GCN 44201: EP260402a/GRB 260402A: refined analysis of the EP-WXT and EP-FXT observations

Y. Q. Zhao (USTC,PRIC), C. L. Guo, J. W. Hu, C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260402a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (GCN #44186), and temporally and spatially consistent with GRB 260402A (GCN #44181). The telemetry WXT data show that the flare started at 2026-04-02T10:29:33 (UTC) and exhibits a double-peaked burst lasting about 200 s. The average WXT 0.5-4 keV spectrum of the burst period can be fitted with an absorbed power law with a hydrogen column density of 0.76（-0.36/+0.41）×10^22 cm^-2 and a photon index of 2.09(-0.77/+0.84). The derived average unabsorbed 0.5-4 keV flux is 2.68 (-0.83/+1.97) ×10^(-9) erg/s/cm^2.
 
The Follow-up X-ray Telescope (FXT) on board EP observed this source at 2026-04-02T12:55:33 (UTC, T0+8760 s). The exposure time of this observation is 3.0ks. On-ground analysis of the FXT data found an uncatalogued source at R.A., Dec. = 170.9607, 63.6128 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 0.40 (-0.13/+0.14) × 10^22 cm^-2 and a photon index of 3.20 (-0.51/+0.55). The derived average unabsorbed 0.5-10 keV flux is 2.91 (-0.73/+1.46) × 10^(-12) erg/s/cm^2.

The optical and near-infrared follow-up observations were performed by Antier et al. (GCN 44183), Carrasco et al. (GCN 44184), Freeberg et al. (GCN 44185), Wu et al. (GCN 44189), Li et al. (GCN 44190), Vijaykumar et al. (GCN 44191), Schneider et al. (GCN 44192) and He et al. (GCN 44199). 

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).