GRB 260402A — All Circulars

Y. Q. Zhao (USTC,PRIC), C. L. Guo, J. W. Hu, C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260402a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (GCN #44186), and temporally and spatially consistent with GRB 260402A (GCN #44181). The telemetry WXT data show that the flare started at 2026-04-02T10:29:33 (UTC) and exhibits a double-peaked burst lasting about 200 s. The average WXT 0.5-4 keV spectrum of the burst period can be fitted with an absorbed power law with a hydrogen column density of 0.76（-0.36/+0.41）×10^22 cm^-2 and a photon index of 2.09(-0.77/+0.84). The derived average unabsorbed 0.5-4 keV flux is 2.68 (-0.83/+1.97) ×10^(-9) erg/s/cm^2.
 
The Follow-up X-ray Telescope (FXT) on board EP observed this source at 2026-04-02T12:55:33 (UTC, T0+8760 s). The exposure time of this observation is 3.0ks. On-ground analysis of the FXT data found an uncatalogued source at R.A., Dec. = 170.9607, 63.6128 (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 Galactic hydrogen column density of 0.40 (-0.13/+0.14) × 10^22 cm^-2 and a photon index of 3.20 (-0.51/+0.55). The derived average unabsorbed 0.5-10 keV flux is 2.91 (-0.73/+1.46) × 10^(-12) erg/s/cm^2.

The optical and near-infrared follow-up observations were performed by Antier et al. (GCN 44183), Carrasco et al. (GCN 44184), Freeberg et al. (GCN 44185), Wu et al. (GCN 44189), Li et al. (GCN 44190), Vijaykumar et al. (GCN 44191), Schneider et al. (GCN 44192) and He et al. (GCN 44199). 

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).

L. B. He, X. Liu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei, H.Z. Wu, W.H. Lei, Y.C. Zou (HUST), J. An, S.Q. Jiang, Z.P. Zhu, D. Xu (NAOC), J.Z. Liu (XAO), Z.N. Dong, B. Ma (SYU) report:

We observed the field of EP260402a/GRB 260402A detected by EP (Zhao et al., GCN 44186) and SVOM (Tan et al., GCN 44181; Gotz et al., GCN 44182), using the JinShan 100C telescope located at Altay, Xinjiang, China. Observation started at 2026-04-02 14:51:48 UT, i.e., 4.36 hrs post-burst, and a series of J-band frames were obtained. 

No new source is detected in the stacked image within the EP/FXT error circle, down to a 5-sigma limiting magnitude of J ~ 17.3 (Vega), calibrated with 2MASS stars in the field. This upper limit is consistent with that reported at WINTER (Schneider et al., GCN 44192).

We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.

Benjamin Schneider (LAM), Geoffrey Mo (Caltech/Carnegie), Robert Stein (UMD), Viraj Karambelkar (Columbia), Tomas Ahumada (NOIRLab), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of GRB 260402A detected by SVOM/ECLAIRs (Tan et al., GCN 44181) and EP/WXT (Zhao et al., GCN Circ. 44186) in the near-infrared with the Palomar 1-m telescope, equipped with the 1-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations started on 2026-04-02 at 10:58:52 UT (28.58 min after the trigger) and consisted of 30 exposures of 120 s in the J-band.

The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565). The photometric calibration was performed using nearby stars from the 2MASS catalog. The magnitude 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 position (Zhao et al., GCN Circ. 44186) or MXT position (Gotz et al., GCN Circ. 44182) and derive the following 5-sigma upper limit:

J > 18.9

Our upper limit is consistent with previously reported optical upper limits from Antier et al. (GCN Circ. 44183), Carrasco et al. (GCN Circ. 44184), Freeberg et al. (GCN Circ. 44185), Wu et al. (GCN Circ. 44189), Li et al. (GCN Circ. 44190), and Vijaykumar et al. (GCN Circ. 44191).

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

V. Vijaykumar, A. Khade, R. Arya, V. Swain, S. Patil, A.P. Saikia, T. Mohan, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and R. Norbu (IAO) report on behalf of the GIT team:

We observed the field of GRB260402A detected by SVOM/ECLAIRs (SVOM, GCN 44181), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2026-04-02 14:10:35 (UTC), i.e., 3.65 hours after the trigger, and obtained multiple exposures in the r' filter. We do not detect any transient in our image within the uncertainty region of source position reported by ECLAIRs and MXT  (Tan et al., GCN Circ. 44181, Gotz et al., GCN Circ.44182). The photometric upper limit is as follows:

| MJD (mid)    | Filter | tmid-t0 (hours) | Exposure Time (sec) | Magnitude (AB) |
| ------------ | ------ | -------------- | ------------------- | -------------- |
| 61132.590| r'    | 3.92           | 5x360               | 20.5 |


The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

Our magnitude for the optical counterpart is consistent with other optical observations (GCN 44190 Li et al.; GCN 44189 Wu et al.; GCN 44185 Freeberg et al.; GCN 44184 Carrasco et al.; GCN 44183 Antier et al.)

