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Published July 1, 2009 | Published
Journal Article Open

The evolution of Swift/BAT blazars and the origin of the MeV background

Abstract

We use three years of data from the Swift/Burst Alert Telescope (BAT) survey to select a complete sample of X-ray blazars above 15 keV. This sample comprises 26 flat-spectrum radio quasars (FSRQs) and 12 BL Lacertae (BL Lac) objects detected over a redshift range of 0.03 < z < 4.0. We use this sample to determine, for the first time in the 15-55 keV band, the evolution of blazars. We find that, contrary to the Seyfert-like active galactic nuclei (AGNs) detected by BAT, the population of blazars shows strong positive evolution. This evolution is comparable to the evolution of luminous optical quasi-stellar objects (QSOs) and luminous X-ray-selected AGNs. We also find evidence for an epoch dependence of the evolution as determined previously for radio-quiet AGNs. We interpret both these findings as a strong link between accretion and jet activity. In our sample, the FSRQs evolve strongly, while our best fit shows that BL Lac objects might not evolve at all. The blazar population accounts for 10%-20% (depending on the evolution of the BL Lac objects) of the cosmic X-ray background (CXB) in the 15-55 keV band. We find that FSRQs can explain the entire CXB emission for energies above 500 keV solving the mystery of the generation of the MeV background. The evolution of luminous FSRQs shows a peak in redshift (z_c = 4.3 ± 0.5) which is larger than the one observed in QSOs and X-ray-selected AGNs. We argue that FSRQs can be used as tracers of massive elliptical galaxies in the early universe.

Additional Information

© 2009 American Astronomical Society. Print publication: Issue 1 (2009 July 1); received 2009 January 12; accepted for publication 2009 May 5; published 2009 June 12. It is a pleasure to thank the referee of his/her comments which improved the paper. M.A. acknowledges very helpful suggestions from A. Comastri, R. Gilli, and A. Reimer and interesting discussions with A. Tramacere about blazars (and his guidance during the first Fermi flare-advocate duty shift). The help of D. Burlon with XMM-Newton data is acknowledged. This research has made use of the NASA/IPAC extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA's Goddard Space Flight Center, and of the SIMBAD Astronomical Database which is operated by the Centre de Données astronomiques de Strasbourg. Facilities: Swift/BAT.

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August 21, 2023
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