The brera multiscale wavelet ROSAT HRI source catalog. I. The algorithm

We present a new detection algorithm based on the wavelet transform for the analysis of high-energy astronomical images. The wavelet transform, becaus e of its multiscale structure, is suited to the optimal detection of pointl ike as well as extended sources,regardless of any loss of resolution with t he off-axis angle. Sources are detected as significant enhancements in the wavelet space, after the subtraction of the nonflat components of the backg round. Detection thresholds are computed through Monte Carlo simulations in order to establish the expected number of spurious sources per held. The s ource characterization is performed through a multisource fitting in the wa velet space. The procedure is designed to correctly deal with very crowded fields, allowing for the simultaneous characterization of nearby sources. T o obtain a fast and reliable estimate of the source parameters and related errors, we apply a novel decimation technique that, taking into account the correlation properties of the wavelet transform, extracts a subset of almo st independent coefficients. We test the performance of this algorithm on s ynthetic fields, analyzing with particular care the characterization of sou rces in poor background situations, where the assumption of Gaussian statis tics does not hold. In these cases, for which standard wavelet algorithms g enerally provide underestimated errors, we infer errors through a procedure that relies on robust basic statistics. Our algorithm is well suited to th e analysis of images taken with the new generation of X-ray instruments equ ipped with CCD technology, which will produce images with very low backgrou nd and/or high source density.