EP260601a, GRB 260601B
GCN Circular 44806
Subject
Konus-Wind detection of GRB 260601B/EP260601a
Date
2026-06-03T16:14:45Z (an hour ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 260601B
(GECAM-B detection: Yu et al., GCN 44767; Wang et al., GCN 44768;
NuSTAR detection: Waratkar et al., GCN 44771;
CALET-GBM detection: Sugita et al., GCN 44773;
Insight-HXMT detection: Wang et al., GCN 44785;
Glowbug detection: Cheung et al., GCN 44801)
triggered Konus-Wind at T0=69173.304 s UT (19:12:53.304).
The burst light curve shows a multipeaked structure
which starts at ~T0-3.2 s and has a total duration of ~230 s.
The emission is seen up to ~8 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260601_T69173/
As observed by Konus-Wind, the burst
had the total fluence of 3.34(-0.17,+0.17)x10^-4 erg/cm2,
and the 64-ms peak flux, measured from T0+27.552 s,
of 1.66(-0.19,+0.19)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+210.688 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.79(-0.10,+0.12),
the high energy photon index beta = -1.74(-0.05,+0.04),
the peak energy Ep = 441(-68,+82) keV
(chi2 = 125/97 dof).
The 'peak-flux' spectrum
(measured from T0+24.832 to T0+31.232 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.89(-0.07,+0.07),
the high energy photon index beta = -1.82(-0.06,+0.05),
the peak energy Ep = 671(-98,+117) keV
(chi2 = 108/82 dof).
Assuming a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimated the burst
isotropic energy release E_iso, the isotropic peak luminosity L_iso, and
the rest-frame peak spectral energy Ep,z of the burst at different redshifts.
With the obtained estimates GRB 260601B is consistent with the 68% prediction band
of the 'Amati' relation for z >~0.2 and with the 68% prediction band of the 'Yonetoku' relation
for z>~1.0 (see http://www.ioffe.ru/LEA/GRBs/GRB260601_T69173/GRB260601B_rest_frame.pdf)
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
GCN Circular 44804
Subject
EP260601a/GRB 260601B: EP-FXT follow-up observation
Date
2026-06-03T14:11:07Z (3 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
G. J. Yang (NAO, CAS), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Wang (PMO, CAS; UCB), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the fast X-ray transient EP260601a (detected by EP-WXT, GCN 44766; followed by Liu et al., GCN 44764, Guelfand et al., GCN 44765, Li et al., GCN 44769, Mo et al., GCN 44777 and Liu et al., GCN 44789). This event is also temporally consistent with GRB 260601B reported by GECAM-B (Yu et al., GCN 44767, Wang et al., GCN 44768), NuSTAR (Waratkar et al., GCN 44771), CALET (Sugita et al., GCN 44773) and Insight-HXMT (Wang et al., GCN 44785).
The observation started at 2026-06-03 01:13:47 UTC, about 30 hours after the WXT trigger, with an exposure time of 3.5 ks. The FXT telemetry data show that an uncatalogued source was detected within the WXT error circle at R.A. = 257.6584, DEC = -1.6447 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT X-ray spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 9 x 10^20 cm^-2. The best-fit photon index is 1.31 (-0.28/+0.29). The unabsorbed 0.5-10 keV flux is 9.2 (-2.2/+3.2) x 10^-13 erg cm^-2 s^-1.
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 44803
Subject
GRB 260601B / EP260601a: Mondy optical upper limit
Date
2026-06-03T13:30:59Z (4 hours ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), 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 GRB 260601B / EP260601a (Yang et al., GCN 44766; Yu et al., GCN 44767; Wang et al., GCN 44768, 44785; Waratkar et al., GCN 44771; Sugita et al., GCN 44773; Cheung GCN 44801) with the AZT-33IK telescope of the Mondy observatory starting on 2026-06-02 UT 15:45:12 and taking several 120-second exposures in the R-band. In the sdtacked frame we do not detect the optical counterpart reported previously (Liu et al., GCN 44764; Guelfand et al., GCN 44765; Mo et al., GCN 44777; Liu et al., GCN 44789 ).
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)
2026-06-02 15:45:12 0.87559 20*120 R n/d n/d 21.1 Mondy/AZT-33IK
The photometry is based on nearby USNO-B1.0 stars (R2-magnitudes) and is not corrected for Galactic extinction.
