Draft version February 23, 2022 Preprint typeset using L A T E X style emulateapj v. 5/2/11 THE INTERNAL STRUCTURE OF MAGNETIZED RELATIVISTIC JETS J. M. Mart´ı 1 and M. Perucho 1 Departamento de Astronom´ıa y Astrof´ısica, Universitat de Val`encia, 46100 Burjassot (Valencia), SPAIN and J. L. G´ omez Instituto de Astrof´ısica de Andaluc´ıa-CSIC, Glorieta de la Astronom´ıa s/n, 18008 Granada, SPAIN Draft version February 23, 2022 Abstract This work presents the first characterization of the internal structure of overpressured steady su- perfast magnetosonic relativistic jets in connection with their dominant type of energy. To this aim, relativistic magnetohydrodynamic simulations of different jet models threaded by a helical magnetic field have been analyzed covering a wide region in the magnetosonic Mach number - specific internal energy plane. The merit of this plane is that models dominated by different types of energy (inter- nal energy: hot jets; rest-mass energy: kinetically dominated jets; magnetic energy: Poynting-flux dominated jets) occupy well separated regions. The analyzed models also cover a wide range of mag- netizations. Models dominated by the internal energy (i.e., hot models, or Poynting-flux dominated jets with magnetizations larger than but close to 1) have a rich internal structure characterized by a series of recollimation shocks and present the largest variations in the flow Lorentz factor (and internal energy density). Conversely, in kinetically dominated models there is not much internal nor magnetic energy to be converted into kinetic one and the jets are featureless, with small variations in the flow Lorentz factor. The presence of a significant toroidal magnetic field threading the jet produces large gradients in the transversal profile of the internal energy density. Poynting-flux dominated models with high magnetization (≈ 10 or larger) are prone to be unstable against magnetic pinch modes, which sets limits to the expected magnetization in parsec-scale AGN jets and/or constrains their magnetic field configuration. Subject headings: Galaxies: active - galaxies: jets - methods: numerical - MHD - shock waves 1. INTRODUCTION How relativistic jets are launched, accelerated, and col- limated is probably one of the most important ques- tions related to AGN jet physics and other astrophys- ical systems involving black hole accretion, such as γ - ray bursts (GRBs) or tidal disruption flares (TDFs). It is thought that dynamically important helical magnetic fields twisted by the differential rotation of the black hole’s accretion disk or ergosphere play an important role (Blandford & Znajek 1977; Blandford & Payne 1982; McKinney & Blandford 2009; Tchekhovskoy, Narayan & McKinney 2011; Zamaninasab et al. 2014). As the jet propagates, part of the magnetic energy of the plasma is converted into kinetic energy, accelerating the jet while maintaining a parabolic shape (see, e.g., Komissarov et al. 2007, and references therein for theoretical approaches to the problem; see Nakamura & Asada 2013, for an in- vestigation of the parabolic jet structure in M87). For initially relativistic hot jets, thermal acceleration can also play a role (see, e.g., G´omez et al. 1995, 1997). Simul- taneous multi-wavelength and Very Long Baseline Inter- ferometry (VLBI) observations of AGN jets suggest that the acceleration and collimation of the jet takes place in the innermost 10 4-6 Schwarzschild radii from the central black hole, upstream of the millimeter VLBI (mm-VLBI) core (Marscher et al. 2008), defined as the bright com- pact feature in the upstream end of the observed VLBI 1 Observatori Astron`omic, Universitat de Val`encia, 46980 Pa- terna (Valencia), SPAIN jet. The simultaneity of multi-wavelength flares (from ra- dio to γ -ray energies) with the passage of a new superlu- minal component through the mm-VLBI core has led to the suggestion that this corresponds to a strong recolli- mation shock (e.g., Marscher et al. 2008, 2010; Casadio et al. 2015a,b). Moreover, in sources as CTA 102, in which this coincidence has not been proven, the presence of a stationary feature close to the VLBI core was claimed to explain the spectral evolution of a radio-flare (Fromm et al. 2011). Multifrequency VLBI observations showed evidence in this direction (Fromm et al. 2013a,b). The interaction of the moving shock associated with the su- perluminal component and the standing shock at or close to the mm-VLBI core would produce the particle accel- eration and burst in particle and magnetic energy densi- ties required to produce the multi-wavelength flares. It should be noted that this association of the mm-VLBI core with a recollimation shock would not be in contra- diction with the predictions from the Blandford & K¨onigl jet model (Blandford & K¨onigl 1979) that establishes the VLBI core as the location at which the jet becomes opti- cally thin, as long as this transition at centimeter wave- lengths takes place downstream of the mm-VLBI core. Relativistic (magneto)hydrodynamical simulations have shown that pressure mismatches between the jet and ambient medium lead to the formation of a pattern of recollimation shocks (e.g., Wilson 1987; Daly & Marscher 1988; Dubal & Pantano 1993; G´omez et al. 1995, 1997, 2016; Mimica et al. 2009; Porth & arXiv:1609.00593v1 [astro-ph.HE] 2 Sep 2016