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[vsnet-grb-info 42267] EP 260329b: COLIBRÍ optical observations
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44146 SUBJECT: EP 260329b: COLIBRÍ optical observations DATE: 26/03/29 14:45:02 GMT FROM: Francesco at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work(a)gmail.com> Francesco Magnani (CPPM), Noémie Globus (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), 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 EP 260329b (Song et al., GCN Circ. 44142) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-29 11:21 to 11:41 UTC (from 45.7 to 65.0 minutes after the trigger) and obtained 16 minutes of exposure in the r and z filters, respectively. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). 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 detected the optical counterpart reported by LCO (Li et al., GCN Circ. 44139, Eyles-Ferris et al., GCN Circ. 44144) and TRT (Zhu et al., GCN Circ 44140), at preliminary magnitudes of: r = 18.85 +/- 0.08 z = 18.53 +/- 0.13 Further observations are planned. 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/44146. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42266] EP260329a: Optical upper limit with Kinder observations
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44145 SUBJECT: EP260329a: Optical upper limit with Kinder observations DATE: 26/03/29 14:40:17 GMT FROM: Janet Chen at National Central University <janetstars(a)gmail.com> A. Sankar.K, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), M. Nicholl (QUB), M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, W.-J. Hou, H.-Y. Hsiao, J.-K. Guo (all NCU), Y. J. Yang (NYUAD), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz M. Huber, K. Chambers (all IfA, Hawaii) report: We observed the field of the EP260329a (Yang et al., GCN 44141) using the 40cm 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 g-band started at 11:03 UTC on 29th of March 2026 (MJD 61128.460), 7.54 hr 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 used the Python-based AutoPhOT package (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction of the Pan-STARRS1 3Pi archive image (Chambers et al., 2016, arXiv:1612.05560) 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 a new or uncatalogued source within the EP-WXT localization circle (3.2'). 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 | 61128.460 | 7.54 | 300 * 9 | >21.1 | 2".36 | 3.02 The presented magnitudes are calibrated using the field stars from the PanSTARRS catalog. The reported upper limit is not corrected for an expected galactic extinction of A_r = 0.44 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/44145. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42265] EP260329b: LCO optical observations of the counterpart
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44144 SUBJECT: EP260329b: LCO optical observations of the counterpart DATE: 26/03/29 13:42:38 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris (Leicester), A. van Hoof (Radboud), G. Corcoran (UCD), J. Chácon (PUC), A. Martin-Carrillo (UCD), J. Quirola-Vasquez (Radboud), A. J. Levan (Radboud), F. E. Bauer (SSI and UTA) and P. G. Jonker (Radboud) report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP260329b (Song et al., GCN 44142) with LCO 1m telescopes located at the Siding Spring Observatory (Australia) equipped with the SINISTRO instrument. We obtained 6 x 300 exposures in each of the r and z filters, starting from 2026-03-29 12:03:17 UT (t_mid = 1.74 hr and 1.78 hr post trigger respectively). We clearly detect the optical counterpart reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140) in our stacked images and measure the following AB magnitudes calibrated to Pan-STARRS and not corrected for Galactic extinction: r = 19.84 +/- 0.05 z = 19.38 +/- 0.05 Our measurements are >1 magnitude fainter than those reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140) indicating the counterpart is fading rapidly. View this GCN Circular online at https://gcn.nasa.gov/circulars/44144. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42264] GRB 260326A: Insight-HXMT/HE detection
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44143 SUBJECT: GRB 260326A: Insight-HXMT/HE detection DATE: 26/03/29 12:55:38 GMT FROM: Xinghao Luo at IHEP <2952704891(a)qq.com> Xing-Hao Luo, Chen-Wei Wang, Chao Zheng, Cheng-Kui Li and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team: At 2026-03-26T01:18:33.250 (T0), Insight-HXMT/HE detected the burst GRB 260326A, which is also detected by Fermi/GBM (Fermi GBM team, GCN #44115). The Insight-HXMT/HE light curve mainly consists of multiple pulses with a T90 of 15 +1/-0.5 s. The 1s peak rate, measured from T0+0.75 s, is 6592 cnts/sec. Insight-HXMT/HE detected a total of 39437 counts from this burst. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260326A.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/44143. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42263] EP260329b: Einstein Probe detection of an X-ray transient
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44142 SUBJECT: EP260329b: Einstein Probe detection of an X-ray transient DATE: 26/03/29 12:07:29 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> F.-F Song (YNAO, CAS), Guojiong Yang, Wei Chen, Yuan Liu (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 EP260329b. The transient triggered EP-WXT (ID: 01709259209) at 2026-03-29T10:36:00 (UTC). The WXT position of the source is R.A. = 181.309 deg, DEC = 4.362 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) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 181.3122 deg, DEC = 4.3526 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). 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/44142. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42262] EP260329a: Einstein Probe detection of an X-ray transient
