In-flight Characteristics and Modelling of the Instrumental Background of EP/FXT

TL;DR

This paper analyzes the in-flight instrumental background of the FXT onboard the Einstein Probe, revealing its spatial, spectral, and temporal characteristics, and establishing a background model based on the correlation between imaging and frame store areas.

Contribution

It introduces a novel background modeling method for FXT using the correlation between imaging and frame store areas, based on in-flight observations.

Findings

Instrumental background rate is ~0.04 counts/s/keV in 0.5-10 keV.

Background distribution is nearly uniform with minor row-based variations.

Background shows periodic variation linked to orbital precession (~57 days).

Abstract

The in-flight instrumental background of the Follow-up X-ray Telescope (FXT) onboard Einstein Probe (EP) mission is analysed in this work by utilizing observations collected during Performance Verification phase and subsequent dedicated filter wheel closed observations. The instrumental backgrounds of the two FXT modules are consistent with each other, with an average rate of $\sim 4\times10^{-2}$\,counts/s/keV in the 0.5--10\,keV band for each module. The background is nearly uniformly distributed across the detector area, with a minor increase ($<8\%$) observed along rows. The spatial distribution shows significant modulation by the geomagnetic field. The spectral shapes remain unchanged in 0.5--10\,keV at different rates. The long-term temporal variation indicates a periodic change associated with the orbital precession ($\sim 57$ days). The innovative design of FXT full-frame readout mode enables simultaneous recording of events in both the imaging area (IMG) and the frame store area (FSA) of the pnCCD. FSA event rates show a strong linear correlation with the IMG, based on which the IMG instrumental background modelling is established.