Swift Burst Alert Telescope (BAT) Spectral Response

The Burst Alert Telescope (BAT) is one of three telescopes aboard the Swift observatory scheduled for launch into a low earth orbit in November 2004. Swift's primary purpose is to localize and observe gamma-ray bursts. BAT will provide the initial burst positions and gamma-ray light curves and spectra within a band of approximately 15-150 keV. BAT is a coded aperture instrument with a wide ( 2 sr) field of view consisting of an array of 32,768 4 mm x 4 mm x 2 mm CZT detectors and a coded mask of lead tiles 1 m above the detector array. The spectra from the 32K individual detectors are combined using a mask-weighting technique that removes background counts. An instrument response function has been developed which transforms the photon spectrum of a source into a counts spectrum as measured by BAT. This function is based on the physical properties of the CZT detectors as well as some empirical adjustment parameters. The model will be described along with the results of fits to calibration source spectra during ground calibration and to gamma-ray sources during the early orbit checkout phase.