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44141 SUBJECT: EP260329a: Einstein Probe detection of an X-ray transient DATE: 26/03/29 11:35:22 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Guojiong Yang (NAO, CAS), F.-F Song(YNAO, CAS), Wei Chen, Yuan Liu (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 EP260329a. The transient triggered EP-WXT (ID: 01709259198) at 2026-03-29T03:30:40 (UTC). The WXT position of the source is R.A. = 5.459 deg, DEC = 82.513 deg (J2000) with an uncertainty of 3.2 arcmin in radius (90% C.L. statistical and systematic). The WXT 0.5-4 keV spectrum can be described by an absorbed power law. Since the hydrogen column density is not well constrained by the current data, it was fixed at the Galactic value 0.1 x 10^22 cm^-2. The best-fit photon index is 2.23 (-0.69/+2.26). The derived 0.5-4 keV flux is 1.43 (-0.33/+0.40) x 10^-10 erg cm^-2 s^-1. A follow-up observation with the Follow-up X-ray Telescope (FXT) is 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/44141. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42261] EP-WXT trigger 01709259209: TRT optical counterpart detection
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44140 SUBJECT: EP-WXT trigger 01709259209: TRT optical counterpart detection DATE: 26/03/29 11:32:38 GMT FROM: Zipei Zhu at NAOC <zpzhu(a)nao.cas.cn> Z.P. Zhu (NAOC), S. Tinyanont, K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), J. An, X. Liu, S.Q. Jiang, L.B. He, N.C. Sun, W.X. Li, D. Xu (NAOC), S.Y. Fu (HUST) report on behalf of a large collaboration: We observed the field of EP260329a detected by the Einstein Probe (EP) (Trigger ID: 01709259209 ), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Sierra Remote Observatories in California. We obtained several exposure frames in the R filter. The optical counterpart located at: R.A. (J2000) = 12:05:14.95 () Dec.( J2000) = +4:21:05.8 with an uncertainty of ~ 0.8 arcsec. The uncatalogued source is clearly detected in the stacked image with the measured brightness of R = 18.4 +/- 0.1 at a mid-time of 10:47:20 UTC on 2026-03-29., consistant with the optical candidate reported by LCO (Li et al., GCN 44139), calibrated using nearby stars from the Pan-STARRS catalog and are not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44140. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42260] The EP-WXT trigger 01709259209 (EP260329b?): Las Cumbres discovery of the optical counterpart
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44139 SUBJECT: The EP-WXT trigger 01709259209 (EP260329b?): Las Cumbres discovery of the optical counterpart DATE: 26/03/29 11:11:13 GMT FROM: Wenxiong Li at NAOC <liwenxiong1992(a)gmail.com> Wenxiong Li, Runduo Liang (NAOC), Iair Arcav, Ido Keinan (TAU), David Sand (U of Arizona) We observed the position of EP-WXT trigger 01709259209 with a Las Cumbres 1m telescope at Siding Spring Observatory, Australia, 6 mins after the Einstein Probe WXT trigger. We took 2x300s exposures in the broad optical w-band. We find an uncataloged source at RA=181.3124, Dec=4.3516 within the EP-WXT error circle and measure the following preliminary photometry calibrated to the r band: Epoch 1: MJD 61128.446 Mag 18.7 Epoch 2: MJD 61128.450 Mag 17.1 The presented magnitudes are calibrated using Pan-STARRS sources in the field and are not corrected for Galactic extinction. We identify a faint source at this position in the Legacy Survey data, with r=24.08 mag. Additional follow-up observations are encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/44139. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42259] GRID detection of GRB 260326A
by GCN Circulars 29 Mar '26

29 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44138 SUBJECT: GRID detection of GRB 260326A DATE: 26/03/29 08:48:36 GMT FROM: GRID Student Team at Tsinghua University <grid(a)tsinghua.edu.cn> Longhao Li, Xiangyi Wang, Chenyu Wang, Zirui Yang report on behalf of the GRID Collaboration: GRID-11B reports the detection of the long-duration GRB 260326A, which was also detected by Fermi-GBM (Fermi GBM team, GCN 44115; Pathak & Bala, GCN 44118). The event was triggered with GRID-11B on 2026-03-26 at 01:18:28 UTC. The measured burst duration (T90) in the 30-2000 keV range is approximately 13.3 +/- 0.6 seconds. At present, only the GRID light curve is available. No spectral analysis is reported at this stage. The GRID light curve of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB260326A/GRID_260… GRID is a student-led project to monitor the transient gamma-ray sky with multiple detectors onboard different nanosatellites in the era of multi-messenger astronomy. For more information about GRID, please refer to the following references: https://doi.org/10.1007/s10686-019-09636-w and https://doi.org/10.1007/s10686-021-09819-4. View this GCN Circular online at https://gcn.nasa.gov/circulars/44138. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42258] GRB 260310A: Gemini-North Spectral Confirmation of Supernova Features
by GCN Circulars 28 Mar '26

28 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44137 SUBJECT: GRB 260310A: Gemini-North Spectral Confirmation of Supernova Features DATE: 26/03/28 14:25:31 GMT FROM: Brendan O'Connor at Carnegie Mellon University <boconno2(a)andrew.cmu.edu> B. O'Connor (CMU), S. Toshikage (Tohoku U.), M. Tanaka (Tohoku U.), X. J. Hall (CMU), D. Gruen (LMU), A. Palmese (CMU) report on behalf of a larger collaboration: We observed the optical counterpart (O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) with GMOS mounted on Gemini-North starting on 2026-03-27 at 14:56:28 UT, corresponding to 17.4 d after the trigger. We obtained a spectral sequence of 2x1060 s exposures using the B480 grating. The observed spectrum has undergone a significant change compared to earlier epochs, and now displays the clear onset of supernova features. We confirm the report of de Ugarte Postigo et al. (GCN 44124) and the association of GRB 260310A to a massive star progenitor. We acknowledge the excellent support from the staff of the Gemini-North telescope, including Siyi Xu, Atsuko Nitta, Teo Mocnik, Jen Andrews, Sunny Stewart, and Denise Hung. View this GCN Circular online at https://gcn.nasa.gov/circulars/44137. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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