On the role of BH spin and accretion in powering relativistic jets in AGN Laura Maraschi M.Colpi,G.Ghisellini,A.Perego,F.Tavecchio INAF - Brera Observatory, Milan, Italy Krakow, May

The core of a radio loud AGN Matter accretes onto the BH dissipating energy in an accretion disk or rad. inefficient accretion flow At the center highly relativistic powerful jets are launched, radiating up to gamma-rays

 A spin threshold for the Blandford-Znajek mechanism ?  The physical link between jet power and accretion power  Relevant results from FERMI observations  Grand Unification prospects for AGN OUTLINE

The Blandford - Znajek mechanism (1977) It concerns the extraction of the free energy associated with the BH spin through a surrounding magnetosphere “Translated” by Macdonald and Thorne (1982) Direct numerical simulations of BH magnetosphere Kommissarov 2001, 2004 (confirms BZ approx. ) From BZ to jet properties Mc Kinney 2005, 2006 (  theta = 5°…..) Efficiency still under investigation: e.g. Tchekhovskoy et al ON THE WHOLE THE MECHANISM APPEARS SUCCESSFUL

 FFFF HHHH The Blandford Znajek mechanism (1977) illustrated by Macdonald and Thorne (1982) plunge

The B – Z “formula” L BZ > 0 only if  F <  H Where  F is the angular velocity of the mag.field lines and  H that of the BH

Indeed the BZ 77 solution finds Ω F = ½ Ω H Confirmed by Kommissarov (2001, 2004) In these papers the magnetosphere configuration is “assumed”

Conjecture about the existence of a threshold for the BZ mechanism

The magnetsphere must be advected by the accretion flow: suppose, before the “plunge” Ω F = Ω ISCO Near the horizon Ω F > Ω ISCO Compare Ω H with Ω ISCO as a function of the BH spin “a”

Comparing Ω ISCO with Ω H Crossing at a=0.4

The analytically predicted B-Z power is negative No BZ for a < 0.4

We are well aware that this argument is qualitative…………………..even simplistic However there is a global physical reason to expect that, if the Black Hole is slowly rotating, the e.m. torque exerted by the faster rotating field lines, frozen in the inflowing matter, will change sign (BZ 1977 MT 1982) The condition for this sign change needs to be explored quantitatively

Mc Kinney and Gammie 2004 performed a GRMHD simulation of a spinning black hole surrounded by a gas torus ! They study the infall of magnetized gas from the torus onto the BH……… Among other interesting results they mention that “in their runs the BZ process does not operate for a < 0.5” This is encouraging !

From Mc Kinney and Gammie 2004 The Net EM Luminosity produced via BZ ! Ratio of EMto mass inflow power (negative)Ratio of EMto mass inflow power (negative)

The power scaling factor for the BZ mech. indicates a physical link between Jet power and Accretion power. Assuming equipartiton between kinetic and magnetic energy densities in the plunging region B 2 r H 2 c ~ 2 M dot c 2 Krolik (1999) Maraschi (2001) Levinson (2010) However the BZ power also depends on the BH spin ( as a 2 up to a 6 ?). A tight connection can result only if the range of BH spin is small….

FERMI surveyed the whole sky in gamma-rays with unprecedented sensitivity The blazar data from the first 3 months already allowed to derive interesting results For FSRQs the acc. disc is directly observed, and the gamma-ray luminosities can be compared with the accretion luminosity, while for BL Lacs an upper limit can be obtained

Jet power vs. accretion disk luminosity (SED modelling) ii) P jet > L disk L disk ~0.1 P acc Pjet ~ Pacc i) For FSRQs the correlation is significant (subtracting z dependence) Ghisellini et al. 2010

Jet power vs. Disk lum. in FSRQs For powerful blazars, Pjet ~ 10 Ldisk ~ Pacc M01, M & Tavecchio 2003, M et al previous smaller independent samples: Different selection,still P jet = 10 L disk

