Gamma-ray Burst 260221C = EP260221a

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #43791
M.-H. Zhang, X. X. Sun, Tian-Yu 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 EP260221a. The transient triggered EP-WXT (ID: 01709258521) at 2026-02-21T11:43:14 (UTC). The WXT position of the source is R.A. = 178.749 deg, DEC = -20.978 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 at 2026-02-21T12:44:34 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 178.7568 deg, DEC = -20.9847 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
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 #43793
EP260221a: COLIBRÍ optical upper limit
Nikos Mandarakas (LAM), Asuka Kuwata (UNAM), Benjamin Schneider (LAM), Jean-Grégoire Ducoin (CPPM), 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), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the EP260221a (Zhang et al., GCN Circ. 43791) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-21T12:22:07 to 2026-02-21T13:05:02 UTC (from 38.87 to 81.80 minutes after the trigger, 30 seconds after the GCN notice) and obtained 32 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the Pan-STARRS-DR2 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 FXT source position (Zhang et al., GCN Circ. 43791) down to the following 5-sigma limit:
```
r > 23.1
z > 21.8
```
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 #43794
A. Kumar, G. Ramsay, D. O’Neill, K. Ulaczyk, S. Moran, B. P. Gompertz, R. Starling, M. Wortley, M. Kennedy, K. Ackley, M. Dyer, J. Lyman, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, M. Pursiainen, on behalf of GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) that serendipitously covered the field of EP/WXT detected EP260221a (WXT01709258521; Zhang et al., GCN 43791). Observations covering the localisation area were taken at 2026-02-21 13:15:35 UT (+1.54h post trigger) utilising GOTO-South. Each observation consisted of 4x45s exposures in the GOTO L-band (400-700 nm).
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.
No new transients that could be credibly associated with EP260221a were detected down to a 3-sigma depth of 20.9 mag (AB); see also Mandarakas et al., GCN 43793.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular #43798
Min-He (NAOC), Junjie-Jin (NAOC), Haiyang-Mu (NAOC), Feng-Xiao (NAOC), Yuguang-Sun (NAOC), Pengliang-Du (NAOC), Zhou-Fan (NAOC), Hong-Wu (NAOC) report on behalf of a large collaboration:
Following the detection of EP260221a (WXT01709258521; Zhang et al., GCN 43791), we observed the field of EP260221a using the 0.8-m and 2.16-m telescope at Xinglong Observatory, NAOC. We obtained 3x600s g-band frames by using 0.8-m with a median time of 2026-02-21T17:18:37.349, 4 hrs after the EP trigger and 3x600s clear-band frames by using 2.16-m with a median time of 2026-02-21T19:09:10, 6 hrs after the EP trigger.
No uncatalogued optical transient is detected in the stacked images within the 20 arcsec EP/FXT error circle (Zhang et al., GCN 43791), down to 3-sigma limiting magnitudes of g-band ~15.48 mag for 0.8-m and clear-band 20.71 mag for 2.16-m, calibrated with Pan-STARRS sources in the field. Also there is no apparent brightening for the catalogued sources within the error circle.
GCN Circular #43802
Tian-Yu Liu, X. X. Sun, M.-H. Zhang, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260221a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 43791). The optical upper limits were reported by COLIBRI telescope, GOTO and the 2.16m Xinglong Telescope (GCN 43793, GCN 43794, GCN 43798). The refined analysis of the WXT data shows that the event started at T0=2026-02-21T11:39:48 (UTC) and lasted for 550 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 3.5 x 10^20 cm^-2 and a photon index of 2.9 (-0.3/+0.4). The derived average unabsorbed 0.5-4 keV flux is 3.1 (-0.7/+0.5) x 10^(-10)erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source at 2026-02-21T12:44:34 (UTC), about 1 hour 5 minutes after T0. The exposure time of this observation is 5997s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 178.7560, DEC = -20.9819 (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 3.5 x 10^20 cm^-2 and a photon index of 1.36 (-0.22/+0.22). The derived average unabsorbed 0.5-10 keV flux is 6.9 (-1.6/+2.0) x 10^(-13)erg/s/cm^2.
