of any further hydrogen absorption at higher redshift implies that the z = 1.910 system is indeed local to the GRB. Our results confirm the tentative redshift measurement reported by He et al. (GCN 44336).
We acknowledge expert support from the ESO observing staff in Paranal, particularly Hannah Osborne and Diego Parraguez. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
GCN Circular 44336
Subject
EP260416a: NOT tentative spectroscopic redshift of z =1.909
Date
2026-04-18T05:02:56Z (6 months ago)
From
L. B. He at NAOC <helb@bao.ac.cn>
Via
Web form
L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, J. An, D. Xu (NAOC), S.Y. Fu (HUST), J. P. U. Fynbo (DAWN/NBI), L. Fuglsang (NOT) report:
We observed the optical counterpart (e.g., Ducoin et al., 44310) of EP260416a (Hu et al., GCN 44307) using the Nordic Optical Telescope (NOT), equipped with the ALFOSC camera and spectrograph.
Observations started at 2026-04-17T03:56:49, and 2x1800 s spectra were obtained, covering the wavelength range 3800-9000 AA.
The trace has quite low signal-to-noise. The continuum is throughout the wavelength range. We identify two relatively prominent absorption lines as Mg II doublet at z = 1.909. Based on it, other weak lines as due to Fe II 2600, Fe II 2374, 2383, Al II 1671 are consistent with this redshift.
We tentatively conclude that this is the redshift of EP260416a.
GCN Circular 44334
Subject
EP260416a: GMG Optical Observation
Date
2026-04-18T01:02:53Z (6 months ago)
From
Rui-Zhi Li at Yunnan Observatories, CAS <liruizhi@ynao.ac.cn>
Via
Web form
R.-Z. Li, B.-T. Wang, F.-F. Song, J. Mao, J.-G Wang and B. Wang (YNAO, CAS) report:
We observed the field of EP260416a (Hu et al., GCN 44307, T0 at 2026-04-16T05:29:32) using the GMG-2.4m telescope at the Lijiang Observatory. The observation began at 2026-04-17T15:58:05, about 34.48 hours after the trigger.
The optical counterpart of EP260416a (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328) was well detected.
The preliminary analysis results are shown as follows:
+---------------+------------+----------+--------------+----------------+
| Tmid-T0 [h] | Exp. [s] | Filter | Mag | 5-sigma U.L. |
+===============+============+==========+==============+================+
| 34.50 | 180 | r | 20.91 ± 0.07 | 22.3 |
+---------------+------------+----------+--------------+----------------+
The given magnitudes are derived based on calibration against Pan-STARRS1 field stars, and are not corrected for the expected Galactic foreground extinction, corresponding to a reddening of E(B-V) = 0.008 mag in the direction of the optical counterpart (Schlafly & Finkbeiner 2011).
GCN Circular 44328
Subject
EP260416a: LCO observations confirm end of flaring phase
Date
2026-04-17T13:52:03Z (6 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
A. Martin-Carrillo (UCD), L. Cotter (UCD), A. J. Levan (Radboud & Warwick), J. A. Chácon (PUC), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the field of EP260416a (Hu et al., GCN Circ. 44307) with the LCO 1m telescope located at the McDonald Observatory (Texas, USA) equipped with the SINISTRO instrument. We obtained 6x300s exposures in the r band starting at 09:58:28.32 UT on the 17th of April 2026, approximately 28.48 hr after the initial trigger. In the stacked images the optical counterpart (Ducoin et al., GCN 44310, Li et al., GCN 44315, Hua et al., GCN 44318, Lee et al., GCN 44319, Zheng et al., GCN 44321) is well detected, and we measure an AB magnitude of
r = 20.78 +/- 0.06,
calibrated using nearby stars from the Pan-STARRS catalogue and not corrected for Galactic extinction.
Compared to the previous observations made by Lee et al. (GCN Circ. 44319) the counterpart seems to have faded by approximately 0.18 AB mag in 5.6 hr.
GCN Circular 44324
Subject
EP260416a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Date
2026-04-17T09:10:30Z (6 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. F. Hu (PMO, CAS), J. Yang (ZZU), M. J. Liu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260416a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 44307), and followed by several optical telescopes (Ducoin et al., GCN 44310, Li et al., GCN 44315, Hua et al., GCN 44318, Lee et al., GCN 44319, Zheng et al., GCN 44321). The refined analysis of the WXT data shows that the event was detected at the beginning of the observation starting at T0=2026-04-16T05:25:24 (UTC), and lasted for about 250 s before the observation was interrupted by 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 1.02 × 10^20 cm^-2, and a photon index of 1.0 +/- 0.8. The derived average unabsorbed 0.5-4 keV flux is 3.2 (-1.4, +2.7) × 10^(-10) erg/s/cm^2.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-04-16T05:33:14 (UTC), about 8 minutes after T0. The exposure time of the observation is 2.4 ks. The on-ground analysis of the FXT data found an uncatalogued source at R.A. = 198.8351, DEC = 31.5972 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT light curve shows no significant variability. The average FXT 0.5-10 keV spectra can be fitted with an absorbed power-law with a free hydrogen column density of 1.62 +/- 0.18 × 10^21 cm^-2, and a photon index of 2.14 +/- 0.08. The derived average unabsorbed 0.5-10 keV flux is 3.4 (-0.1, +0.1) × 10^(-10) erg/s/cm^2. No significant variability is observed from T0 to T0+6 ks. The uncertainties are at the 90% confidence level for the above parameters.
