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[vsnet-grb-info 42339] Fermi trigger No 797039833: Global MASTER-Net observations report
by GCN Circulars 05 Apr '26

05 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44218 SUBJECT: Fermi trigger No 797039833: Global MASTER-Net observations report DATE: 26/04/05 11:00:42 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-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB260404.99 (trigger No 797039833,12h 09m 43.20s , -35d 10m 48.0s, R=16.55) errorbox 21767 sec after notice time and 21798 sec after trigger time at 2026-04-05 06:00:27 UT, with upper limit up to 19.5 mag. The observations began at zenith distance = 19 deg. The sun altitude is -58.3 deg. The galactic latitude b = 27 deg., longitude l = 294 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3232826 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 21859 | 2026-04-05 06:00:27 | MASTER-OAFA | (12h 59m 32.15s , -41d 01m 29.5s) | C | 120 | 18.4 | 23083 | 2026-04-05 06:21:21 | MASTER-OAFA | (12h 37m 40.74s , -42d 05m 50.7s) | C | 60 | 17.6 | 23143 | 2026-04-05 06:21:21 | MASTER-OAFA | (12h 37m 40.75s , -42d 05m 50.7s) | C | 180 | 18.7 | Coadd 23163 | 2026-04-05 06:22:42 | MASTER-OAFA | (12h 48m 26.86s , -42d 07m 29.9s) | C | 60 | 17.5 | 23240 | 2026-04-05 06:23:58 | MASTER-OAFA | (12h 37m 42.77s , -42d 05m 40.4s) | C | 60 | 18.3 | 23334 | 2026-04-05 06:25:32 | MASTER-OAFA | (12h 48m 27.14s , -42d 06m 12.0s) | C | 60 | 19.2 | 23409 | 2026-04-05 06:26:48 | MASTER-OAFA | (12h 37m 39.06s , -42d 07m 24.1s) | C | 60 | 19.1 | 23525 | 2026-04-05 06:28:43 | MASTER-OAFA | (12h 35m 41.78s , -40d 12m 31.0s) | C | 60 | 18.5 | 23585 | 2026-04-05 06:28:43 | MASTER-OAFA | (12h 35m 41.77s , -40d 12m 31.0s) | C | 180 | 19.5 | Coadd 23600 | 2026-04-05 06:29:59 | MASTER-OAFA | (12h 46m 10.91s , -40d 11m 40.1s) | C | 60 | 19.2 | 23676 | 2026-04-05 06:31:14 | MASTER-OAFA | (12h 35m 48.78s , -40d 12m 30.1s) | C | 60 | 19.2 | 23761 | 2026-04-05 06:32:39 | MASTER-OAFA | (12h 46m 18.29s , -40d 12m 02.0s) | C | 60 | 19.1 | 23837 | 2026-04-05 06:33:55 | MASTER-OAFA | (12h 35m 41.12s , -40d 12m 28.9s) | C | 60 | 19.1 | 23932 | 2026-04-05 06:35:30 | MASTER-OAFA | (12h 49m 34.20s , -47d 48m 23.1s) | C | 60 | 19.1 | 24013 | 2026-04-05 06:36:51 | MASTER-OAFA | (13h 01m 25.31s , -47d 50m 17.4s) | C | 60 | 19.1 | 24089 | 2026-04-05 06:38:07 | MASTER-OAFA | (12h 49m 34.19s , -47d 49m 47.0s) | C | 60 | 19.1 | 35699 | 2026-04-05 09:51:38 | MASTER-OAFA | (12h 18m 08.46s , -19d 48m 02.5s) | C | 60 | 16.7 | 35759 | 2026-04-05 09:51:38 | MASTER-OAFA | (12h 18m 08.45s , -19d 48m 02.5s) | C | 180 | 17.1 | Coadd 35765 | 2026-04-05 09:52:43 | MASTER-OAFA | (12h 18m 08.49s , -19d 48m 02.4s) | C | 60 | 16.6 | 35830 | 2026-04-05 09:53:49 | MASTER-OAFA | (12h 18m 08.48s , -19d 48m 02.4s) | C | 60 | 16.5 | 35896 | 2026-04-05 09:54:55 | MASTER-OAFA | (12h 18m 08.55s , -19d 48m 02.6s) | C | 60 | 16.5 | 35956 | 2026-04-05 09:54:55 | MASTER-OAFA | (12h 18m 08.55s , -19d 48m 02.7s) | C | 180 | 16.6 | Coadd 35962 | 2026-04-05 09:56:00 | MASTER-OAFA | (12h 18m 08.56s , -19d 48m 02.5s) | C | 60 | 16.4 | 36028 | 2026-04-05 09:57:06 | MASTER-OAFA | (12h 18m 08.63s , -19d 48m 02.7s) | C | 60 | 16.2 | 36095 | 2026-04-05 09:58:13 | MASTER-OAFA | (12h 18m 08.72s , -19d 48m 03.5s) | C | 60 | 16.0 | 36155 | 2026-04-05 09:58:13 | MASTER-OAFA | (12h 18m 08.72s , -19d 48m 03.6s) | C | 180 | 16.3 | Coadd 36160 | 2026-04-05 09:59:18 | MASTER-OAFA | (12h 18m 08.73s , -19d 48m 03.4s) | C | 60 | 16.0 | 36225 | 2026-04-05 10:00:24 | MASTER-OAFA | (12h 18m 08.80s , -19d 48m 03.7s) | C | 60 | 15.8 | 36291 | 2026-04-05 10:01:30 | MASTER-OAFA | (12h 18m 08.89s , -19d 48m 03.9s) | C | 60 | 15.7 | 36357 | 2026-04-05 10:02:35 | MASTER-OAFA | (12h 18m 08.96s , -19d 48m 04.3s) | C | 60 | 15.5 | 36423 | 2026-04-05 10:03:41 | MASTER-OAFA | (12h 18m 09.09s , -19d 48m 04.8s) | C | 60 | 15.3 | 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/44218. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42338] GRB 260403A: GECAM-B observation of a long burst
by GCN Circulars 05 Apr '26

