EP260602d
GCN Circular 44841
Subject
EP260602d: EP-FXT follow-up observation
Date
2026-06-05T12:47:27Z (5 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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J.Y. Cao, Z. X. Li, G. L. Huang, (IHEP, CAS), J. W. Hu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260602d was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Cao et al., GCN 44796) at 2026-06-02T21:13:25 (UTC).
Follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-04T04:27:19(UTC), ~31 hours after the WXT detection. The exposure time of the observation is around 4 ks. The on-ground analysis of the FXT data found a source within the error circle of WXT at R.A. = 267.9681 deg, DEC = 18.1658 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic) which have no historical X-ray source within 10 arcsec. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 6.7×10^20 cm^-2, and a photon index of 0.8 (-0.3, +0.4). The derived average unabsorbed 0.5-10 keV flux is 3.2 (-1.2, +1.8) ×10^(-14) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Since this source is below the RASS upper limit and shows no significant fading, we cannot identify it as the counterpart of EP260602d at present.
The optical and infrared follow-up observations were performed by A. Aryan et al. (GCN 44813) and Tanishk Mohan et al. (GCN 44830) without detection.
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 44830
Subject
EP260602d: J-band upper limit with WINTER
Date
2026-06-05T01:19:18Z (16 hours ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
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Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (NOIRLab), Benjamin Schneider (LAM), Robert Stein (UMD), Viraj Karambelkar (Columbia), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP260602d (Cao et al., GCN 44796) 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 at 2026-06-03T08:06:13 UTC in the J band (10.88 hr after the EP trigger), consisting of 15*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 WXT localization (Aryan et al., GCN 44813; Cao et al., GCN 44796) after image subtraction performed relative to J-band images from the UKIRT Hemisphere survey (Dye et al., 2017). We obtain the following 5-sigma upper limit: J = 19.4 mag (AB).
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 44813
Subject
EP260602d: Optical Upper limits with kinder observations
Date
2026-06-04T02:54:17Z (2 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
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A. Aryan, T.-W. Chen, W.-J. Hou, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang (HNAS), Y.-H. Lee, A. Sankar.K, M.-H. Lee, A. Dutta, Y.-C. Pan, C.-C. Ngeow, K. N.-T. Ho, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, K. W. Smith, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the FXT EP260602d (Cao et al., GCN 44796) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in r-band started at 14:34 UTC on the 3rd of June 2026 (MJD 61194.6066), 17.33 hours after the EP-WXT detection.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We next used the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. Neither in the stacked nor in the difference image did we detect any signature of any new or uncataloged source within the WXT region of uncertainty.
Moreover, we further employed AutoPhOT to perform PSF photometry. The details of the observations and the 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude| avg. Seeing | Med. Airmass LOT | r | 61194.6066 | 17.33 | 300 * 6 | >22.8 | 1".28 | 1.18
The presented upper limit is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and is not corrected for an expected galactic extinction of A_r = 0.19 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 44796
Subject
EP260602d: Einstein Probe detection of a fast X-ray transient
Date
2026-06-03T09:46:29Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
J.Y. Cao, Z. X. Li, G. L. Huang, (IHEP, CAS), J. W. Hu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260602d. The source did not trigger the WXT on-board trigger unit. A GCN Notice was sent manually (trigger ID: 13616654987). The ground analysis of the WXT data shows that the event started at T0=2026-06-02T21:13:25 (UTC) and lasted for about 200 s. The WXT position of the source is R.A. = 267.958 deg, DEC = 18.173 deg (J2000) with an uncertainty of 2.3 arcmin in radius (90% C.L. statistical and systematic).
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 6.7 × 10^20 cm^-2, and a photon index of 0.8 (-0.5, +0.6). The derived average unabsorbed 0.5-4 keV flux is 1.4 (-0.9, +0.7)×10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
A Target-of-Opportunity observation with the Follow-up X-ray Telescope (FXT) has been scheduled. 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).