XRT Operations

The XRT supports four readout modes to enable it to cover the dynamic range and rapid variability expected from GRB afterglows, and autonomously determines which readout mode to use.

Imaging Mode

Imaging Mode produces an integrated image measuring the total energy deposited per pixel and does not permit spectroscopy.

Photodiode Mode

This mode is used to obtain high timing accuracy for the rapidly varying flux from a bright source. Spectroscopy is possible in this mode. The timing resolution in photodiode mode is 0.14 ms. This mode is not available in Cycle 8.

Windowed Timing Mode

This mode is used to obtain high resolution timing (2.2 ms) with one-dimensional position information. Spectroscopy is possible for sources with fluxes less than 600 mCrab.

Photon-Counting Mode

This mode uses sub-array windows to permit full spectral and spatial information to be obtained for source fluxes ranging from the XRT sensitivity limit of $2 \times 10^{-14}$ to $3 \times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$.

The two timing modes (photodiode and windowed timing) sacrifice position information to achieve high time resolution and bright source spectroscopy through rapid CCD readouts.