GRB 260601B — All Circulars

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.

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.

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.

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.

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.

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.

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.

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).

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).