A Connection Between Apparent VLBA Jet Speeds and Initial AGN Detections Made by the Fermi Gamma-ray Observatory

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  • 12 February 2009
Abstract
In its first three months of operations, the Fermi Gamma-ray Observatory has detected approximately one quarter of the radio-flux limited MOJAVE sample of bright flat-spectrum AGN at energies above 100 MeV. We have investigated the apparent parsec-scale jet speeds of 26 MOJAVE AGN measured by the VLBA that are in the LAT Bright AGN Sample (LBAS). We find that the gamma-ray bright quasars have faster jets on average than the non-LBAS quasars, with a median of 15 c, and values ranging up to 34 c. The LBAS AGN in which the LAT has detected significant gamma-ray flux variability generally have faster jets than the non-variable ones. These findings are in overall agreement with earlier results based on non-uniform EGRET data which suggested that gamma-ray bright AGN have preferentially higher Doppler boosting factors than other blazar jets. However, the relatively low LAT detection rates for the full MOJAVE sample (24%) and previously-known MOJAVE EGRET-detected blazars (43%) imply that Doppler boosting is not the sole factor that determines whether a particular AGN is bright at gamma-ray energies. The slower apparent jet speeds of LBAS BL Lac objects and their higher overall LAT detection rate as compared to quasars suggest that the former are being detected by Fermi because of their higher intrinsic (unbeamed) gamma-ray to radio luminosity ratios.

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