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Our present knowledge of the properties of blazars mostly comes from smallsamples of bright objects, especially regarding studies on their cosmologicalevolution. Statistically well defined and completely identified samples offaint blazars are very difficult to obtain. We present a new X-ray selectedsample of 62 blazars and blazar candidates reaching deep X-ray fluxes. Werelied on the availability of large catalogs of astronomical objects combinedwith on-line services offering simple access to finding charts and magnitudeestimates. We built the sample cross-matching X-ray sources in the SwiftSerendipitous Survey in deep XRT GRB Fields catalog with data from deep radioand optical surveys. Our sample can probe populations of sources 10 timesweaker in the X-ray flux with respect previous studies, thus allowing for amore detailed comparison between data and simulated counts. We use the sampleto calculate the radio and X-ray LogN-LogS of blazars down to fluxes at leastone order of magnitude fainter than previous studies. We show that, consideringthat our sample may be somewhat contaminated by sources other than blazars, weare in agreement with previous observational and theoretical estimations.

A new sample of X-ray selected Swift/SDSS faint blazars and blazar candidates