Blazar flares from plasma blobs crossing the broad-line region

TL;DR

This paper models a rapid, asymmetric gamma-ray flare in blazar 3C 279 using a two-zone scenario with a stationary region and an accelerating plasma blob crossing the broad-line region, reproducing observed features.

Contribution

It introduces a detailed, time-dependent model of blazar flares incorporating bulk acceleration and external photon field variations, providing a new explanation for asymmetric gamma-ray light curves.

Findings

A stationary region outside the dusty torus and an accelerating blob reproduce the flare's features.

The model explains the asymmetric gamma-ray flux evolution without ad hoc particle injection.

Predicted delayed EUV/X-ray enhancement after the blob exits the BLR.

Abstract

The blazar 3C 279 is well known for its rapid and large-amplitude variability. On 20 December 2013, the source exhibited an orphan {\gamma}-ray flare characterized by a flux-doubling timescale of a few hours, a very hard spectrum, a time-asymmetric light curve with a slow decay, and no significant optical variability. We propose a new interpretation of this event based on a two-zone scenario in which a stationary emission region produces the quiescent emission, while a second zone accelerates within the broad-line region(BLR). We compute the time-dependent radiative output of both zones with the EMBLEM code, including synchrotron, synchrotron self-Compton, and external inverse-Compton processes, as well as bulk acceleration, adiabatic expansion, and a Fokker-Planck treatment of the electron distribution. This is the first attempt to precisely model the asymmetric {\gamma}-ray flux evolution during this flare. A model with a stationary region outside the dusty torus and an accelerating plasma blob reproduces the main features of the event: a short and intense {\gamma}-ray flare with a hard spectrum and no optical counterpart. The flare results from the variation of the external photon field in the blob frame as the blob crosses the BLR and reaches its terminal Lorentz factor not far from the inner radius of the BLR. Bulk acceleration and the propagation of a plasma blob within the jet provide a natural mechanism for producing high-energy flares and asymmetric light curves without requiring an ad hoc time-dependent particle injection. The model predictsa delayed EUV/X-ray enhancement once the blob exits the BLR. No very-high-energy data are available for this event, but if {\gamma}-rays were emitted in this band, a delay would be expected with respect to the Fermi-LAT flare.