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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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[vsnet-grb-info 42475] GOTO26djh/AT2026jpw: CrAO and Mondy optical observations of the transient
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44353 SUBJECT: GOTO26djh/AT2026jpw: CrAO and Mondy optical observations of the transient DATE: 26/04/20 06:51:39 GMT FROM: Nicolai Pankov at IKI <colinsergesen(a)gmail.com> N. Pankov (IKI), V. Rumyantsev (CrAO), E. Klunko (ISTP), A. Pozanenko (IKI), A. Volnova (IKI) report on behalf of IKI-GRB-FuN: We continued optical observations of the field of AT2026jpw (O'Neill et al., GCN 44301; Corcoran et al., GCN 44303; Moskvitin and Spiridonova, GCN 44304; Xu et al., GCN 44305; The COLIBRÍ team, GCN 44306; Sosnovskij et al., GCN 44309; Ma et at., GCN 44314; Pankov et al., GCN 44316) in the R band with the Shajn 2.6-meter telescope (ZTSh) of the Crimean Astrophysical Observatory (CrAO) and the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy). The observations began on 2026-04-16 20:53 UT, i.e. ~2.92 days since GOTO detection at CrAO and on 2026-04-18 14:47 UT (~4.67 days since GOTO detection) at Mondy. The series of 38x120s exposures were taken by ZTSh/CrAO and 35x120s exposures were taken by AZT-33IK/Mondy. The optical counterpart is clearly detected in the co-add image from CrAO, but not detected by AZT-33IK. The preliminary photometry and observation details are provided below: Date UT start t-T0 Exp. Filter OT Err. UL(3sigma) Telescope/Site (mid, days) (s) 2026-04-16 20:53:30 2.92638 38x120 R 22.11 0.02 23.7 ZTSH/CrAO 2026-04-18 14:47:18 4.66999 35x120 R n/d n/d 22.8 AZT-33IK/Mondy The photometry is calibrated using nearby stars from USNO-B1.0 (see Moskvitin and Spiridonova, GCN 44304) and has not been corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44353. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42474] EP-WXT trigger 01709260303: KAIT optical observations
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44352 SUBJECT: EP-WXT trigger 01709260303: KAIT optical observations DATE: 26/04/20 06:33:01 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 01709260303 (Yin et al., GCN 44350) starting at 10:53 UT, about 3.5 hours after the trigger. A set of B,V,R,I and clear (roughly R) filter images were obtained and lasted for ~0.9 hours. We performed photometry on the suggested flare star Gaia DR3 4524306802880186496 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/44352. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42473] EP260416a: 1.5m OSN and 2.2m CAHA optical monitoring
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44351 SUBJECT: EP260416a: 1.5m OSN and 2.2m CAHA optical monitoring DATE: 26/04/20 04:03:59 GMT FROM: Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct(a)iaa.es> S. Guziy, F. J. Aceituno, A. J. Castro-Tirado, M.D. Caballero-García, M. Gritsevich, I. Pérez-García, R. Sánchez-Ramírez, S.-Y. Wu (IAA-CSIC) and A. Fernández-Martin and M. M. Sánchez-Andújar (CAHA), on behalf of a larger collaboration,report: We observed EP260416a (Hu et al., GCN 44307) with both the 1.5m telescope at Observatorio de Sierra Nevada (OSN) and the 2.2m telescope at Calar Alto Observatory (CAHA), in Southern Spain. A series of 300 s BVRI exposures were obtained at the 1.5m OSN starting on Apr 17, 21:04 UT, complemented by one 100s i-band image at the 2.2m CAHA, on Apr 18, 01:04 UT., i.e. 39.6 h and 43.6 h respectively, after the initial trigger. 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) is well detected in our images. We measure B = 21.08 +/- 0.14 and R = 20.99 +/- 0.07 on Apr 17, 21:15 UT and I = 19.82 +/-0.18 on Apr 18, 01:04 UT (none corrected for Galactic extinction). The night after, a second epoch BVRI observation was conducted at the 1.5m OSN, for which measure R = 21.59 +/- 0.10 on Apr 18, 20:14 UT. Our measurement is consistent with the continued fading of the afterglow. Further optical monitoring is being carried out. View this GCN Circular online at https://gcn.nasa.gov/circulars/44351. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42472] The EP-WXT trigger 01709260303 is likely a flaring star
by GCN Circulars 20 Apr '26