The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

H. L. Li, C. Wu, Y. L. Qiu, L. P. Xin, Y. N. Ma, 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), W. Tan (IHEP) and R. Li (YNAO) report on behalf of the SVOM/VT team.

SVOM/VT performed observations with automatic slew to the field of GRB 260402A triggered by SVOM/ECLAIRs (sb26032123, Tan et al., GCN 44181), SVOM/GRM (Tan et al., GCN 44181) and Einstein Probe (EP260402a, Zhao et al., GCN 44186). The observation started at 2026-04-02T10:44:07 UTC, approximately 13.8 minutes post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. 

No uncatalogued sources are detected in single or stacked images within the error box of SVOM/MXT (Gotz et al., GCN 44182) or EP-FXT (Zhao et al., GCN 44186), compared to the Legacy Survey. The 3-sigma limit magnitudes are VT_B ~ 23.1 mag and VT_R ~ 23.0 mag at 31.3 minutes post trigger.

Our result is consistent with the non-detection reports from SVOM/COLIBRÍ (Antier et al., GCN 44183), MASTER (Carrasco et al., GCN 44184), Kilonova-Catcher (Freeberg et al., GCN 44185) and SVOM/C-GFT (Wu et al., GCN 44189). 

Given the early optical non-detection, near-infrared follow-up observations are encouraged.

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.

C. Wu (NAOC), Z. Kang (CHO), L.P. Xin, X.H. Han, P.P. Zhang, X.M. Lu (NAOC), Z.W. Li, Y. Lv (CHO), R.S. Zhang, Y.J. Xiao, Y.L. Qiu, J. Wang, J.S. Deng, L. Huang, J.Y. Wei (NAOC) ,  W. Tan, R. Li (IHEP) report on behalf of the SVOM/C-GFT team:

We observed the field of GRB 260402A (SVOM burst-id sb26040203) detected by SVOM/ECLAIRs (Tan et al., GCN 44181) with LATIOS on SVOM/C-GFT. Observations started at 2026-04-02T12:50:51 UTC, ~2.34 hr after the trigger. 

We obtained  i-band follow-up imaging.  After preliminary analysis，no credible candidate was detected within the error box provided by  SVOM/MXT (Gotz et al., GCN 44182) and EP/MXT (Zhao et al., GCN 44186) in our stacked image. The 3-sigma upper limit is: 

| Mid_t - T0 (hr) | Exposure Time (s) | Band | Upper Limit (AB) |
|----------------|-------------------|------|------------------|
| 2.69            | 30×30              | i    | 19.0            |

The photometry was calibrated against Pan-STARRS1 stars  and no correction for Galactic dust extinction was applied.

This result is consistent with  Antier et al. (GCN 44183) and Freeberg et al.(GCN 44185).
 

We thank the observation assistants Ying-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.

Y. Q. Zhao (USTC,PRIC), C. L. Guo, J. W. Hu, C. Jin (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 EP260402a. The transient triggered EP-WXT (ID: 01709259488) at 2026-04-02T10:30:41 (UTC). The WXT position of the source is R.A. = 170.994 deg, DEC = 63.604 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). EP260402a is temporally and spatially consistent with GRB 260402A (GCN #44181).

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically triggered by SVOM GRB detection, began at 2026-04-02T12:55:33Z. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 170.9607 deg, DEC = 63.6128 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).

M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN), S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 260402A detected by SVOM/ECLAIRs (Tan et al., GCN 44181) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the TEC160FL telescope operated by M. Freeberg. Our observations started at TGRB+20.8 min and were taken with the sdssr filter.

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect any optical counterpart in the SVOM/ECLAIRs error radius.

