on the role of bh spin and accretion in powering relativistic jets in agn laura maraschi...
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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
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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
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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
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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
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FFHH
The Blandford Znajek mechanism (1977)
illustrated by Macdonald and Thorne (1982)
plunge
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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
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Indeed the BZ 77 solution finds Ω F = ½ Ω H
Confirmed by Kommissarov (2001, 2004)
In these papers the magnetosphere configuration is “assumed”
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Conjecture about the
existence
of
a threshold for the BZ
mechanism
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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”
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Comparing
ΩISCO with ΩH
Crossingat a=0.4
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The analytically predicted B-Z power is negative
No BZ
for a < 0.4
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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
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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 !
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From Mc Kinney and Gammie 2004
The Net EM Luminosity produced via BZ
!
Ratio of EMto mass inflow power (negative)
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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….
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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
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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
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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
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Model independent:
Jet vs. Disk observed luminositiesof brightFERMI blazars
For FSRQs
LLd
Ltrue)Ld
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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
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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”
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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
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From Sikora et al. 2007
These general propertiescan be understood:
The radio loudness – accretion rate plane
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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
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The critical divide:
L (ob) ~ 1047
L (em)~ 1045
~ 0.01 L(Edd)for M ~ 109GMT 09
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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)
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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
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The FRII – FRI divide
The power limit that separates thetwo morfology classes depends onthe luminosty of the host galaxy
Ledlow and Owen
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FRI—FRII: a mass dependent luminosity and morphology division
G&C2001
Ledlowand Owendiagramin termsof jetpower
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Jet vs Disk luminosity
(FERMI 3 m)
LLd
Both are“observed”quantities but
Lis beamedfactor LLd
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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)
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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)
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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
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Gamma-ray luminosity vs disk luminosity
Lg ~10-100Ld
(model independentbut Lg is beamed, Ld is not)
modelling needed
(2 observables)
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Gamma-ray luminosity vs disk luminosity
LLd
(model independentbut Lis beamed, Ld is not)
modelling needed
(2 observables)
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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
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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
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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 !!
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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
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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
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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)
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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
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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)
••
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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
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AssumingPjet~Paccalways andusing massestimates we deriveaccretion rates inEddington Units
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Jet power to disk Luminosity ratio for FSRQs
Pjet~10 Ldisk~Pacc
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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
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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
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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
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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
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Jet power vs. accretion disk luminosity
Pjet > LdiskLdisk~PaccPjet ~ Pacc
For FSRQs the correlationis signficant(subtracting zdependence)
Ghisellini et al.2009
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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.