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[vsnet-grb-info 43262] EP260626a: EP-WXT detection of a fast X-ray transient
by GCN Circulars 26 Jun '26

26 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45047 SUBJECT: EP260626a: EP-WXT detection of a fast X-ray transient DATE: 26/06/26 15:13:06 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J.P. Chen (SYSU), K. J. Zhang, D. Zhu (YNU), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of a fast X-ray transient, designated EP260626a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was identified in the telemetry data. The transient was detected in the WXT observation starting at 2026-06-26T05:25:04 (UTC). A significant flare was detected in the source light curve, which started before the beginning of the observation and ended approximately 100 seconds after the start of the observation. The position of the transient is R.A. = 277.4417 deg, Dec. = 21.2403 deg (J2000), with an uncertainty radius of 2.7 arcmin in radius (90% confidence level, including both statistical and systematic uncertainties). There are no known X-ray sources in the WXT error circle. The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 2.04×10^21 cm^-2 and a photon index of 1.93 (-0.4/+0.42). The derived average unabsorbed 0.5-4 keV flux is 8.02 (-1.7/+2.1) x 10^(-11) erg cm^-2 s^-1. The uncertainties are at the 90% confidence level for the above parameters. Follow-up observations by EP-FXT will be arranged. Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/45047. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43261] Konus-Wind detection of GRB 260623B / EP260623a
by GCN Circulars 26 Jun '26

26 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45046 SUBJECT: Konus-Wind detection of GRB 260623B / EP260623a DATE: 26/06/26 15:11:51 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> D. Svinkin, D. Frederiks, A.Lysenko, A. Ridnaia, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long-duration GRB 260623B was detected by Konus-Wind (KW) in the waiting mode at about T0 = T0(EP) = 10445 s UT (02:54:05). A Bayesian block analysis of the KW waiting mode data in the 20-300 keV band reveals a ~10 sigma count-rate increase in the interval from T0-259 s to T0+327 s. The burst overlaps in time with EP260623a (EP-WXT detection: Wang et al., GCN 45020; Wang et al., GCN 45035) and the KW ecliptic latitude response is consistent with the EP260623a localization. The positional and temporal coincidence of GRB 260623B with the EP260623a supports the conclusion that both events have a common origin. The KW light curve of this burst is available at http://www.ioffe.ru/LEA/GRBs/GRB260623B The total burst fluence is 8.2(-1.2,+1.2)x10^-6 erg/cm^2, and the 2.944 s peak energy flux, measured from T0+3.366 s, is 0.7(-0.3,+0.4)x10^-7 erg/cm^2/s. (both in the 20 keV - 1500 keV energy range). The time-integrated spectrum of the burst, measured from T0-258.650 s to T0+327.206 s, can be described by a power law with exponential cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha > -1.16 and Ep = 76(-8,+28) keV. The spectrum near the maximum count rate (measured from T0-2.522 to T0+38.694 s) can be described with the CPL model with alpha > -1.42 and Ep = 87(-15,+83) keV. Assuming the redshift z=0.703 (Izzo et al., GCN 45022) and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimate the following rest-frame parameters: the isotropic energy release E_iso is 1.13(-0.16,+0.24)x10^52 erg, the peak luminosity L_iso is 1.71(-0.72,+0.97)x10^50 erg/s, the rest-frame peak energy of the time-averaged spectrum Ep,i,z is 130(-14,+47) keV and the spectrum near the maximum count rate Ep,p,z is 148(-26,+143) keV. With the obtained estimates, GRB 260623B is inside the 68% prediction band for the 'Amati' and inside the 90% prediction band for the 'Yonetoku' relations derived for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021), see http://www.ioffe.ru/LEA/GRBs/GRB260623B/GRB260623B_rest_frame.pdf All the quoted errors are estimated at the 68% confidence level. All the presented results are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/45046. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43260] EP260623a: TNG NIR afterglow detection
by GCN Circulars 25 Jun '26

