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Sikora with Therein) Draft version January 25, 2013 A Preprint typeset using LTEX style emulateapj v. 5/2/11 MAGNETIC FLUX PARADIGM FOR RADIO-LOUDNESS OF AGN Marek Sikora1 and Mitchell C. Begelman2,3 1 Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland 2 JILA, University of Colorado and National Institute of Standards and Technology, 440 UCB, Boulder, CO 80309, USA and 3 Department of Astrophysical and Planetary Sciences, University of Colorado, 391 UCB, Boulder, CO 80309, USA Draft version January 25, 2013 ABSTRACT We argue that the magnetic flux threading the black hole, rather than black hole spin or Eddington ratio, is the dominant factor in launching powerful jets and thus determining the radio loudness of active galactic nuclei (AGN). Most AGN are radio quiet because the thin accretion disks that feed them are inefficient in depositing magnetic flux close to the black hole. Flux accumulation is more likely to occur during a hot accretion (or thick disk) phase, and we argue that radio-loud quasars and strong emission-line radio galaxies occur only when a massive, cold accretion event follows an episode of hot accretion. Such an event might be triggered by the merger of a giant elliptical galaxy with a disk galaxy. This picture supports the idea that flux accumulation can lead to the formation of a so-called magnetically choked accretion flow (MCAF). The large observed range in radio loudness reflects not only the magnitude of the flux pressed against the black hole, but also the decrease in UV flux from the disk, due to its disruption by the “magnetosphere” associated with the accumulated flux. While the strongest jets result from the secular accumulation of flux, moderate jet activity can also be triggered by fluctuations in the magnetic flux deposited by turbulent, hot inner regions of otherwise thin accretion disks, or by the dissipation of turbulent fields in accretion disk coronae. These processes could be responsible for jet production in Seyferts and low-luminosity AGN, as well as jets associated with X-ray binaries. Subject headings: accretion, accretion disks — black hole physics — galaxies: active — galaxies: jets — magnetohydrodynamics (MHD) — X-rays: binaries 1. INTRODUCTION second parameter, also fails to explain the observed range Half a century after the discovery of quasars, we are of radio loudness (Sikora et al. 2007). This suggests that still struggling to understand the enormous diversity of the main parameter driving the diversity of jet produc- their jet activities. In most quasars the jet power, P , is tion efficiencies is the magnetic flux (Sikora et al. 2013). j In the case of the most powerful jets, with P ∼ P , a factor ∼ 104 times smaller than the accretion power, j acc the required magnetic flux is too large to be confined by P , while others have jet powers covering the range ex- acc the “static” pressure of the accretion disk, even for max- ˙ 2 tending up to Pj ∼ Pacc = ǫMc (Rawlings & Saunders imal black hole spins (Moderski & Sikora 1996; Ghosh 1991; Ghisellini et al. 2010; Fernandes et al. 2011; Punsly & Abramowicz 1997). However, this limitation can be 2011), where M˙ is the accretion rate and ǫ the accretion overcome if very large magnetic fluxes can be forced efficiency. Such a broad range of jet production efficien- onto the black hole by the ram pressure of the magnet- cies, Pj /Pacc, can be explained via the Blandford-Znajek ically affected accretion flow. Such a scenario is pre- mechanism for powering jets (Blandford & Znajek 1977) dicted by models that involve the central accumulation if there is a sufficiently large spread of black hole spins of large magnetic fluxes and the formation of magneti- and/or magnetic fluxes that thread the holes. arXiv:1301.5638v1 [astro-ph.HE] 23 Jan 2013 cally arrested/choked accretion flows (MCAFs: Narayan The broad range of jet production efficiencies is seen et al. 2003; Reynolds et al. 2006; McKinney et al. 2012). not only in quasars, but also in AGN with luminosi- The idea that a large net magnetic flux can be accu- ties much smaller than the Eddington limit (Sikora et mulated around a black hole was already in place in the al. 2007, and references therein). Radio-selected AGN 1970s (Bisnovatyi-Kogan & Ruzmaikin 1976). However have been found to produce jets with efficiency typi- as Lubow et al. (1994) pointed out, large-scale poloidal cally 3 orders of magnitude larger than radio-detected, magnetic fields cannot be dragged to the center by stan- optically-selected AGN. To explain such a difference dard, geometrically thin accretion disks, because of the solely in terms of black hole spin (i.e., according to the outward diffusion of magnetic field in the turbulence trig- “spin