Confronting observations of VHE gamma-ray blazar flares with reconnection models

Abstract

Several models have been suggested to explain the fast gamma-ray variability observed in blazars, but its origin is still debated. One scenario is magnetic reconnection, a process that can efficiently convert magnetic energy to energy of relativistic particles accelerated in the reconnection layer. In our study, we compare results from state-of-the-art particle-in-cell simulations with observations of blazars at Very High Energy (VHE, E > 100 GeV) gamma-rays. Our goal is to test our model predictions on fast gamma-ray variability with data and to constrain the parameter space of the model, such as the magnetic field strength of the unreconnected plasma and the reconnection layer orientation in the blazar jet. For this first comparison, we used the remarkably well-sampled VHE gamma-ray light curve of Mrk 421 observed with the MAGIC and VERITAS telescopes in 2013. The simulated VHE light curves were generated using the observable parameters of Mrk 421, such as the jet power, bulk Lorentz factor, and the jet viewing angle, and sampled as real data. Our results pave the way for future model-to-data comparison with next-generation Cherenkov telescopes, which will help further constrain the different variability models.

Quantitative comparisons of very-high-energy gamma-ray blazar flares with relativistic reconnection models

“…In Jormanainen et al (2021), we concluded that the simulations we produced with the bulk Lorentz factor Γ j = 4 (Piner & Edwards 2018) that was based on the literature value and a smaller half-length of the layer L the resulting fluxes were often 100−1000 times lower than those of the observed data. Because of this, we reran our simulations for this study with an increased Γ j and L to obtain fluxes closer to the observed values.…”

“…Γ j = 4). This choice was based on our previous work, reported in Jormanainen et al (2021), where the lower Γ j and a shorter halflength of the reconnection layer L yielded us significantly lower fluxes than in the observed dataset of Mrk 421. For the simulations presented in this work, the initial values of Γ j and L were multiplied by 3 to obtain simulated fluxes in the observed range (see Figs.…”

“…We noticed in our earlier work (Jormanainen et al 2021, where lower Γ j and L were used) that some simulations have a tendency of showing extremely fast variability (see Sect. 6.1.1).…”