Further FXT follow-up observations have been arranged.
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 44321
Subject
EP260416a: KAIT optical upper limit
Date
2026-04-17T06:10:43Z (6 months ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
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 EP260416a (Hu et al.,
GCN 44307) starting at 05:36, Apr 16 UT, about 7.2 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 the
OT (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al.,
GCN 44318; Lee et al., GCN 44319) down to a limiting mag of 21.0
mag (Vega) at a mid-time of 1.3 hours after the burst. Our result
is consistent with the OT reported at a fatiner magnitude by COLIBRÍ
at a similar time (Ducoin et al., GCN 44310).
GCN Circular 44319
Subject
EP260416a: COLIBRÍ confirmation of the optical rebrightening
Date
2026-04-17T05:36:55Z (6 months ago)
From
William H. Lee at UNAM <wlee@astro.unam.mx>
Via
Web form
William H. Lee (UNAM), Stéphane Basa (UAR Pytheas), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Diego López-Cámara (UNAM), Nikos Mandarakas (LAM), Enrique Moreno Méndez (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We reimaged the field of the EP260416a (Hu et al., GCN Circ. 44307) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-17 04:20 to 04:43 UTC (from 22.85 to 23.24 hours after the trigger) and obtained 4/11/3/18 minutes of exposure in the g/r/i/z filters, respectively.
The data were reduced and coadded with COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
The candidate reported previously by Ducoin et al. (GCN Circ. 44310) is clearly detected in all our filters with preliminary magnitudes of:
r = 20.60 +/- 0.02
z = 19.88 +/- 0.02
Our values confirm the rebrightening reported by the WFST Collaboration (Hua et al. GCN Circ. 44318), and suggests a photo-z <2.5 (90% conf.).
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
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 44318
Subject
EP260416a: WFST optical observations and rebrightening
Date
2026-04-17T03:55:29Z (6 months ago)
From
ylhua@pmo.ac.cn
Via
Web form
Yan-Long Hua, Ding-Fang Hu, Jin-Jun Geng, Xue-Feng Wu, Hui Sun, Ye Li, Zhi-Ping Jin, Tian-Rui Sun, Yi-Fang Liang, Yuan-Tai Yang, Ji-An Jiang report on behalf of the WFST team:
Following the detection of EP260416a by Einstein Probe (Hu et al., GCN 44307). We performed follow-up observations using the Wide Field Survey Telescope (WFST Collaboration; arXiv:2306.07590) at the Lenghu Astronomical Observation Base in Qinghai Province, China. Observations in the r-band began at 2026-04-16T17:54:47 UTC, approximately 12.4208 hours after the trigger.
The optical candidate reported by COLIBRÍ (Ducoin et al., GCN 44310) and SVOM/VT (Li et al., GCN 44315) was clearly detected in images in r band and showed significant rebrightening. The preliminary measurements are given in the AB magnitudes and are not corrected for Galactic extinction:
r = 20.27 ± 0.01 (AB)
We encourage further follow-up observations.
We thank the WFST staff for supporting these observations.
GCN Circular 44315
Subject
EP260416a: SVOM/VT optical observation
Date
2026-04-16T14:48:47Z (6 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed a ToO observation of EP260416a, triggered by Einstein Probe (Hu et al., GCN 44307). The observation started at 2026-04-16T06:17:46 UTC, i.e., approximately 46.8 minutes in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical candidate reported by COLIBRÍ (Ducoin et al., GCN 44310) was clearly detected in stack images in both channels. The preliminary measurements are given in the AB magnitudes and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
1.93 hr VT_B 99*50 sec 23.36+/-0.25 mag
1.93 hr VT_R 97*50 sec 22.91+/-0.20 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 44310
Subject
EP260416a: COLIBRÍ optical counterpart candidate
Date
2026-04-16T09:22:19Z (6 months ago)
From
J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr>
Via
Web form
Jean-Grégoire Ducoin (CPPM), Enrique Moreno Méndez (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the EP260416a (Hu et al., GCN Circ. 44307) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-16 05:36:04 to 06:51:03 UTC (from 0.11 to 1.36 hours after the trigger) and obtained 60 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with both the COLIBRÍ and COLIBRÍ ASU 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 r band, we detect a source revealed by image subtraction using Legacy Survey as template and consistent with the FXT 20 arcsec error circle (Hu et al., GCN Circ. 44307) at:
RA(J2000) = 13:15:20.49 = 198.8354 degrees
Dec(J2000) = +31:35:49.4 = 31.5971 degrees
with an uncertainty of 0.5 arcsec.
