Testing two-component models on very high-energy gamma-ray-emitting BL Lac objects

Abstract

Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very high-energy (VHE) gamma-ray-emitting blazars. While two-component models perform better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt at taking into account the observational constraints from very long baseline interferometry (VLBI) data, long-term light curves (radio, optical, and X-rays), and optical polarisation to limit the parameter space for a two-component model and test whether or not it can still reproduce the observed spectral energy distribution (SED) of the blazars. Methods. We selected five TeV BL Lac objects based on the availability of VHE gamma-ray and optical polarisation data. We collected constraints for the jet parameters from VLBI observations. We evaluated the contributions of the two components to the optical flux by means of decomposition of long-term radio and optical light curves as well as modelling of the optical polarisation variability of the objects. We selected eight epochs for these five objects based on the variability observed at VHE gamma rays, for which we constructed the SEDs that we then modelled with a two-component model. Results. We found parameter sets which can reproduce the broadband SED of the sources in the framework of two-component models considering all available observational constraints from VLBI observations. Moreover, the constraints obtained from the long-term behaviour of the sources in the lower energy bands could be used to determine the region where the emission in each band originates. Finally, we attempt to use optical polarisation data to shed new light on the behaviour of the two components in the optical band. Our observationally constrained two-component model allows explanation of the entire SED from radio to VHE with two co-located emission regions.

Constraining Axion-Like Particles with HAWC Observations of TeV Blazars

“…A two-zone SSC model of the SED was performed by the MAGIC collaboration in 2020 [94]. The zone responsible for the VHE emission has the following best-fit parameters: B = 0.1 G, δ = 12 and radius R = 1.3 × 10 16 cm.…”

A spectral hardening in the Fermi-LAT Data of 1ES 0502+675

“…The disadvantage of the two-component model is that there are a large number of free parameters. Intensive multiwavelengths observations are required to constrain model parameters (e.g., Acciari et al 2020). Here, we interpret the SED with a one-zone SSC model.…”

Time domain astronomy with the THESEUS satellite

“…As multi-wavelength observing campaigns on blazars become more complete and sophisticated, and the astroparticle angle of their investigation becomes more effective, thanks to the improved sensitivity of present high-energy neutrino detectors, fundamentally new observational and theoretical issues are emerging, critically driven by X-ray data. These include the finding that blazar variability may be occasionally due to a multiplicative process originated in the accretion disk, rather than in the jet [302]; the evidence that multi-component models describe the blazar spectral energy distributions and variations thereof more comfortably than homogeneous, single zone models [303][304][305][306], even down to sub-hour X-ray variability timescales [307]; the presence of a hadronic component in the jet, either suggested by a model-driven approach [308] or proven by the direct detection of high-energy (> TeV) neutrinos, so far achieved with the IceCube array in a few cases, with attendant multi-wavelength outbursts [309][310][311][312][313].…”