arXiv:1301.5638v1 [astro-ph.HE] 23 Jan 2013 crto”prdg,aotn h digo ai sa as ratio reproduce Eddington “spin- to the phenomenological adopting difficult A paradigm, rather 2012). accretion” is 2007, range al. et a (Volonteri such black spins, of evolution hole cosmological the of simulations merical oeyi em fbakhl pn(.. codn othe to according range the (i.e., difference over parameters spin spread spin a hole dimensionless requires black paradigm”) such of “spin typi- radio-detected, terms explain efficiency in than To solely with larger AGN. magnitude jets of optically-selected produce AGN orders Radio-selected to 3 luminosi- et cally found (Sikora with therein). limit been AGN references Eddington have in and the also 2007, than al. but smaller , much ties in spins only hole not black holes. the of thread spread that large fluxes sufficiently magnetic a 1977) and/or is Znajek & there (Blandford if jets powering for mechanism hi e ciiis nms usr h e power, jet the quasars of most diversity In enormous activities. the jet understand their to struggling still ffiiny uhabodrneo e rdcinefficien- production jet of range cies, broad a Such efficiency. 91 hsliie l 00 enne ta.21;Punsly 2011; al. where et Fernandes 2011), 2010; al. et Ghisellini 1991; edn pto up tending P factor a acc ioacm.d.l [email protected] [email protected]; h ra ag fjtpouto ffiinisi seen is efficiencies production jet of range broad The afacnuyatrtedsoeyo usr,w are we quasars, of discovery the after century a Half rpittpstuigL using 2013 typeset 25, Preprint January version Draft hl teshv e oescvrn h ag ex- range the covering powers jet have others while , P 2 j /P IA nvriyo ooaoadNtoa nttt fStan of Institute National and Colorado of University JILA, ol ersosbefrjtpouto nSyet n low-luminosit headings: and Subject Seyferts in accre binaries. production in m X-ray fields jet turbulen with turbulent flux, for of by responsible of dissipation deposited be the accumulation by flux could or magnetic secular disks, the accretion the thin in associated als from fluctuations “magnetosphere” but by result the hole, triggered by black jets disruption the strongest its against to the pressed due flux ran disk, the observed the of large magnitude The the (MCAF). o only flow merger accretion the choked by magnetically triggered be might acc event ar F cold an we massive, . Such and hole. a disk phase, when black accretion. disk) only the hot thick occur t (or of to accretion the close radio hot emission-line because flux a de strong quiet during magnetic thus occur radio depositing and to are in likely jets AGN inefficient powerful Most are launching (AGN). them in nuclei factor galactic dominant active the is ratio, ∼ 3 acc hspcuespot h data u cuuaincnla othe to lead can accumulation flux that idea the supports picture This th rather hole, black the threading flux magnetic the that argue We eateto srpyia n lntr cecs Unive Sciences, Planetary and Astrophysical of Department 10 a eepandvateBlandford-Znajek the via explained be can , M 4 ˙ P 1 ie mle hnteaceinpower, accretion the than smaller times j oencsAtooia etr oihAaeyo Science of Academy Polish Center, Astronomical Copernicus steaceinrt and rate accretion the is ∼ 1. P A INTRODUCTION acc T a E ANTCFU AAIMFRRDOLUNS FAGN OF RADIO-LOUDNESS FOR PARADIGM FLUX MAGNETIC ∼ tl mltajv 5/2/11 v. emulateapj style X = crto,aceindss—bakhl hsc aais cie—ga — active galaxies: — physics hole black — disks accretion accretion, 0 anthdoyais(H)—Xry:binaries X-rays: — (MHD) magnetohydrodynamics — . ǫ 03 Mc ˙ − 2 .Btacrigt nu- to according But 1. Rwig Saunders & (Rawlings ae Sikora Marek ǫ h accretion the rf eso aur 5 2013 25, January version Draft 1 P n icelC Begelman C. Mitchell and j ABSTRACT is , ad n ehooy 4 C,Budr O839 S and USA 80309, CO Boulder, UCB, 440 Technology, and dards inecece stemgei u Skr ta.2013). al. et (Sikora produc- flux jet magnetic of the diversity that is the suggests efficiencies This driving tion parameter 2007). al. main et the (Sikora range observed loudness the radio explain of to fails also parameter, second vn.W xlr hsie eo n r overify to try and below idea this low- accretion explore cold hot, We current, a large the et to event. the during Sikora prior AGN accumulated phase 2012), radio-loud were accretion-rate Ogilvie in fluxes & that magnetic Guilet suggested et Livio (2013) 2011; 1994; al. Cao al. geomet- et 1999; of (Lubow case al. flows the accretion in applies thick limitation rically Not- such no (MRI). that instability ing magneto-rotational trig- the the turbulence the by of in gered field because magnetic stan- disks, of by diffusion accretion center outward poloidal the thin large-scale to geometrically dragged out, dard, be pointed cannot (1994) fields