The preliminary magnitude derived for that source is:
r = 23.4 +/- 0.2
z > 22.8 (3 sigma-limit)
The position of the source is coincident with a galaxy in the Legacy Survey catalog (Dey et al. 2019) with a photometry of g = 24.01 +/- 0.17 r = 24.05 +/- 0.30 z = 23.35 +/- 0.34. In our observations, we find evidence for brighter emission in the r band relative to the catalog value, suggesting tentative excess emission relative to the cataloged galaxy flux. However, the non-detection in the z band, combined with the lack of clear evidence for variability, prevents a firm confirmation .
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
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 44308
Subject
EP260416a: Global MASTER-Net observations report
Date
2026-04-16T06:18:42Z (6 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 EP260416a ( EP Team et al., GCN 44307) errorbox 436 sec after trigger time at 2026-04-16 05:36:48 UT, with upper limit up to 20.7 mag. The observations began at zenith distance = 66 deg. The sun altitude is -64.5 deg.
The galactic latitude b = 83 deg., longitude l = 71 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3254155
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
482 | 2026-04-16 05:36:48 | MASTER-OAFA | (13h 14m 35.21s , +31d 29m 47.4s) | C | 90 | 20.1 |
602 | 2026-04-16 05:36:48 | MASTER-OAFA | (13h 14m 35.21s , +31d 29m 47.4s) | C | 330 | 20.7 | Coadd
588 | 2026-04-16 05:38:25 | MASTER-OAFA | (13h 14m 34.93s , +31d 29m 48.7s) | C | 110 | 20.2 |
714 | 2026-04-16 05:40:21 | MASTER-OAFA | (13h 14m 34.60s , +31d 29m 49.8s) | C | 130 | 20.2 |
865 | 2026-04-16 05:42:37 | MASTER-OAFA | (13h 14m 34.22s , +31d 29m 50.9s) | C | 160 | 20.2 |
925 | 2026-04-16 05:42:37 | MASTER-OAFA | (13h 14m 34.22s , +31d 29m 50.8s) | C | 280 | 20.4 | Coadd
981 | 2026-04-16 05:45:23 | MASTER-OAFA | (13h 14m 33.92s , +31d 29m 51.3s) | C | 60 | 20.0 |
1047 | 2026-04-16 05:46:28 | MASTER-OAFA | (13h 14m 33.76s , +31d 29m 51.3s) | C | 60 | 20.0 |
1112 | 2026-04-16 05:47:34 | MASTER-OAFA | (13h 14m 33.59s , +31d 29m 51.5s) | C | 60 | 20.0 |
1172 | 2026-04-16 05:47:34 | MASTER-OAFA | (13h 14m 33.59s , +31d 29m 51.5s) | C | 180 | 20.3 | Coadd
1178 | 2026-04-16 05:48:40 | MASTER-OAFA | (13h 14m 33.42s , +31d 29m 51.9s) | C | 60 | 20.0 |
1244 | 2026-04-16 05:49:45 | MASTER-OAFA | (13h 14m 33.23s , +31d 29m 52.2s) | C | 60 | 20.0 |
1309 | 2026-04-16 05:50:51 | MASTER-OAFA | (13h 14m 33.04s , +31d 29m 52.8s) | C | 60 | 20.0 |
1369 | 2026-04-16 05:50:51 | MASTER-OAFA | (13h 14m 33.04s , +31d 29m 52.8s) | C | 180 | 20.3 | Coadd
1375 | 2026-04-16 05:51:57 | MASTER-OAFA | (13h 14m 32.87s , +31d 29m 52.7s) | C | 60 | 20.0 |
1441 | 2026-04-16 05:53:02 | MASTER-OAFA | (13h 14m 32.72s , +31d 29m 52.6s) | C | 60 | 20.0 |
1506 | 2026-04-16 05:54:08 | MASTER-OAFA | (13h 14m 32.54s , +31d 29m 52.7s) | C | 60 | 20.0 |
1566 | 2026-04-16 05:54:08 | MASTER-OAFA | (13h 14m 32.54s , +31d 29m 52.7s) | C | 180 | 20.4 | Coadd
1573 | 2026-04-16 05:55:15 | MASTER-OAFA | (13h 14m 32.35s , +31d 29m 52.6s) | C | 60 | 20.0 |
1639 | 2026-04-16 05:56:20 | MASTER-OAFA | (13h 14m 32.16s , +31d 29m 52.8s) | C | 60 | 20.0 |
1704 | 2026-04-16 05:57:26 | MASTER-OAFA | (13h 14m 32.01s , +31d 29m 52.6s) | C | 60 | 20.0 |
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 44307
Subject
EP260416a: Einstein Probe detection of an X-ray transient
Date
2026-04-16T06:15:34Z (6 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. F. Hu (PMO, CAS), J. Yang (ZZU), M. J. Liu, Y. Liu (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 EP260416a. The transient triggered EP-WXT (ID: 01709260236) at 2026-04-16T05:29:32 (UTC). The WXT position of the source is R.A. = 198.836 deg, DEC = 31.590 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. = 198.8353 deg, DEC = 31.5986 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).