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[vsnet-grb-info 42485] GRB 260421B: TRT optical counterpart detection
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44363 SUBJECT: GRB 260421B: TRT optical counterpart detection DATE: 26/04/21 05:26:32 GMT FROM: L. B. He at NAOC <helb(a)bao.ac.cn> L.B. He (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), X. Liu, Z.P. Zhu, S.Q. Jiang, J. An, D. Xu (NAOC) report: We observed the field of GRB 260421B detected by Fermi (GCN 44360) and SVOM (zhao et al., GCN 44362), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile (CTO). We obtained a series of frames in the r-band. An uncatalogued source is detected within the Fermi error circle at coordinates R.A. (J2000) = 12:40:00.5 Dec. (J2000) = -17:44:24.5 with an uncertainty of ~ 1.0 arcsec. The source had r = 19.14 +/- 0.07 mag, calibrated with Pan-STARRS 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 is the optical counterpart of GRB 260421B. View this GCN Circular online at https://gcn.nasa.gov/circulars/44363. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42484] GRB 260421B: SVOM detection of a long burst
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44362 SUBJECT: GRB 260421B: SVOM detection of a long burst DATE: 26/04/21 05:20:23 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> D.Zhao (NAOC), H.Goto (RIKEN), W. L. Zhang (PMO), Z. Y. Liu (GZNU) report on behalf of the SVOM mission team: At 2026-04-21T04:13:25 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260421B (SVOM burst-id sb26042105). The Fermi GBM also detected this burst (GCN 44360 & 44361). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network. The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 13 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 30.71 in the [8-120] keV energy band over a time window of 20.48 seconds starting at 2026-04-21T04:13:17. The localization of the best alert is R.A., Dec. 189.9811, -17.7676 degrees (J2000) with a 90% confidence level (C.L.) radius of 3.21 arcmin (including systematic error of 2 arcmin added in quadrature). The SVOM/ECLAIRs light curve showed a multiple peak structure with a T90 duration of about 52.824 (-0.172 +0.169). This burst also triggered SVOM/GRM at 2026-04-21T04:13:21 on a timescale of 1 seconds with an SNR of 7.90. The SVOM/GRM light curve showed a multiple peak structure with a T90 duration of about 20.04 (-0.93 +3.07). SVOM slewed to the burst. SVOM/MXT began observing the field at 2026-04-21T04:18:40 UTC, 315 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 190.0318, -17.7566 degrees: R.A. (J2000) = 12h40m07.62s Dec. (J2000) = -17d45m23.69s with a 90% C.L. radius of 97.27 arcseconds. This location is 2.97 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received. VT began observing the field after the slew. The analysis of the data will be published in a future circular. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE. The Burst Advocate (BA) on shift for this alert is W. L. Zhang: wlzhang(a)pmo.ac.cn Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/44362. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42483] GRB 260421B: BALROG localization (Fermi Trigger 798437606 / GRB 260421176)
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44361 SUBJECT: GRB 260421B: BALROG localization (Fermi Trigger 798437606 / GRB 260421176) DATE: 26/04/21 04:54:24 GMT FROM: Jochen Greiner at MPE <jcgrog(a)mpe.mpg.de> T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report: The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger 798437606 at 04:13:21 on 21 April 2026 were automatically fitted for spectrum and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427; Berlato et al. 2019, ApJ 873, 60). The best-fit position is: RA(2000.0) = 190.5 deg Decl.(2000.0) = -16.5 deg The 1 sigma statistical error radius is 2.2 deg. We estimate an additional systematic error of 1 deg. Further details are available at: https://grb.mpe.mpg.de/grb/GRB260421176/ The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB260421176/healpix The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB260421176/json View this GCN Circular online at https://gcn.nasa.gov/circulars/44361. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42482] GRB 260421B: Fermi GBM Final Real-time Localization