the al. magnetic However in et place 1976). Lubow in Ruzmaikin as already & was (Bisnovatyi-Kogan hole 1970s 2012). black al. a et around McKinney mulated 2006; al. et magneti- Reynolds of 2003; Narayan formation al. pre- (MCAFs: et the flows is and accretion accumulation arrested/choked fluxes scenario central cally the magnetic a involve magnet- large Such that forced the of models of be flow. by pressure ram can dicted accretion the fluxes affected by be hole ically magnetic black can large the Ghosh limitation onto very this 1996; However, if Sikora & overcome 1997). (Moderski max- Abramowicz for spins by & even confined hole disk, be accretion black to the imal of large pressure too “static” is the flux magnetic required the st fClrd,31UB ole,C 00,USA 80309, CO Boulder, UCB, 391 Colorado, of rsity ntecs ftems oefljt,with jets, powerful most the of case the In h data ag e antcflxcnb accu- be can flux magnetic net large a that idea The ,u.Bryk 8 076Wra,Poland Warsaw, 00-716 18, Bartycka ul. s, inds ooa.Teeprocesses These coronae. disk tion ihteacmltdflx While flux. accumulated the with G,a ela esassociated jets as well as AGN, y drt e ciiycnas be also can activity jet oderate in litclglx iha with galaxy elliptical giant a f ,htinrrgoso otherwise of regions inner hot t, ei ai odesrflcsnot reflects loudness radio in ge emnn h ai odesof loudness radio the termining u htrdolu usr and quasars radio-loud that gue nbakhl pno Eddington or spin hole black an h eraei Vflxfrom flux UV in decrease the o eineetflosa episode an follows event retion 2 i crto ik htfeed that disks accretion hin , 3 u cuuaini more is accumulation lux omto faso-called a of formation ais jets laxies: P j ∼ P acc , 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

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 rg is formed and the MCAF scenario proceeds if cold accre- to be tion at a rate M˙ was preceded by hot accretion (satis- 4/3 2/3 d r Φ (M˙ ) r M˙ fying condition [1]) at a rate ǫ ∼ g ≃ BH,max d ≃ g d , d ˙ ˙ rm(Md) Φd ! rd Mh ! ˙ −3/2 ˙ Mh > (rd/rg) Md . (6) (11) Magnetic flux paradigm for radio-loudness 3 and then would probably arise in a corona above the thin accre- 2/3 tion disk, and the jet could be propelled thermally by ηj rd M˙ h efficient heating of the coronal plasma due to magnetic R≃ 104 . (12) 3/2 ˙ reconnection (Heinz & Begelman 2000). In sources where (f/10) rg Md ! the inner region of the accretion flow becomes hot or geo- Replacing M˙ d by the Eddington-ratio parameter, λ ≡ metrically thick for any reason, large magnetic field fluc- 2 tuations could develop with coherence length ∼ r and Pd/LEdd = ǫdM˙ dc /LEdd, in equations (11) and (12) gives strength of order the ram pressure. These flows could deposit transient flux onto the black hole, leading to in- r 3/5 λ 2/5 λ 2/5 termittent jet production. We tentatively associate these ǫ = g ∼ (13) d r m˙ λMCAF stochastic jets with the radio activity detected in radio-  d   h   max  intermediate quasars, low-luminosity AGN, and X-ray and binaries. − η λ 2/5 R≃ 104 j , (14) 4.2. Strong-line radio galaxies (SLRGs) (f/3)3/2 λMCAF  max  Strong, broad-line and narrow-line radio galaxies 2 wherem ˙ h ≡ M˙ hc /LEdd, and (BLRGs and NLRGs, respectively) form together with radio-selected quasars an Eddington-ratio sequence that 3/2 −4 MCAF rd extends down to λ ∼ 10 (Sikora et al. 2007, 2012). λmax = m˙ h (15) rg Sikora et al. (2013) showed that their radio loudness an-   ticorrelates with the Eddington ratio, in accordance with is the maximal Eddington-ratio achievable in the MCAF the MCAF model prediction (see equation [14]). for given values ofm ˙ h and rd (see equation 6). 4.3. Seyferts 4.1. Quasars Seyferts dominate the population of radio-quiet AGN. Studies of nearby quasars indicate that their radio- Though covering a similar Eddington-ratio range as loudness distribution is bimodal, with the majority of SLRGs, they are 2–4 orders less radio-loud, with R∼ 1 quasars narrowly clustered around R ∼ 1 and others 4 like radio-quiet quasars (Sikora et al. 2007). According to forming a tail extending up to R ∼ 10 (Kimball et the magnetic flux paradigm, this implies very inefficient al. 2011; Balokovi´cet al. 2012; Singal et al. 2012). While flux accumulation prior to the start of the Seyfert activ- radio emission of quasars with R < 10 is likely to be as- ity phase. This inefficiency can be attributed to the lack sociated with stellar processes in star formation regions, of a hot accretion pre-phase. In the case of objects hosted radio emission of quasars with R > 10 must involve dis- by disk galaxies, this conjecture is supported by studies sipation of jet energy. Values of R > 100 are achiev- of the nuclei of disk galaxies at epochs of