05 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44217 SUBJECT: GRB 260403A: GECAM-B observation of a long burst DATE: 26/04/05 07:34:52 GMT FROM: Xinghao Luo at IHEP <2952704891(a)qq.com> Xing-Hao Luo, Chen-Wei Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team: GECAM-B was triggered in-flight by GRB 260433A, at 2026-04-03T02:31:29.500 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #44195) and AstroSat CZTI (A. Goyal et.al., GCN#44202). According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a single pulse with a duration (T90) of 24.5 +3.0/-4.0 s. The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecambgrb260403A.png Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (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/44217. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42337] EP260324A / AT 2026hir: COLIBRÍ further optical observations
by GCN Circulars 05 Apr '26

05 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44216 SUBJECT: EP260324A / AT 2026hir: COLIBRÍ further optical observations DATE: 26/04/05 05:38:58 GMT FROM: Alan Watson at UNAM <alan(a)astro.unam.mx> Alan M. Watson (UNAM), Massimiliano Lincetto (CPPM), Rosa L. Becerra (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Camila Angulo (UNAM), 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), 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 observed AT2026hir/ZTF26aapviim, the optical counterpart (Anumarlapudi et al., GCN Circ. 44120, Stein et al., GCN Circ. 44194, Globus et al., GCN Circ. 44196) of EP260324a (Wu et al., Atel #17728), with the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-05 03:10 to 03:26 UTC (from 11.95 to 11.96 days after the discovery time reported in the TNS record and from 11.84 to 11.85 days after the EP trigger) and obtained 5 minutes of exposure in each of the g, r, i filters, and 15 minutes of simultaneous exposure in the z filter. 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 detect the optical counterpart. The preliminary PSF-fitting magnitudes after performing image subtraction using PanSTARRS DR1 images as a template are: r = 18.01 +/- 0.01, z = 18.54 +/- 0.02. Compared to our previous observations from 2026-04-03 (Globus et al., GCN Circ. 44196), we see an apparently achromatic fading of: delta r = 0.28 +/- 0.01, delta z = 0.28 +/- 0.03. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44216. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42336] Fermi Gamma-ray Burst Monitor trigger 796972356/260404217 is not a GRB
by GCN Circulars 04 Apr '26

04 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44215 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 796972356/260404217 is not a GRB DATE: 26/04/04 07:18:24 GMT FROM: Rushikesh Sonawane at IISER, TVM <rushikesh23(a)iisertvm.ac.in> R. Sonawane (IISER, TVM) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 796972356/260404217 at 05:12:31.90 UT on 04 April 2026, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to Local Particles." View this GCN Circular online at https://gcn.nasa.gov/circulars/44215. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42335] EP260403a: refined analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 04 Apr '26

04 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44214 SUBJECT: EP260403a: refined analysis of the EP-WXT and EP-FXT observations DATE: 26/04/04 06:09:17 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> C.-Y. Dai (NJU), R.-Z. Li (YNAO), C.-L. Guo, and W.-D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260403a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Guo et al., GCN 44204). Optical follow-up of EP260403a revealed a fading counterpart, first identified by COLIBRÍ (Antier et al., GCN 44206) and subsequently confirmed by LCO (Pérez-Fournon et al., GCN 44207; Quirola-Vasquez et al., GCN 44209). The burst beginning at T0 = 2026-04-03 10:27:45 (UTC). The WXT observation lasted for approximately 50 seconds, and was interrupted due to the autonomous follow-up observation. The WXT light curve exhibits a single-peaked structure. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at the Galactic value of 5.6×10^20 cm^-2 and a photon index of 0.3 (-0.3/+0.3). The derived average unabsorbed flux in the 0.5-4 keV band is 1.6 (-0.3/+0.3) × 10^-9 erg s^-1 cm^-2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-04-03 10:33:22 (UTC, T0+337 s). The exposure time of this observation is 3.6 ks. On-ground analysis of the FXT data found an uncatalogued fading source at R.A., Dec. = 219.1556, -25.5045 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at the Galactic value and a photon index of 1.94 (-0.02/+0.02). The derived average unabsorbed flux in the 0.5-10 keV band is 6.4 (-0.1/+0.1)×10^-11 erg s^-1 cm^-2. A follow-up observation with the EP/FXT was planned, and further information will be updated when the telemetry data are 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/44214. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42334] The EP-WXT trigger 01709259586 is likely a flaring star
by GCN Circulars 04 Apr '26

