A Test of External Compton Models for Gamma‐Ray Active Galactic Nuclei

There is clear evidence that the $\gamma$-ray emission from active galacticnuclei (AGNs) is attributed to the inverse Compton scatterings in therelativistic blobs near the massive black holes. If the soft seed photons arefrom the regions outside the blobs, a linear relation between $(\nuF_{\nu,\gamma}/\nu F_{\nu, \rm syn} u^{*})^{1/(1+\alpha)}$ and Doppler factor$\delta$ is expected, where $\nu F_{\nu,\gamma}$ and $\nu F_{\nu, \rm syn}$ aremonochromatic $\gamma$-ray and synchrotron fluxes, respectively, and $u^{*}$ isthe energy density of soft seed photons \citep{D97}. We estimate the softphoton energy density in the relativistic blobs contributed by the broad lineregion (BLRs) in these $\gamma$-ray AGNs using their broad-line emission data.The Doppler factors $\delta$ are derived from their radio core and X-rayemission data, based on the assumption that the X-ray emission is producedthrough synchrotron self-Compton (SSC) scatterings. We find two nearly linearcorrelations: $(\nu F_{\nu,\gamma}/\nu F_{\rm opt} u^{*})^{1/(1+\alpha)}\propto \delta^{1.09}$, and $(\nu F_{\nu,\gamma}/\nu F_{\rm IR}u^{*})^{1/(1+\alpha)} \propto \delta^{1.20}$, which are roughly consistent withthe linear correlation predicted by the theoretical model for external Comptonscatterings. Our results imply that the soft seed photons are dominantly fromthe BLRs in these $\gamma$-ray AGNs.Comment: 18 pages, accepted by Ap