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[vsnet-grb-info 42372] EP260409a: refined analysis of the EP-WXT and autonomous EP-FXT observations
by GCN Circulars 10 Apr '26

10 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44251 SUBJECT: EP260409a: refined analysis of the EP-WXT and autonomous EP-FXT observations DATE: 26/04/10 06:06:36 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), S. Q. Jiang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260409a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Jiang et al., GCN 44240). The refined analysis of the WXT data shows that the event started at T0=2026-04-09T00:16:43 (UTC). WXT coverage of the transient lasted for approximately 100 seconds after trigger, and was interrupted due to the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.74 × 10^(20) cm^-2, and a photon index of 0.6 +/- 0.7. The derived average unabsorbed 0.5-4 keV flux is 3.2 (-1.2, +1.7) × 10^(-10) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP autonomously observed this source a few minutes after the on board trigger. The FXT observation started at 2026-04-09T00:22:08 (UTC, T0+325 s). The effective exposure time of the observation is around 2 ks. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 172.9096, DEC = -9.3457 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectra can be fitted with an absorbed power-law with a fixed Galactic hydrogen column density of 2.74 × 10^(20) cm^-2, and a photon index of 1.9 +/- 0.4. The derived average unabsorbed 0.5-10 keV flux is 6.4 (-1.8, +2.8) × 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. Further FXT follow-up observations have been arranged. The optical follow-up observations were performed by Lipunov et al. (GCN 44241), Noémie Globus et al. (GCN 44243), Li et al. (GCN 44244). 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/44251. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42371] EP260321a: Chandra X-ray Non-detection
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44250 SUBJECT: EP260321a: Chandra X-ray Non-detection DATE: 26/04/09 22:11:06 GMT FROM: Brendan O'Connor at Carnegie Mellon University <boconno2(a)andrew.cmu.edu> Brendan O'Connor (CMU) and Eleonora Troja (U Rome) on behalf of a larger collaboration: We observed the position of EP260321a (Huang et al., GCN 44068) with the Chandra X-ray Observatory (CXO) under GO program 27400339 (PI: O'Connor). Observations were carried out with ACIS-S starting on 2026-04-05 at 22:00 UT, corresponding to ~15.4 d after the EP trigger, for a total of 19.81 ks. We do not detect any coincident X-ray source at this time. With the exception of GW170817, this observation is capable of excluding all known GRB X-ray afterglows from the Swift era, including GRB 060218 which is also located at ~150 Mpc. Further observations are planned. We thank the staff of the Chandra X-ray Observatory, including Pat Slane, Dan Schwartz, Jack Steiner, and Vinay Kashyap, for rapidly scheduling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/44250. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42370] GRB 260409A: Fermi GBM Final Real-time Localization
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44249 SUBJECT: GRB 260409A: Fermi GBM Final Real-time Localization DATE: 26/04/09 17:43:22 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 17:32:45 UT on 9 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260409A (trigger 797448770.671026 / 260409731). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 298.8, Dec = 61.6 (J2000 degrees, equivalent to J2000 19h 55m, 61d 36'), with a statistical uncertainty of 2.0 degrees. The angle from the Fermi LAT boresight is 121.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260409731/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260409731/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260409731/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44249. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42369] GRB 260310A/AT2026fgk: Kilonova-Catcher SN peak identification
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44248 SUBJECT: GRB 260310A/AT2026fgk: Kilonova-Catcher SN peak identification DATE: 26/04/09 13:57:11 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro(a)hotmail.com> M. Freeberg, M. Serrau (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 have continued to observe the field of GRB 260310A detected by Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat CZTI (Salunke et al., GCN 43958) with the GRANDMA citizen science project Kilonova-catcher (KNC) over the past days mainly in Rc and sdssr filters. Our last night observations were performed with a TEC180FL telescope operated by M. Freeberg. Our observations started at TGRB+29.9 days and were taken with Rc filters. In our stacked frames, subtracted from the PanSTARRS DR2 template image, we still detect the optical counterpart at Rc = 20.39+/-0.18 at a midtime T-TGRB = 29.97 days. Despite the moderate Signal-to-Noise ratio (SNR) in our image series, we tentatively identified the supernova peak brightness at Rc = 19.9+/-0.1 around T-TGRB = 21.8 days. At z = 0.153 (de Ugarte Postigo et al., GCN 43984), such a brightness and peak time seem in good agreement with the expected peak brightness and time of the redshifted canonical GRB/SNe SN 1998bw. The KNC members will keep monitoring the source as long as it stays detectable. All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-Cousins filters were calibrated using the GAIA DR3 Synphot 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/44248. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42368] The EP-WXT trigger 01709259773 is likely a flaring star
