EP260602c

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).

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 EP260602c. The source did not trigger the WXT on-board trigger unit. A GCN Notice was sent manually (trigger ID: 13616654964). The ground analysis of the WXT data shows that the event started at T0=2026-06-02T09:06:44 (UTC) and lasted for about 200 s. The WXT position of the source is R.A. = 134.673 deg, DEC = -75.673 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 8 × 10^20 cm^-2, and a photon index of 2.14 (-0.41, +0.46). The derived average unabsorbed 0.5-4 keV flux is 1.9 (-0.5, +0.6)×10^(-10) erg/s/cm^2. 
 
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).