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[vsnet-grb-info 42944] EP260602d: Optical Upper limits with kinder observations
by GCN Circulars 04 Jun '26

04 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 04 Jun '26

04 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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[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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