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Published September 1, 2015 | Published + Submitted
Journal Article Open

Why Have Many of the Brightest Radio-loud Blazars Not Been Detected in Gamma-Rays by Fermi?


We use the complete MOJAVE 1.5 Jy sample of active galactic nuclei (AGNs) to examine the gamma-ray detection statistics of the brightest radio-loud blazars in the northern sky. We find that 23% of these AGNs were not detected above 0.1 GeV by the Fermi-LAT during the four-year 3FGL catalog period partly because of an instrumental selection effect and partly due to their lower Doppler boosting factors. Blazars with synchrotron peaks in their spectral energy distributions located below 10^(13.4) Hz also tend to have high-energy peaks that lie below the 0.1 GeV threshold of the LAT, and are thus less likely to be detected by Fermi. The non-detected AGNs in the 1.5 Jy sample also have significantly lower 15 GHz radio modulation indices and apparent jet speeds, indicating that they have lower than average Doppler factors. Since the effective amount of relativistic Doppler boosting is enhanced in gamma-rays (particularly in the case of external inverse-Compton scattering), this makes them less likely to appear in the 3FGL catalog. Based on their observed properties, we have identified several bright radio-selected blazars that are strong candidates for future detection by Fermi.

Additional Information

© 2015 The American Astronomical Society. Received 2015 July 6; accepted 2015 July 21; published 2015 August 25. The authors thank Ken Kellermann for helpful discussions. The MOJAVE program is supported under NASA-Fermi grant NNX12A087G. M.F.A. was supported by NASA-Fermi GI grants NNX09AU16G, NNX10AP16G, and NNX11AO13G and NSF grant AST-0607523. T.H. was supported by the Academy of Finland project number 267324. E.R. acknowledges support by the MINECO grant AYA-2012-38491-C02- 01 and Generalitat Valenciana grant PROMETEOll/2014/057. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of the Swinburne University of Technology software correlator (Deller et al. 2011), developed as part of the Australian Major National Research Facilities Programme and operated under license.

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Published - Lister_2015.pdf

Submitted - 1507.05953v1.pdf


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