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Register a new userThe ablation of gas clouds by blazar jets and the long-lasting flare in CTA 102
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Long-lasting, very bright multiwavelength flares of blazar jets are a curious phenomenon. The interaction of a large gas cloud with the jet of a blazar may serve as a reservoir of particles entrained by the jet. The size and density structure of the cloud then determine the duration and strength of the particle injection into the jet and the subsequent radiative outburst of the blazar. In this presentation, a comprehensive parameter study is provided showing the rich possibilities that this model offers. Additionally, we use this model to explain the 4-months long, symmetrical flare of the flat spectrum radio quasar CTA 102 in late 2016. During this flare, CTA 102 became one of the brightest blazars in the sky despite its large redshift of $z=1.032$.
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Flaring activity in blazars can last for vastly different time-scales, and may be the result of density enhancements in the jet flow that result from the intrusion of an interstellar cloud into the jet. We investigate the lightcurves expected from the ablation of gas clouds by the blazar jet under various cloud and jet configurations. We derive the semi-analytical formulae describing the ablation process of a hydrostatic cloud, and perform parameter scans of artificial set-ups over both cloud and jet parameter spaces. We then use parameters obtained from measurements of various cloud types to produce lightcurves of these cloud examples. The parameter scans show that a vast zoo of symmetrical lightcurves can be realized. Both cloud and emission region parameters significantly influence the duration, and strength of the flare. The scale height of the cloud is one of the most important parameters, as it determines the shape of the lightcurve. In turn, important cloud parameters can be deduced from the observed shape of a flare. The example clouds result in significant flares lasting for various time scales.
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.
The flat spectrum radio quasar CTA 102 ($z = 1.032$) went through a tremendous phase of variability. Since early 2016 the gamma-ray flux level has been significantly higher than in previous years. It was topped by a four month long giant outburst, where peak fluxes were more than 100 times higher than the quiescence level. Similar trends are observable in optical and X-ray energies. We have explained the giant outburst as the ablation of a gas cloud by the relativistic jet that injects additional matter into the jet and can self-consistently explain the long-term light curve. Here, we argue that the cloud responsible for the giant outburst is part of a larger system that collides with the jet and is responsible for the years-long activity in CTA 102.
The flat spectrum radio quasar CTA 102 (redshift 1.037) exhibited a tremendously bright 4-months long outburst from late 2016 to early 2017. In a previous paper, we interpreted the event as the ablation of a gas cloud by the relativistic jet. The multiwavelength data have been reproduced very well within this model using a leptonic emission scenario. Here we expand that work by using a hadronic scenario, which gives us greater freedom with respect to the location of the emission region within the jet. This is important, since the inferred gas cloud parameters depend on the distance from the black hole. While the hadronic model faces the problem of invoking super-Eddington jet luminosities, it reproduces well the long-term trend and also days-long subflares. While the latter result in inferred cloud parameters that match those expected for clouds of the broad-line region, the long-term trend is not compatible with such an interpretation. We explore the possibilities that the cloud is from the atmosphere of a red giant star or comes from a star-forming region that passes through the jet. The latter could also explain the much longer-lasting activity phase of CTA 102 from late 2015 till early 2018.
The blazar CTA 102 underwent a major radio flare in April 2006. We used several 15 GHz VLBI observations from the MOJAVE program to investigate the influence of this extreme event on jet kinematics. The result of modeling and analysis lead to the suggestion of an interaction between traveling and standing shocks 0.2 mas away from the VLBI core.
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