The angular resolution is about 5 arcmin (FWHM) for the central focal spot (as shown in Figure 3), limited by the manufacturing precision of the MPO plates for the current technology, and possibly by misalignment and distortion of the plates introduced in the process of assembling.
Figure 1 Simulated effective area curves of WXT for the central focal spot and plus the cruciform arms. The MPO arrays are coated with Iridium. The focal plane detectors are back-illuminated CMOSs (layer thickness of 20um and the surface is coated with 200 nm-thick Aluminum).
Figure 2 Grasp of EP/WXT (effective area times field of view) and comparison with the current and future missions with focusing X-ray optics.
Figure 3 Profile of the simulated point-spread-function of WXT of the central spot on the focal plane. The PSF is ~5 arcmin (FWHM).
The simulated X-ray background generated on the WXT detectors, including the incident diffuse X-ray emission and background generated by charged particles in orbit (out of the SAA), is shown in Figure 4. The total background level is estimated to be ~0.19 counts s-1 cm-2 (for CMOS with a thickness of 10micron). Based on this result, the estimated detection sensitivity of WXT for a point-like source with a typical X-ray spectrum is shown in Figure 5. In a 1000s exposure, a sensitivity of approximately ~10-11 ergs s-1 cm-2 in the 0.5-4 keV band (or ~0.3 mCrab) can be achieved at the 5-sigma level. Such a sensitivity improves by one order of magnitude or more upon the previous and current wide-field X-ray monitors, including MAXI and Swift/BAT. Figure 6 shows a simulated X-ray sky image of 400 sq. deg. observed by WXT with an accumulated exposure of 10 kilo-seconds.
Figure 4 Simulated background on the detector of WXT in orbit, including the incident diffuse X-ray emission and background generated by charged particles in space. The detectors are back-illuminated CMOSs (layer thickness of 20um and the surface is coated with 200 nm-thick Aluminum).
Figure 5 Detection limiting fluxes (sensitivity at 5 sigma) of WXT and its dependence on accumulative exposure time for cosmic X-ray sources with typical spectral shape (power-law photon index of -2 and -3, respectively, assuming a Galactic ISM absorption column density 3×1020 cm-2). The shaded area indicates the typical sensitivity of the ROSAT All-sky Survey.
Figure 6 Simulated X-ray sky image of 400 sq. deg. observed by WXT with an accumulated exposure of 10 kilo-seconds (based on the ROSAT All-sky Survey catalogue).