The Palermo BAT survey

In such telescopes, the radiation coming from the sky sources is spatially modulated by a mask consisting of an array of opaque and transparent elements and recorded by a position sensitive detector. In order to avoid ambiguities in the reconstruction of the sky image, the mask pattern is designed in such a way that the radiation from each sky direction casts a unique shadow on the detector. In contrast to focusing imaging systems, where source photons are concentrated on a small detector area, in a coded-mask instrument the photons emitted from a cosmic source are spread over the whole detector area and the recorded shadowgram is the additive contribution from all the sources inside the field of view (FOV) and from the instrumental background. By knowing the orientation of the satellite in space and by analyzing the data from the detector it is possible, through image reconstruction algorithms, to evaluate the position and the intensity of all cosmic sources and of the background level, thus reproducing the image of the observed sky. Several algorithms are used to reconstruct the sky observed with coded-mask instruments. The most widely used method for image reconstruction is the cross-correlation of the recorded data pattern with the mask pattern.

The BAT is a coded-mask detector sensitive in the 15-150 keV band used as a GRB detector on board Swift. While waiting for new GRBs, it continuously collects spectral and imaging information in hard X-rays and accumulates data in survey mode covering a fraction between 50 and 80% of the sky every day. Data are immediately made available to the scientific community in the public Swift data archive (Swift Quick-Look Data), together with all housekeeping and spacecraft attitude information. The Swift-BAT entrance window is a 2.7 m² coded-aperture mask of 5x5 mm² elements placed 1 m above the detector plane, with an on-axis point spread function (PSF) of 22 arcmin FWHM. The mask is designed with a pseudo-random pattern that has the advantage (with respect to cyclic patterns like e.g. the one used for ISGRI) of being quite insensitive to the possible turning off of a limited number of detector elements and to the presence of detector gaps; moreover the reconstructed images do not suffer from the presence of ghost sources caused by the periodic design of the cyclic pattern. On the other hand, it is characterized by a PSF with an extended plateau that spans over the whole image and induces cross-contaminations among sources in the same FOV. The BAT detector plane consists of 5200 cm² array of 32,768 4x4 mm² CdZnTe elements organized in 128 elements sub-arrays (modules) separated by thin gaps and mounted in 16 mechanical structures (blocks).

The BAT survey data are collected in the form of Detector Plane Histograms (DPH). A DPH consists of a three-dimensional array where an 80 channel spectrum is accumulated for each detector pixel over the integration period. Typical DPH integration time is five minutes, but longer integration times are sometimes found (e.g. when telemetry reduction is needed) as well as shorter integration times since survey mode is always interrupted when the spacecraft begins a slew to a new target or when entering the South Atlantic Anomaly (SAA).

The 1st Palermo BAT catalog