GCN Circular 44801
Subject
GRB 260601B/EP260601a: Glowbug gamma-ray detection
Date
2026-06-03T12:57:03Z (4 hours ago)
From
C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil>
Via
Web form
C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 260601B, which was detected initially by Einstein Probe as EP260601a (GCN 44766). The GRB was also detected by GECAM-B (GCN 44767, 44768), NuSTAR (GCN 44771), CALET (GCN 44773), and Insight-HXMT (GCN 44785).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-06-01 19:12:50.296 with a duration of 39.7 s and a total significance of about 17.3 sigma. The light curve comprises an initial 2.05-s duration burst followed by a long-duration episode from ~T0+16.4s to +39.7s. Note that data from ~T0-2s to +1s, ~T0+5s to +10s, and ~T0+37s to +40s suffered from deadtime in various detectors.
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12.
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular 44789
Subject
GRB260601B/EP260601a: NOT near-infrared observations
Date
2026-06-03T06:41:27Z (10 hours ago)
From
liuxing@nao.cas.cn
Via
Web form
X. Liu, J. An, L.B. He, S.Q. Jiang, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu (HUST), J.P.U. Fynbo (DAWN/NBI), A. M. Kadela report (NOT):
We observed the field of GRB260601B/EP260601a (Yang et al., GCN 44766; Yu et al., GCN 44767; Waratkar et al., GCN 44771; Sugita et al., 44773), using the 2.56m Nordic Optical Telescope (NOT) equipped with the NOTCam near-infrared camera. Observation started at 2026-06-03 00:53:22 UT, i.e., 29.67 hr post-burst, and 25x60s J-band frames were obtained.
The previously reported optical counterpart (Liu et al., GCN 44764; Guelfand et al., GCN 44765; Li et al., GCN 44769) was detected in our stacked image with a brightness of J = 20.7 +/- 0.1 (Vega) at a median time of 29.94 hr post-trigger, calibrated with the 2MASS catalog and not corrected for Galactic extinction.
We acknowledge the excellent support from the NOT staff, in particular A. M. Kadela.
GCN Circular 44785
Subject
GRB 260601B/EP260601a: Insight-HXMT detection
Date
2026-06-03T06:12:43Z (11 hours ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Zheng-Hang Yu, Shao-Lin Xiong, Cheng-Kui Li, and Chao Zheng report on behalf of the Insight-HXMT team:
At 2026-06-01T19:12:49.900 (T0), Insight-HXMT/HE detected the possible long duration Type I burst GRB 260601B/EP260601a, which is also detected by GECAM-B (Yu et al., GCN #44767; Wang et al., GCN #44768), NuSTAR (Waratkar et al., GCN #44771), CGBM (S. Sugita et al., GCN #44773) and EP (Yang et al., GCN #44766).
The Insight-HXMT/HE light curve mainly consists of a possible precursor and then a short pulse followed by a long emission with a T90 of 76.0 +4.5/-4.0 s.
The total counts from this burst is 175433 counts.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb260601B.png
All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.
GCN Circular 44777
Subject
GRB 260601B/EP260601a: J-band upper limit with WINTER
Date
2026-06-02T23:04:15Z (18 hours ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form
Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (NOIRLab), Benjamin Schneider (LAM), Robert Stein (UMD), Viraj Karambelkar (Columbia), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of GRB 260601B/EP260601a (Yang et al., GCN 44766; Yu et al., GCN 44767; Wang et al., GCN 44768; Waratkar et al., GCN 44771; Sugita et al., GCN 44773) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began at 2026-06-02T11:12:20 UTC in the J band (~16 hr after the GRB trigger), consisting of 15 x 120 s exposures. 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).
We do not detect a source at the optical counterpart location (Liu et al., GCN 44764; Guelfand et al., GCN 44765; Li et al., GCN 44769). We obtain the following 5-sigma upper limit: J = 18.8 mag (AB).
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.
GCN Circular 44773
Subject
GRB 260601B: CALET Gamma-Ray Burst Monitor detection
Date
2026-06-02T18:35:44Z (a day ago)
From
Yuta Kawakubo at Aoyama Gakuin University <kawakubo@phys.aoyama.ac.jp>
Via
Web form
S. Sugita, A. Yoshida, T. Sakamoto, Y. Kawakubo (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii,
Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U),
N. Cannady (GSFC), M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:
The CALET Gamma-ray Burst Monitor (CGBM) detected GRB 260601B
at 19:12:46.66 UTC on 1 June 2026 (trigger #1464376270;
https://cgbm.calet.jp/cgbm_trigger/flight/1464376270/index.html).
A CGBM Notice was distributed in near real time.
This event was also reported by GECAM-B (Yu et al., GCN #44767;
Wang et al., GCN #44768) and NuSTAR (Waratkar et al., GCN #44771).
The burst signal was seen by all CGBM detectors.
The burst light curve shows a multi-peaked structure starting
at T+0.1 s, peaking at T+4.9 s, and ending at T+101.4 s.