very low accre- able for jets produced via the Blandford-Znajek mecha- tion rates. Broad-band spectra of such low-luminosity nism in the MCAF scenario. The radio-loudness of such AGN (LLAGN) indicate that at least the outer portions “MCAF”–quasars is expected to cover a range from R∼ 4 of their accretion disks are cold and geometrically thin tens up to R ∼ 10 , with a spread determined by the (Nemmen et al. 2012; Yu et al. 2011). range of black hole spins and of the Eddington-ratio pa- Nevertheless, LLAGNs are found to be moderately rameter λ (see equations [9] and [14]). radio-loud (Ho 2002). Their radio activity, like that in Observations show a bottom-heavy distribution of low/hard states of XRBs, is presumably associated with radio-loudness, with only a few percent of quasars a presence of a hot, radiatively inefficient accretion zone having R > 100 (de Vries et al. 2006; Lu et al. 2007; extending out to some distance from the black hole. As Balokovi´cet al. 2012). According to the magnetic flux discussed in §4.1, geometrically thick flows enable a sig- paradigm, this may indicate a bottom-heavy distribution nificant poloidal magnetic field to impinge on the black ˙ of the hot accretion rate Mh (or the absence of a hot ac- hole and thus generate jets or winds (Livio et al. 1999). cretion phase altogether), leading to a small fraction of quasars that satisfy the MCAF condition (6). Alterna- 4.4. X-ray binaries (XRBs) tively, some systems could fail to meet the flux threshold The MCAF scenario was suggested by Igumenshchev condition (1), perhaps because the hot accretion phase (2009) to explain jet activity and state transitions in is too brief or the external field too disordered, imply- XRBs. However, noting that the outer portions of the ing that the accumulated magnetic flux is too weak to accretion disks in such objects are geometrically thin, support the magnetosphere and is entirely crushed into it is not clear how magnetic flux can be advected ef- the hole by the disk flow. These “MCAF–failed” quasars ficiently to the vicinity of the black hole. An alterna- produce less powerful jets. Such jets are likely to be effi- tive scenario may involve fluctuating magnetic fields, as ciently decelerated within the galactic optical cores and mentioned earlier (§4.1). In low/hard states this could most of them become subsonic with Fanaroff-Riley type I involve a central hot, geometrically thick accreting re- or other centrally dominated morphologies (Komissarov gion with large–scale fluctuating poloidal magnetic fields, 1994; Bicknell 1995; Heywood et al. 2007). This can while in high/hard states the dominant mechanism could explain why radio emission in quasars with R < 100 is be thermal propulsion following dissipation of tangled dominated by compact sources (Lu et al. 2007). magnetic fields in the disk corona. Radio emission at a level corresponding to R ∼< 100 could also be contributed by jets associated with fluctu- 5. EVOLUTIONARY AND OBSERVATIONAL ating magnetic fields. In the case of quasars, such fields CONSIDERATIONS 4 Sikora & Begelman

The proposed flux paradigm explains the relative rarity ever, that significant reduction of a black hole’s spin by of radio-loud AGN because it requires a special sequence zero-angular-momentum inflow requires its mass to be of events to occur in order to trigger powerful jet activ- approximately doubled, and that such a doubling is not ity. This contrasts with the spin paradigm, which ap- achievable during the typical lifetime of a classical double 7 peals to a bottom-heavy distribution of black-hole spins radio-source, tFRII ∼ 3 × 10 years (O’Dea et al. 2009), that seems implausible in the light of recent evolutionary we regard the latter option as rather unrealistic unless modeling. M˙ d > M˙ Edd. Furthermore, an evolutionary connection The flux paradigm fits well with the observationally between radio-loud and radio-quiet AGN seems to be ex- supported idea that the most luminous AGN are trig- cluded by observations showing that the hosts of radio- gered by mergers (Hirschmann et al. 2012; Treister et loud AGN are on average located in denser environments al. 2012; Ramos Almeida et al. 2012). To produce a than those of radio-quiet AGN (Shen et al. 2009; Donoso radio-loud quasar or SLRG, a disk galaxy would pre- et al. 2010; Sabater et al. 2012). sumably merge with a giant elliptical where hot accre- We have focused mainly on the possible role of MCAFs tion from the has been going on for in explaining the most luminous, radio-loud AGN, but some time. The precondition of hot accretion is actually emphasize that other processes may trigger jets as well, a dual requirement, because the hot accreting gas must albeit giving a lower level