GCN 44209: EP260403a: LCO optical observations of the counterpart
2026-04-03T14:56:19.153Z | rev 0
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.
GCN 44207: EP260403a: LCO optical counterpart detection
2026-04-03T13:19:20.877Z | rev 2
I. Pérez-Fournon (IAC and ULL), F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz,
I. Correa-Plasencia, E. Lekaroz-Urriza, M. Quintana-Ansaldo (all ULL), A. López-Oramas (IAC and ULL), and D. Aguado (IAC and ULL)
Following the detection of the fast X-ray transient EP260403a (Guo et al., GCN circ. 44204) by the Wide-field X-ray Telescope (WXT) and the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission, we observed the field with one of the two Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 180 sec in the SDSS r' filter, started on 2026-04-03 at 11:28:01 UT, about 58.8 minutes after the EP WXT trigger. The optical counterpart, first reported by Antier et al. (GCN circ. 44206), is detected in our image with an AB magnitude of r' = 19.09 +/- 0.21, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction.
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2026A-011, SGLF and Superluminous Supernovae surveys).
This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).
GCN 44207: EP260403a: LCO optical counterpart detection
2026-04-03T13:19:20.877Z | rev 2
I. Pérez-Fournon (IAC and ULL), F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz,
I. Correa-Plasencia, E. Lekaroz-Urriza, M. Quintana-Ansaldo (all ULL), A. López-Oramas (IAC and ULL), and D. Aguado (IAC and ULL)
Following the detection of the fast X-ray transient EP260403a (Guo et al., GCN circ. 44204) by the Wide-field X-ray Telescope (WXT) and the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission, we observed the field with one of the two Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 180 sec in the SDSS r' filter, started on 2026-04-03 at 11:28:01 UT, about 58.8 minutes after the EP WXT trigger. The optical counterpart, first reported by Antier et al. (GCN circ. 44206), is detected in our image with an AB magnitude of r' = 19.09 +/- 0.21, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction.
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2026A-011, SGLF and Superluminous Supernovae surveys).
This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).
GCN 44206: EP260403a: COLIBRÍ optical counterpart candidate
2026-04-03T12:26:56.945Z | rev 0
Sarah Antier (IJCLAB), Noémie Globus (UNAM), Benjamin Schneider (LAM), Dalya Akl (NYUAD), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Rosa L. Becerra (UNAM), 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), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the EP260403a (Guo et al., GCN Circ. 44204) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-04 11:06:21 to 11:45:02 (from 37.20 to 76.87 min after the trigger) and obtained 29 min of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the ASU, COLIBRÍ pipelines and 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 detect an uncatalogued and fading source (revealed by image subtraction using PanSTARRS as template) consistent with the FXT 20 arcsec error circle (Guo et al., GCN Circ. 44204) at:
RA(J2000) = 14:36:37.44 = 219.1560 degrees
Dec(J2000) = -25:30:14.2 = -25.5039 degrees
The preliminary magnitude derived for that source is:
r = 18.92 +/- 0.02
z = 18.70 +/- 0.02
Further observations are ongoing.
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.
GCN 44204: EP260403a: Einstein Probe detection of an X-ray transient
2026-04-03T11:41:00.852Z | rev 0
C.-L. Guo (NAO, CAS), R.-Z. Li (YNAO) and C.-Y. Dai (NJU) 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 EP260403a. The transient triggered EP-WXT (ID: 01709259505) at 2026-04-03T10:29:10 (UTC). The WXT position of the source is R.A. = 219.150 deg, DEC = -25.502 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. = 219.1572 deg, DEC = -25.5069 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 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).