We report our follow-up results in the table below:
+----------------+----------+---------+---------------+--------------+
| Tmid-TGRB (min)| Exp (s)  | Filter  | Magnitude     | Instrument   |
+================+==========+=========+===============+==============+
| 29.97          | 5 x 180s | r (AB)  | 19.5 (5 sigma)| TEC160FL     | 
+----------------+----------+---------+---------------+--------------+

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022).  Images obtained with the Sloan filters were calibrated using the PanSTARRS DR1 catalog.

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). 

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez, A.R.Corella,
J.Tanori, L. Villalobos, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
V.Lipunov, A.Kuznetsov, I.Panchenko, P.Balanutsa, G.Antipov, E.Gorbovskoy, N.Tiurina,
V.Senik, A.Chasovnikov, Ya.Kechin, V.Topolev, D.Vlasenko, A.Sankovich,Yu.Tselik (Lomonosov MSU),
M.J. Segura, C.Francile,  F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
N.Budnev, O.Gress (ISU),
A.Sosnovskij (CrAO),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)

MASTER-OAGH robotic telescope of MASTER Global Robotic Net [1-4]
started SVOM GRB 260302A (Tan et al. GCN 44181, Gotz et al. GCN 44182 Ttrigger=2026-04-02 10:30:17UT)
at 2026-04-02 10:55:46UT (25min after trigger time) with m_lim=19.1m(unfiltered) at 1st image. We didn't find new object inside error-box at single images.Observations and reduction will be continued.


Error-box altitude  was 34deg, the sun_alt=-28deg, the FULL moon (0.99) alt=27deg (in 75 deg from erro-box) Cover map is available here
https://master.sai.msu.ru/site/master2/event.php?id=3227188


[1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L
[2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271
[3] Lipunov et a. 2019, ARep, vol.63, 293
[4] Lipunov et al. 2023, Astronomical Robotic Networks and Operative Multichanel
Astrophysics,Lomonosov MSU PRESS, 591pp.
 http://www.pereplet.ru/lipunov/625.html#625

Sarah Antier (IJCLAB), 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), 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), Nikos Mandarakas (LAM), 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), Antonio de Ugarte Postigo (LAM), W. Tan (IHEP), R. Li (IHEP) report:

We imaged the field of the SVOM GRB 260402A (Tan et al., GCN Circ. 44181) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-04-02 10:37:04 to 11:52:05 UTC (from 7 minutes after the trigger) and obtained about 45 min of exposure in the r/z filters.

The data were reduced and coadded with the ASU and Colibri 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.

In the stacked image, we do not detect any new source at the ECLAIRs and MXT source position (Tan et al., GCN Circ. 44181, Gotz et al., GCN Circ.44182) and down to the following 3-sigma limit:

r > 22.6
z > 22.0

Further observations are ongoing.

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.

D. Gotz (CEA Saclay) on behalf of the SVOM MXT team reports:

MXT observed the field of GRB 260402A at 10:53:24 U.T., about 23 minutes after the ECLAIRs trigger.
The automatic on board software detects a faint source at

R.A.= 170.806 deg (J2000)
Dec.= +63.627 deg (J2000)

with a R90 uncertainty (incdluding systematics) of 114 arc sec.

This position is at 4.7 arc min from the ECLAIRs position reported by Tan et al. (GCN 44181).

Further data will be needed to confirm the afterglow nature of this candidate source.

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this alert is Wenjun Tan: tanwj at ihep.ac.cn.
Please contact the BA by email if you require additional information.

W. Tan, R. Li (IHEP), M. Brunet, O. Godet (IRAP), D. Gotz (CEA), report on behalf of the SVOM mission team:

At 2026-04-02T10:30:17 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260402A (SVOM burst-id sb26040203).

The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 17 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 18.74 in the [5-20] keV energy band over a time window of 10.20 seconds starting at 2026-04-02T10:30:17.

The localization of the best alert is R.A., Dec. 170.982, 63.622 degrees:
R.A. (J2000) = 11h23m55s
Dec. (J2000) = 63d37m18s
 with a 90% confidence level (C.L.) radius of 4.57 arcmin (including systematic error of 2 arcmin added in quadrature).

The SVOM/ECLAIRs light curve showed a broad peak structure with a T90 duration of 15.17 (-1.27 / +5.86) s in the 5-120 keV energy band.

This burst was also detected by SVOM/GRM with a significance of 7.20.

SVOM slewed to the burst.

No prompt X-ray observation could be performed by SVOM/MXT for the time being.
 
VT began observing the field after the slew. The analysis of the data will be published in a future circular.


The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this alert is Wenjun Tan: tanwj at ihep.ac.cn.
Please contact the BA by email if you require additional information.