EP260403a
GCN Circular 44231
Subject
EP260403a: Mephisto optical upper limits
Date
2026-04-07T15:11:25Z (7 months ago)
From
Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn>
Via
Web form
Chenxi Shang, Chenxu Liu, Guowang Du (SWIFAR, YNU), Jinghang Xue (NJU), Yuhui Zhang, Jialei Zheng, Tao Wang, Brajesh Kumar, Xueling Du, Xufeng Zhu, Yu Pan, Xingzhu Zou, Xinlei Chen, Yuan Fang, Jinghua Zhang (SWIFAR, YNU), Chao Wu (NAOC), Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team:
The 1.6-m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory, was triggered at 2026-04-03 17:53:31 UT (~7.4 hours after the Einstein Probe/WXT trigger; Guo et al., GCN 44204) to observe the field of EP260403a. A set of simultaneous multi-band (u, v, i, z) images were collected with individual exposure times of 45 s per frame. The observations consist of multiple exposures in each band, which were later stacked for analysis. In the stacked images, we did not detect the optical counterpart reported by Antier et al. (GCN 44206), Pérez-Fournon et al. (GCN 44207) and Quirola-Vasquez (GCN 44207). Our non-detection is consistent with the continued fading of the optical source at later epochs. The estimated 5-sigma limiting magnitudes (AB system, not corrected for Galactic extinction) are listed below:
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Start_Time(UT) | Band | Exp(s) | LimMag (AB)
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2026-04-03T17:53:31 | u | 45.0*2 | >19.62
2026-04-03T17:55:57 | v | 45.0*2 | >18.79
2026-04-03T17:53:33 | i | 45.0*2 | >19.72
2026-04-03T17:56:00 | z | 45.0*2 | >19.16
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Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. The facility is operated by the South-Western Institute for Astronomy Research (SWIFAR), Yunnan University. It provides real-time, high-quality colors of stellar objects. The Mephisto mosaic cameras were installed in October 2025. The first light was achieved in all three channels on 10 October 2025 and presently, these are under the commissioning phase. All the data have been reduced by the Mephisto data processing pipeline. We note that the current data-processing pipeline is still at a preliminary stage, with flux calibration precision of about 2% in the u and v bands, about 1% in the g and r bands, and better than 1% in the i and z bands.
GCN Circular 44214
Subject
EP260403a: refined analysis of the EP-WXT and EP-FXT observations
Date
2026-04-04T06:09:17Z (7 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
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).
GCN Circular 44209
Subject
EP260403a: LCO optical observations of the counterpart
Date
2026-04-03T14:56:19Z (7 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
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 Circular 44207
Subject
EP260403a: LCO optical counterpart detection
Date
2026-04-03T13:19:20Z (7 months ago)
Edited On
2026-04-03T15:35:48Z (7 months ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
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 Circular 44206
Subject
EP260403a: COLIBRÍ optical counterpart candidate
Date
2026-04-03T12:26:56Z (7 months ago)
From
antier@ijclab.in2p3.fr
Via
Web form
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 M ártir, 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 Circular 44204
Subject
EP260403a: Einstein Probe detection of an X-ray transient
Date
2026-04-03T11:41:00Z (7 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
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).