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[vsnet-grb-info 42950] GRB 260603A: Simeiz Zeiss-1000 optical upper limit
by GCN Circulars 04 Jun '26

04 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44819 SUBJECT: GRB 260603A: Simeiz Zeiss-1000 optical upper limit DATE: 26/06/04 09:46:23 GMT FROM: Nicolai Pankov at IKI <colinsergesen(a)gmail.com> N. Pankov (IKI), A. Pozanenko (IKI), A. Novichonok (KIAM), A. Volnova (IKI) report on behalf of IKI-GRB-FuN We observed the field of GRB 260603A (Zhao et al., GCN 44787; Li et al., GCN 44805) with the Zeiss-1000 1-meter telescope of Simeiz Observatory starting on 2026-06-03 UT 19:38:36 (~0.6 days since trigger). A series of 20x180 sec exposures was acquired in the R-filter using a CCD-photometer. We do not detect the optical afterglow (Guelfand et al., GCN 44791, 44797; Li et al., GCN 44799; Freeberg et al., GCN 44800). The preliminary upper limit and the observation details are summarized below: Date UTstart t-T0 Exp. Filter Mag Err. UL (mid,days) (n*s) (3-sigma) 2026-06-03 19:38:36 0.60443 20*180 R n/d n/d 22.3 The photometry is based on nearby USNO-B1.0 stars and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44819. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42949] GRB 260604B: Fermi GBM Final Real-time Localization
by GCN Circulars 04 Jun '26

04 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44818 SUBJECT: GRB 260604B: Fermi GBM Final Real-time Localization DATE: 26/06/04 08:58:18 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely SHORT GRB At 08:45:55 UT on 4 Jun 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260604B (trigger 802255560.356374 / 260604365). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 173.4, Dec = -19.9 (J2000 degrees, equivalent to J2000 11h 33m, -19d 53'), with a statistical uncertainty of 10.8 degrees. The angle from the Fermi LAT boresight is 60.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604365/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604365/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604365/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44818. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42948] GRB 260601A: CALET Gamma-Ray Burst Monitor detection
by GCN Circulars 04 Jun '26

04 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44817 SUBJECT: GRB 260601A: CALET Gamma-Ray Burst Monitor detection DATE: 26/06/04 08:23:39 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University <kawakubo(a)phys.aoyama.ac.jp> K. Yamaoka (Nagoya U), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU), 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 the short GRB 260601A at 11:00:12.042 UTC on 1 June 2026 (trigger #1464346716; https://cgbm.calet.jp/cgbm_trigger/flight/1464346716/index.html) A CGBM Notice was distributed in near real time. This event was also reported by Fermi GBM (The Fermi GBM team, GCN #44755; Holzmann Airasca et al., GCN #44760), Glowbug (Cheung et al., GCN #44763), GECAM-B (Wang et al., GCN #44778), SVOM/GRM (Wang et al., GCN #44793), and KONUS-Wind (IPN_RAW Notice at 2026-06-01 11:00:09.16 UTC). The burst signal was seen by all CGBM detectors. The burst light curve shows a multi-peaked structure, starting at T+0.40 s, peaking at T+0.50 s, and ending at T+0.59 s. The T90 and T50 durations measured with the SGM data are 0.18 +/- 0.01 s and 0.05 +/- 0.03 s in the 40-1000 keV band, respectively. The ground-processed light curve is available at: https://cgbm.calet.jp/cgbm_trigger/ground/1464346716/ The CALET data used in this analysis were provided by the Waseda CALET Operation Center located at Waseda University. View this GCN Circular online at https://gcn.nasa.gov/circulars/44817. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42947] EP260602a: EP-FXT follow-up observation
by GCN Circulars 04 Jun '26

04 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44816 SUBJECT: EP260602a: EP-FXT follow-up observation DATE: 26/06/04 07:09:04 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Wang (PMO, CAS; UCB), G. J. Yang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: The X-ray transient EP260602a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 44770) at 2026-06-02T08:11:20 (UTC). Follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-03T14:02:47(UTC), ~30 hours after the WXT detection. The exposure time of the observation is around 6 ks. The on-ground analysis of the FXT B data found an uncatalogued source at R.A. = 240.8343 deg, DEC = 50.4636 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is approximately consistent with the position of the optical counterpart (Xu et al., GCN 44781; Watson et al., GCN 44790; Malesani et al., GCN 44807). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.03×10^20 cm^-2, and a photon index of 1.6 (-0.9, +1.2). The derived average unabsorbed 0.5-10 keV flux is 7.1 (-4.3, +13.0) ×10^(-14) 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/44816. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42946] EP260603a: Global MASTER-Net observations report
by GCN Circulars 04 Jun '26

