EP260602b
GCN Circular 44829
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 EP260602b (Wu et al., GCN 44786; Wu et al., GCN 44802) 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 2026-06-03T04:53:16 UTC in the J band (~11 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 a source at the optical counterpart location (He et al., GCN 44780; Eyles-Ferris et al., GCN 44809; Bochenek et al., GCN 44811). We obtain the following 5-sigma upper limit: J = 19.1 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 44811
A. Bochenek, D. A. Perley (LJMU) report:
We observed the field of EP260602b (Hu et al., GCN 44786) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x150s exposures in SDSS i and z filters, starting at 2026-06-02 02:53:23 UT, approximately 8.99h hours after trigger.
We report an i-band detection at the position of the counterpart (He et al., GCN 44780; Hu et al., GCN 44802; Eyles-Ferris et al., GCN 44809), with i = 20.7 ± 0.25, and a tentative z-band detection of z = 20.8 ± 0.4 (or a 3 sigma upper limit of z > 20.4 mag).
The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 44809
R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), J. N. D. van Dalen (Radboud), J. Chacón (PUC), F. E. Bauer (SSI and UTA), R. L. C. Starling (Leicester), P. G. Jonker (Radboud), A. J. Levan (Radboud & Warwick), report on behalf of a larger collaboration:
We observed the field of EP260602b (Hu et al., GCNs 44786, 44802) with the 2m Liverpool Telescope (LT) on La Palma using the IO:O instrument and an LCO 1m telescope located at Cerro Tololo Interamerican Observatory (Chile) and equipped with the SINISTRO instrument. We obtained exposures in the Sloan r and g filters. We detect the optical counterpart (He et al., GCN 44780) in our LT and LCO r band stacked images.
We obtain the following photometry for the counterpart calibrated to Pan-STARRS and not corrected for Galactic extinction:
| t_mid - t_0 (hours) | Telescope/Instrument | Filter | Exposure (s) | AB magnitude |
|---|---|---|---|---|
| 8.2 | LT/IO:O | r | 6x150 | 21.61 +/- 0.28 |
| 8.5 | LT/IO:O | g | 6x150 | >21.80 |
| 8.8 | LCO/SINISTRO | r | 5x300 | 21.68 +/- 0.23 |
GCN Circular 44802
J. W. Hu (NAO, CAS), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260602b was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 44786) at 2026-06-02T17:53:12 (UTC).
Follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-03T04:30:31(UTC), ~10.5 hours after the WXT detection. The exposure time of the observation is around 4 ks. The on-ground analysis of the FXT data found an uncatalogued source at R.A. = 273.4737 deg, DEC = 7.5702 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the position of the optical counterpart (He et al., GCN 44780). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.9×10^21 cm^-2, and a photon index of 2.0 (-0.7, +0.8). The derived average unabsorbed 0.5-10 keV flux is 1.4 (-0.6, +1.1)×10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above 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 44786
J. W. Hu (NAO, CAS), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), 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 EP260602b. The transient triggered EP-WXT (ID: 01709266414) at 2026-06-02T17:54:02 (UTC). The WXT position of the source is R.A. = 273.487 deg, DEC = 7.595 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
The refined analysis of the WXT data shows that the event was detected at R.A. = 273.483 deg, DEC = 7.579 deg (J2000) with an uncertainty of 2.6 arcmin in radius. The event started at T0=2026-06-02T17:53:12 (UTC), and lasted for about 60 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 1.9 × 10^21 cm^-2, and a photon index of 1.36 (-0.80, +0.87). The derived average unabsorbed 0.5-4 keV flux is 1.8 (-0.6, +0.7)×10^(-9) erg/s/cm^2.
The optical counterpart of EP260602b was reported by He et al. (GCN 44780).
No autonomous follow-up observation with Follow-up X-ray Telescope (FXT) was performed. A Target-of-Opportunity observation with the 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).
GCN Circular 44780
L.B. He, X. Liu, Z.P. Zhu, D. Xu, J. An, S.Q. Jiang (NAOC), S.Y. Fu, A.D. Zhu, L. Lei (HUST), J.Z. Liu (XAO) report:
We observed the field of EP-WXT Trigger 01709266414 (presumably EP260602b) detected by Einstein Probe (EP), using the JinShan 100B and 100C telescopes located at Altay, Xinjiang, China. A series of r-band frames were obtained.
An uncatalogued and decaying source is detected within the EP/WXT error circle at coordinates
R.A. (J2000) = 18:13:53.7 (273.4738 deg)
Dec. (J2000) = +07:34:16.4 (7.5712 deg)
with an uncertainty of ~ 1.0 arcsec. The source had r ~ 19.0 mag at a median time of ~ 1.0 hr post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction of A_V ~ 0.94 mag. We thus think the source is the optical counterpart.
We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.