25 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45045 SUBJECT: EP260623a: TNG NIR afterglow detection DATE: 26/06/25 15:50:24 GMT FROM: Riccardo Brivio at INAF-OAB <riccardo.brivio(a)inaf.it> R. Brivio (INAF-OAB), M. Ferro (INAF-OAB), L. Izzo (INAF-OACN and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), A. Melandri (INAF-OAR), V. Lorenzi (INAF-TNG), A. G. de Gurtubai Escudero (INAF-TNG) report on behalf of the CIBO collaboration: We observed the field of EP260623a detected by EP/WXT (Wang et al., GCN 45020) with the Italian 3.6m TNG telescope equipped with the near-infrared camera NICS to follow up its afterglow. A series of images were obtained with the K filter starting on 2026-06-25 at 01:29:13 UT (i.e. 1.94 days after the trigger). The afterglow (Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee et al., GCN 45030; Saikia et al., GCN 45034; Bochenek & Perley, GCN 45040; Busmann et al., GCN 45042) is detected in the co-added image with a preliminary magnitude of K = 19.9 +/- 0.2 mag (Vega; calibrated against the 2MASS catalogue), at a mid-time of 1.96 days after the trigger. View this GCN Circular online at https://gcn.nasa.gov/circulars/45045. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43259] GRB 260618A: Fermi GBM Observation
by GCN Circulars 25 Jun '26

25 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45044 SUBJECT: GRB 260618A: Fermi GBM Observation DATE: 26/06/25 13:02:58 GMT FROM: Christian Malacaria at INAF-OAR <cmalacaria.astro(a)gmail.com> C. Malacaria (INAF-OAR) and R. Sonawane (IISER, TVM) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 06:45:34.28 UT on 18 June 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260618A (trigger 803457939/260618282). Final Real-time localization was reported in GCN 44973. A possible afterglow is detected by EP (Yang et al., GCN 44978, Jiang et al., GCN 44991). The GBM light curve consists of a variable emission episode with a duration (T90) of about 140 s (50-300 keV). The time-averaged spectrum from T0-17 to T0+150 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.4 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 43 +/- 2 keV. The event fluence (10-1000 keV) in this time interval is (5.9 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+25 s in the 10-1000 keV band is 2.7 +/- 0.3 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/" View this GCN Circular online at https://gcn.nasa.gov/circulars/45044. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43258] MASS-Cube (GRID 2.0) detection of GRB 260616B
by GCN Circulars 25 Jun '26

25 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45043 SUBJECT: MASS-Cube (GRID 2.0) detection of GRB 260616B DATE: 26/06/25 08:24:21 GMT FROM: GRID Student Team at Tsinghua University <grid(a)tsinghua.edu.cn> Jiahuan Zhu, Chenyu Wang, Xutao Zheng, Hao Chang, Zirui Yang report on behalf of the MASS-Cube Collaboration: MASS-Cube reports the detection of the long-duration GRB 260616B, which was also detected by Fermi/GBM (GCN #44953), EP (GCN #44956 and #44976), CALET (GCN #44966), SVOM (GCN #45011), HXMT (GCN #45023) and GRID-09. The event was triggered by MASS-Cube on 2026-06-16 at 07:05:14.5 UTC. The measured burst duration (T90) in the 50-10000 keV range is approximately 14.1 +/- 1.6 seconds. The MASS-Cube light curve of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB260616B/MASS_Cub…. MASS-Cube is a pathfinder for a 3D position-sensitive Compton telescope in space, currently considered the next generation of the GRID constellation in the era of multi-messenger astronomy. For more information about MASS-Cube, please refer to the following reference: https://doi.org/10.1007/s10686-024-09920-4. View this GCN Circular online at https://gcn.nasa.gov/circulars/45043. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43257] EP260623a: FTW optical and NIR observations
by GCN Circulars 25 Jun '26

25 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45042 SUBJECT: EP260623a: FTW optical and NIR observations DATE: 26/06/25 08:19:33 GMT FROM: Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann(a)physik.lmu.de> Malte Busmann (LMU), Julius Gassert (LMU/CMU), Xander Hall (CMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report: We observed the counterpart of EP260623a (Wang et al., GCNs 45020 and 45035; Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee et al., GCN 45030; Saikia et al., GCN 45034; Bochenek & Perley, GCN 45040) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 20 x 180 s starting at 2026-06-24T23:13:34 UT (1.85 days after the trigger). We detect the counterpart in all bands and measure an r-band magnitude of r = 23.93 ± 0.22 AB mag. The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction. We thank Christoph Ries from the Wendelstein Observatory for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/45042. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43256] GRB 260619A: SVOM/ECLAIRs refined analysis
by GCN Circulars 24 Jun '26

