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[vsnet-grb-info 42906] EP260530a/AT 2026nwl: COLIBRÍ optical follow up
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44776 SUBJECT: EP260530a/AT 2026nwl: COLIBRÍ optical follow up DATE: 26/06/02 20:15:16 GMT FROM: Camila Angulo Valdez at UNAM <camiangulo(a)astro.unam.mx> Marion Guelfand (CPPM), Camila Angulo (UNAM), Noémie Globus (UNAM), Alan M. Watson (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We observed the field of the optical source AT 2026nwl, discovered by GOTO (Godson et al., TNS Astronomical Transient Report No. 306622), associated with EP260530a (Wu et al., ATel #17824), further observed by COLIBRÍ (Globus et al., GCN Circ. 44759) and KAIT (Zheng et al., GCN Circ. 44775), and classified as a CV (Pursiainen et al., TNS Classification Report No. 23885), with the DDRAGO two-channel wide-field imager mounted on the COLIBRÍ telescope. We observed from 2026-06-02 04:19 to 07:11 UTC (from 79.21 to 82.15 hours after the GOTO discovery) and obtained 24 minutes of exposure in each of the g, r, and i filters, and 72 minutes of exposure in the z filter. The data were reduced and coadded with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. We continue to detect the source at the following preliminary magnitudes: g = 15.77 +/- 0.01, r = 15.87 +/- 0.01, i = 15.98 +/- 0.01, z = 16.07 +/- 0.01. We observe a quasi-periodic modulation of the light curve in the different photometric bands with a characteristic timescale of approximately 1 hour, typical of certain classes of accreting binary stars, and a peak-to-valley amplitude of about 0.1 mag Further observations and analysis are ongoing. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/44776. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42905] EP260530a/AT 2026nwl: KAIT optical observations
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44775 SUBJECT: EP260530a/AT 2026nwl: KAIT optical observations DATE: 26/06/02 18:47:59 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of EP260530a (Wu et al., ATel #17824) on May 31 and again on Jun 02 UT. A set of clear (roughly R) filter images were obtained. We clearly detected the bright CV outburst associated with AT 2026nwl (TNS Astronomical Transient Report 306622, 306654; TNS Classification Reports 23885; Globus et al., GCN 44759). We measure its brightness decayed from 14.4 +/- 0.1 mag (Vega) at 0.98d after the burst, to 15.6 +/- 0.1 mag at 2.95d after the burst. View this GCN Circular online at https://gcn.nasa.gov/circulars/44775. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42904] GRB 260602A: Fermi GBM Final Real-time Localization
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44774 SUBJECT: GRB 260602A: Fermi GBM Final Real-time Localization DATE: 26/06/02 18:40:24 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 18:29:53 UT on 2 Jun 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260602A (trigger 802117798.896848 / 260602771). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 26.3, Dec = 48.7 (J2000 degrees, equivalent to J2000 01h 45m, 48d 42'), with a statistical uncertainty of 2.0 degrees. The angle from the Fermi LAT boresight is 28.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260602771/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260602771/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260602771/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44774. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42903] GRB 260601B: CALET Gamma-Ray Burst Monitor detection
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44773 SUBJECT: GRB 260601B: CALET Gamma-Ray Burst Monitor detection DATE: 26/06/02 18:35:44 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University <kawakubo(a)phys.aoyama.ac.jp> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44773. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42902] EP260602a: Optical upper limits with Kinder observations
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44772 SUBJECT: EP260602a: Optical upper limits with Kinder observations DATE: 26/06/02 17:23:06 GMT FROM: Janet Chen at National Central University <janetstars(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 EP260602a (Li et al., GCN 44770) using the 40cm Seisi Lulin Telescope (SLT) 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 SLT epoch of observations in r-band started at 15:31 UTC on the 2nd of June 2026 (MJD 61140.5068), 7.34 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 utilized 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 uncatalogued source. Moreover, we further used 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 SLT | r | 61140.5068 | 7.34 | 300 * 6 | >20.0 | 1".77 | 1.13 The presented upper limit is calibrated using the field stars from the PanSTARRS catalog and is not corrected for an expected galactic extinction of A_r = 0.06 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/44772. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42901] GRB 260601B / EP260601a: NuSTAR detection of prompt emission
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44771 SUBJECT: GRB 260601B / EP260601a: NuSTAR detection of prompt emission DATE: 26/06/02 16:42:17 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar(a)iitb.ac.in> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44771. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42900] EP260602a: Einstein Probe detection of a fast X-ray transient
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44770 SUBJECT: EP260602a: Einstein Probe detection of a fast X-ray transient DATE: 26/06/02 16:35:29 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: 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 EP260602a. The transient triggered EP-WXT (ID: 01709266359) at 2026-06-02T08:11:20 (UTC). The WXT position of the source is R.A. = 240.886 deg, DEC = 50.563 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The WXT spectrum can be fitted with an absorbed power-law, with the absorption fixed at the Galactic value of 2.03 x 10^20 cm^-2, and a photon index of 1.7 (-0.5, +0.5). The unabsorbed 0.5-4 keV flux is 9.0 (-2.4, +3.2) x 10^-11 erg/s/cm2. A follow-up observation with the Follow-up X-ray Telescope (FXT) onboard EP is scheduled. 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/44770. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42899] GRB 260601B/EP260601a: SVOM/VT optical observations
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44769 SUBJECT: GRB 260601B/EP260601a: SVOM/VT optical observations DATE: 26/06/02 15:23:39 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44769. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42898] GRB 260601B/EP260601a: GECAM-B analysis suggests a possible long duration Type I burst
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44768 SUBJECT: GRB 260601B/EP260601a: GECAM-B analysis suggests a possible long duration Type I burst DATE: 26/06/02 15:04:42 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44768. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42897] GRB 260601B: GECAM-B detection of a burst probably associated with EP260601a
by GCN Circulars 02 Jun '26

02 Jun '26

TITLE: GCN CIRCULAR NUMBER: 44767 SUBJECT: GRB 260601B: GECAM-B detection of a burst probably associated with EP260601a DATE: 26/06/02 13:22:14 GMT FROM: yzh807926(a)163.com 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: -5.48 deg Err: 2.66 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/44767. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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