The T90 and T50 durations measured with the SGM data are 70.0 +/- 4.7 s
and 21.9 +/- 1.0 s in the 40-1000 keV band, respectively.
Einstein Probe/WXT detected an X-ray transient, EP 260601a
(Yang et al., GCN #44766), about 35 s after the CGBM trigger.
The position of EP 260601a was above the Earth horizon as seen from
CALET, and was observable by CGBM at the trigger time, with an incident
angle of 83 degrees.
The ground-processed light curve is available at:
https://cgbm.calet.jp/cgbm_trigger/ground/1464376270/
The CALET data used in this analysis were provided by
the Waseda CALET Operation Center located at Waseda University.
GCN Circular 44771
Subject
GRB 260601B / EP260601a: NuSTAR detection of prompt emission
Date
2026-06-02T16:42:17Z (a day ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:
The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 260601B, associated with EP260601a, in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper.
The NuSTAR SINGS algorithm, triggered at 2026-06-01T19:12:46.65 UTC, shows a detection of GRB 260601B, also detected by CALET-GBM (Trigger 1464376270), GECAM-B (Yu et al., GCN 44767), and is associated with EP260601a detected by EP-WXT (Yang et al., GCN 44766).
The NuSTAR CsI shield data are recorded at 1 Hz. We detect a short peak followed by a long burst lasting for a total of about 75-s, consistent with the CALET-GBM & GECAM-B lightcurve. The peak count rate is ~2500-cps with a baseline rate of ~1000-cps during this time period. We also see marginal evidence in the signal above 100 keV in the CZT detectors.
The EP/WXT localization (GCN 44766) at RA = 257.649, Dec = -1.647 implies an offset from the NuSTAR boresight of 98-deg (i.e. from the side) and an offset from the geocenter of 120-deg.
Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260601B
Information on NuSTAR SINGS can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/
NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.
GCN Circular 44769
Subject
GRB 260601B/EP260601a: SVOM/VT optical observations
Date
2026-06-02T15:23:39Z (a day ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
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) and J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations to the field of GRB 260601B/EP260601a triggered by Einstein Probe (Yang et al., GCN 44766), which was also detected by GECAM-B (Yu et al., GCN 44767; Wang et al., GCN 44768). The VT observation started at 2026-06-02T08:56:14 UTC, 13.72 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical counterpart (Liu et al., GCN 44764; Guelfand et al., GCN 44765) was detected by VT in both channels. The following measurements are in the AB magnitude without correction for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
14.65 h VT_B 76*50 s 23.4 +/- 0.3 mag
14.63 h VT_R 62*50 s 21.43 +/- 0.15 mag
The VT red color suggests a moderate redshift ~ 4 (Wang et al.,2020).
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 44768
Subject
GRB 260601B/EP260601a: GECAM-B analysis suggests a possible long duration Type I burst
Date
2026-06-02T15:04:42Z (a day ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Zheng-Hang Yu, Shao-Lin Xiong, Chao Zheng, Cheng-Kui Li (IHEP) report on behalf of GECAM team:
With the event-by-event data downloaded through the ground station, we conducted the standard analysis of GRB 260601B/EP260601a. The GECAM-B light curve shows that this burst exhibits a temporal profile similar to that of GRB 060614, together with an exceptionally hard spectrum.
The initial pulse is clearly detected above 500 keV, while the subsequent “extended” emission, which is temporally consistent with the EP/WXT trigger, is even harder than the initial pulse and is clearly detected above 1 MeV.
Using the localization reported by EP/WXT (R.A. = 257.649 deg, Dec = -1.647 deg, GCN#44766) , the time-averaged spectrum from T0-8 s to T0+90 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.36 +0.05/-0.05 and the cutoff energy, parameterized as Epeak, is 3600 +830/-770 keV. The event fluence (10-1000 keV) in this time interval is (1.03 +/-0.02)E-04 erg/cm^2.
GRB 260601B is therefore consistent with Type I GRBs in the Amati-relation diagram, as shown at:
https://www.bursthub.cn//admin/static/gecambgrb260601B_amati.png
Moreover, the main pulse, corresponding to T0+1 s to T0+5 s, can also be fitted by a power law function with an exponential high-energy cutoff, with power law index of -1.10 +0.14/-0.15, and Epeak of 715 +249/-162 keV.
Considering its GRB 060614-like light-curve pattern, exceptionally hard spectrum, and location in the Amati-relation diagram, GRB 260601B/EP260601a could be a possible long-duration Type I GRB.
Follow-up observations are strongly encouraged.
