Why Haven't Many of the Brightest Radio Loud Blazars Been Detected by Fermi ?

Abstract

We use the complete MOJAVE 1.5 Jy sample of active galactic nuclei (AGN) to examine the gamma-ray detection statistics of the brightest radio-loud blazars in the northern sky. We find that 23% of these AGN were not detected above 0.1 GeV by the Fermi LAT during the 4-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 AGN 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.

Accepted for publication in Astrophysical Journal Letters

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