1. Why only a small fraction of quasars are radio loud?
Xinwu Cao ( 曹新伍 )
Shanghai Astronomical Observatory, CAS

2. 1. Radio dichotomy of quasars
Quasars can be divided into two categories, i.e., radio-loud (RL) and radio-quiet (RQ) quasars, according to their ratios of radio emission to optical emission. Their appearance is quite similar to the RQ counterparts in almost all wavebands except radio wavebands.

3. Observational differences:
1. the massive black holes in RL quasars are systematically a few times heavier than those in their RQ counterparts.
2. RL nuclei are invariably hosted by core galaxies.
Power law galaxies (filled histogram)
Core galxies (empty histogram)

4. Jet formation mechanisms
The jets are definitely related with
accretion disks
Possible energy sources of jets:
1. kinetic energy of the gases in the accretion disks (BP)
2. rotational energy of the rapidly spinning black hole (BZ)
A mechanism is needed to channel the gases in the disk to the jet, of which the motion is perpendicular to the disk ,
Large scale magnetic field is a crucial ingredient in jet formation mechanisms (either Blandford-Payne or Blandford-Znajek mechanism).

5. The Blandford-Payne (BP) mechanism
(Spruit 1996, astro-ph/ )

6. (Spruit H. C., 2010, Lecture Notes in Physics, 794, 233)
BP mechanism:
BZ mechanism
(Spruit H. C., 2010, Lecture Notes in Physics, 794, 233)

7. BP: magnetic field
BZ: magnetic field+black hole spin
A large scale magnetic field is a crucial ingredient in jet formation mechansims!

8. Previous scenarios for radio dichotomy
Black hole spin?
The black hole spin is regarded as an intrinsic difference between RL and RQ quasars (Wilson & Colbert 1995).
However, the numerical simulations show that almost all massive black holes will soon be spun up to rapidly spinning holes through accretion (Volonteri et al. 2007).
Magnetic field?
It was suggested that a strong magnetic field is a crucial factor causing the radio dichotomy in quasars (Sikora & Begelman 2013).

9. 2. Formation of large scale magnetic field
collapse!

10. Diffusivity
Parker (1979) argued that (l is the largest eddy size, and is turnover velocity), and the magnetic Prandtl number , is expected in isotropic turbulence.
Disk accretion:
Bondi accretion:
Beckwith et al., 2009, ApJ, 707, 428

11. Different accretion modes
Slim disk: optically thick, geometrically thick
Standard thin disk: optically thick, geometrically thin
Advection dominated accretion flow (ADAF)
optically thin, hot, geometrically thick
Accretion mode transition occurs while

12. (Lubow et al. 1994, MNRAS, 267, 235) Typical values:
P~1, H/R~0.05, D~20

13. So, it is difficult to form a strong field in a thin disk!
The external field can be dragged efficiently inward by the hot corona above the disc, i.e., the so called “coronal mechanism” (Beckwith, Hawley, & Krolik 2009).

14. The power of the jets accelerated by the coronal field
(Cao 2018, MNRAS)
p_m<p_gas in the corona, because the field is dragged inwards by the corona.
The maximal jet power is always less than 0.05 Eddington luminosity, which is insufficient for the observed strong jets in some blazars with jet power ~0.1-1 Eddington luminosity!

15. Large-scale magnetic fields can be formed in the thin disks with outflows (Cao & Spruit, 2013, ApJ, 765, 149)
We consider the angular momentum of the disk is removed predominantly by the magnetically driven outflows.
The radial velocity of the disk is significantly increased due to the presence of the outflows, and the field can be dragged in more efficiently than the conventional viscous disk.
We find that a weak field can be dragged inwards efficiently by the thin disk with magnetic outflows.

16. Model The radial velocity of an accretion disk with outflows is
The magnetic torque exerted by the outflows is
The mass loss rate is
In the cold Weber-Davis model, the Alfven radius is a function of mass loss rate and magnetic field strength.
The inclination of the field line can be derived with the balance between field advection and diffusion

17. Model parameters:

18. Even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion of the field.

19. Circularization radius
A model for radio dichotomy of quasars
(Cao, 2016, ApJ, 833, 30)
The gas falls almost freely toward the black hole, if the angular momentum of the gas is significantly lower than the Keplerian value at the Bondi radius. The angular momentum of the gas is roughly conserved until it approaches the circularization radius.
Bondi radius
Circularization radius

20. At the circularization radius, the gas is rotating at the Keplerian velocity. This leads to
Suppose a weak vertical magnetic field threading the gas at the Bondi radius, we can estimate the field strength at the circularization radius as

21. Field advection in a thin disk with magnetic outflows
The radial velocity of the disk is significantly increased due to the presence of the outflows:
We use a parameter to describe the relative importance of the outflows on the radial velocity of the disk.
We derive the first condition for efficient field advection in an accretion disk with magnetic outflows as

22. Circularization radius
In the region beyond a critical radius , the disk may become a clumpy disk due to the gravitational instability.
A clumpy disk region would be an obstacle for accumulation of external magnetic field. To avoid such a clumpy region in the disk, one requires
, i.e.,
Bondi radius
Circularization radius
For

23. A radio-loud quasar should satisfy two conditions:

24. Conditions for RL quasars: rotating velocity is lower than a critical value
External field strength:
solid: 0.01 mGauss,
dashed: 0.1 mGauss,
dashed–dotted: 1 mGauss
In the central region of our galaxy, the field strength of the gas can be as high as ~mGauss;
Field strengths of galaxy cluster atmospheres are

25. For a fixed gas temperature: 1 keV

26. A quasar will appear as a radio-loud (RL) one if the angular velocity of the circumnuclear gas is lower than a critical value at the Bondi radius, otherwise, it will appear as a radio-quiet (RQ) quasar.
Our model is consistent with two observational features;
Summary
a. RL nuclei are invariably hosted by core galaxies. The core galaxies are slowly rotating, while the power-law galaxies rotate rapidly and are disky.
b. The massive black holes in RL quasars are systematically a few times heavier than those in their RQ counterparts.

27. Thanks!