Gamma-ray Burst 260302B = EP260302a

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #43898
Wenxiong Li (NAOC), Iair Arcavi (TAU), Ido Keinan (TAU), Runduo Liang (NAOC), David Sand (U of Arizona)
We observed the position of the EP/WXT trigger 01709258736 with a Las Cumbres 1m telescope at Cerro Tololo Interamerican Observatory, Chile, 1 hour after the Einstein Probe WXT trigger. We took 2x300s exposures in each visit in the broad optical w band.
We find an uncataloged source at RA=132.9739, Dec=-65.9837 within the EP/WXT error circle and measure the following preliminary photometry calibrated to the R band: MJD 61102.042 Mag 19.8
GCN Circular #43899
Y. J. Zhang (THU), Y. H. Cheng, D. Zhu, K. J. Zhang (YNU), H. W. Pan (NAOC) 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 EP260302a. The transient triggered EP-WXT (ID: 01709258736) at 2026-03-02T23:39:22 (UTC). The WXT position of the source is R.A. = 133.015 deg, DEC = -66.007 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. = 132.9655 deg, DEC = -65.9859 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).
GCN Circular #43903
I. Pérez-García, S.-Y. Wu, E. Fernandez-García, M. D. Caballero-García, M. Gritsevich, R. Sánchez-Ramírez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Pérez del Pulgar (Univ. de Málaga), Y.-D. Hu (GXU), D. R. Xiong (Yunnan Observatories of CAS), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP260302a by the Einstein Probe (Zhang et al. GCN 42899), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) observed the fast X-ray transient location starting on Mar 3, 00:04:05 UT (11.1 min after trigger) in different optical bands. On a coadd of clear-filter images (4x60s, 00:10:26 UT, mid exposure time) we detect at early times the optical counterpart reported by LCO 1 hr after the trigger (Li et al. GCN 43898), within the EP/FXT 20 arcsec radius error circle, with 19.1 +/- 0.2 mag.
We thank the staff at San Pedro de Atacama Celestial Explorations observatory for their excellent support.
GCN Circular #43904
G. Corcoran (UCD), L. Cotter (UCD), J. Chacón (PUC), J. A. Quirola-Vasquez (Radboud), A. Martin-Carrillo (UCD), F. E. Bauer (UTa), P. G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260302a (Zhang et al., GCN 43899) with the LCO 1m telescope located at the Cerro Tololo Inter-American Observatory (Chile) equipped with the SINISTRO instrument. We obtained 6 x 300 exposures in each of the r and z filters, starting on 2026-03-03 at 02:20:48 UT (~2.46 hr post the EP trigger).
In our stacked images for both filters we detect the optical counterpart reported by Li et al. (GCN 43898) and Perez-Garcia et al. (GCN 43903), with the following magnitudes:
```
r = 20.84+/-0.11 
z = 20.38+/-0.14
```
The counterpart has faded significantly since the previous reports. These values are in the AB system and are calibrated to Pan-STARRS. These values are not corrected for Galactic extinction.
GCN Circular #43906
Y. J. Zhang (THU), Y. H. Cheng, D. Zhu, K. J. Zhang (YNU), H. W. Pan (NAOC) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260302a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-03-02T23:52:58 (UTC) (Zhang et al., GCN 43899), and was followed by several telescopes (Li et al., GCN 43898, Pérez-García et al., GCN 43903, Corcoran et al., GCN 43904). The X-ray source detected by WXT exhibited a flare within the first 100 seconds of the observation. The X-ray source detected by WXT has a 0.5--4 keV flux of approximately 1.4 x 10^-10 erg s^-1 cm^-2, with an power-law index of 1.1. The NH value is fixed to galactic value (8.6 x 10^20 cm^-2).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically, starting at 2026-03-02T23:54:17(UTC), about 2 minutes after the trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 132.9717 deg, Dec. = -65.9839 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The uncatalogued X-ray source revealed a decline in its light curve during the FXT observation. Its X-ray spectrum has a 0.5–10 keV flux of 1.84(+0.05/-0.04) x 10^-10 erg s^-1 cm^-2, with a power-law index of 1.46(+0.04/-0.04) and a NH value of 8.6 x 10^20 (fixed galactic value) + 5.7 (+1.5/-1.4) x 10^20 cm^-2. All errors are performed with 90% significance.
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 #43913
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-SAAO robotic telescope [1] located in South Africa (South African Astronomical Observatory) was pointed to the EP260302a ( EP Team et al., GCN 43899) errorbox 57834 sec after notice time and 69373 sec after trigger time at 2026-03-03 18:55:35 UT, with upper limit up to 18.3 mag. The observations began at zenith distance = 38 deg. The sun altitude is -22.0 deg.
The galactic latitude b = -14 deg., longitude l = 282 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3165840
We obtain a following upper limits.