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44247 SUBJECT: The EP-WXT trigger 01709259773 is likely a flaring star DATE: 26/04/09 12:34:09 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> S.Q. Jiang (NAO, CAS), G. L. Huang, Z. X. Li, J. Y. Cao (IHEP, CAS), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709259773 at the time of 2026-04-09T11:45:11, is likely a stellar flare associated with 1RXS J121842.8-060921. The estimated flux of the flare is around 1.4 × 10^-10 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 8.6 × 10^30 erg/s. 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/44247. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42367] GRB 260407A: GECAM-A detection
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44246 SUBJECT: GRB 260407A: GECAM-A detection DATE: 26/04/09 11:19:26 GMT FROM: guohx(a)ihep.ac.cn Hao-Xuan Guo, Shao-Lin Xiong, Peng Zhang, Yan-Qiu Zhang, Chen-Wei Wang (IHEP) report on behalf of the GECAM team: GECAM-A detected a long burst, GRB 260407A, at 2026-04-07T09:11:07.950 UTC (denoted as T0), which was also observed by Fermi GBM (GCN #44226), Glowbug (C.C. Cheung et al., GCN #44237), CALET (M. L. Cherry et al., GCN #44242) and Insight-HXMT/HE (Hao-Xuan Guo et al., GCN #44245). According to the GECAM-A light curves in about 40-6000 keV, this burst mainly consists of a single pulse with a duration (T90) of 19.5 +4.5/-6.5 s. The GECAM-A light curve can be found here: https://www.bursthub.cn//admin/static/gecamagrb260407A.png Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two microsatellites (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/44246. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42366] GRB 260407A: Insight-HXMT/HE detection
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44245 SUBJECT: GRB 260407A: Insight-HXMT/HE detection DATE: 26/04/09 11:05:54 GMT FROM: guohx(a)ihep.ac.cn Hao-Xuan Guo, Chen-Wei Wang, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng (IHEP) report on behalf of the Insight-HXMT team: At 2026-04-07T09:11:07.000 (T0), Insight-HXMT/HE detected a long burst, GRB 260407A, which is also detected by Fermi GBM (GCN #44226), Glowbug (C.C. Cheung et al., GCN #44237) and CALET (M. L. Cherry et al., GCN #44242). The Insight-HXMT/HE light curve mainly consists of a single pulse with a total duration (T90) of 23.5 +16/-9 s. The 1s peak rate, measured from T0+0.25 s, is 493 cnts/sec. Insight-HXMT/HE detected a total of 4651 counts from this burst. The Insight-HXMT /HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260407A.png All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://hxmten.ihep.ac.cn/ View this GCN Circular online at https://gcn.nasa.gov/circulars/44245. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42365] EP260409a: SVOM/VT optical upper limits
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44244 SUBJECT: EP260409a: SVOM/VT optical upper limits DATE: 26/04/09 09:55:42 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, 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, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team. SVOM/VT performed ToO observations of EP260409a triggered by Einstein Probe (Jiang et al., GCN 44240). The observation started at 2026-04-09T01:35:09 UTC, approximately 1.28 hours post trigger, in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. No uncatalogued sources were detected within the error box of EP-WXT (Jiang et al., GCN 44240), compared to the Legacy Survey. The following measurements are in the AB magnitude and are not corrected for Galactic extinction: Mid time | Band | Exposure Time | 3 sigma upper limit 3.04 hr VT_B 106*50 sec > 23.7 mag 3.04 hr VT_R 101*50 sec > 23.6 mag Our non-detection is consistent with the result from COLIBRÍ (Globus et al., GCN 44242). 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/44244. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42364] EP260409a: COLIBRÍ optical upper limits
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44243 SUBJECT: EP260409a: COLIBRÍ optical upper limits DATE: 26/04/09 04:04:15 GMT FROM: globus(a)astro.unam.mx Noémie Globus (UNAM), Marion Guelfand (CPPM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (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), Jean-Grégoire Ducoin (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 imaged the field of the EP260409a (Jiang et al., GCN Circ. 44240) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-09 02:56 to 03:41 (from 2.6 to 3.4 hours after the trigger) and obtained 34 min of simultaneous exposure in the r and z filters. The data were reduced and coadded with the 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. In the stacked image, we do not detect any new source at the FXT source position down to the following 5-sigma limit: r > 23.14, z > 21.84. 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/44243. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42363] GRB 260407A: CALET Gamma-Ray Burst Monitor detection
by GCN Circulars 09 Apr '26

09 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44242 SUBJECT: GRB 260407A: CALET Gamma-Ray Burst Monitor detection DATE: 26/04/09 03:43:44 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University <kawakubo(a)phys.aoyama.ac.jp> M. L. Cherry (LSU), A. Yoshida, T. Sakamoto, S. Sugita, 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), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration: The long GRB 260407A (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 44226; Glowbug gamma-ray detection: Cheung et al., GCN Circ 44237) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 09:11:06.11 UTC on 7 April 2026 (https://cgbm.calet.jp/cgbm_trigger/flight/1459588274/index.html) The burst signal was seen by all CGBM detectors. The burst light curve shows a single pulse that starts at T-0.5 sec, peaks at T+1.6 sec, and ends at T+27.1 sec. The T90 and T50 durations measured by the SGM data are 22.6 +/- 5.0 sec and 7.0 +/- 1.0 sec (40-1000 keV), respectively. The ground-processed light curve is available at https://cgbm.calet.jp/cgbm_trigger/ground/1459588274 The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University. View this GCN Circular online at https://gcn.nasa.gov/circulars/44242. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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