The Phoenix Fire Model for Jet Creation: The Role of Magnetic Fields in the Production and Propagation of Relativistic Jets

Using recent observational and theoretical results, I outline the latest ideas on how relativistic jets are produced by rotating magnetic fields, along with the physical processes that result in the FR I/II dichotomy. Simply put, at the end of the acceleration and collimation phase the now super-magnetosonic jet (the “Phoenix”) passes through a master recollimation shock (the “fire”) and is reborn as a new and more stable collimated MHD flow. Depending on whether the magnetic dissipation in the shock is negligible or substantial, the final jet will be either trans-magnetosonic (a BL Lac/FR I source) or super-(magneto)sonic with a weak magnetic field (an FSRQ/FR II source). Predictions are made about the behavior of additional observational and theoretical studies that can test this hypothesis more completely. 1 Motivation: The Fanaroff and Riley Class Division Puzzle 1.1 Kiloparsec-scale Clues to its Physical Origin The Fanaroff and Riley division of extragalactic radio source morphologies into two distinct classes identifies FR I sources as those with bright radio emission near the jet origin or “core” (galactic center or quasar) and diffuse emission far from the origin, while FR IIs are those with bright emission far from the jet origin and diffuse emission closer to the core. Furthermore, the FR Is are identified with less luminous radio sources and the FR IIs with the most powerful sources observed [9]. Understanding the physical processes that produce this division has long been a vexing puzzle in this field. Bicknell [3] argued that FR II sources are those that propagate through the interstellar medium (ISM) at speeds greater than the jet internal sound speed, while FR Is propagate at trans-sonic speeds that lead to an eventual slowing of the jet to subsonic flow. Meier [25], on the other hand, argued that processes near the black hole (BH), where the jets are produced, could result in jets being ejected with very high speeds if the jet-production power were high or with slower speeds if the power fell below a “magnetic switch” threshold. However, the discovery of sources with hybrid FR I/II morphology or “HYMORs” (one jet with FR I structure and a counterjet with FR II structure) [13] essentially eliminated any scenario that assumes the FR I/II division is rooted in processes very near the BH: the dynamical time at, say, 10 rS (Schwarzschild radii) from a 109 M BH is of order a few days, and any asymmetry in jet production there probably would disappear on time scales of a month or so. HYMORs, on the other hand, ae-mail: [email protected] clearly have existed in that asymmetric state for at least the jet propagation time from core to lobes (1 − 10 Myr), making a near-BH origin of the FR division extremely unlikely. However, the dynamical time scale in a typical 1012 M elliptical galaxy is of order 50−100 Myr, making the Bicknell model more consistent with the existence of HYMORs. Nevertheless, there are indications that the origin of the FR I/II break occurs on a kiloparsec-scale, or smaller, rather than on a multi-kpc scale. In M87 (an FR I) the jet is seen to accelerate from the core to a distance of order the stationary component HST-1 (∼ 0.3 kpc deprojected), whereupon the jet continually decelerates to eventual sub-luminal speeds [2]. Jets from somewhat lowermass BHs, therefore, might be expected to have their FR nature determined at a similar galactic radius of rFR ∼ 0.1 kpc (M•/10 M ) from the BH or so. The dynamical time scale at this distance [26] is τdyn ∼ rFR/σV ≈ 0.3 Myr (where σV ∼ 340 km s−1 is the velocity dispersion of the stars in a 1012 M galaxy). This is just the sort of minimum FR I/II origin distance that is still allowable by the HYMOR test (assuming ten dynamical times for a jet asymmetry to last in a galactic nucleus). One final kpc-scale clue to the FR I/II division nature comes from measurements of the relative magnetic field strength in the outer jet compared to the equipartition value there (the threshold where the field itself is strong enough to affect or control the jet dynamics in the lobes). Werner et al. [39] used Spitzer data to pin down the infrared portion of the synchro-self-Compton spectrum of FR II hot spots and concluded that most have B/Beq values well below unity, indicating that in FR II lobes the magnetic field plays essentially no dynamical role. This conclusion is consistent with that from early numerical simulations DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2013 ,