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

TL;DR

This work characterizes the in-orbit instrumental background of EP/FXT using Performance Verification and filter-wheel-closed data, revealing a consistent per-module rate of about $4×10^{-2}$ counts s$^{-1}$ keV$^{-1}$ in the $0.5–10$ keV band and a near-uniform detector distribution with a small row gradient. The background shows geomagnetic modulation and a long-term ~57-day precession, while the innovative FF readout enables a robust IMG–FSA linear relationship that is used to model the instrumental background with a spectral template scaled by the predicted rate. Fluorescence lines from Al, Fe, Cu, and Ni provide in-situ energy calibration references, and a detailed background estimation tool (fxtbkggen) is validated against dedicated observations, enabling reliable analysis of extended sources. The study finds in-orbit backgrounds to be ~5× lower than SRG/eROSITA, consistent with solar-cycle differences, and demonstrates a practical pathway for accurate background subtraction in EP/FXT time-domain X-ray astronomy.

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.

Figures (10)

• Figure 1: EP structure before the solar panels are deployed.
• Figure 2: EP/FXT two modules.
• Figure 3: EP/FXT filter wheel and the filters. (a) The six positions on the filter wheel. (b) The schematic design diagrams of the closed, thin, medium and hole filters. (c) The large hexagonal frame structures and the tiny hexagonal grids in the thin/medium filter. (d) The small aperture hole filter, surrounded by the nickle annulus.
• Figure 4: Top: The distribution of instrumental background events in the energy range of 0.5 to 10 keV on the IMG pixels of the pnCCD for FXTA and FXTB, respectively. Bottom: The 1-D projection distribution to the columns and rows, FXTA in red circles and FXTB in green crosses.
• Figure 5: The instrumental background spectra of FXTA and FXTB using SCD and FWC data. Also plotted is the pre-launch estimate instrumental background level of one FXT module (in dotted gray), and the SRG/eROSITA instrumental background level derived from 2021AA...647A...1P which is rescaled by a factor of $1/5$ for comparison in this plot.