EP260731a
GCN Circular 45416
Subject
EP260731a: SVOM/ECLAIRs detection through offline targeted search
Date
2026-08-21T13:10:00Z (6 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
H. Yang, S. Guillot, O. Godet (IRAP)
report on behalf of the SVOM team:
The SVOM/ECLAIRs telescope detected EP260731a (EP team, GCN 45261), starting at 2026-07-31T00:53:19 UTC (T0), through an offline targeted search with the event-by-event data downloaded via the X-band ground station. The ECLAIRs detection, at T0, occurred 31 s before the EP-WXT detection that began at 2026-07-31T00:53:50 UTC (EP team, GCN 45301).
The burst was detected within several timescales and energy ranges. The best detection is obtained by the image trigger with a signal-to-noise ratio of 6.4 within 8-120 keV over a time window of 82 s starting at T0. This event was not detected onboard because it was too faint.
The localization of the source is RA, Dec = 344.500, -11.872 degrees:
RA (J2000) = 22h58m00s
Dec (J2000) = -11d52m19s
with a 90% C.L. radius of 12.14 arcmin (including systematic error of 2 arcmin added in quadrature). This position is consistent with that measured by EP/FXT, with an angular separation of 7.8 arcmin.
The ECLAIRs lightcurve shows a multi-peaked structure lasting around 82 s in the 4 - 120 keV band.
The time-averaged spectrum from T0 to T0+82 s in the 5-120 keV is well fitted by a power law with a photon index of -1.4 +/- 0.2, a 4-120 keV photon flux of 0.22 +0.02/-0.06 photons/cm^2/s, and a corresponding 4-120 keV fluence of (7 +/- 1)e-7 erg/cm^2.
Using the same extraction time window from T0+31 s to T0+66 s as EP/WXT, the time-averaged spectrum in the 5-120 keV is also well fitted by a power law with a photon index of -1.4 +/- 0.2, a 4-120 keV photon flux of 0.28 +0.02/-0.08 photons/cm^2/s, and a corresponding 4-120 keV fluence of (3.7 +/- 0.8)e-7 erg/cm^2. The spectral properties measured with SVOM/ECLAIRs are broadly consistent with those reported by EP/WXT (EP team, GCN 45301).
All quoted errors are at the 68% confidence level.
The Space 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 APC, CEA, CNES, and IRAP.
The SVOM/ECLAIRs point of contact for this event is : Hui Yang (hui.yang@cnrs.fr)
Please contact him by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular 45301
Subject
EP260731a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Date
2026-08-07T03:41:59Z (21 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
K.R. Ni (CCNU), Y.F. Liang (PMO, CAS), H.Y. Liu, and H.W. Pan (NAO, CAS), on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260731a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Ni et al., GCN 45261), and followed up by NOT/ALFOSC and COLIBRÍ/DDRAGO (Liu et al., GCN 45263; de Ugarte Postigo et al., GCN 45264). We report the refined analysis of the EP-WXT and autonomous EP-FXT observations.
The refined analysis of the WXT data shows that the event started at T0 = 2026-07-31T00:53:50 (UTC) and lasted for approximately 35 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.10 x 10^20 cm^-2 and a photon index of 0.89 (-0.50/+0.52). The derived average unabsorbed 0.5-4 keV flux is 2.30 (-0.70/+0.94) x 10^-9 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously starting at 2026-07-31T00:56:34 (UTC, T0+164 s). The exposure time of this observation was 2982 s. On-ground analysis of the FXT data found an uncatalogued X-ray source at R.A. = 344.4246 deg, DEC = -11.7626 deg (J2000), with an uncertainty of 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 with a free hydrogen column density of 1.83 (-1.83/+2.06) x 10^21 cm^-2 and a photon index of 2.34 (-0.68/+0.75). The derived average unabsorbed 0.5-10 keV flux is 1.15 (-0.21/+0.33) x 10^-12 erg s^-1 cm^-2.
The uncertainties are at the 90% confidence level for the above spectral parameters.
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 45264
Subject
EP260731a: COLIBRÍ optical upper limit
Date
2026-07-31T07:24:07Z (a month ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
Antonio de Ugarte Postigo (LAM), Fredd Sánchez Álvarez (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), 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), Noémie Globus (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), Benjamin Schneider (LAM), Alan M. Watson (UNAM) report:
We imaged the field of EP260731a (Ni et al., GCN Circ. 45261) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-07-31 05:52:50 to 07:10:03 UTC (from 4.9739 to 6.2776 hours after the trigger) and obtained 39x60 s of simultaneous exposure in the z-band filter. Some further images obtained during this time window were lost due to passing clouds.
The data were reduced, coadded and analysed with the COLIBRÍ ASU and CPPM pipelines. The photometry was calibrated using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the FXT source position (Ni et al., GCN Circ. 45261) down to the following 5-sigma limit:
z > 20.9
This upper limit is consistent with the one reported by Liu et al. (GCN Circ. 45263).
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 Circular 45263
Subject
EP260731a: NOT optical upper limit
Date
2026-07-31T04:39:27Z (a month ago)
From
liuxing@nao.cas.cn
Via
Web form
X. Liu, J. An, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J. P. U. Fynbo (DAWN/NBI), A. L. Bouquin, A. Schenone-Zanuzzi (NOT) report:
We observed the field of EP260731a detected by EP (Ni et al., GCN 45261), using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observation started at 2026-07-31 03:16:25.90 UT, i.e., 2.37 hours post-trigger, and a series of z-band frames were obtained. The target was 16 deg from the moon at 98% illumination at the time of the observation.
No uncatalogued optical transient is detected in the stacked image within the EP-FXT error circle (Ni et al., GCN 45261) down to a 5-sigma limiting magnitude of z ~ 19.8 at a median time of 2.46 hr post-trigger, calibrated with Pan-STARRS DR2 stars in the field.
GCN Circular 45261
Subject
EP260731a: Einstein Probe detection of an X-ray transient
Date
2026-07-31T02:15:53Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
K.R. Ni (CCNU) , Y.F. Liang (PMO, CAS), H.Y. Liu, and H.W. Pan (NAO, CAS) 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 EP260731a. The transient triggered EP-WXT (ID: 01709277303) at 2026-07-31T00:54:24 (UTC). The WXT position of the source is R.A. = 344.424 deg, DEC = -11.764 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. = 344.4279 deg, DEC = -11.7623 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 is 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).