Model independent: Jet vs. Disk observed luminosities of bright FERMI blazars For FSRQs L  L d L  true)  L d 

FSRQs disappear and BL Lacs appear below a disk luminosity of erg s -1 = L Edd for M = 10 9 BLLacs are subEddington and radiatively inefficient accretors L acc prop to m 2 in this regime HOWEVER L jet prop to m, according to the B-Z formula, thus L jet prop to L acc 1/2 as indicated by the grey stripe in the figure

The existence of a spin threshold for the BZ process would allow to understand: i) the close relationship between jet power and accretion power found in the FERMI survey ii)the existence of two AGN populations radio-loud and radio quiet respectively with spin above and below the threshold (Sikora 2007) leading to a Grand Unification of AGN on the basis of 2 parameters “a” and “Mdot”

m Optically thin hot flow Optically thick disc No jet a 0.5 Powerful jet The fundamental AGN plane FSRQ FR II FR I BL Lac a

From Sikora et al These general properties can be understood: The radio loudness – accretion rate plane

Conclusions The two populations are distinguished by different values of “a” above and below ≈ 0.5 respectively. The R parameter increases at low λ due to reduced optical efficiency of disk accretion. A prediction is that radio emission in “radio quiet” objects is due to the Blandford Payne process which does not lead to highly relativistic jets

The critical divide: L  (ob) ~ L  (em)~ ~ 0.01 L(Edd) for M ~ 10 9 GMT 09

An important consequence from the BZ mechanism also for BL Lacs If the jet power and accretion power are related (as BZ predicts) BL Lacs, which appear at lower gamma-ray luminosity, must also have accretion power lower than FSRQ (in particular largely sub Eddington)

Interpretation of the “divide” For a “typical” mass of 10 9 solar masses, an average beaming factor of order 100, the dividing luminosity corresponds to 0.01 Eddington. Jets in AGN accreting above this limit (FSRQs) have “steep” gamma-ray spectra. AGN accreting below this limit turn into BL Lacs with “hard” gamma-ray spectra because accretion becomes radiatively inefficient and does not provide enough photons for External Compton scattering

The FRII – FRI divide The power limit that separates the two morfology classes depends on the luminosty of the host galaxy Ledlow and Owen

FRI—FRII: a mass dependent luminosity and morphology division G&C 2001 Ledlow and Owen diagram in terms of jet power

Jet vs Disk luminosity (FERMI 3 m) L  L d Both are “observed” quantities but L  is beamed  factor  L  L d 

Back to the BZ formula: Omega H must be larger than Omega F B must be carried by infalling matter and is frozen into the accretion disk Suppose, before “the plunge”, Omega F = Omega (ISCO)

The division between FRIs and FRIIs in the Ledlow and Owen diagram also corresponds to a limit of ~ 1% Eddington accretion (deriving the jet power from the extended radio emission and the BH mass from the galaxy magnitude)

The coincidence between these two totally independent limits confirms in a “model independent” way the scenario in which the different properties and SEDs of FSRQs and BL Lac objects are due to different Eddington ratios

Gamma-ray luminosity vs disk luminosity Lg ~ L d (model independent but Lg is beamed, L d is not) modelling needed (2 observables)

Gamma-ray luminosity vs disk luminosity L  L d (model independent but L  is beamed, L d is not)  modelling needed (2 observables)

Average SED models of the FSRQs and BLLacs in the 3 months Fermi Blazar sample The blue bump is directly observed in FSRQs  the accretion disk lumnosity can be derived For BL Lacs upper limits can be derived

What are these results telling about the jet production mechanism ? The Blandford Znajek (BZ) mechanism is at present the most popular model for the production of relativistic jets: It is purely el.mag., thus in some sense “simple” ! But it does not specify the origin of the field thus it is not completely realistic….. Present theoretical work concerns mostly GRMHD simulations to study its astrophysical realization

GRMHD simulations of a Poynting dominated jet with selfconsistent interaction between the BH and the accretion disk (Mc Kinney 06) show that   is reached at R ~ 100 Rs --Theta ~ 5 deg surrounded by a wider cone with smaller   consistent with “spine – layer” jet structure inferred from observations Winds from the inner regions of the acc. Disk reach  < 3 (not adequate) BZ (ideal) is described by a simple formula !!

