4 The Burst Alert Telescope

The Burst Alert Telescope (BAT) is a highly sensitive, large FoV instrument designed to provide critical GRB triggers and 4-arcminute positions. It is a coded aperture imaging instrument with a 1.4 steradian field-of-view (half coded). Figure 4 shows the field of view of the BAT. The energy range is 15-150 keV for imaging with a non-coded response up to approximately 500 keV. Within approximately the first ten seconds of detecting a burst, the BAT will calculate an initial position, decide whether the burst merits a spacecraft slew and, if worthy, send the position to the spacecraft.

Figure 4: The FoV of the BAT superimposed onto an Aitoff projection of the sky. Color represents the coding fraction with white indicating fully coded and blue indicating partially coded.

In order to study bursts with a variety of intensities, durations, and temporal structures, the BAT has a large dynamical range and trigger capabilities. The BAT uses a two-dimensional coded aperture mask and a large area solid state detector array to detect weak bursts, and has a large FoV to detect a good fraction of bright bursts. Since the BAT coded aperture FoV always includes the XRT and UVOT FoVs, long duration gamma-ray emission from a burst can be studied simultaneously in the X-ray and UV/optical regimes. The data from the BAT will also produce a sensitive hard X-ray all-sky survey over the course of Swift's mission. Table 5 lists the BAT's parameters.

Table 5: Characteristics of the BAT. The effective area listed above is the maximum on-axis effective area assuming that all 32000 detectors are enabled.

BAT Parameter	Value
Energy Range	approximately 15-150 keV (imaging)
Energy Resolution	approximately 5 keV at 60 keV
Aperture	Coded mask, random pattern, 50% open
Effective Area	approximately 1400 cm$^2$ (see caption)
Detection Area	5240 cm$^2$
Detector Material	CdZnTe (CZT)
Detector Operation	Photon counting
Field of View	1.4 sr (half-coded)
Detector Elements	256 modules of 128 elements/module
Detector Element Size	$4 \times 4 \times 2$ mm
Coded Mask Cell Size	$5 \times 5 \times 1$ mm Pb tiles
Telescope PSF	22 arcminutes FWHM
Source Localization	1-3 arcminutes radius
15-150 keV Sensitivity (5$\sigma$)	$2 \times 10^{-8} \sqrt{(T/1 \mathrm{s})}$ erg cm$^{-2}$ s$^{-1}$
Number of Bursts Detected	approximately 300 detections per year
	approximately 100 localizations per year