Using

Context.While waiting for new gamma-ray burst detections, the Burst Alert Telescope (BAT) onboard Swift covers each day ~50% of the sky in the hard X-ray band (“Survey data”). The large field of view (FOV), high sensitivity and good angular resolution make BAT a potentially powerful all-sky hard X-ray monitor, provided that mask-related systematics can be properly accounted for. Aims.We have developed and tested a complete procedure entirely based on public Swift/BAT software tools to analyse BAT Survey data, aimed at assessing the flux and spectral variability of bright sources in the 15–150 keV energy range. Methods.Detailed tests of the capabilities of our procedure were performed focusing, in particular, on the reliability of spectral measurements over the entire BAT FOV. First, we analyzed a large set of Cra