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 #43803
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 EP260221a ( EP Team et al., GCN 43791) errorbox 44530 sec after notice time and 51263 sec after trigger time at 2026-02-22 01:57:37 UT, with upper limit up to 13.5 mag. The observations began at zenith distance = 76 deg. The sun altitude is -22.8 deg.
MASTER-OAFA robotic telescope located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260221a errorbox 53402 sec after notice time and 60136 sec after trigger time at 2026-02-22 04:25:30 UT, with upper limit up to 20.7 mag. The observations began at zenith distance = 27 deg. The sun altitude is -48.3 deg.
The galactic latitude b = 40 deg., longitude l = 286 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3149283
We obtain a following upper limits.
```
Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   51354 | 2026-02-22 01:57:37 |   MASTER-Kislovodsk | (11h 54m 51.01s , -20d 28m 04.3s) |   C |   180 | 13.5 |        
   60167 | 2026-02-22 04:25:30 |         MASTER-OAFA | (11h 50m 53.67s , -20d 40m 02.3s) |   C |    60 | 20.3 |        
   60227 | 2026-02-22 04:25:30 |         MASTER-OAFA | (11h 50m 53.67s , -20d 40m 02.4s) |   C |   180 | 20.5 |  Coadd 
   60243 | 2026-02-22 04:26:46 |         MASTER-OAFA | (11h 50m 48.37s , -20d 40m 39.3s) |   C |    60 | 20.3 |        
   60317 | 2026-02-22 04:28:00 |         MASTER-OAFA | (11h 50m 49.21s , -20d 39m 36.9s) |   C |    60 | 18.6 |        
   60395 | 2026-02-22 04:29:18 |         MASTER-OAFA | (11h 59m 28.08s , -20d 41m 19.6s) |   C |    60 | 19.8 |        
   60455 | 2026-02-22 04:29:18 |         MASTER-OAFA | (11h 59m 28.07s , -20d 41m 19.6s) |   C |   180 | 20.7 |  Coadd 
   60474 | 2026-02-22 04:30:37 |         MASTER-OAFA | (11h 59m 24.96s , -20d 40m 07.1s) |   C |    60 | 20.2 |        
   60551 | 2026-02-22 04:31:54 |         MASTER-OAFA | (11h 59m 25.03s , -20d 41m 43.3s) |   C |    60 | 20.2 |        
   75471 | 2026-02-22 08:39:34 |             MASTER- | (11h 56m 08.31s , -21d 31m 03.3s) |   C |   180 | 19.7 |        
```
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 #43805
Andrew J. Levan (Radboud), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. A. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (Leicester), J. Sanchez-Sierras (Radboud), D. Xu (NAOAC), report on behalf of a larger collaboration
We observed the field of the fast X-ray transient EP260221a (Zhang et al., GCN 43791; Liu et al., GCN 43802) using the GMOS-S instrument mounted on the Gemini South telescope (Cerro Pachon, Chile). A single 300 s z-band observation was secured starting on 2026 Feb 22 at 08:20 UT (20.62 hr after trigger).
No new source is detected within the EP/FXT error circle, down to a limiting magnitude z = 24.8 (AB, calibrated against nearby sources from the Pan-STARRS catalog, and not corrected for Galactic extinction). A number of faint sources are visible consistent with the EP/FXT position, but they are all present in archival imaging from the Legacy survey and from DECam.
Our results are consistent with other non-detection reports (Mandarakas et al., GCN 43793; Kumar et al., GCN 43794; He et al., GCN 43798; Lipunov et al., GCN 43803).
We thank excellent support from the Gemini staff, in particular Lindsay Magill.
GCN Circular #43806
Alan M. Watson (UNAM), Antonio de Ugarte Postigo (LAM), Rosa L. Becerra (UNAM), Benjamin Schneider (LAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Asuka Kuwata (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Fredd Sánchez Álvarez (UNAM) report:
We reobserved the field of the EP260221a (Zhang et al., GCN Circ. 43791) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-22 05:55 to 10:54 UTC (from 18.20 to 23.20 hours after the trigger) and obtained 71 minutes of simultaneous exposure in the r and z filters.
The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the Pan-STARRS-DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.
