Radio-Loud AGN ModelRadio-Loud AGN Model

(Credit: C.M. Urry and P. Padovani )(Credit: C.M. Urry and P. Padovani )

These objects also have hot, These objects also have hot, ADAF-typeADAF-type accretion flows, where the accretion flows, where the radiative radiative cooling is very inefficientcooling is very inefficient and most of the dissipated energy is advected into the black hole

Hot, tenuous disks are favorable sites for Hot, tenuous disks are favorable sites for relativistic particle accelerationrelativistic particle acceleration because because the gas is collisionlessthe gas is collisionless

Up to the present date, the precise nature Up to the present date, the precise nature of the mechanism responsible for of the mechanism responsible for transferring the gravitational potential transferring the gravitational potential energy from the infalling matter to the energy from the infalling matter to the small population of nonthermal particles small population of nonthermal particles that escape to form the jet is not yet clearthat escape to form the jet is not yet clear

Blandford-Znajek MechanismBlandford-Znajek Mechanism

– Rotation of black hole drags Rotation of black hole drags the inertial framethe inertial frame

– This results in twisting of the This results in twisting of the magnetic field lines supported magnetic field lines supported by the surrounding diskby the surrounding disk

– The resulting magnetic stress The resulting magnetic stress is then released as a Poynting is then released as a Poynting flux away from the holeflux away from the hole

– In this mechanism, the power In this mechanism, the power of the jets is provided by the of the jets is provided by the rotating holerotating hole

Is it possible to explain the outflows in terms of well-understood microphysical processes Is it possible to explain the outflows in terms of well-understood microphysical processes operating in the hot, tenuous disk, such as the possible acceleration of the jet particles at a operating in the hot, tenuous disk, such as the possible acceleration of the jet particles at a standing accretion shock?standing accretion shock?

Connection with cosmic-ray Connection with cosmic-ray accelerationacceleration

• The discovery of the high-energy cosmic-ray spectrum prompted work on the acceleration of The discovery of the high-energy cosmic-ray spectrum prompted work on the acceleration of cosmic rays in SN shock waves via the first-order Fermi mechanism (Krymsky 1977; Bell cosmic rays in SN shock waves via the first-order Fermi mechanism (Krymsky 1977; Bell 1978; Blandford and Ostriker 1978)1978; Blandford and Ostriker 1978)

• These models were developed in the test-particle approximation (this must be abandoned if These models were developed in the test-particle approximation (this must be abandoned if the compression ratio equals or exceeds 4)the compression ratio equals or exceeds 4)

• We apply the same picture to understand particle acceleration in We apply the same picture to understand particle acceleration in accretion disks containing standing, centrifugally-supported shocksaccretion disks containing standing, centrifugally-supported shocks

• In our disk/outflow model the liberated energy and entropy are thought to be lost from the In our disk/outflow model the liberated energy and entropy are thought to be lost from the disk in the vicinity of the shock via the escape of high-energy particles in ADAFs disks.disk in the vicinity of the shock via the escape of high-energy particles in ADAFs disks.

ShockShock

UpstreamUpstreamDownstreaDownstreammB.H.B.H.

ShockShock

OutflowsOutflows

Particle acceleration in accretion Particle acceleration in accretion disksdisks

• In this case there are In this case there are two groups of particlestwo groups of particles: the thermally-distributed : the thermally-distributed background background particlesparticles, and the higher-energy, , and the higher-energy, relativistic “test particles”relativistic “test particles”

• Since we are employing the Since we are employing the test particle approximationtest particle approximation, the pressure of the accelerated , the pressure of the accelerated particles is not included in the dynamicsparticles is not included in the dynamics

• In In ADAF disksADAF disks, the mean free path , the mean free path λλiiii for ion-ion collisions is much longer than the disk for ion-ion collisions is much longer than the disk height – the gas is collisionlessheight – the gas is collisionless

• The mean free path The mean free path λλmagmag for collisions with magnetic waves is much shorter than for collisions with magnetic waves is much shorter than λλiiii for the for the thermal particles, and much longer than thermal particles, and much longer than λλiiii for the relativistic particles – we assume for the relativistic particles – we assume collective processescollective processes thermalize the background thermalize the background

B.H.B.H.

ShockShock

OutflowsOutflows

B.H.B.H.

