The Brera Multiscale WaveletROSATHRI Source Catalog. I. The Algorithm

We present a new detection algorithm based on the wavelet transform for theanalysis of high energy astronomical images. The wavelet transform, due to itsmulti-scale structure, is suited for the optimal detection of point-like aswell as extended sources, regardless of any loss of resolution with theoff-axis angle. Sources are detected as significant enhancements in the waveletspace, after the subtraction of the non-flat components of the background.Detection thresholds are computed through Monte Carlo simulations in order toestablish the expected number of spurious sources per field. The sourcecharacterization is performed through a multi-source fitting in the waveletspace. The procedure is designed to correctly deal with very crowded fields,allowing for the simultaneous characterization of nearby sources. To obtain afast and reliable estimate of the source parameters and related errors, weapply a novel decimation technique which, taking into account the correlationproperties of the wavelet transform, extracts a subset of almost independentcoefficients. We test the performance of this algorithm on synthetic fields,analyzing with particular care the characterization of sources in poorbackground situations, where the assumption of Gaussian statistics does nothold. For these cases, where standard wavelet algorithms generally provideunderestimated errors, we infer errors through a procedure which relies onrobust basic statistics. Our algorithm is well suited for the analysis ofimages taken with the new generation of X-ray instruments equipped with CCDtechnology which will produce images with very low background and/or highsource density.Comment: 8 pages, 6 figures, ApJ in pres