A sample of Swift/SDSS faint blazars and blazar candidates

We aim here to provide a complete sample of faint (fr ≳ 1 mJy, fx ≳ 10−15 erg cm−2 s−1) blazars and blazar candidates serendipitously discovered in deep Swift images centered on Gamma-ray bursts (GRBs). By stacking all available images, we obtain exposures ranging from 104 to more than a million seconds. Since GRBs are thought to explode randomly across the sky, this set of deep fields can be considered as an unbiased survey of approximately 12 square degrees of extragalactic sky, with sensitivities reaching a few 10−15 erg cm−2 s−1 in the 0.5-2 keV band. We then derive the x-ray Log N Log S and show that, considering that our sample may be contaminated by sources other than blazars, we are in agreement with previous estimations based on data and simulations.