20 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44350 SUBJECT: The EP-WXT trigger 01709260303 is likely a flaring star DATE: 26/04/20 02:32:00 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y.-H. I. Yin (HKU), Z.-C. Zou (NJU), H.Q. Cheng and H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709260303 at the time of 2026-04-19T07:22:35, is likely a stellar flare associated with Gaia DR3 4524306802880186496. The estimated flux of the flare is around 8e-11 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 2.1e31 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/44350. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42471] GRB 260417A: SVOM/GRM observation
by GCN Circulars 19 Apr '26

19 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44349 SUBJECT: GRB 260417A: SVOM/GRM observation DATE: 26/04/19 16:33:14 GMT FROM: zhangjinpeng(a)ihep.ac.cn SVOM/GRM team: Jin-Peng Zhang, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Sebastien Guillot (IRAP) Report on behalf of the SVOM team: SVOM/GRM detected GRB 260417A at 2026-04-17T00:10:11.500 (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44317), AstroSat CZTI (S. Salunke et al., GCN #44330) and possibly by EP (K. R. Ni et al., GCN #44337). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst mainly consists of a pulse with a T90 of 38 +15/-10 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb260417A.png In addition, the position of this burst, according to Fermi/GBM localization (RA = 252.0, Dec = -19.8, error(stat) = 3.4, Fermi GBM Team, GCN #44317), is located at about 90.3 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-20 to T0+20 s is fitted by a power law function. The power law index is -1.77 +0.09/-0.09. The event fluence (10-1000 keV) in this time interval is (9.1 +1.0/-1.1)E-06 erg/cm^2. 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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM/GRM point of contact for this burst is: Jin-Peng Zhang (IHEP) (zhangjinpeng(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/44349. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42470] EP260321A/AT2026gzf: Optical observation from 1.3m DFOT
by GCN Circulars 19 Apr '26

19 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44348 SUBJECT: EP260321A/AT2026gzf: Optical observation from 1.3m DFOT DATE: 26/04/19 09:06:27 GMT FROM: ANSHIKA GUPTA at ARIES <anshika05180(a)gmail.com> Anshika Gupta, Pankaj Pawar, Divyanshu Janghel, Dhruv Jain, Debalina Kar, and Kuntal Misra (ARIES) report: We observed the field of EP260321A/AT2026gzf detected by EP-WXT (Huang, GCN 44068) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. Observations began on 2026-04-16 at 14:22:58 UT, i.e., ~26.07 days after the EP-WXT trigger. We have taken multiple frames with an exposure time of 300s in the R filter. We stacked the images after the alignment. We detect an optical afterglow in our stacked image. We obtain the following preliminary magnitude in the stacked image: Date Start_UT T_start-T0 (days) Filter Exp time (s) Magnitude ================================================================= 2026-04-16 14:22:58 ~26.07 R 300s*4 17.14 +- 0.01 The optical detection of the burst is consistent with Lipunov et al., GCN 44069; Lee et al., GCN 44070; Aryan et al., GCN 44081; Tanvir et al., GCN 44082; Moran et al., GCN 44083; Ahumada et al., GCN 44084; Liu et al., GCN 44087; Sankar et al. GCN 44089; de Ugarte Postigo et al., GCN 44091; Xu et al., GCN 44092; Pankov et al. GCN 44103; Corcoran et al., GCN 44105; Rastinejad et al., GCN 44107; Gao et al., GCN 44110; He et al., GCN 44117; Zhang et al., GCN 44171; Li et al., GCN 44284; Volnova et al., GCN 44295. The magnitude is not corrected for the Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the USNO-B1.0 catalog. View this GCN Circular online at https://gcn.nasa.gov/circulars/44348. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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