of radio-loudness. We sug- also be carrying sufficient magnetic flux. Thus we con- gest that any hot or geometrically thick inner region of jecture that the hot interstellar medium in at least some an accretion flow can develop magnetic field fluctuations ellipticals contains a relatively coherent magnetic field of large enough spatial coherence and strength to pro- that can be dragged inward. While the origin of large- duce an intermittent jet. The magnetic fields in these scale magnetic fields in galaxies is still under debate, it is jets would undergo frequent reversals of polarity, possi- plausible that these fields are seeded by stellar processes, bly leading to enhanced dissipation through reconnection through winds and supernova explosions, which suggests across the current sheets (Sikora et al. 2003) and result- that the occurrence of powerful jets in AGN should be ing radiative signatures that could be used to distinguish correlated with the cosmic history of star formation, sep- them observationally from the jets produced by coher- arately from any generic correlation that links the growth ent magnetic flux in MCAFs. We tentatively associate of supermassive black holes to star formation. In the these stochastic jets with the radio activity detected in central galaxies of cool-core clusters and groups, jet pro- radio-intermediate quasars, low-luminosity AGN, and X- duction is ubiquitous, suggesting that the gas in these ray binaries in low/hard states. In quasars, Seyferts and relatively dense hot environments generally carries net XRBs in high/hard states, where flows proba- magnetic flux (Burns 1990; Hardcastle et al. 2007; Tasse bly extend all the way to the black hole, radio-emitting et al. 2008; Dunn et al. 2010). jets or winds could be produced through the dissipation The same cold accretion flow responsible for amplify- of fluctuating coronal magnetic fields. ing radio-loudness could also serve as the circumbinary As for testing the MCAF picture directly, we note that disk needed to drive the black holes in merging galax- a cold accretion flow, disrupted by magnetic stresses in- ies through the “final ” toward merger (Begelman side the magnetospheric radius rm, may have a unique et al. 1980; Cuadra et al. 2009; Dotti et al. 2012, and structure that would allow it to be distinguished from references therein). In this case, we could relate the ra- a hot inner accretion flow or a thin disk enveloped by dius within which the cold disk gathers up the trapped a hot corona. We expect the MCAF to consist of cold flux, rd, to the circumbinary radius, which could be blobs or filaments dropping through the magnetosphere 3 4 ∼ 10 − 10 rg , or larger. via interchange instabilities. These blobs might repro- To explain powerful jets, the MCAF scenario requires cess the surrounding disk emission more effectively than a not only a large magnetic flux but also a substantial black thick disk or corona, and also provide more optical depth hole spin. Because accreting matter gives up its angu- across the inner region surrounding the black hole. Fur- lar momentum to the dynamically dominant magnetic thermore,the power-density spectrum of X-ray variations fields as it falls through the magnetosphere, it reaches should have a high-frequency break at lower frequency in the black hole with almost no angular momentum. As a MCAFs than in viscous accretion flows. The best objects result, angular momentum extracted from the black hole to look for signatures of MCAFs would be those with the during the MCAF phase is not replenished by the ac- most extended magnetospheres, primarily radio galaxies, creted matter, and the black hole spin decreases. Thus, both strong-line and weak-line. an additional requirement for a high level of radio loud- ness is that the black hole should have a large spin prior to the start of the MCAF phase. ACKNOWLEDGMENTS Considering the above requirements for producing We acknowledge support from the Polish Ministry radio-loud quasars, one might envisage two evolutionary of Science and Higher Education through the grant tracks leading to radio-quiet quasars. One track would NCN DEC-2011/01/B/ST9/04845, from the National involve the merger of two disk galaxies, where magnetic Science Foundation through grant AST-0907872, and flux has never been collected through an extended period from NASA’s Astrophysics Theory Program through of hot accretion. Another track would represent the last grant NNX09AG02G. MCB thanks the Copernicus As- phases of MCAF activity where the black hole has been tronomical Center for its hospitality during the early spun-down to extremely small values of a. Noting, how- stages of this project.

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