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Register a new userThe ablation of gas clouds by blazar jets
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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.
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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 $z=1.032$.
In this paper we propose a way to use optical polarisation observations to provide independent constraints and guide to the modelling of the spectral energy distribution (SED) of blazars, which is particularly useful when two-zone models are required to fit the observed SED. As an example, we apply the method to the 2008 multiwavelength campaign of PKS 2155-304, for which the required polarisation information was already available. We find this approach succesful in being able to simultaneously describe the SED and variability of the source, otherwise difficult to interpret. More generally, by using polarisation data to disentangle different active regions within the source, the method reveals otherwise unseen correlations in the multiwavelength behaviour which are key for the SED modelling.
Blazars - active galaxies with the jet pointing at Earth - emit across all electromagnetic wavelengths. The so-called one-zone model has described well both quiescent and flaring states, however it cannot explain the radio emission. In order to self-consistently describe the entire electromagnetic spectrum, extended jet models are necessary. Notably, kinetic descriptions of extended jets can provide the temporal and spatial evolution of the particle species and the full electromagnetic output. Here, we present the initial results of a recently developed hadronic extended-jet code. As protons take much longer than electrons to lose their energy, they can transport energy over much larger distances than electrons and are therefore essential for the energy transport in the jet. Furthermore, protons can inject additional leptons through pion and Bethe-Heitler pair production, which can explain a dominant leptonic radiation signal while still producing neutrinos. We will present a detailed parameter study and provide insights into the different blazar sub-classes.
We present multi-epoch, parsec-scale core brightness temperature observations of 447 AGN jets from the MOJAVE and 2cm Survey programs at 15 GHz from 1994 to 2019. The brightness temperature of each jet over time is characterized by its median value and variability. We find that the range of median brightness temperatures for AGN jets in our sample is much larger than the variations within individual jets, consistent with Doppler boosting being the primary difference between the brightness temperatures of jets in their median state. We combine the observed median brightness temperatures with apparent jet speed measurements to find the typical intrinsic Gaussian brightness temperature of (4.1 +- 0.6)*10^10 K, suggesting that jet cores are at or below equipartition between particle and magnetic field energy in their median state. We use this value to derive estimates for the Doppler factor for every source in our sample. For the 309 jets with both apparent speed and brightness temperature data, we estimate their Lorentz factors and viewing angles to the line of sight. Within the BL Lac optical class, we find that high-synchrotron-peaked (HSP) BL Lacs have smaller Doppler factors, lower Lorentz factors, and larger angles to the line of sight than intermediate and low-synchrotron-peaked (LSP) BL Lacs. We confirm that AGN jets with larger Doppler factors measured in their parsec-scale radio cores are more likely to be detected in gamma rays, and we find a strong correlation between gamma-ray luminosity and Doppler factor for the detected sources.
The concept of highly relativistic electrons confined to blobs that are moving out with modestly relativistic speeds is often invoked to explain high energy blazar observations. The important parameters in this model such as the bulk Lorentz factor of the blob ($Gamma$), the random Lorentz factor of the electrons ($gamma$) and the blob size are typically observationally constrained, but its not clear how and why the energetic electrons are held together as a blob. Here we present some preliminary ideas based on scenarios for cosmic ray electron self-confinement that could lead to a coherent picture.
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