A numerical study on the role of instabilities on multi-wavelength emission signatures of blazar jets

¹ Department of Astronomy, Astrophysics and Space Engineering, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, India
e-mail: sriya.acharya@gmail.com
² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
³ Department of Physics, Indian Institute of Science, Bengaluru, Karnataka 560012, India
⁴ Tsung-Dao Lee Institute, Shanghai Jiao-Tong University, 520 Shengrong Road, Shanghai 201210, PR China

Received: 13 June 2022
Accepted: 27 November 2022

Abstract

Context. Blazars, a class of active galaxies whose jets are relativistic and collimated flows of plasma directed along the line of sight, are prone to a slew of magnetohydrodynamic (MHD) instabilities. These jets show characteristic multi-wavelength and multi-timescale variabilities.

Aims. We aim to study the interplay of radiation and particle acceleration processes in regulating the multi-band emission and variability signatures from blazars. In particular, the goal is to decipher the impact of shocks arising due to MHD instabilities in driving the long-term variable emission signatures from blazars.

Methods. To this end, we performed relativistic MHD (RMHD) simulations of a representative section of a blazar jet. The jet was evolved using a hybrid Eulerian-Lagrangian framework to account for radiative losses due to synchrotron process as well as particle acceleration due to shocks. Additionally, we incorporated and validated radiative losses taking into consideration the external Compton (EC) process that is relevant for blazars. We further compared the effects of different radiation mechanisms through numerical simulation of 2D slab jet as a validation test. Finally, we carried out a parametric study to quantify the effect of magnetic fields and external radiation field characteristics by performing 3D simulations of a plasma column. The synthetic light curves and spectral energy distribution (SEDs) were analyzed to qualitatively understand the impact of instability driven shocks.

Results. We observed that shocks produced with the evolution of instabilities give rise to flaring signatures in the high-energy band. The impact of such shocks is also evident from the instantaneous flattening of the synchrotron component of the SEDs. At later stages, we observed the transition in X-ray emission from the synchrotron process to that dominated by EC. The inclusion of the EC process also gives rise to γ-ray emission and shows signatures of mild Compton dominance that is typically seen in low-synchrotron peaked blazars.