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[vsnet-grb-info 42450] EP260416a: LCO observations confirm end of flaring phase
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44328 SUBJECT: EP260416a: LCO observations confirm end of flaring phase DATE: 26/04/17 13:52:03 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo(a)ucd.ie> A. Martin-Carrillo (UCD), L. Cotter (UCD), A. J. Levan (Radboud & Warwick), J. A. Chácon (PUC), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud) report on behalf of a larger collaboration: We observed the field of EP260416a (Hu et al., GCN Circ. 44307) with the LCO 1m telescope located at the McDonald Observatory (Texas, USA) equipped with the SINISTRO instrument. We obtained 6x300s exposures in the r band starting at 09:58:28.32 UT on the 17th of April 2026, approximately 28.48 hr after the initial trigger. In the stacked images the optical counterpart (Ducoin et al., GCN 44310, Li et al., GCN 44315, Hua et al., GCN 44318, Lee et al., GCN 44319, Zheng et al., GCN 44321) is well detected, and we measure an AB magnitude of r = 20.78 +/- 0.06, calibrated using nearby stars from the Pan-STARRS catalogue and not corrected for Galactic extinction. Compared to the previous observations made by Lee et al. (GCN Circ. 44319) the counterpart seems to have faded by approximately 0.18 AB mag in 5.6 hr. View this GCN Circular online at https://gcn.nasa.gov/circulars/44328. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42449] EP260324a / AT2026hir: Further multiband optical follow-up with the PKU 60cm Telescope
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44327 SUBJECT: EP260324a / AT2026hir: Further multiband optical follow-up with the PKU 60cm Telescope DATE: 26/04/17 13:01:53 GMT FROM: Qiang Wang at Peking University <2501110280(a)stu.pku.edu.cn> Qiang Wang, Yacheng Kang, Ruize Shi, Xinmiao Zhao, and Fangzhou Ren (PKU) report on behalf of the PKU HiTF (High-energy Transients Follow-up) group: We observed the field of EP260324a (Wu et al., ATel 17728; T0 = 2026-03-24T07:04:20) using the PKU 60cm telescope at Xinglong Observatory, NAOC. Observations were conducted over multiple epochs between 2026 Mar 30 and Apr 11, corresponding to ~ 6.3–18.3 days after the EP-WXT detection, in the g, r, and i bands. In the stacked images, we clearly detect the optical counterpart in the difference images at a position consistent with that of ZTF26aapviim / AT2026hir (Anumarlapudi et al., GCN 44120; Mohan et al., GCN 44128; Kang et al., GCN 44156; Stein et al., GCN 44194; Globus et al., GCN 44196; Watson et al., GCN 44216). Preliminary photometry is calibrated against nearby Pan-STARRS field stars and reported in the AB magnitude system, without correction for Galactic extinction. Combining these data with our earlier observations on 2026 March 27 (Kang et al., GCN 44156), we find that the optical emission likely peaked at ~ 1 week after the EP trigger. Following the peak, we estimate the temporal decay using a simple power-law model, yielding indices of alpha_g ~ 1.13 and alpha_r ~ 0.93. The observation log is summarized below following the time sequence: | Date | Start_UT | T_mid − T0 (days) | Filter | Exposure (s) | Magnitude (AB) | | :--------: | :---------: | :---------------: | :----: | :----------: | :------------: | | 2026-03-30 | 14:03:01.11 | 6.30 | g | 2 * 600 | 17.40 +/- 0.03 | | 2026-03-30 | 14:25:47.00 | 6.31 | r | 2 * 600 | 17.62 +/- 0.04 | | 2026-03-31 | 13:25:47.93 | 7.27 | g | 2 * 600 | 17.31 +/- 0.03 | | 2026-03-31 | 13:48:41.86 | 7.29 | r | 2 * 600 | 17.64 +/- 0.04 | | 2026-03-31 | 14:11:31.90 | 7.34 | i | 12 * 600 | 17.87 +/- 0.03 | | 2026-04-06 | 14:02:29.94 | 13.30 | i | 4 * 600 | 18.11 +/- 0.03 | | 2026-04-06 | 14:45:34.47 | 13.34 | g | 4 * 600 | 17.97 +/- 0.03 | | 2026-04-07 | 13:49:09.03 | 14.30 | g | 4 * 600 | 18.06 +/- 0.03 | | 2026-04-07 | 14:34:49.59 | 14.33 | r | 4 * 600 | 18.27 +/- 0.04 | | 2026-04-10 | 14:10:56.15 | 17.31 | g | 5 * 600 | 18.42 +/- 0.03 | | 2026-04-10 | 15:04:02.07 | 17.35 | r | 5 * 600 | 18.54 +/- 0.04 | | 2026-04-11 | 13:05:42.64 | 18.28 | r | 6 * 600 | 18.57 +/- 0.05 | Further analysis and follow-up observations are ongoing. The PKU 60cm Telescope is operated by the Department of Astronomy, Peking University. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients. View this GCN Circular online at https://gcn.nasa.gov/circulars/44327. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42448] EP260417a: TRT optical counterpart detection
