XRT Instrument Performance

The mirror point-spread function (PSF) has an 18 arcsecond half-energy width. Given sufficient photons the centroid of a point source image can be determined to sub-arcsec accuracy in detector coordinates. Figure 6 shows the XRT's PSF.

**Figure 7:** The left panel shows the XRT's PSF at different on- and off-axis positions. It was produced by summing different images taken at different times. The right panel shows a mosaic of images of PSFs for different energies and positions. The energies are 0.28 keV (C-K), 1.49 keV (Al-K), 4.51 keV (Ti-K), 6.40 keV (Fe-K), and 8.05 keV (Cu-K).
$\begin{figure}%\htmlimage{thumbnail=0.5} \begin{center} %\includegraphics[scal... ...sf_rot.eps} \includegraphics[scale=0.3]{xrt_psf.eps} \end{center} \end{figure}$

The XRT resolution at launch was approximately 140 eV at 6 keV, with spectra similar to that shown in Figure 7. The spectral resolution has degraded during the mission due to the development of charge traps in the detector. We have been measuring these traps and developing software corrections for the charge loss in these traps. Therefore, we expect significantly improved effective energy resolution during Cycle 7. Photometric accuracy will be good to 10% or better for source fluxes from the XRT's sensitivity limit of between approximately $2 \times 10^{-14}$ erg cm $^{-2}$ s $^{-1}$ and $8 \times 10^{-7}$ erg cm $^{-2}$ s $^{-1}$ (about twice as bright as the brightest X-ray burst observed to date).

**Figure 8:** A 13.7 ks observation of the supernova remnant Cas A taken in February 2005 with the XRT.
$\begin{figure}%\htmlimage{thumbnail=0.5} \begin{center} %\includegraphics[scal... ...ot.eps} \includegraphics[scale=0.5]{cas_a_lines.eps} \end{center} \end{figure}$