When we subtract the r-band image from our second epoch from the one from our first (Mandarakas et al., GCN Circ. 43793), we detect a residual source with a SNR of about 5 at:
```
RA (J2000) = 11:55:01.22 = 178.7551 deg 
Dec (J2000) = -20:58:57.6 = -20.9827 deg
```
The uncertainty on this position is 0.5 arcsec. The source is also seen in subtractions against PS1 images, but at a lower significance, and lies within the enhanced FXT uncertainty region (Liu et al., GCN Circ. 43802). We suggest it is the optical counterpart of the EP source.
We notice that these coordinates are consistent with the position of a galaxy present in the Legacy Survey catalog (Dey et al., 2019) with magnitudes g = 23.78 +/- 0.13, r = 23.68 +/- 0.14, i = 23.86 +/- 0.27 and z = 23.56 +/- 0.37 and with a photo-z = 1.22 +/- 0.55.
In the r band, the total magnitude of the galaxy and the candidate counterpart faded from 22.54 +/- 0.08 in our first epoch to 23.39 +/- 0.15 in our second epoch (a difference of 0.85 +/- 0.17) and in the z band it faded from 22.03 +/- 0.17 to 22.69 +/- 0.16 (a difference of 0.66 +/- 0.23). These fadings have significances of 5.0- and 2.8-sigma, respectively.
The detection of this candidate in r as well as z is consistent with the photo-z of the coincident galaxy Therefore, we suggest that this galaxy might be the host.
The non-detection of this source in deep imaging by Levan et al. (GCN Circ. 43805) suggests that it faded very quickly. This is perhaps consistent with temporal decay estimated from the two values reported in X-rays by (Liu et al., GCN Circ. 43802) alpha of approximately 2.2 and suggests the presence of an early jet break.
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 #43807
R. A. J. Eyles-Ferris and P. T. O’Brien (Leicester) report on behalf of a larger collaboration:
We observed the field of EP260221a (Zhang et al., GCN 43791; Liu et al., GCN 43802) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 6x200s exposures in the SDSS i’ and 8x200s exposures in the SDSS z’ filters starting at 2026-02-22 00:47:58 UT, approximately 14.1 hours after the X-ray detection.
Within the EP/FXT localisation (Liu et al., GCN 43802), we detect no new sources in our stacked images consistent with the results of Mandarakas et al. (GCN 43793), Kumar et al. (GCN 43794), He et al. (GCN 43798), Lipunov et al. (GCN 43803), and Levan et al. (GCN 43805).
At the position of the candidate counterpart identified by Watson et al. (GCN 43806), we measure i’ = 22.21 +/- 0.12 and z’ = 22.29 +/- 0.18 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction. As pointed out by Watson et al. (GCN 43806), this is significantly brighter than the magnitudes catalogued in Legacy DR10 and suggests this could indeed be the counterpart of EP260221a.
We derive 3-sigma upper limits of i’ > 23.4 and z’ > 23.0 for our stacked images.
GCN Circular #43813
Y.-H. Lee, A. Aryan, H.-Y. Hsiao, T.-W. Chen (all NCU), A. K. H. Kong (NTHU), J. H. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, M.-H. Lee, C.-H. Lai, Y.-C. Pan, C.-C. Ngeow, W.-J. Hou, C.-S. Lin, 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 EP260221a (Zhang et al., GCN 43791 and Liu et al., GCN 43802) using the 40cm SLT at the Lulin observatory, 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 in r-band started at 13:51 UTC on 21st of February 2026 (MJD 61092.577), 2.13 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. 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. Neither in the stacked frame nor in the difference image do we detect the optical counterpart candidate proposed by Watson et al. (GCN 43806) and confirmed by Eyles-Ferris et al. (GCN 43807). Clearly, our non-detection is attributable to the shallower detection limits in our observations.
Moreover, we used AutoPhOT to perform PSF photometry. The details of the observations and the measured 3-sigma upper limits (in the AB system) are as follows:
```
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT       | r      | 61092.577   | 2.13      | 300 * 24     | >20.7     | 3".42       | 2.11                    
```
The non-detection in our observations is consistent with the reports by Mandarakas et al. (GCN 43793), Kumar et al. (GCN 43794), and Min-He et al. (GCN 43798).
The presented magnitude is calibrated using the field stars from the ATLAS-refcat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_r = 0.075 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.

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