The Burst Alert Telescope

The Burst Alert Telescope (BAT; Barthelmy, et al. 2005, SSRv, 120 143) is a highly sensitive, large FoV instrument designed to provide critical gamma-ray 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 5: 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.
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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 4: Characteristics of the BAT. The effective area listed above is the maximum on-axis effective area assuming that all 32768 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 100 localizations per year

Eleonora 2017-08-16