The power of the jets accelerated by the coronal magnetic field

Abstract

It was suggested that the large scale magnetic field can be dragged inwards efficiently by the corona above the disc, i.e., the so called "coronal mechanism" (Beckwith, Hawley, & Krolik 2009), which provides a way to solve the difficulty of field advection in a geometrically thin accretion disc. In this case, the magnetic pressure should be lower than the gas pressure in the corona. We estimate the maximal power of the jets accelerated by the magnetic field advected by the corona. The Blandford-Payne (BP) jet power is found always to be higher than the Blandford-Znajek (BZ) jet power, except for a rapidly spinning black hole with a 0.8. The maximal jet power is always low, less than 0.05 Eddington luminosity, even for an extreme Kerr black hole, which is insufficient for the observed strong jets in some blazars with jet power ∼ 0.1 − 1 Eddington luminosity (or even higher). It implies that these powerful jets cannot be accelerated by the coronal field. We suggest that, the magnetic field dragged inward by the accretion disc with magnetically outflows may accelerate the jets (at least for the most powerful jets, if not all) in the blazars.

The dynamical structure of the outflows driven by a large-scale magnetic field

“…In the field advection scenario of this work, the gas-to-magnetic ratio at the upper surface of the corona should be greater than unity, the field line is easily sheared into toroidal component in the outflow, and the resultant Alfvén surface is close to the surface of the corona. Thus, the outflows driven by the large-scale magnetic field advected by the corona are in general not very powerful (relative low mass loss rate and/or low terminal speed), which are consistent with the simple estimate in Cao (2018). Sheikhnezami et al (2012) also carried out time-dependent simulations on the outflows/jets launched magnetically from diffusive accretion disks, and they confirmed that the magnetocentrifugal acceleration is more efficient in a strong magnetic case (i.e., a lower plasma β), which may imply the outflow accelerated from a MAD (magnetically arrested disks) can reach a very high velocity with a low mass loss rate.…”

“…We assume that the outflows are launched from the surface of the corona (i.e., z = z h ), which serves as a of the gas to feed the outflows. In this case, the magnetic flux is mainly dragged inward by the corona, thus the gas-to-magnetic ratio at the corona surface should be greater than unity, which means the outflows are driven by a relatively weak magnetic field (compared with gas pressure) (Cao 2018). The derived slow sonic points of the outflows are therefore very close to the surface of the corona (see red-dashed line in Figure 3).…”

“…In this work, for simplicity, we neglect the impact of the outflows on the dynamics of the disk, which is a reasonable approximation for weak or moderately strong outflows. As pointed by Cao (2018), the strength of the large-scale field dragged inward by the corona is always limited by the gas pressure of the corona, and therefore only outflows with low or moderate power can be formed in this case.…”

“…Such a sandwich disk-corona model has been well developed, and it can successfully reproduce main features of the X-ray spectra of accretion disk-corona systems in different scales, i.e., AGNs, X-ray binaries or even ultrluminous X-ray sources (e.g. Di Matteo 1998;Di Matteo et al 1999;Wu & Gu 2008;Cao 2009;Merloni & Fabian 2001You et al 2012;Liu et al 2016;Lyu & Rieke 2017;Qiao & Liu 2018;Arcodia et al 2019;Yang et al 2019;Liu et al 2019;Cheng et al 2020;Zou et al 2020;Mondal et al 2020;You et al 2021;Liu et al 2021a,b). The disk-corona system is usually described as a two-phase accretion flow with its temperature changing sharply from the cold disk to the hot corona in the vertical direction (Haardt & Maraschi 1991, 1993, which is confirmed by the elaborate calculation of the vertical disk-coronal structure with detailed vertical energy transportation included (e.g., Liu, Mineshige, Meyer, Meyer-Hofmeister, & Kawaguchi 2002).…”

“…Thus, the external field is sufficiently dragged inward by the fast moving gas in the disk, and a strong large-scale magnetic field can be formed due to the counterbalance of outward flux diffuse by the efficient inward field transport (Cao & Spruit 2013;Li & Begelman 2014;Li 2014;Li & Cao 2019;Scepi et al 2020). Another possibility is the presence of highly conducting fast inward moving layers above a thin disk that substantially suppress the diffusion of the magnetic flux, which may also lead to a rather strong large-scale magnetic field (Lovelace et al 2009;Beckwith et al 2009;Guilet & Ogilvie 2012, though the strength of the field is not very strong (limited by the gas pressure of the layers above the disk) (Cao 2018). Recently, the timedependent global numerical simulations suggested that for the disk-corona system the mass accretion occurs mainly in the corona region, of which the magnetic flux is efficiently transported inward, and a quasi-static large-scale magnetic field with field lines strongly inclined towards the disk surface is formed.…”