on the role of bh spin and accretion in powering relativistic jets in agn laura maraschi...

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 23-26 2011

The core of a radio loud AGN

Matter accretesonto the BHdissipating energy inan accretion disk or rad. inefficientaccretion flow

At the center highly relativisticpowerful jets arelaunched, radiatingup 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. 2010

ON THE WHOLE THE MECHANISM APPEARS SUCCESSFUL

FFHH

The Blandford Znajek mechanism (1977)

illustrated by Macdonald and Thorne (1982)

plunge

The B – Z “formula”

LBZ > 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” ΩF = ΩISCO

Near the horizon ΩF > ΩISCO

Compare ΩH with ΩISCO as a function of the BH spin “a”

Comparing

ΩISCO with ΩH

Crossingat 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, frozenin the inflowing matter, will change sign(BZ 1977 MT 1982)

The condition for this sign change needsto 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 theymention 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)

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

B2 rH2 c ~ 2 Mdot c2

Krolik (1999) Maraschi (2001) Levinson (2010)

However the BZ power also depends on the BH spin ( as a2 up to a6 ?). 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 monthsalready 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 beobtained

Jet power vs. accretion disk luminosity (SED modelling)

ii) Pjet > LdiskLdisk~0.1 PaccPjet ~ Pacc

i) For FSRQs the correlationis significant(subtracting zdependence)

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. 2008

previoussmallerindependentsamples:Differentselection,still

Pjet = 10 Ldisk

Model independent:

Jet vs. Disk observed luminositiesof brightFERMI blazars

For FSRQs

LLd

Ltrue)Ld

FSRQs disappear and BL Lacs appear below a disk luminosity of

10 45 erg s -1 = 10 -2 LEdd for M = 109

BLLacs are subEddington and radiatively inefficient accretors

Lacc

prop to m2 in this regime

HOWEVER Ljet prop to m, according to the B-Z formula, thus Ljet 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 AGNon the basis of 2 parameters “a” and “Mdot”

m

Optically thin hot flow

Opticallythick disc

No jet a < 0.5 a > 0.5 Powerful

jet

The fundamental AGN planeThe fundamental AGN plane

FSRQ

FR II

FR IBL Lac

a

From Sikora et al. 2007

These general propertiescan 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 notlead to highly relativistic jets

The critical divide:

L (ob) ~ 1047

L (em)~ 1045

~ 0.01 L(Edd)for M ~ 109GMT 09

An important consequence from the BZ mechanism also for BL Lacs

If the jet power and accretion power arerelated (as BZ predicts) BL Lacs , whichappear at lower gamma-ray luminosity, must also have accretion power lowerthan FSRQ (in particular largely sub Eddington)

Interpretation of the “divide”

For a “typical” mass of 109 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 enoughphotons for External Compton scattering

The FRII – FRI divide

The power limit that separates thetwo morfology classes depends onthe luminosty of the host galaxy

Ledlow and Owen

FRI—FRII: a mass dependent luminosity and morphology division

G&C2001

Ledlowand Owendiagramin termsof jetpower

Jet vs Disk luminosity

(FERMI 3 m)

LLd

Both are“observed”quantities but

Lis beamedfactor LLd

Back to the BZ formula:

Omega H must be larger than Omega FB must be carried by infalling matterand is frozen into the accretion diskSuppose, before “the plunge”, Omega F = Omega (ISCO)

The division between FRIs and FRIIs in theLedlow and Owen diagram also correspondsto a limit of ~ 1% Eddington accretion

(deriving the jet power from the extendedradio emission and the BH mass from the galaxy magnitude)

The coincidence between these twototally independent limits confirms in a “model independent” way

the scenario in which the differentproperties and SEDs of FSRQs andBL Lac objects are due to different Eddington ratios

Gamma-ray luminosity vs disk luminosity

Lg ~10-100Ld

(model independentbut Lg is beamed, Ld is not)

modelling needed

(2 observables)

Gamma-ray luminosity vs disk luminosity

LLd

(model independentbut Lis beamed, Ld is not)

modelling needed

(2 observables)

Average SED models of the FSRQs and BLLacs in the 3 months Fermi Blazar sample

The blue bumpis directly observed in FSRQsthe accretiondisk lumnosity can be derived

For BL Lacs upper limits canbe 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 mostlyGRMHD simulations to study its astrophysicalrealization

GRMHD simulations of a Poynting dominatedjet with selfconsistent interaction betweenthe 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 byGarofalo (09) considering a special mechanismof field amplification in the plunging regionAND “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, LBZ is negative and no jet can be produced

This argument is qualitative but stronglySuggests the existence of a threshold in abelow which the B-Z process cannot occur

The existence of a threshold for the BZmechanism 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 fromradiative models, protons need assumption. Power can be estim. at diff. scales along the jet

AssumingPjet~Paccalways andusing massestimates we deriveaccretion rates inEddington 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. 2001

RED

BLUE

Th

th

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 disappearsbecause the accretion flow becomes radiatively inefficient: the jet propagates in a photon poor ambient and its SED is “blue”

JET POWERS AND SEDs

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 variability Fast variability • HE and VHE spectraHE and VHE spectra

Blazar jets are special jets only with regard to orientation

Due to priviledged orientation theyare brighter and can be best studied

Their “intrinsic” properties are representative of the jet population in AGN

Jet power vs. accretion disk luminosity

Pjet > LdiskLdisk~PaccPjet ~ Pacc

For FSRQs the correlationis signficant(subtracting zdependence)

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 aresteep (> 1.2) and apparent Luminosities high,while for BL Lacs spectral indices are hard (< 1.2) and apparent Luminosities are lower.