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 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
OUTLINE
 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
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
(G = 10 theta = 5°…..)
Efficiency still under investigation:
e.g. Tchekhovskoy et al. 2010
ON THE WHOLE THE MECHANISM APPEARS SUCCESSFUL
The Blandford Znajek mechanism (1977)
H
F
plunge
illustrated by Macdonald and Thorne (1982)
The B – Z “formula”
Where F is the angular velocity of the
mag.field lines and H that of the BH
LBZ > 0
only if
F < H
Indeed the BZ 77 solution finds
ΩF=½ΩH
Confirmed by Kommissarov (2001, 2004)
In these papers the magnetosphere
configuration is “assumed”
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
Crossing
at a=0.4
The analytically predicted B-Z power
is negative
for a < 0.4
No BZ
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 !
The Net EM Luminosity produced via BZ
R
a
t
i
o
!
o
f
E
M
t
From Mc Kinney and Gammie 2004
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 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)
i) For FSRQs
the correlation
is significant
(subtracting z
dependence)
ii) Pjet > Ldisk
Ldisk~0.1 Pacc
Pjet ~ Pacc
Ghisellini et al.
2010
Jet power vs. Disk lum. in FSRQs
previous
smaller
independent
samples:
Different
selection,still
Pjet = 10 Ldisk
For powerful blazars, Pjet ~ 10 Ldisk ~ Pacc
M01, M & Tavecchio 2003, M et al. 2008
Model
independent:
Jet vs. Disk
observed
luminosities
of bright
FERMI blazars
For FSRQs
Lg ~10-100Ld
Lg (true) ~ 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 acc1/2
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”
The fundamental AGN plane
No jet
Optically
thick disc
a < 0.5
a
a > 0.5 Powerful jet
FSRQ
FR II
m
FR I
Optically thin
hot flow
BL Lac
The radio loudness – accretion rate plane
From
Sikora et
al. 2007
These
general
properties
can be
understood:
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 g (ob) ~ 1047
L g (em)~ 1045
~ 0.01 L(Edd)
for M ~ 109
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 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 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
Ledlow
and
Owen
diagram
in terms
of jet
power
G&C
2001
Jet vs Disk
luminosity
(FERMI 3 m)
Lg ~10-100Ld
Both are
“observed”
quantities but
Lg is beamed
(factor 100)
Lg ~ Ld
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
(2 observables)
Lg ~10100Ld
(model
independent
but Lg is
beamed,
Ld is not)
modelling
needed
Gamma-ray luminosity vs disk luminosity
(2 observables)
Lg ~10-100Ld
(model
independent
but Lg is
beamed,
Ld is not)
modelling
needed
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 
G ~ 10 is reached at R ~ 100 Rs --Theta ~ 5 deg
surrounded by a wider cone with smaller G
consistent with “spine – layer” jet structure
inferred from observations
Winds from the inner regions of the acc. Disk
reach G < 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, LBZ 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)
Conclusions
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
The accretion rate in Eddington units,
• is a fundamental parameter for:
m
- 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
Fossati et al. 1998; Donato et al. 2001
RED
FSRQ
BL Lacs
BLUE
The Spectral Energy
Distributions of
t
blazars Tshow
h
systematic
h trends:
peaks at higher
frequencies
with decreasing
luminosity
JET POWERS AND SEDs
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”
Outline
•
•
•
•
•
•
The AGN core
Relativistic jets and blazars
Population properties of blazars
First FERMI results
Fast variability
HE 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
For FSRQs
the correlation
is signficant
(subtracting z
dependence)
Pjet > Ldisk
Ldisk~0.1 Pacc
Pjet ~ Pacc
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.