Revisiting the EC/CMB model for extragalactic large scale jets

TL;DR

This paper revisits the EC/CMB model for large-scale AGN jets, showing that electron cooling can resolve gamma-ray overproduction issues and predict a new class of X-ray bright, radio-quiet jets at intermediate redshifts.

Contribution

It demonstrates that electron cooling effects can address gamma-ray overproduction and predicts a new class of jets that are X-ray bright but radio and optical faint.

Findings

Electron cooling suppresses high-energy gamma-ray emission.

A new class of X-ray bright, radio-quiet jets is predicted.

Jets at intermediate redshifts are more likely to be detected in X-rays.

Abstract

One of the most outstanding results of the Chandra X-ray Observatory was the discovery that AGN jets are bright X-ray emitters on very large scales, up to hundreds of kpc. Of these, the powerful and beamed jets of Flat Spectrum Radio Quasars are particularly interesting, as the X-ray emission cannot be explained by an extrapolation of the lower frequency synchrotron spectrum. Instead, the most common model invokes inverse Compton scattering of photons of the Cosmic Microwave Background (EC/CMB) as the mechanism responsible for the high energy emission. The EC/CMB model has recently come under criticism, particularly because it should predict a significant steady flux in the MeV-GeV band which has not been detected by the Fermi/LAT telescope for two of the best studied jets (PKS 0637-752 and 3C273). In this work we revisit some aspects of the EC/CMB model and show that electron cooling plays an important part in shaping the spectrum. This can solve the overproduction of gamma-rays by suppressing the high energy end of the emitting particle population. Furthermore, we show that cooling in the EC/CMB model predicts a new class of extended jets that are bright in X-rays but silent in the radio and optical bands. These jets are more likely to lie at intermediate redshifts, and would have been missed in all previous X-ray surveys due to selection effects.