BAT Instrument Performance

A typical spectrum of the 60 keV gamma-ray line from an Am-241 radioactive source for an individual pixel is shown in Figure 5. It has a full-width-half maximum (FWHM) at 60 keV of 3.3 keV ( $\Delta E / E = 5$%), which is typical of CZT detectors. The typical FWHM resolution based on on-orbit performance is approximately 5 keV at 60 keV. The typical BAT background event rate in the full array above threshold is about 10 000 counts per second. The region around the SAA produces exceptionally high rates, and within the SAA itself BAT event processing is disabled. The background rate yields a GRB sensitivity of approximately (8 mCrab) ${(T/20 \mathrm{ks})}^{0.5}$ (15-150 keV) where 8 mCrab $= 2 \times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$. The burst sensitivity depends on burst duration. For example, the approximate observed 5-sigma sensitivities for bursts of duration 100 ms, 1 s, and 100 s are $9 \times 10^{-8}$, $2.8 \times 10^{-8}$, and $2.8 \times 10^{-9}$ erg cm$^{-2}$ s$^{-1}$ respectively. These estimate are based on good observing conditions. If the particle background is high, or if there are nearby bright sources such as Sco X-1 or the Crab, the sensitivity will be degraded. More detailed sensitivity calculations can be done using the full BAT response matrix inPIMMS (the Portable Interactive Multi-Mission Simulator).

The combination of the 4 mm square CZT pieces, plus the 5 mm square mask cells and the 1-m detector-to-mask separation gives an instrumental angular resolution of 22 arcminutes FWHM, yielding a conservative 3 arcminute centroiding capability for bursts and steady-state sources given an 8-sigma burst threshold.

Figure 6: A typical BAT spectrum of Am-241 for a single CZT pixel.

The BAT team maintains a BAT Digest, which provides up-to-date information about issues that affect BAT observations and data analysis.