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44360 SUBJECT: GRB 260421B: Fermi GBM Final Real-time Localization DATE: 26/04/21 04:22:36 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 04:13:21 UT on 21 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260421B (trigger 798437606.166588 / 260421176). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 188.8, Dec = -17.8 (J2000 degrees, equivalent to J2000 12h 35m, -17d 48'), with a statistical uncertainty of 2.4 degrees. The angle from the Fermi LAT boresight is 58.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44360. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42481] Fermi GRB 260421A: Global MASTER-Net observations report
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44359 SUBJECT: Fermi GRB 260421A: Global MASTER-Net observations report DATE: 26/04/21 03:16:41 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAGH robotic telescope [1] located in Mexico (OAGH National Institute for Astrophysics, Optics and Electronics) started inspect of the Fermi GRB 260421A ( Fermi GBM team, GCN 44358) errorbox 3101 sec after notice time and 3131 sec after trigger time at 2026-04-21 03:04:55 UT, with upper limit up to 19.4 mag. Observations started at twilight. The observations began at zenith distance = 47 deg. The sun altitude is -16.0 deg. The galactic latitude b = 54 deg., longitude l = 118 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3262778 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 3162 | 2026-04-21 03:04:55 | MASTER-OAGH | (13h 20m 14.14s , +62d 00m 28.6s) | C | 60 | 18.8 | 3162 | 2026-04-21 03:04:55 | MASTER-OAGH | (13h 22m 34.25s , +61d 26m 33.6s) | C | 60 | 19.2 | 3234 | 2026-04-21 03:06:08 | MASTER-OAGH | (13h 37m 23.83s , +62d 00m 57.1s) | C | 60 | 18.8 | 3234 | 2026-04-21 03:06:08 | MASTER-OAGH | (13h 39m 43.36s , +61d 26m 60.0s) | C | 60 | 19.2 | 3426 | 2026-04-21 03:09:20 | MASTER-OAGH | (13h 07m 30.22s , +63d 21m 09.2s) | C | 60 | 19.4 | 3426 | 2026-04-21 03:09:20 | MASTER-OAGH | (13h 04m 59.80s , +63d 55m 03.4s) | C | 60 | 19.0 | 3497 | 2026-04-21 03:10:31 | MASTER-OAGH | (13h 25m 34.30s , +63d 20m 22.6s) | C | 60 | 19.4 | 3497 | 2026-04-21 03:10:31 | MASTER-OAGH | (13h 23m 04.46s , +63d 54m 18.8s) | C | 60 | 19.0 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/44359. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42480] GRB 260421A: Fermi GBM Final Real-time Localization
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44358 SUBJECT: GRB 260421A: Fermi GBM Final Real-time Localization DATE: 26/04/21 02:23:18 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 SHORT GRB At 02:12:44 UT on 21 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260421A (trigger 798430369.324586 / 260421092). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 198.8, Dec = 62.9 (J2000 degrees, equivalent to J2000 13h 15m, 62d 53'), with a statistical uncertainty of 8.2 degrees. The angle from the Fermi LAT boresight is 60.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421092/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421092/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421092/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44358. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42479] EP260321a: VLA radio follow-up
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44357 SUBJECT: EP260321a: VLA radio follow-up DATE: 26/04/20 21:56:06 GMT FROM: todwyer1(a)jh.edu Tanner O'Dwyer (JHU), Alessandra Corsi (JHU), Gokul Srinivasaragavan (UMD), S. Bradley Cenko (NASA GSFC), Shreya Anand (Stanford.U), Anna Ho (Cornell), Jesper Sollerman (U.Stockholm), Bei Zhou (FemiLab & U.Chicago), Marc Kamionkowski (JHU), Jillian Rastinejad (UMD) Following GCN 44239 we re-observed the field of EP260321a (Huang et al., GCN 44068) with the Karl G. Jansky Very Large Array (VLA) in its A-configuration. Our observations were conducted at about 22 days post trigger at a nominal central frequency of 6 GHz. We report no significant variation of the radio source at a position compatible with that of SN 2026gzf (Corcoran et al., GCN 44105, Rastinejad et al., GCN 44107). View this GCN Circular online at https://gcn.nasa.gov/circulars/44357. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42478] GRB 260412A: LCOGT Optical Upper Limits