24 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45041 SUBJECT: GRB 260619A: SVOM/ECLAIRs refined analysis DATE: 26/06/24 20:31:19 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Nicolas Dagoneau (CEA), Marius Brunet (IRAP), Wenjin Xie (NAOC) Using the event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 260619A (SVOM burst-id sb26061901). The burst that triggered ECLAIRs onboard (GCN 44985) consists of a faint single peak lightcurve with a duration of about 40 sec in the 8-50 keV energy band. The time-averaged spectrum (from T0-40.96 s to T0) in the energy range 5-120 keV is best fit by a blackbody model with a temperature kT = 6.6 -1.3/+1.6 keV. With this model, the 4-120 keV fluence is (2.03 +/- 0.63)*E-7 erg/cm² and the 4-120 keV photon flux is (2.02 +/- 0.33)*E-1 ph/cm²/s. All the quoted errors are at the 68% confidence level. 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. The SVOM/ECLAIRs point of contact for this burst is: N. Dagoneau (nicolas.dagoneau at cea.fr) View this GCN Circular online at https://gcn.nasa.gov/circulars/45041. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43255] EP260623a: Liverpool Telescope optical observations
by GCN Circulars 24 Jun '26

24 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45040 SUBJECT: EP260623a: Liverpool Telescope optical observations DATE: 26/06/24 17:59:07 GMT FROM: A. Bochenek at Liverpool John Moores University <a.m.bochenek(a)2023.ljmu.ac.uk> A. Bochenek, D. A. Perley (LJMU), report: We observed the field of EP260623a (Waang et al., GCN 45020; Wang et al., GCN 45035) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x120s exposures in SDSS griz filters, starting at 2026-06-24 01:15:57 UT, approximately 22.4 hours after EP trigger. Multiple exposures across all filters had to be discarded prior to stacking due to an issue with the telescope rotator. We detect a source in the images in all filters at the position reported by Li et al. (GCN 45021), also Izzo et al. (GCN 45022); Angulo et al. (GCN 45024); Brivio et al. (GCN 45025); Jiang et al. (GCN 45026); Li et al. (GCN 45027); Jelinek et al. (GCN 45029); Lee et al. (GCN 45030); Saikia et al. (GCN 45034). The preliminary photometry is: | MJD (mid) | T_mid-T_0 | Filter | Mag. (AB) | |---------------|-------------|-----------|---------------| | 61215.05694 | 22.49 h | g | > 21.44 | | 61215.06840 | 22.76 h | r | 21.96 ± 0.23 | | 61215.08921 | 23.26 h | i | 21.59 ± 0.13 | | 61215.09872 | 23.50 h | z | 21.06 ± 0.16 | The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/45040. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43254] Fermi GBM trigger 804010507/260624677 is not a GRB
by GCN Circulars 24 Jun '26

24 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45039 SUBJECT: Fermi GBM trigger 804010507/260624677 is not a GRB DATE: 26/06/24 17:44:48 GMT FROM: rhamburg(a)usra.edu R. Hamburg (USRA) reports report on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 804010507/260624677 at 16:15:02.91 UT on 24 June 2026, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to particle activity." View this GCN Circular online at https://gcn.nasa.gov/circulars/45039. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 43253] Konus-Wind detection of GRB 260618B
by GCN Circulars 24 Jun '26

24 Jun '26

TITLE: GCN CIRCULAR NUMBER: 45038 SUBJECT: Konus-Wind detection of GRB 260618B DATE: 26/06/24 17:14:43 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 260618B (Fermi-GBM detection: Fermi GBM team, GCN 44979; Veres & Meegan, GCN 45010; Fermi-LAT detection: Gupta et al., GCN 44983) triggered Konus-Wind at T0=60907.637 s UT (16:55:07.637). The burst light curve shows a multipeaked structure which starts at ~T0-1.6 s and has a total duration of ~61.2 s. The emission is seen up to ~4 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB260618_T60907/ As observed by Konus-Wind, the burst had the total fluence of 2.26(-0.34,+0.43)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+0.378 s, of 4.90(-0.98,+1.01)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+57.600 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.83(-0.16,+0.19) and Ep = 1066(-241,+365) keV (chi2 = 94/98 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.7 (chi2 = 94/97 dof). The spectrum near the maximum count rate (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.39(-0.14,+0.15) and Ep = 817(-97,+119) keV (chi2 = 96/88 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.2 (chi2 = 96/87 dof). 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/45038. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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