GCN Circular 44767
Subject
GRB 260601B: GECAM-B detection of a burst probably associated with EP260601a
Date
2026-06-02T13:22:14Z (a day ago)
Edited On
2026-06-02T16:16:07Z (a day ago)
From
yzh807926@163.com
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of yzh807926@163.com
Via
Web form
Zheng-Hang Yu, Chen-Wei Wang, Shao-Lin Xiong, Chao Zheng (IHEP) report on behalf of GECAM team:
GECAM-B was triggered in-flight by a burst GRB 260601B at 2026-06-01T19:12:48.050 UTC (denoted as T0). According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a possible precursor and then a short pulse followed by a long emission with a duration (T90) of 66.5 +2.5/-3.5 s.
The on-ground localization of GECAM-B is:
Ra: 271.2 deg
Dec: -5.5 deg
Err: 2.7 deg (1-sigma, statistical only)
Considering systematic uncertainties, this localization is consistent with the EP/WXT localization of EP260601a (Yang et al., GCN#44766). The time coincidence and localization coincidence between GRB 260601B and EP260601a strongly support the association of these two events.
The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb260601B.png
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
GCN Circular 44766
Subject
EP260601a: Einstein Probe detection of an X-ray transient
Date
2026-06-02T11:52:06Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
G. J. Yang (NAO, CAS), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Wang (PMO, CAS; UCB), W. D. Zhang (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 EP260601a. The transient triggered EP-WXT (ID: 01709266307) at 2026-06-01T19:13:22 UTC. The WXT position of the source is R.A. = 257.649 deg, Dec = -1.647 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The preliminary analysis of the WXT data shows that the burst started at T_0 = 2026-06-01T19:12:41 UTC. The light curve shows an initial flare lasting approximately 100 s, followed by tail emission. During the flare, the unabsorbed 0.5-4 keV flux was 5.2 (-1.7/+3.6) x 10^-08 erg cm^-2 s^-1.
The time-averaged WXT 0.5-4 keV spectrum over the entire detected emission can be described by an absorbed power law, with the hydrogen column density left as a free parameter. The best-fit hydrogen column density is 6.7 (-1.5/+1.6) x 10^21 cm^-2. The best-fit photon index is 1.2 (-0.3/+0.3). The unabsorbed 0.5-4 keV flux is 2.4 (-0.2/+0.3) x 10^-10 erg cm^-2 s^-1.
A follow-up observation with the Follow-up X-ray Telescope (FXT) onboard EP is 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).
GCN Circular 44765
Subject
EP-WXT trigger 01709266307 (EP260601a): COLIBRÍ optical observations
Date
2026-06-02T11:05:28Z (a day ago)
From
Marion G <marion.guelfand@gmail.com>
Via
Web form
Marion Guelfand (CPPM), Camila Angulo (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), 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) and Antonio de Ugarte Postigo (LAM) report:
We observed the field of the EP-WXT Trigger 01709266307 (EP260601a) with the DDRAGO two-channel wide-field imager mounted on the COLIBRÍ telescope. We observed from 2026-06-02 08:36 UTC to 09:00 UTC (from 13.38 to 13.39 hours after the trigger) and obtained 20 minutes of simultaneous exposure in the r and z filters, respectively.
The data were reduced and coadded with the COLIBRI 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.
We detected the optical counterpart reported by Liu et al., GCN Circ. 44764.
The preliminary magnitude derived for that source is:
r = 21.93 +/- 0.15
z = 21.07 +/- 0.12
The source is fading between the two observations.
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.
GCN Circular 44764
Subject
EP-WXT trigger 01709266307 (EP260601a): JinShan optical counterpart detection
Date
2026-06-02T10:23:04Z (a day ago)
From
liuxing@nao.cas.cn
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X. Liu, Z.P. Zhu, D. Xu, J. An, S.Q. Jiang, L.B. He (NAOC), S.Y. Fu, A.D. Zhu, L. Lei (HUST), J.Z. Liu (XAO) report:
We observed the field of EP-WXT Trigger 01709266307 (EP260601a) detected by Einstein Probe (EP), using the JinShan 100B telescope located at Altay, Xinjiang, China. A series of r-band frames were obtained.
An uncatalogued and decaying source is detected within the EP/WXT error circle at coordinates
R.A. (J2000) = 17:10:38.24
Dec. (J2000) = -01:38:42.4
with an uncertainty of ~ 1.0 arcsec. The source had r ~ 18.4 mag at a median time of ~ 0.73 hr post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction of A_V ~ 1.15 mag. We thus think the source is the optical counterpart of EP260601a.
We also note the presence of a faint source with g ~ 25.2 mag in the Legacy Survey DR10 image at the above position, which could be the host of EP260601a.
We acknowledge the excellent support from T.Q. Chen, and J.F. Zhang for enabling these observations.