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44326 SUBJECT: EP260417a: TRT optical counterpart detection DATE: 26/04/17 11:48:23 GMT FROM: L. B. He at NAOC <helb(a)bao.ac.cn> L.B. He (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Q. Jiang, X. Liu, Z.P. Zhu, J. An, D. Xu (NAOC), S.Y. Fu (HUST) report: We observed the field of EP260417a detected by EP (Wang et. al, GCN 44325), using the 0.7-m telescope of the Thai Robotic Telescope network, located at Cerro Tololo Inter-American Observatory, Chile (TRT-CTO). We obtained a series of frames in the Sloan-r band. An uncatalogued source is detected within the EP/WXT error circle at coordinates R.A. (J2000) = 16:21:17.47 (245.3228), Dec. (J2000) = -22:01:26.31 (-22.0240) with an uncertainty of ~ 1.0 arcsec. The source had r = 20.30 +/- 0.03 mag at a median time of ~ 7.92 hr post-trigger, calibrated with Pan-STARRS1 DR2 and not corrected for Galactic extinction. No NEO would be at the above position at the observational time by checking MPC. We thus think the source could be the optical counterpart of EP260417a. View this GCN Circular online at https://gcn.nasa.gov/circulars/44326. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42447] EP260417a: Einstein Probe detection of an X-ray transient
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44325 SUBJECT: EP260417a: Einstein Probe detection of an X-ray transient DATE: 26/04/17 11:44:47 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. L. Wang (NAO, CAS; ICE, CSIC), Y. Q. Zhao (USTC; PRIC), K. R. Ni (CCNU) and Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260417a. The source was detected at 2026-04-17 00:23:02 (UTC). The WXT light curve showed a slow decreasing trend. The WXT position of the source is R.A. = 245.324 deg, DEC = -22.023 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). Preliminary spectral analysis with an absorbed power-law model gave a best-fit photon index of 0.75 (-0.38/+0.97) and an unconstrained hydrogen column density of ~2.3e-20. The unabsorbed 0.5-4 keV flux is 1.3(-0.3/+0.4)e-10 erg/s/cm^2. Quoted uncertainties are at 90% confidence level. A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP has been scheduled. Further information will be updated when the telemetry data is received. 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/44325. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42446] EP260416a: refined analysis of the EP-WXT and autonomous EP-FXT observations
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44324 SUBJECT: EP260416a: refined analysis of the EP-WXT and autonomous EP-FXT observations DATE: 26/04/17 09:10:30 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> D. F. Hu (PMO, CAS), J. Yang (ZZU), M. J. Liu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: The X-ray transient EP260416a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 44307), and followed by several optical telescopes (Ducoin et al., GCN 44310, Li et al., GCN 44315, Hua et al., GCN 44318, Lee et al., GCN 44319, Zheng et al., GCN 44321). The refined analysis of the WXT data shows that the event was detected at the beginning of the observation starting at T0=2026-04-16T05:25:24 (UTC), and lasted for about 250 s before the observation was interrupted by 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 1.02 × 10^20 cm^-2, and a photon index of 1.0 +/- 0.8. The derived average unabsorbed 0.5-4 keV flux is 3.2 (-1.4, +2.7) × 10^(-10) erg/s/cm^2. The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-04-16T05:33:14 (UTC), about 8 minutes after T0. The exposure time of the observation is 2.4 ks. The on-ground analysis of the FXT data found an uncatalogued source at R.A. = 198.8351, DEC = 31.5972 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT light curve shows no significant variability. The average FXT 0.5-10 keV spectra can be fitted with an absorbed power-law with a free hydrogen column density of 1.62 +/- 0.18 × 10^21 cm^-2, and a photon index of 2.14 +/- 0.08. The derived average unabsorbed 0.5-10 keV flux is 3.4 (-0.1, +0.1) × 10^(-10) erg/s/cm^2. No significant variability is observed from T0 to T0+6 ks. The uncertainties are at the 90% confidence level for the above parameters. Further FXT follow-up observations have been arranged. 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/44324. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42445] EP260417b: Einstein Probe detection of a fast X-ray transient