04 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44815 SUBJECT: EP260603a: Global MASTER-Net observations report DATE: 26/06/04 06:18:16 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), M. Segura, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAGH robotic telescope [1] located in Mexico (OAGH National Institute for Astrophysics, Optics and Electronics) was pointed to the EP260603a ( EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> et al., GCN 44814) errorbox 436 sec after notice time and 43770 sec after trigger time at 2026-06-04 06:00:42 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 58 deg. The sun altitude is -33.4 deg. The galactic latitude b = 25 deg., longitude l = 340 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3359661 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 43860 | 2026-06-04 06:00:42 | MASTER-OAGH | (15h 18m 40.19s , -26d 59m 12.9s) | C | 180 | 17.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/44815. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42945] EP260603a: Einstein Probe detection of a fast X-ray transient
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44814 SUBJECT: EP260603a: Einstein Probe detection of a fast X-ray transient DATE: 26/06/04 05:48:48 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. F. Liang (PMO), J. W. Hu, Y. L. Wang, Z. X. Ling(NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260603a. The ground analysis of the WXT data shows that the event started at 2026-06-03T17:51:12 (UTC) and lasted for about 350 s. The WXT position of the source is R.A. = 229.721 deg, DEC = -26.940 deg (J2000) with an uncertainty of 2.8 arcmin in radius (90% C.L. statistical and systematic). The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.56 × 10^21 cm^-2, and a photon index of 1.11 (-0.77, +0.80). The derived average unabsorbed 0.5-4 keV flux is 1.2 (-0.4, +0.6)×10^(-10) erg/s/cm^2. The peak flux is about 9.5×10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. A Target-of-Opportunity observation with the Follow-up X-ray Telescope (FXT) has been scheduled. Further information will be updated when the telemetry data is received. Further multi-wavelength observations are encouraged to determine the nature of this source. 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/44814. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42944] EP260602d: Optical Upper limits with kinder observations
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44813 SUBJECT: EP260602d: Optical Upper limits with kinder observations DATE: 26/06/04 02:54:17 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941(a)gmail.com> A. Aryan, T.-W. Chen, W.-J. Hou, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang (HNAS), Y.-H. Lee, A. Sankar.K, M.-H. Lee, A. Dutta, Y.-C. Pan, C.-C. Ngeow, K. N.-T. Ho, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, K. W. Smith, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report: We observed the field of the FXT EP260602d (Cao et al., GCN 44796) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in r-band started at 14:34 UTC on the 3rd of June 2026 (MJD 61194.6066), 17.33 hours after the EP-WXT detection. We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We next used the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. Neither in the stacked nor in the difference image did we detect any signature of any new or uncataloged source within the WXT region of uncertainty. Moreover, we further employed AutoPhOT to perform PSF photometry. The details of the observations and the 3-sigma upper limit (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude| avg. Seeing | Med. Airmass LOT | r | 61194.6066 | 17.33 | 300 * 6 | >22.8 | 1".28 | 1.18 The presented upper limit is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and is not corrected for an expected galactic extinction of A_r = 0.19 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69. View this GCN Circular online at https://gcn.nasa.gov/circulars/44813. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42943] GRB 260604A: Fermi GBM Final Real-time Localization
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44812 SUBJECT: GRB 260604A: Fermi GBM Final Real-time Localization DATE: 26/06/04 02:36:52 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 02:24:27 UT on 4 Jun 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260604A (trigger 802232672.54125 / 260604100). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 60.4, Dec = -12.8 (J2000 degrees, equivalent to J2000 04h 01m, -12d 48'), with a statistical uncertainty of 9.3 degrees. The angle from the Fermi LAT boresight is 94.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604100/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604100/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604100/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44812. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42942] EP260602b: Liverpool Telescope observations
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44811 SUBJECT: EP260602b: Liverpool Telescope observations DATE: 26/06/03 20:01:36 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 EP260602b (Hu et al., GCN 44786) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x150s exposures in SDSS i and z filters, starting at 2026-06-02 02:53:23 UT, approximately 8.99h hours after trigger. We report an i-band detection at the position of the counterpart (He et al., GCN 44780; Hu et al., GCN 44802; Eyles-Ferris et al., GCN 44809), with i = 20.7 ± 0.25, and a tentative z-band detection of z = 20.8 ± 0.4 (or a 3 sigma upper limit of z > 20.4 mag). 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/44811. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42941] EP260602a: Liverpool Telescope optical upper limits
by GCN Circulars 03 Jun '26

03 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44810 SUBJECT: EP260602a: Liverpool Telescope optical upper limits DATE: 26/06/03 19:58:47 GMT FROM: A. Bochenek at Liverpool John Moores University <a.m.bochenek(a)2023.ljmu.ac.uk> A. Bochenek, D. A. Perley, J. L. Wise (LJMU), report: We observed the field of EP260602a (Li et al., GCN 44770) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained multiple 150s exposures in SDSS gri filters, starting at 2026-06-02 21:15:26 UT, approximately 13.07 hours after trigger. The observations were interrupted and consequently resumed due to weather conditions and telescope tracking issues; some exposures needed to be discarded. We do not detect any sources at the position of the counterpart (Xu et al., GCN 44781, Li et al., GCN 44794; also Watson et al., GCN 44790; He et al., GCN 44798, Malesani et al., GCN 44807) down to the 3 sigma limiting magnitudes of: | MJD (mid) | T_mid-T_0 | Exp. Time | Filter | Mag limit. (AB) | | ------------ | ------------ | ------------ | ------------ | --------------- | | 61193.93925 | 14.34 h | 600 s | g | > 21.8 | | 61193.91413 | 13.75 h | 960 s | r | > 22.1 | | 61194.11373 | 18.54 h | 720 s | i | > 21.9 | The stacked exposures were subtracted from PanSTARRS reference imaging using custom PSF-matching image subtraction code utilising Swarp, SourceExtractor, and PSFex. 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/44810. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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