GCN Circular 44840

EP260602c: EP-FXT follow-up observation

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 EP260602c was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Cao et al., GCN 44792
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 ) at 2026-06-02T09:06:44 (UTC). 
 
Follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-03T18:37:43(UTC), ~33 hours after the WXT detection. The exposure time of the observation is around 4 ks. The on-ground analysis of the FX-A data found three sources within the error circle of WXT.

Source 1 is at R.A. = 134.8551 deg, DEC = -75.673 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). This position is 12 arcsec from a known X-ray source 1eRASS J085924.9-754010 with the historical flux of 1.8 ×10^(-14) erg/s/cm^2 in 0.3-2.3 keV band. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 8×10^20 cm^-2, and a photon index of 1.9 (-1.2, +1.2). The observed 0.5-10.0 keV flux is 9.1 (+/-2.9) ×10^(-14) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

Source 2 is at R.A. = 134.5839 deg, DEC = -75.7147 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). This position is 5 arcsec from a known X-ray source 1eRASS J085820.6-754257 with the historical flux of 2.0 ×10^(-14) erg/s/cm^2 in 0.3-2.3 keV band. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 8×10^20 cm^-2, and a photon index of 0.13 (-0.68, +0.61). The observed 0.5-10.0 keV flux is 5.1 (+/-2.0) ×10^(-14) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

Source 3 is at R.A. = 134.5079 deg, DEC = -75.6493 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 8×10^20 cm^-2, and a photon index of 3.1 (-1.3, +1.7). The observed 0.5-10.0 keV flux is 3.51 (+/-1.6) ×10^(-14) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 
 
Since none of them is significantly above the eRASS1 upper limit or shows fading trends, we cannot identify the counterpart of EP260602c at present.

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).