paradigm”) requires dimensionless spin parameters gered by the magneto-rotational instability (MRI). Not- spread over the range a ∼ 0.03 − 1. But according to nu- ing that no such limitation applies in the case of geomet- merical simulations of the cosmological evolution of black rically thick accretion flows (Lubow et al. 1994; Livio et hole spins, such a range is rather difficult to reproduce al. 1999; Cao 2011; Guilet & Ogilvie 2012), Sikora et (Volonteri et al. 2007, 2012). A phenomenological “spin- al. (2013) suggested that in radio-loud AGN the large accretion” paradigm, adopting the Eddington ratio as a magnetic fluxes were accumulated during a hot, low- accretion-rate phase prior to the current, cold accretion [email protected]; [email protected] event. We explore this idea below and try to verify 2 Sikora & Begelman whether magnetic flux can be the main parameter re- 3. JET PRODUCTION EFFICIENCY sponsible for the observed spread in radio loudness. If the condition given by equation (6) is satisfied, then Our paper is organized as follows. In §2 we de- ˙ rive the conditions that must be satisfied by the hot Φd > ΦBH,max(Md) and the rate of energy extraction and cold accretion rates and by the initial inner ra- from the rotating black hole via the Blandford-Znajek dius of the cold accretion disk (within which the flux mechanism is is trapped), in order to initiate a magnetically-choked 2 (MCAF ) −3 2 ˙ ΩBH accretion flow (MCAF). We next (§3) estimate the re- P ≃ 4 × 10 Φ (Md) fa(ΩBH ) BZ BH,max c sulting efficiency of black-hole rotational energy extrac- 2 ˙ 2 tion via the Blandford-Znajek mechanism, and investi- =10(φ/50)xafa(xa)Mdc , (7) gate whether the Blandford-Znajek mechanism can ex- plain the observed broad range and distribution of radio where ΩBH is the angular velocity of the black hole, § § loudness of AGN ( 4). In 5 we speculate about why the 2 −1 flux accumulation in most AGN is inefficient, discuss pos- xa = rgΩBH /c = [2(1 + 1 − a )] a , (8) sible cosmological scenarios for black hole evolution that ≃ 2 − 4 p may lead to the observed radio-demographics of AGN, fa(xa) 1+1.4xa 9.2xa, and a is the dimension- and suggest additional observational consequences. less angular momentum parameter (Tchekhovskoy et al. 2010). Although energy is also extracted from the mag- 2. MAGNETIC FLUX ACCUMULATION netospheric region outside the event horizon, the latter can dominate the jet power only for very slowly rotat- Suppose a “hot” (geometrically thick) accretion flow, ing holes (a < 0.1: McKinney et al. 2012). Assuming with mass flux M˙ h, drags a net poloidal magnetic flux a > 0.1, we find that the efficiency of jet production in Φh,tot into the central region of an AGN. Provided that the MCAF scenario is this flux is larger than the maximum that can be confined ˙ 2 2 on the BH by the ram pressure of the accreting plasma, ηj ≡ Pj /Mdc ≃ 10(φ/50)xafa(xa) . (9) Φ > Φ (M˙ )= φ(M˙ cr2)1/2 , (1) This gives ηj ≃ 1.9(φ/50) for a = 1 and ηj ≃ 0.0063 for h,tot BH,max h h g a =0.1, and therefore the jet efficiency varies by a factor such an accumulation will result in the formation of a ≃ 300 over the spin range 0.1 <a< 1.0. For the spin “magnetosphere” extending out to a radius range 0.4 <a< 0.9, however, where most black holes are expected to lie, the efficiency varies by less than a 4/3 factor 10. Φ r (M˙ ) ≃ h,tot r , (2) m h ˙ g ΦBH,max(Mh)! 4. RADIO LOUDNESS 2 According to the analysis by Willott et al. (1999), the (Narayan et al. 2003), where rg = GMBH/c is the radio luminosity of a typical extended radio source is gravitational radius and φ is a dimensionless factor approximately proportional to the jet power. Adopt- which, according to numerical simulations by McKinney ing the modifications and approximations discussed by et al. (2012), is typically of order 50. The amount of 2 3/2 Sikora et al. (2013), we use Pj ∼ 10 ν1.4Lν1.4 (f/3) , magnetic flux enclosed within a radius r < rm(M˙ h) is where ν1.4 =1.4 GHz and f is a factor that is predicted 3/4 to be in the range 1 <f < 20 (Willott et al. 1999). ˙ r Then, using the standard definition of the radio-loudness, Φh(r) ≃ ΦBH,max(Mh) . (3) 14 rg R = Lν5 /LνB , where ν5 = 5 GHz and νB = 6.8 × 10 Hz (Kellermann et al. 1989), we obtain that the radio If the hot accretion phase is followed by a cold accretion loudness predicted by the MCAF model is event initiated by the formation of a cold disk at rd < ˙ ν ν L P η rm(Mh), that disk will trap the flux R≡ B 5 ν5 ≃ 4 j ≃ 4 j 10 3/2 10 3/2 ν5 νBLνB (f/3) Pd ǫd(f/3) Φd =Φh(r = rd) (4) (10) 2 and squeeze it to a radius where ǫd ≡ Pd/M˙ dc is the radiative efficiency of the accretion disk, Pd ∼ 10νBLνB , and Lν5 is related to Lν1.4 4/3 2/3 assuming a radio spectral index αr =0.8.
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