Powerful extragalactic jets dissipate their kinetic energy far from the central black hole

Accretion onto supermassive black holes in some active galactic nuclei (AGN) drives relativistic jets whose spectral energy distributions (SEDs) show synchrotron emission peaking in the infrared–optical and inverse Compton emission peaking in the gamma-rays. In powerful jets, gamma-rays dominate the radiative output, and the high-energy emission is thought to arise from external Compton (EC) scattering of seed photon fields external to the jet. The principal candidate seed fields are the sub-parsec broad-line region (BLR) and the parsec-scale molecular torus (MT). Prior attempts to localize the gamma-ray emission region have yielded contradictory results, ranging from inside the BLR to beyond the torus. Here we introduce a simple diagnostic—the seed factor (SF)—that depends only on observable SED quantities and is uniquely determined by the seed photon population at the gamma-ray emission site. Applying this to 62 quasi-simultaneous multiwavelength SEDs of gamma-ray quasars, we find the SF distribution peaks at the value expected for EC on the MT, implying that powerful jets dissipate their kinetic energy predominantly at ~1 pc from the black hole (~10^4 Schwarzschild radii for a 10^9 M☉ black hole).

Adam Leah W. Harvey, Markos Georganopoulos, Eileen T. Meyer