GRB 260429B, EP260429a
GCN Circular 44738
Subject
GRB 260429B / EP260429a: COLIBRÍ optical counterpart candidate
Date
2026-05-29T15:17:32Z (4 months ago)
From
F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Via
Web form
Francis Fortin (IRAP), 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), Jean-Grégoire Ducoin (CPPM), 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 reimaged the field of GRB 260429B / EP260429a (Wang et al., GCN Circ. 44426, Arya et al., GCN Circ. 44443) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-27 06:34 to 08:42 UTC (from 27.7 to 27.8 days after the trigger) and obtained 1.57 hours of exposure in the r and z filters.
The data were reduced and coadded with the ASU COLIBRÍ 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.
By comparing these observations with our first epoch reported by Fortin et al. (GCN Circ. 44434), we found that an uncatalogued optical source located at:
RA (J2000) = 240.58812 (16:02:21.15)
Dec (J2000) = -9.60116 (-09:36:04.2)
and with an uncertainty of 0.5”, is no longer detected in this second epoch down to the following (3-sigma) limits:
r > 23.90
z > 23.07
In our first epoch, for this source, we measured:
r = 21.95 +/- 0.05
z = 21.08 +/- 0.04
Given the fading nature of the source, the spatial coincidence with the source EPF J160221.2-093607 reported in by the EP team (Wang et al., GCN Circ. 44445), and the absence of any known minor planet at this position, we suggest this is the optical counterpart of GRB 260429B / EP260429a.
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 44445
Subject
GRB 260429B / EP260429a: EP-FXT follow-up observation
Date
2026-05-02T16:46:33Z (5 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. Wang (PMO, CAS; UCB), Y. L. Wang (NAO, CAS; ICE, CSIC), G. J. Yang, C.C Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed two follow-up observations of the EP-WXT detected transient GRB 260429B / EP260429a (Wang et al., GCN), also triggered by AstroSat CZTI (Arya et al., GCN 44443). The observations started at 2026-04-30T07:26:44 UTC and 2026-05-01T21:53:58 UTC, corresponding to approximately 16.8 and 55.2 hours after the EP-WXT trigger, with total exposure times of 9064 s and 5262 s, respectively.
Two X-ray sources were detected by FXT within the WXT error region:
EPF_J160221.2-093607
RA (J2000): 240.5884 deg
Dec (J2000): -9.6018 deg
with an uncertainty of 10 arcsec (radius, 90% C.L., statistical and systematic).
EPF_J160226.1-093337
RA (J2000): 240.6088 deg
Dec (J2000): -9.5602 deg
with an uncertainty of 10 arcsec (radius, 90% C.L., statistical and systematic).
The 0.5-10 keV flux of EPF_J160221.2-093607 decreased from 2.1e-12 to 5.8e-13 erg cm^-2 s^-1 between the two observations. In contrast, the flux of EPF_J160226.1-093337 remained nearly constant at about 6.6e-13 erg cm^-2 s^-1 in the same energy band. In addition, the position of EPF_J160226.1-093337 is 1.264 arcsec from Gaia DR3 4346772233753744896, suggesting that this Gaia source may be its counterpart. Therefore, EPF_J160221.2-093607 is the more likely X-ray afterglow of GRB 260429B / EP260429a. We note that follow-up observations were carried out by Eyles-Ferris et al. (GCN 44433), Fortin et al. (GCN 44434) and Lipunov et al. (GCN 44437), but no optical counterpart was detected in these observations. Further follow-up observations are encouraged.
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 44443
Subject
GRB 260429B / EP260429a: AstroSat CZTI detection
Date
2026-05-02T12:54:35Z (5 months ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
A. Arya (IITB), A. Goyal (IITB), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), S. Salunke (IUCAA), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long GRB 260429B which is also detected by Konus-Wind (priv. comm), and is associated with EP260429a (Wang et al., GCN Circ. 44426).
