Modeling non-thermal emission from the jet-launching region of M 87 with adaptive mesh refinement

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
e-mail: j.davelaar@astro.ru.nl
² Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
³ Institut für Theoretische Physik, Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany
⁴ Anton Pannekoek Instituut, Universiteit van Amsterdam, PO Box 94249 1090 GE Amsterdam, The Netherlands
⁵ Max-Planck Institute for Radio Astronomy, Auf dem Huegel 69, 53115 Bonn, Germany

Received: 21 June 2019
Accepted: 21 August 2019

Abstract

Context. The galaxy M 87 harbors a kiloparsec-scale relativistic jet, whose origin coincides with a compact source thought to be a supermassive black hole. Observational millimeter very long baseline interferometry campaigns are capable of resolving the jet-launching region at the scale of the event horizon. In order to provide a context for interpreting these observations, realistic general-relativistic magnetohydrodynamical (GRMHD) models of the accretion flow are constructed.

Aims. Electrons in the jet are responsible for the observed synchrotron radiation, which is emitted in frequencies ranging from radio to near-infrared (NIR) and optical. The characteristics of the emitted radiation depend on the shape of the electrons’ energy-distribution function (eDF). The dependency on the eDF is omitted in the modeling of the first Event Horizon Telescope results. In this work, we aim to model the M 87 spectral-energy distribution from radio up to optical frequencies using a thermal-relativistic Maxwell–Jüttner distribution, as well as a relativistic κ-distribution function. The power-law index of the eDF is modeled based on sub-grid, particle-in-cell parametrizations for sub-relativistic reconnection.

Methods. A GRMHD simulation in Cartesian–Kerr–Schild coordinates, using eight levels of adaptive mesh refinement (AMR), forms the basis of our model. To obtain spectra and images, the GRMHD data was post-processed with the ray-tracing code RAPTOR, which is capable of ray tracing through GRMHD simulation data that is stored in multi-level AMR grids. The resulting spectra and images maps are compared with observations.

Results. We obtain radio spectra in both the thermal-jet and κ-jet models consistent with radio observations. Additionally, the κ-jet models also recover the NIR and optical emission. The images show a more extended structure at 43 GHz and 86 GHz and more compact emission at 228 GHz. The models recover the observed source sizes and core shifts and obtain a jet power of ≈10⁴³ ergs s⁻¹. In the κ-jet models, both the accretion rates and jet powers are approximately two times lower than the thermal-jet model. The frequency cut-off observed at ν ≈ 10¹⁵ Hz is recovered when the accelerator size is 10⁶ − 10⁸ cm, this could potentially point to an upper limit for plasmoid sizes in the jet of M 87.