```
Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   69433 | 2026-03-03 18:55:35 |         MASTER-SAAO | (08h 50m 51.10s , -66d 07m 10.9s) |   C |   120 | 18.3 |        
   69433 | 2026-03-03 18:55:35 |         MASTER-SAAO | (08h 46m 26.64s , -65d 51m 45.5s) |   C |   120 | 18.1 |        
   69556 | 2026-03-03 18:58:07 |         MASTER-SAAO | (08h 50m 47.34s , -66d 05m 34.5s) |   C |    60 | 18.1 |        
   69556 | 2026-03-03 18:58:07 |         MASTER-SAAO | (08h 46m 22.97s , -65d 50m 10.3s) |   C |    60 | 18.0 |        
```
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 #43928
G. Corcoran (UCD), L. Cotter (UCD), J. Quirola-Vasquez (Radboud), A. Martin-Carrillo (UCD), R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the optical counterpart (Li et al., GCN 43898; Pérez-García et al., GCN 43903; Corcoran et al., GCN 43904) of the fast X-ray transient EP260302a (Zhang et al., GCN 43899; Zhang et al., GCN 43906) on two epochs. Our first observation was carried out with the Gemini South telescope located in Cerro Pachon (Chile); and the second with the LCO 1m telescope located in Sutherland (South Africa). The exposure times were 1x30 and 9x300 s, respectively, both in the SDSS r band, at mean epochs 2026-03-03 at 07:25:03 UT and 2026-03-05 at 19:05:14 UT (~7.8 and ~67.4 hr after trigger, respectively).
The counterpart is clearly detected on both epochs. We measure magnitudes r = 22.06 +- 0.12 (Gemini) and r = 21.07 +/- 0.06 (LCO). Both values are calibrated against nearby stars from the SkyMapper catalog, and are not corrected for Galactic extinction.
The observed fluxes imply a rebrightening by ~1 mag, which is uncommon for FXT and GRB afterglows around this epoch, and possibly indicates the emergence of an extra component (for another example, see e.g. van Dalen et al. 2025, doi:10.3847/2041-8213/adbc7e).
We encourage further multiwavelength follow up of this unusual source.
GCN Circular #43932
A. J. Levan (Radboud), G. Corcoran (UCD), J. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud), F. E. Bauer (UTa), J. Chácon (PUC), L. Cotter (UCD), R. A. J. Eyles-Ferris (Leicester), A. Martin-Carrillo (UCD), R. Ruiz (Gemini), V. Firpo (Gemini) report on behalf of a larger collaboration:
Taking advantage of its rebrightening (Corcoran et al., GCN 43928), we observed the optical counterpart (Li et al., GCN 43898; Pérez-García et al., GCN 43903; Corcoran et al., GCNs 43904, 43928) of the fast X-ray transient EP260302a (Zhang et al., GCNs 43899, 43906) using Gemini-South equipped with the GMOS spectrograph.
In the acquisition image (mid time 3.18 days after trigger) we measure r = 21.30 +- 0.07 AB, suggesting some fading compared with our previous observation (Corcoran et al., GCN 43928).
Four exposures of 600 s each were obtained, using grating B480, a slit width of 1 arcsec, and covering the wavelength range 4470-8500 Å. The observation mid time was 2026 March 6.182 UT (3.20 days after the EP/WXT trigger).
Continuum is significantly detected over the entire covered wavelength range. From the lack of Hydrogen absorption, we infer a firm upper limit z < 2.67. A number of clear absorption features are also visible, which we interpret as Fe II (2344, 2374, 2383, 2586, 2600 AA) and Mg II (2796, 2803 AA), all at a common redshift z = 1.535. Given the good S/N of the continuum, and the lack of any further unidentified features, we therefore conclude that EP260302a likely lies at this redshift.
GCN Circular #43935
Y. N. Ma, L. P. Xin, H. L. Li, Z. H. Yao, Y. L. Qiu, C. Wu, X. H. Han, Y. Xu, 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 a Target of Opportunity observation of EP260302a detected by EP/WXT (Zhang et al., GCN 43899; Zhang et al., GCN 43906). SVOM/VT began observing the field at 2026-03-05T07:05:29 UTC, 55.435 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data available, the optical counterpart (Li et al., GCN 43898; Pérez-García et al., GCN 43903; Corcoran et al., GCN 43904; Corcoran et al., GCN 43928; Levan et al., GCN 43932) was detected in both VT_B and VT_R bands. The AB magnitudes are:
```
mid time (h) | exposure time (s) | band | mag (AB) 
-------------|-------------------|------|------------
    58.03    |      53*50        | VT_B |  21.4+-0.1
    58.03    |      50*50        | VT_R |  21.2+-0.1 
```
Our photometry was not corrected for Galactic extinction.
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 #43962
F. Carotenuto (INAF-OAR), J. Bright (Oxford), P. G. Jonker (Radboud), on behalf of a larger collaboration
We observed the field of EP260302a detected by the Einstein Probe (GCN 43899, 43906, 43932) with the MeerKAT radio telescope on 2026-03-10, from 18:00 UTC to 18:46 UTC (with a total time on source of 44 minutes) using the S-band receiver at a central frequency of 3.06 GHz. Analysing the SARAO Science Data Processor output continuum image we detect a source at the position of EP260302a (RA 08:51:53.7, DEC -65:59:01) with a flux density of around 170 uJy/beam, and with a typical image noise of around 8 uJy/beam.
Further observations are planned.
We thank the SARAO staff for rapidly scheduling these observations. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. This work has made use of the "MPIfR S-band receiver system" designed, constructed and maintained by funding of the MPI für Radioastronomie and the Max-Planck-Society.

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Jochen Greiner, last update: 11-Mar-2026 [Disclaimer]