The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2026-04-29 14:42:59.27 UTC. The measured peak count rate associated with the burst is 147 (+31, -33) counts/s above the background in the combined data of all quadrants, with a total of 1802 (+485, -554) counts. The local mean background count rate was 1313 (+3, -3) counts/s. We measure a T90 of 34 (+6, -9) s from the cumulative Veto light curve.
The source was also faintly detected in the CZT detectors in the 20-200 keV energy range.
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI data products like interactive and downloadable light curves for this GRB can be found at:
https://astrosat.iucaa.in/cift/cift_products/515169782.27/S515169782.27_details.html
CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb
GCN Circular 44437
Subject
EP260429a: Global MASTER-Net observations report
Date
2026-05-01T10:09:31Z (5 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-OAGH robotic telescope [1] located in Mexico (OAGH National Institute for Astrophysics, Optics and Electronics) was pointed to the EP260429a ( EP Team et al., GCN 44426) errorbox 1 days 24768 sec after notice time and 1 days 69627 sec after trigger time at 2026-05-01 10:01:38 UT, with upper limit up to 18.6 mag. The observations began at zenith distance = 44 deg. The sun altitude is -29.1 deg.
The galactic latitude b = 30 deg., longitude l = 2 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3281864
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
156117 | 2026-05-01 10:01:38 | MASTER-OAGH | (16h 00m 56.52s , -09d 05m 48.0s) | C | 180 | 18.6 |
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 44434
Subject
GRB 260429B / EP260429a: COLIBRÍ optical upper limit
Date
2026-04-30T13:54:36Z (5 months ago)
From
F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Via
Web form
Francis Fortin (IRAP), Enrique Moreno Méndez (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), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), 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 EP GRB 260429A (Wang et al., GCN Circ. 44426) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-30 07:47:50 to 11:42:06 UTC (from 17.1 to 21.0 hours after the trigger) and obtained 139 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the ASU COLIBRÍ 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 the stacked image, we do not detect any new source at the WXT source position (Wang et al., GCN Circ. 44426) down to the following 5-sigma limit:
r > 23.56
z > 22.84
These upper limits are consistent with the ones reported by Eyles-Ferris et al. GCN Circ. 44433.
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 44433
Subject
EP260429a: LCO optical upper limits
Date
2026-04-30T11:20:40Z (5 months ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris (Leicester), J. A. Chácon (PUC), F. E. Bauer (SSI and UTA), J. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. J. Levan (Radboud & Warwick), P. G. Jonker (Radboud), report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP26029a (Wang et al., GCN 44426) using LCO 1m telescopes located at the Cerro Tololo Inter-American Observatory (Chile) equipped with the SINISTRO instrument. A series of 6x300 s exposures were taken in each of the SDSS-r and SDSS-z filters, starting at 2026-04-30 08:23:54 UT (~18.0 hr after the trigger).
No new source is detected within or at the border of the EP/WXT uncertainty region (Wang et al., GCN 44426) nor do we identify any significant residuals in subtractions using PanSTARRS DR1 templates. From the stacked images, we obtain the following 3-sigma upper limits:
r > 22.5
z > 20.9
These upper limits are in AB magnitudes, calibrated using nearby stars from the Pan-STARRS catalogue, and are not corrected for Galactic extinction.
GCN Circular 44426
Subject
EP260429a: EP-WXT detection of a fast X-ray transient
Date
2026-04-30T03:05:55Z (5 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. Wang (PMO), Y. L. Wang (NAO, CAS; ICE, CSIC), C.C Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260429a. The WXT light curve shows a single pulse with a duration of about 250 s. The transient started at 2026-04-29 14:41:11 UTC. The WXT position of the source is R.A. = 240.629 deg, DEC = -9.588 deg (J2000) with an uncertainty of 1.9 arcmin in radius (90% C.L. statistical and systematic).
The average WXT 0.5 - 4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 4.31 (-0.08/+0.09) x 10^21 cm^-2 and a photon index of 1.44 (-0.04/+0.04). The derived average unabsorbed 0.5-4 keV flux is 3.96 (-0.32/+0.37) x 10^-9 erg/s/cm^2.
Follow-up observation with the Follow-up X-ray Telescope on board EP has been scheduled. 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).