The near equality Pjet ~ Pacc requires high efficiency This seems to be a problem(Mc Kinney 05) Possible interesting solution is proposed by Garofalo (09) considering a special mechanism of field amplification in the plunging region AND “counterrotation” ….  The plunging region is wider

Suppose  F corresponds to the rotation frequency at the innermost stable orbit Omega (ISCO) of the accretion disc (could be larger but not smaller in the plunge) If the BH rotation frequency is SMALLER than  ISCO, L BZ is negative and no jet can be produced This argument is qualitative but strongly Suggests the existence of a threshold in a below which the B-Z process cannot occur

The existence of a threshold for the BZ mechanism would allow to understand i) The correlation between jet power and accretion power in jetted (radio-loud) AGN ii)The Radio Loud/Radio Quiet dicothomy (contrary to the continuous dependence on “a” discussed by Tchekovskoy Narayan Mc Kinney 2009)

For FSRQs jet power and accretion power are indeed correlated and are comparable This is consistent with the Blandford & Znajek mechanism for the origin of jets, but requires very high efficiency. Previous estimates (Gammie 04, Mc Kinney 05) are relatively low unless a~1. An attractive possibility, proposed by Garofalo 09 is that in the brightest blazars the BH and disk are counterrotating Conclusions

The accretion rate in Eddington units, m is a fundamental parameter for: -the radiative properties of the accretion flow associated with the jets (bright disks or RIAF) -the shape of the jet SED through the intensity of the radiation field surrounding the jet (radiative energy losses and em. mech.) The shape and luminosity of the gamma-ray emission! -the jet power and its survival to large scales, that is the main morphological difference between FRI and FRII radio sources (Celotti and Ghisellini 2001)

Jet power where Electrons, mag. field and bulk Lorentz factor from radiative models, protons need assumption. Power can be estim. at diff. scales along the jet

Assuming Pjet~Pacc always and using mass estimates we derive accretion rates in Eddington Units

Jet power to disk Luminosity ratio for FSRQs Pjet~10 Ldisk~Pacc

The spectral sequence of blazar SEDs FSRQ BL Lacs Fossati et al. 1998; Donato et al RED BLUE ThTh thth The Spectral Energy Distributions of blazars show systematic trends: peaks at higher frequencies with decreasing luminosity

In FSRQs the jet’s SEDs are “red” due to the high photon density provided by the optically thick accretion disk. At accretion rates below some threshold (10 -2,10 -3 Edd. ) the optical disk disappears because the accretion flow becomes radiatively inefficient: the jet propagates in a photon poor ambient and its SED is “blue” JET POWERS AND SEDs FFFF

Outline The AGN coreThe AGN core Relativistic jets and blazarsRelativistic jets and blazars Population properties of blazarsPopulation properties of blazars First FERMI resultsFirst FERMI results Fast variabilityFast variability HE and VHE spectraHE and VHE spectra

Blazar jets are special jets only with regard to orientation Due to priviledged orientation they are brighter and can be best studied Their “intrinsic” properties are representative of the jet population in AGN

Jet power vs. accretion disk luminosity P jet > L disk L disk ~  P acc Pjet ~ Pacc For FSRQs the correlation is signficant (subtracting z dependence) Ghisellini et al. 2009

The FERMI Blazar sample : a critical divide More than 100 blazars after 3 months FIRST GAMMA-SELECTED BLAZAR SAMPLE For FSRQs gamma-ray spectral indices are steep (> 1.2) and apparent Luminosities high, while for BL Lacs spectral indices are hard (< 1.2) and apparent Luminosities are lower.