Constraints on the location of $\gamma$-ray emitting region for a sample of blazars with radio core-shift measurements

TL;DR

This study models the spectral energy distributions of 25 blazars with radio core-shift data to locate gamma-ray emission regions, finding they are likely outside the broad line region, based on multi-band fitting and magnetic field analysis.

Contribution

It introduces a method combining SED fitting with radio core-shift measurements to estimate gamma-ray emission locations in blazars, emphasizing the external torus as seed photon source.

Findings

Gamma-ray emission regions are outside the BLR, around 10 times its size.

SED fitting favors torus seed photons over BLR photons.

Gamma-ray regions are approximately 20,000 Schwarzschild radii from the black hole.

Abstract

We model simultaneous or quasi-simultaneous multi-band spectral energy distributions (SEDs) for a sample of 25 blazars that have radio core-shift measurements, where a one-zone leptonic model and Markov chain Monte Carlo technique are adopted. In the SED fitting for 23 low-synchrotron-peaked (LSP) blazars, the seed photons from broad line region (BLR) and molecular torus are considered respectively in the external Compton process. We find that the SED fitting with the seed photons from the torus are better than those utilizing BLR photons, which suggests that the $\gamma$-ray emitting region may be located outside the BLR. Assuming the magnetic field strength in the $\gamma$-ray emitting region as constrained from the SED fitting follows the magnetic field distribution as derived from the radio core-shift measurements (i.e., $B(R)\simeq B_{\rm 1 pc}(R/\rm 1pc)^{-1}$, $R$ is the distance from the central engine and $B_{\rm 1 pc}$ is the magnetic field strength at 1 pc), we further calculate the location of the $\gamma$-ray emitting region, $R_{\gamma}$, for these blazars. We find that $R_{\gamma}\sim2\times10^{4}R_{\rm S}\simeq10 R_{\rm BLR}$ ($R_{\rm S}$ is the Schwarzschild radius and $R_{\rm BLR}$ is the BLR size), where $R_{\rm BLR}$ is estimated from the broad line luminosities using the empirical correlations obtained using the reverberation mapping methods.