EP261003a
GCN Circular 45808
Subject
EP#01709320865: Swift-XRT observations
Date
2026-10-05T08:58:28Z (a day ago)
Edited On
2026-10-05T13:50:46Z (a day ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
email
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP#01709320865, collecting 847 s of Photon Counting (PC) mode data between
T0+15 ks and T0+16 ks after the trigger. We have detected 1 source. This has been
automatically classified as an uncatalogued X-ray source.
Uncatalogued X-ray sources
--------------------------
Source 1 (SWIFT J203008.6-125641):
==================================
RA (J2000.0): 307.5359 = 20 30 08.62
Dec (J2000.0): -12.9448 = -12 56 41.3
Error: 6.2 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 52 arcsec from the Einstein Probe/WXT position.
Mean rate: 0.0156 [+0.0064, -0.0051] ct s^-1
Mean flux: (6.0 [+2.4, -1.9])e-13 erg cm^-2 s^-1
Peak rate: 0.0156 [+0.0064, -0.0051] ct s^-1
Peak flux: (6.0 [+2.4, -1.9])e-13 erg cm^-2 s^-1
ECF: 3.82e-11 erg cm^-2 ct^-1
assuming NH=4.73e+20 cm^-2, gamma=1.84
determined from a spectral fit.
RASS UL: 5.5e-02 ct s^-1 (converted to XRT; 0.3-10 keV)
so the source is not above this 3-sigma upper limit.
There is no evidence for fading.
All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP/EP_FIELD00091.
This circular is an official product of the Swift-XRT team.
GCN Circular 45804
Subject
EP261003a: J-band upper limit with WINTER
Date
2026-10-03T13:29:38Z (3 days ago)
From
tanishk.mohan@iitb.ac.in
Via
Web form
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP261003a (Lao et al., GCN 45798) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began on 2026-10-03 at 02:46:15.793 UTC in the J band (2.53 hr after the EP-WXT trigger), consisting of 29*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We do not detect any new source in the refined FXT localization (Lao et al., GCN 45803) in our stacked image.
We obtain the following 5-sigma upper limit: J = 19.43 mag (AB).
We also note deep upper limit by Globus et al., GCN 45801.
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 45803
Subject
EP261003a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Date
2026-10-03T09:45:53Z (3 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q.H. Lao, H.Y. Liu (NAO, CAS), Y.H. Jiang (NJU), Y.H. Cheng (YNU) and Y. Liu (NAO, CAS), on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP261003a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-10-03T00:14:43 (UTC) (Lao et al. GCN 45798). Subsequent follow-up observations were reported by Segura et al. (GCN 45796) and Globus et al. (GCN 45801). The refined analysis of the WXT data shows that the event started at T0 = 2026-10-03T 00:12:15 (UTC), approximately 150 s before the WXT trigger.
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at Galactic NH = 4.15 x 10^21 cm^-2 at the source position. The best-fit photon index is 1.52 (-0.54/+0.58), and the derived average unabsorbed 0.5-4 keV flux is 8.14 (-2.39/+3.37) x 10^-10 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed the source autonomously following the WXT trigger at 2026-10-03T00:17:27(UTC), approximately 160 s after the trigger. With the telemetry data received, the refined on-ground analysis gives an X-ray source position of RA=307.5337 deg, DEC=-12.9423 deg (J2000) , with an uncertainty of about 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows a rapid declining trend during the first 700s of observation.
We jointly fitted the FXT-A and FXT-B spectra in the 0.5-10 keV band with an absorbed power-law model, with the hydrogen column density fixed at Galactic NH = 4.15 x 10^21 cm^-2 at the source position. The best-fit photon index is 1.69 (-0.22/+0.23). The derived average unabsorbed 0.5-10 keV flux is 7.06 (-1.30/+1.59) x 10^-12 erg s^-1 cm^-2.
The uncertainties are at the 90% confidence level for the above spectral parameters.
Further multi-wavelength follow-up observations are encouraged to explore the nature of EP261003a.
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 45801
Subject
EP261003a: COLIBRÍ optical upper limits
Date
2026-10-03T05:04:14Z (3 days ago)
From
globus@astro.unam.mx
Via
Web form
Noémie Globus (UNAM), Jean-Grégoire Ducoin (CPPM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (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), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) report:
We imaged the field of the EP261003a (Lao et al., GCN Circ. 45798) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-10-03 03:41:26 to 04:47:19 UTC (from 3.45 to 4.56 hours after the trigger) and obtained 51 minutes of simultaneous exposure in the r and z filters.
The data were reduced and analyzed with the COLIBRÍ 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 do not detect any new source at the FXT source position down to the following 5-sigma limit:
r > 23.88,
z > 22.40.
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 45798
Subject
EP261003a: Einstein Probe detection of an X-ray transient
Date
2026-10-03T01:40:53Z (3 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q.H. Lao, H.Y. Liu (NAO, CAS), Y.H. Jiang (NJU), Y.H. Cheng (YNU) and Y. Liu (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 EP261003a. The transient triggered EP-WXT (ID: 01709320865) at 2026-10-03T00:14:43 (UTC). The WXT position of the source is R.A. = 307.524 deg, DEC = -12.936 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. = 307.5318 deg, DEC = -12.9422 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).