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44323 SUBJECT: EP260417b: Einstein Probe detection of a fast X-ray transient DATE: 26/04/17 08:45:52 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> K. R. Ni (CCNU), Y. Q. Zhao (USTC; PRIC), Y. L. Wang (NAO, CAS; ICE, CSIC) and 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 EP260417b. The ground analysis of the WXT data shows that the event started at T0 = 2026-04-17T01:21:04 (UTC) and lasted for around 230 s. The WXT position of the source is R.A. = 151.114 deg, DEC = 51.964 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP has been scheduled. Further information will be updated when the telemetry data is received. 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/44323. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42444] EP-WXT trigger 01709259586: KAIT optical observations
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44322 SUBJECT: EP-WXT trigger 01709259586: KAIT optical observations DATE: 26/04/17 06:57:45 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, automatically responded to the EP-WXT trigger 01709259586 (Wu et al., GCN 44213) starting at 05:46 UT, about 27 minutes after the trigger. A set of B,V,R,I and clear (roughly R) filter images were obtained and lasted for ~1.5 hours. We performed photometry on the suggested flare star UCAC4 498-055345 and did not find obvious variability on its brightness at 0.1 mag level. View this GCN Circular online at https://gcn.nasa.gov/circulars/44322. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42443] EP260416a: KAIT optical upper limit
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44321 SUBJECT: EP260416a: KAIT optical upper limit DATE: 26/04/17 06:10:43 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 EP260416a (Hu et al., GCN 44307) starting at 05:36, Apr 16 UT, about 7.2 minutes after the burst. A set of 20s, 40s and 60s clear (roughly R) filter images were obtained. In our coadd image, we did not detect the OT (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319) down to a limiting mag of 21.0 mag (Vega) at a mid-time of 1.3 hours after the burst. Our result is consistent with the OT reported at a fatiner magnitude by COLIBRÍ at a similar time (Ducoin et al., GCN 44310). View this GCN Circular online at https://gcn.nasa.gov/circulars/44321. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42442] GRB 260414A: Insight-HXMT/HE detection
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44320 SUBJECT: GRB 260414A: Insight-HXMT/HE detection DATE: 26/04/17 05:57:10 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 2026-04-14T06:32:07.400 (T0), Insight-HXMT/HE detected a long burst, GRB 260414A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44293) and Glowbug (C.C. Cheung et al., GCN #44297). The Insight-HXMT/HE light curve mainly consists of multiple pulses with a total duration (T90) of 34.5 +7/-11 s. The 1s peak rate, measured from T0+0.25 s, is 818 cnts/sec. Insight-HXMT/HE detected a total of 6318 counts from this burst. The Insight-HXMT /HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260414A.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/44320. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42441] EP260416a: COLIBRÍ confirmation of the optical rebrightening
by GCN Circulars 17 Apr '26

17 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44319 SUBJECT: EP260416a: COLIBRÍ confirmation of the optical rebrightening DATE: 26/04/17 05:36:55 GMT FROM: William H. Lee at UNAM <wlee(a)astro.unam.mx> William H. Lee (UNAM), Stéphane Basa (UAR Pytheas), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Diego López-Cámara (UNAM), Nikos Mandarakas (LAM), Enrique Moreno Méndez (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We reimaged the field of the EP260416a (Hu et al., GCN Circ. 44307) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-17 04:20 to 04:43 UTC (from 22.85 to 23.24 hours after the trigger) and obtained 4/11/3/18 minutes of exposure in the g/r/i/z filters, respectively. The data were reduced and coadded with 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. The candidate reported previously by Ducoin et al. (GCN Circ. 44310) is clearly detected in all our filters with preliminary magnitudes of: r = 20.60 +/- 0.02 z = 19.88 +/- 0.02 Our values confirm the rebrightening reported by the WFST Collaboration (Hua et al. GCN Circ. 44318), and suggests a photo-z <2.5 (90% conf.). 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/44319. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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