FIRST J090935.7+454736 , the SIMBAD biblio

Our present knowledge of the properties of blazars mostly comes from small samples of bright objects, especially regarding studies on their cosmological evolution. Statistically well-defined and completely identified samples of faint blazars are very difficult to obtain. We present a new X-ray selected sample of 62 blazars and blazar candidates reaching deep X-ray fluxes. We relied on the availability of large catalogues of astronomical objects combined with online services offering simple access to finding charts and magnitude estimates. We built the sample cross-matching X-ray sources in the Swift Serendipitous Survey in deep XRT GRB Fields catalogue with data from deep radio and optical surveys. Our sample can probe populations of sources 10 times weaker in the X-ray flux with respect to previous studies, thus allowing for a more detailed comparison between data and simulated counts. We use the sample to calculate the radio and X-ray LogN-LogS of blazars down to fluxes at least one order of magnitude fainter than previous studies. We show that, considering that our sample may be somewhat contaminated by sources other than blazars, we are in agreement with previous observational and theoretical estimations.