Spectral Performance of the Follow-up X-Ray Telescope on Board the EP Satellite

The Follow-up X-ray Telescope (FXT) is one of the two main scientific instruments on board the Einstein Probe astronomical satellite, which was launched in 2024 January. FXT focuses on the energy range of 0.3-10 keV and mainly conducts follow-up observations of transients and burst sources. It consists of two units of completely independent optical system and detector system (FXT-A and FXT-B). The focal plane detector adopts PNCCD provided by Max Planck Institute for Extraterrestrial Physics. FXT was designed to have three operating modes with different integration times and readout schemes, namely full-frame mode, partial-window mode and timing mode. We conducted a detailed calibration for PNCCD at the Institute of High Energy Physics before launch. Our results demonstrate that both FXT-A and FXT-B exhibit excellent spectral performance. The energy resolution (Full Width at Half Maximum) of FXT-A and FXT-B are both better than 85 eV at 1.487 keV. We determined a mean equivalent noise charge around 2.8 e- for FXT-A and FXT-B in three operating modes at -90 degrees C +/- 0.5 degrees C, except for a few noisy pixels in full-frame mode. In addition, we measured the relation of charge transfer inefficiency as function of photon energy and confirmed the ability to detect photons in the energy range of 0.3-10 keV. These calibration results have been ingested into the initial version of calibration database and applied to the analysis of scientific data acquired by FXT.