ShockShock

OutflowsOutflows • Therefore the background particles cross the Therefore the background particles cross the shock shock ONCEONCE, and the relativistic test , and the relativistic test particles cross the shock particles cross the shock MULTIPLEMULTIPLE times times

• The The maximum particle energymaximum particle energy that can be that can be produced in this model depends on the produced in this model depends on the magnetic wave distribution magnetic wave distribution via the recoil via the recoil effecteffect

Isothermal shocks (TIsothermal shocks (T++=T_)=T_)• In In isothermal shocksisothermal shocks radiative cooling is very efficient radiative cooling is very efficient

– More energy is lost than in the isentropic or RH shocksMore energy is lost than in the isentropic or RH shocks

– The entropyThe entropy decreasesdecreases as the gas crosses the shockas the gas crosses the shock

– This implies that the sound speed and the thickness of the This implies that the sound speed and the thickness of the flow remain unchanged through the shockflow remain unchanged through the shock

– This type of shock TThis type of shock T++=T_, but =T_, but εε++< < εε_ and K_ and K++<K_<K_

• The The compression ratiocompression ratio is is maximizedmaximized for a given Mach number, for a given Mach number, enhancing particle accelerationenhancing particle acceleration

• We focus on isothermal shocks here and therefore we assume We focus on isothermal shocks here and therefore we assume that that particles escape particles escape from the diskfrom the disk only at the shock location only at the shock location

• We will show the gas is We will show the gas is stronglystrongly bound in the post-shock region bound in the post-shock region

• Equations describing structure of adiabatic, Equations describing structure of adiabatic, inviscid accretion flows with inviscid accretion flows with isothermalisothermal shocks shocks

• Sonic Point AnalysisSonic Point Analysis

• Shock Point AnalysisShock Point Analysis

• Transport equationTransport equation that governs the relativistic that governs the relativistic particle energy/space distributionparticle energy/space distribution

• Assumption about Spatial Diffusion CoefficientAssumption about Spatial Diffusion Coefficient

• Assumption about Vertical EscapeAssumption about Vertical Escape

• Solutions for the Relativistic Number & Energy Solutions for the Relativistic Number & Energy DensitiesDensities

Disk-Jet Disk-Jet ConnectionConnectionB.H.B.H.

ShockShock

OutflowsOutflows

B.H.B.H.

ShockShock

OutflowsOutflows

ConstrainConstrain ΓΓescesc

AssumeAssume EE00

2)( cMLshock

200 )( cMEN

escjetL

escescesc NEL

0A0

MinimizeMinimize EEescescConstrainConstrain κκ00

ConstrainConstrain AA00

Powering the jet from the diskPowering the jet from the disk

0N

Heating the seed particlesHeating the seed particles

ConstrainConstrain NN00 ..

parameter spaceparameter spaceparameter spaceparameter space

Flow structure with/without shockFlow structure with/without shock

shock-free profile

shocked profile

Number and Energy Density DistributionNumber and Energy Density DistributionNumber and Energy Density DistributionNumber and Energy Density DistributionGlobal relativistic number (fig a) and energy density (fig b) distributions obtained in a shocked disk (solid line) and

associated with M87 (left panel) and Sgr A (right panel). *

shock- free (smooth) disk (dash line)

In each case the densities decrease monotonically with increasing radius.

shock-free profile shocked profile

Mean Energy of the Relativistic Particles in the DisksMean Energy of the Relativistic Particles in the DisksMean Energy of the Relativistic Particles in the DisksMean Energy of the Relativistic Particles in the Disks T he results demonstrate that when a is present in the flow, the relativistic particle

at the shock location(solid line).

By contrast, we find that in the models with the same values for , , and

(dash line).

shock

shock- free

energy is boosted by a factor of ~ 5- 6

_ l 0 the energy is boosted by a factor of only ~1.4- 1.5

Results: M87 & Sgr A*Results: M87 & Sgr A*

• Our model then gives for the Our model then gives for the escape energyescape energy at the shock radius at the shock radius rr = = rr* * , which is the jet radius , which is the jet radius rrjetjet ~ 22 ~ 22 rrgg and and rrjetjet ~ 16 r ~ 16 rgg for M87 and Sgr A*, respectively. for M87 and Sgr A*, respectively.

• Our results indicate that the shock acceleration mechanism can produce relativistic outflows with Our results indicate that the shock acceleration mechanism can produce relativistic outflows with terminal Lorentz factor of ~ 8 (M87) and ~ 7 (Sgr A*), and the total powers comparable to those terminal Lorentz factor of ~ 8 (M87) and ~ 7 (Sgr A*), and the total powers comparable to those estimated in M87 and Sgr A*.estimated in M87 and Sgr A*.

• From From observationsobservations, Biretta et al. (2002) suggest that the M87 jet forms in a region no larger than , Biretta et al. (2002) suggest that the M87 jet forms in a region no larger than rrjetjet < < 30 r30 rg g ; Biretta et al. (1999) ; Biretta et al. (1999) estimateestimate for the bulk flow in the jet of M87. for the bulk flow in the jet of M87.

• In the case of Sgr A*, our disk-jet model indicates that the jet forms at In the case of Sgr A*, our disk-jet model indicates that the jet forms at rrjetjet ~ 16 r ~ 16 rgg which is fairly close which is fairly close to the value suggested by Yuan (2000) model. However, future observational work will be needed to to the value suggested by Yuan (2000) model. However, future observational work will be needed to test our prediction for the asymptotic Lorentz factor of Sgr A*, since no reliable observational test our prediction for the asymptotic Lorentz factor of Sgr A*, since no reliable observational estimate for that quantity is currently available.estimate for that quantity is currently available.

6~

The EndThe End