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[vsnet-grb-info 42940] EP260602b: Liverpool Telescope and LCO observations
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44809 SUBJECT: EP260602b: Liverpool Telescope and LCO observations DATE: 26/06/03 18:45:03 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), J. N. D. van Dalen (Radboud), J. Chacón (PUC), F. E. Bauer (SSI and UTA), R. L. C. Starling (Leicester), P. G. Jonker (Radboud), A. J. Levan (Radboud & Warwick), report on behalf of a larger collaboration: We observed the field of EP260602b (Hu et al., GCNs 44786, 44802) with the 2m Liverpool Telescope (LT) on La Palma using the IO:O instrument and an LCO 1m telescope located at Cerro Tololo Interamerican Observatory (Chile) and equipped with the SINISTRO instrument. We obtained exposures in the Sloan r and g filters. We detect the optical counterpart (He et al., GCN 44780) in our LT and LCO r band stacked images. We obtain the following photometry for the counterpart calibrated to Pan-STARRS and not corrected for Galactic extinction: | t_mid - t_0 (hours) | Telescope/Instrument | Filter | Exposure (s) | AB magnitude | | --- | --- | --- | --- | --- | | 8.2 | LT/IO:O | r | 6x150 | 21.61 +/- 0.28 | | 8.5 | LT/IO:O | g | 6x150 | >21.80 | | 8.8 | LCO/SINISTRO | r | 5x300 | 21.68 +/- 0.23 | View this GCN Circular online at https://gcn.nasa.gov/circulars/44809. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42939] GRB 260603B: Fermi GBM Final Localization
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44808 SUBJECT: GRB 260603B: Fermi GBM Final Localization DATE: 26/06/03 18:16:31 GMT FROM: Eva M. Palafox at INAOE <eva.palafox(a)gmail.com> The Fermi GBM team reports the detection of a likely LONG GRB "At 16:44:46.61 UT on 03 June 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260603B (trigger 802197891/260603698). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 350.27, Dec = 39.20 (J2000 degrees, equivalent to J2000 +23h 21m, +39d 12'), with a statistical uncertainty of 3.16 degrees. The angle from the Fermi LAT boresight is 63 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260603698/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260603698/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260603698/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/44808. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42938] EP260602a: GTC and Liverpool Telescope optical observations
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44807 SUBJECT: EP260602a: GTC and Liverpool Telescope optical observations DATE: 26/06/03 16:41:18 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), R. A. J. Eyles-Ferris (Leicester), J. N. D. van Dalen (Radboud), P. G. Jonker (Radboud), R. L. C. Starling (Leicester), A. J. Levan (Radboud & Warwick), J. Chacón (PUC), I. Yanes (GTC), A. Tejero (GTC), S. Geier (GTC) report on behalf of a larger collaboration: We observed the field of EP260602a (Li et al., GCN 44770) with the Gran Telescopio Canarias (GTC) and the Liverpool Telescope (LT), both on the island of La Palma, equipped with the OSIRIS+ and IO:O instruments, respectively. Observations were performed in the Sloan r and z filters. We detect the optical/NIR counterpart (Xu et al., GCN 44781; Watson et al., GCN 44790; Li, GCN 44794; He et al. GCN 44798) in the GTC r and z stacked images, and marginally in the LT r image. We obtain the following photometry for the counterpart, calibrated to Pan-STARRS and not corrected for Galactic extinction: | t_mid - t_0 (hours) | Telescope/instrument | Filter | Exposure (s) | AB magnitude | | --- | --- | --- | --- | --- | | 16.7 | LT/IO:O | r | 7x150 | 22.62 +/- 0.29 | | 17.0 | GTC/OSIRIS+ | r | 10x60 | 23.20 +/- 0.10 | | 17.1 | GTC/OSIRIS+ | z | 10x45 | 22.70 +/- 0.10 | | 17.4 | LT/IO:O | z | 4x150 | >22.15 | View this GCN Circular online at https://gcn.nasa.gov/circulars/44807. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42937] Konus-Wind detection of GRB 260601B/EP260601a
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44806 SUBJECT: Konus-Wind detection of GRB 260601B/EP260601a DATE: 26/06/03 16:14:45 GMT FROM: Anna Ridnaia at Ioffe Institute <ridnaia(a)mail.ioffe.ru> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44806. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42936] GRB 260603A: EP-FXT follow-up observation