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44356 SUBJECT: GRB 260412A: LCOGT Optical Upper Limits DATE: 26/04/20 17:36:58 GMT FROM: Robert Strausbaugh at Eastern Illinois University <rstrausbaugh(a)eiu.edu> R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration: We observed the GRB260412A field (Fermi GBM Team, GCN 44275; Wang et al., GCN 44276) with the LCOGT 1-meter Sinistro instrument at the South African Astronomical Observatory, South Africa site, on April 12, from 17:13 to 17:26 UT (corresponding to 11.0 to 11.2 hours after the trigger time) and with the LCOGT 1-meter Sinistro instrument at the Cerro Tololo Interamerican Observatory, Chile site, from 23:20 to 23:52 (17.1 to 17.7 hours after the trigger time) with the Bessel I and R filters. We performed a series of 3x300s exposures in I-band and 1x100s exposure in R-band due to a scheduling interruption, and then 3x300s in I-band and R-band for the second epoch. We do not detect an uncatalogued source within the SVOM error region (Wang et al., GCN 44276), consistent with other optical upper limits (Lipunov et al., 44280). The following 5-sigma upper limits are calculated using the USNO-B1.0 catalog as reference: Epoch 1: I>20.7 R>17.3 Epoch 2: I>21.0 R>22.6 These magnitudes are not corrected for galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44356. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42477] The EP-WXT trigger 01709260314 is likely a flaring star
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44355 SUBJECT: The EP-WXT trigger 01709260314 is likely a flaring star DATE: 26/04/20 09:50:59 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. L. Wang (NAO, CAS; ICE, CSIC), J. P. Chen (SYSU), J. W. Hu, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709260314 at the time of 2026-04-20T09:25:12, is likely a stellar flare associated with L 691-74. The estimated flux of the flare is around 1.6e-10 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 6.2e30 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/44355. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42476] EP260416a: Optical follow-up observations with Kinder
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44354 SUBJECT: EP260416a: Optical follow-up observations with Kinder DATE: 26/04/20 08:42:49 GMT FROM: Janet Chen at National Central University <janetstars(a)gmail.com> K. N.-T. Ho, A. Aryan, T.-W. Chen, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), A. Sankar.K, M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, 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, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report: We observed the field of the fast X-ray transient EP260416a (Hu et al., GCN 44307) 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 r-band started at 11:48 UTC on the 16th of April 2026 (MJD 61146.4918), 6.32 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. Owing to the faint optical counterpart candidate (Ducoin et al., GCN 44310), the transient was not detected in our stacked image. After the reported optical rebrightening (Hua et al., GCN 44318), we performed another SLT epoch of observations starting at UTC 12:33 on April 18th, 2026 (61148.5228), 55.05 hr post the WXT discovery. 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; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334, He et al. GCN 44336, An et al. GCN 44339, Jelinek et al. GCN 44341; and Guziy et al., GCN 44351) was detected in the stacked images. We observed the transient for one more epoch, starting at UTC 11:30 on April 19th, 2026 (61149.4790), about 78.0 hr since the WXT discovery. The optical counterpart candidate was not detected in the stacked image. Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and measured photometry/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 | 61146.4918 | 6.31 | 300 * 24 | >21.3 | 2".13 | 1.26 SLT | r | 61148.5228 | 55.05 | 300 * 19 | 21.16 +/- 0.08 | 2".05 | 1.13 SLT | r | 61149.4790 | 78.00 | 300 * 18 | >20.9 | 2".66 | 1.35 The presented magnitudes are calibrated using the field stars from ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitudes/upper-limits are not corrected for an expected galactic extinction of A_r = 0.04 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/44354. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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