04 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44213 SUBJECT: The EP-WXT trigger 01709259586 is likely a flaring star DATE: 26/04/04 06:05:02 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Q.Y. Wu, X.X. Sun, W. Chen (NAO,CAS), Y.J. Yi (BNU), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709259586 at the time of 2026-04-04T05:19:38, is likely a stellar flare associated with UCAC4 498-055345. The estimated flux of the flare is around 8 x 10^-11 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 1 x 10^33 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/44213. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42333] GRB 260330B: SVOM/VT further optical observation
by GCN Circulars 04 Apr '26

04 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44212 SUBJECT: GRB 260330B: SVOM/VT further optical observation DATE: 26/04/04 05:24:23 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H. L. Li, L. P. Xin, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team. SVOM/VT performed a second ToO observation of the field of GRB 260330B detected by SVOM/ECLAIRs (sb26033003, Lan et al., GCN 44157), SVOM/GRM (Lan et al., GCN 44157) and Fermi/GBM with sub-threshold detection (Trigg et al., GCN 44159). The observations started at 2026-04-03T12:56:07 UTC, approximately 3.94 days post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. The optical candidate (Li et al., GCN 44205) was detected in VT_R. The measurements in AB magnitude are as follows: Mid time | Band | Exposure Time | Brightness 4.019 d VT_R 82*70 sec 22.7+/-0.2 mag 4.019 d VT_B 87*70 sec >24.0 mag Our photometry was in the AB magnitude and was not corrected for Galactic extinction. We note the current brightness may be contaminated by the catalogued extended source (Li et al., GCN 44205). Combined with our previous observations (Li et al., GCN 44205), the candidate brightened and then faded in our observations. Further observations are encouraged to confirm the nature of this source. 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/44212. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42332] Fermi Gamma-ray Burst Monitor trigger 796931884/260403749 is not a GRB
by GCN Circulars 03 Apr '26

03 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44211 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 796931884/260403749 is not a GRB DATE: 26/04/03 19:38:24 GMT FROM: Suman Bala at USRA <sumanbala2210(a)gmail.com> S. Bala (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 796931884/260403749 at 17:57:59.85 UT on 03 April 2026, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to charge particles. View this GCN Circular online at https://gcn.nasa.gov/circulars/44211. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42331] GRB 260330B: SVOM/ECLAIRs refined analysis
by GCN Circulars 03 Apr '26

03 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44210 SUBJECT: GRB 260330B: SVOM/ECLAIRs refined analysis DATE: 26/04/03 19:30:32 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Authors: S. Guillot, M. Brunet, F. Triot, H. Yang, O. Godet, G. Lan (IRAP) report on behalf of the SVOM/ECLAIRs team Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260330B (SVOM burst-id sb26033003 – GCN 44157, trigger time T0 = 2026-03-30T14:19:57 UTC), which was also detected by Fermi GBM as a sub-threshold event (GCN 44159). The burst that triggered ECLAIRs onboard shows a multiple-peak lightcurve. The burst duration is T90 = 58.7 (+9.4)(-4.9) s s in the 4-120 keV energy band. The time-averaged spectrum from T0-23.0 s to T0+43.04 s in the energy range 4-120 keV is best fitted by a cutoff powerlaw model with alpha = -1.21 (+0.18)(-0.170), with Epeak = 24.9 (+11.5)(-8.8) keV. With this model, the 4-120 keV fluence is 1.32 (0.02)(-0.32) e-6 erg/cm^2 and the 4-120 keV photon flux is 0.85 +0.01/-0.14 ph/cm^2/s. All the quoted errors are at the 68% confidence level. 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. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this burst is: Sebastien Guillot (IRAP) (sebastien.guillot1 at utoulouse.fr) View this GCN Circular online at https://gcn.nasa.gov/circulars/44210. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42330] EP260403a: LCO optical observations of the counterpart
by GCN Circulars 03 Apr '26

03 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44209 SUBJECT: EP260403a: LCO optical observations of the counterpart DATE: 26/04/03 14:56:19 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo(a)ucd.ie> J. Quirola-Vasquez (Radboud), A. Martin-Carrillo (UCD), J. N. D. van Dalen (Radboud), J. Chácon (PUC) and P. G. Jonker (Radboud) report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP260403a (Guo et al., GCN 44204) with the LCO 1m telescope located at Siding Spring Observatory equipped with the SINISTRO instrument. We obtained 6 x 60 exposures in r-band, starting on 2026-04-03 at 12:19:37.245 UT (~1.84 hr post the EP trigger). In our stacked images, we detect the optical counterpart reported by Antier et al., GCN 44206 and Pérez-Fournon et al. GCN 44207 with an AB magnitude of r = 19.35 +/- 0.05, calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction. Our observations confirm that the counterpart is fading rapidly. View this GCN Circular online at https://gcn.nasa.gov/circulars/44209. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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