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44805 SUBJECT: GRB 260603A: EP-FXT follow-up observation DATE: 26/06/03 14:21:26 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), J. W. Hu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed an automatic follow-up observation of GRB 260603A detected by SVOM/ECLAIRs (SVOM/sb26051504, ZHAO et al., GCN 44787). The follow-up observation started at 2026-06-03T07:38:06 UTC, approximately 2 hours after the SVOM trigger, with a total exposure time of 2.4 ks. On-ground analysis of the FXT data found an uncatalogued source within the ECLAIRs error circle at R.A., Dec. = 233.1435, 40.0258 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). This position is consistent with the optical counterpart (Guelfand et al., GCN 44791). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density of 1.7 (-0.8/+0.8) × 10^21 cm^-2 and a photon index of 2.4 (-0.3/+0.3). The derived average unabsorbed flux in the 0.5-10 keV band is 4.0 (-0.4, +0.5) x 10^(-12) 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/44805. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42935] EP260601a/GRB 260601B: EP-FXT follow-up observation
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44804 SUBJECT: EP260601a/GRB 260601B: EP-FXT follow-up observation DATE: 26/06/03 14:11:07 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/44804. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42934] GRB 260601B / EP260601a: Mondy optical upper limit
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44803 SUBJECT: GRB 260601B / EP260601a: Mondy optical upper limit DATE: 26/06/03 13:30:59 GMT FROM: Alina Volnova at IKI RAS <alinusss(a)gmail.com> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44803. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42933] EP260602b: EP-FXT follow-up observation
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44802 SUBJECT: EP260602b: EP-FXT follow-up observation DATE: 26/06/03 13:21:53 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J. W. Hu (NAO, CAS), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: The X-ray transient EP260602b was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 44786) at 2026-06-02T17:53:12 (UTC). Follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-03T04:30:31(UTC), ~10.5 hours after the WXT detection. The exposure time of the observation is around 4 ks. The on-ground analysis of the FXT data found an uncatalogued source at R.A. = 273.4737 deg, DEC = 7.5702 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the position of the optical counterpart (He et al., GCN 44780). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.9×10^21 cm^-2, and a photon index of 2.0 (-0.7, +0.8). The derived average unabsorbed 0.5-10 keV flux is 1.4 (-0.6, +1.1)×10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/44802. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42932] GRB 260601B/EP260601a: Glowbug gamma-ray detection
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44801 SUBJECT: GRB 260601B/EP260601a: Glowbug gamma-ray detection DATE: 26/06/03 12:57:03 GMT FROM: C.C. Cheung at Naval Research Lab <Teddy.Cheung(a)nrl.navy.mil> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44801. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42931] GRB 260603A: Kilonova-Catcher optical upper limit
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44800 SUBJECT: GRB 260603A: Kilonova-Catcher optical upper limit DATE: 26/06/03 12:36:36 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro(a)hotmail.com> 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 260603A detected by SVOM/ECLAIRs (Zhao et al., GCN 44787) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the T11 itelescope located at the Utah Desert Remote Observatory (USA) and operated by M. Freeberg. Our observations started at TGRB+1.78 hours and were taken with the sdssr filter. In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect the optical counterpart reported by SVOM/COLIBRÍ (FM-GFT) (Guelfand et al., GCN 44791; GCN 44797) and SVOM/VT (Li et al., GCN 44799). We report our follow-up results in the table below: | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | | ------------ | ------------ |------------ |--------------- |------------ | | 2.25 | 11 x 300s | r (AB) | 20.1 (3 sigma) | iT11 | 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/) View this GCN Circular online at https://gcn.nasa.gov/circulars/44800. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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