Cloud ablation by a relativistic jet and the extended flare in CTA 102 in 2016 and 2017

TL;DR

This paper explains the 2016-2017 outburst of CTA 102 as caused by a gas cloud ablation in a relativistic jet, resulting in a long-term flux increase and short-term flares observed across multiple energy bands.

Contribution

It introduces a model of gas cloud ablation to explain the long-term and short-term variability in CTA 102's outburst, providing a novel physical mechanism for such events.

Findings

Successful fit of the long-term flux trend using the cloud ablation model

Explanation of short-term flares as a consequence of particle injection dynamics

Quantitative estimates of cloud parameters consistent with observations

Abstract

In late 2016 and early 2017 the flat spectrum radio quasar CTA 102 exhibited a very strong and long-lasting outburst. The event can be described by a roughly 2 months long increase of the baseline flux in the monitored energy bands (optical to $\gamma$ rays) by a factor 8, and a subsequent decrease over another 2 months back to pre-flare levels. The long-term trend was superseded by short but very strong flares, resulting in a peak flux that was a factor 50 above pre-flare levels in the $\gamma$-ray domain and almost a factor 100 above pre-flare levels in the optical domain. In this paper we explain the long-term evolution of the outburst by the ablation of a gas cloud penetrating the relativistic jet. The slice-by-slice ablation results in a gradual increase of the particle injection until the center of the cloud is reached, after which the injected number of particles decreases again. With reasonable cloud parameters we obtain excellent fits of the long-term trend.