Search for high-redshift blazars with Fermi/LAT

Abstract

High-redshift blazars (z ≥ 2.5) are the most powerful class of persistent γ-ray sources in the Universe. These objects possess the highest jet powers and luminosities and have black hole masses often in excess of 109 solar masses. In addition, high-z blazars are important cosmological probes and serve as test objects for blazar evolution models. Due to their large distance and the resulting electron/positron cascading on the extragalactic background light, their high-energy emission typically peaks below the GeV range. This makes these blazars difficult to study with Fermi/LAT and only the very brightest objects are detectable. Hence, only a small number of high-z blazars could be detected with Fermi/LAT so far. In this work, we present a strategy to increase the detection statistics at redshift z ≥ 2.5 via a search for flaring events in high-z γ-ray blazars whose long-term averaged flux is just below the sensitivity limit of Fermi/LAT. Seven previously GeV undetected high-z blazars have been identified with a test statistic of TS ≥ 25 from their bright monthly outburst periods, while more detections are expected in the future.