Astrophysical jets

Yuri LyubarskyBen-Gurion University

Universality of relativistic jet phenomenon 1 .Radio galaxies and quasars

M 87

Universality of relativistic jet phenomenon 1 .Radio galaxies and quasars (cont)

Apparent superluminal

motion (Rees 1967)

v

c

towards observer

sinctx

cos)/v(1 ctt

cctx

cos)/v(1

sinapparentv

Blazars - AGNs with jet pointing towards the observer. The observed emission is dominated by the jet.

Universality of relativistic jet phenomenon 1 .Radio galaxies and quasars (cont)

The flux above 200 GeV) Aharonian et al 07 (

Rapid (few-minute) variability in TeV flux of blazars implies Levinson 06;Begelman etal 08(

PKS 2155-304

synchrorton IC

Universality of relativistic jet phenomenon 2 .Microquasars

Superluminal motionin microquasar

GRS 1915+105 ; Vapparent =1.5c

Microquasar is a scaled down (by a factor of 106)

version of active galactic nuclei

Scaling of X-ray binaries to AGNs

Merloni et al 03; Falcke et al 04

Distribution of Lorentz factors inferred from superluminal motionBlue – AGNs; red – microquasars

) Fender 05(

min

time, s

Optical afterglow Optical afterglow of of GRB 990510GRB 990510

Universality of relativistic jet phenomenon3 .Gamma-ray bursts

GRBs apparently involve ultrarelativistic (=100-1000), highlycollimated (=1o-5o) outflows. They likely arise during the

collapse of star’s core.

Universality of relativistic jet phenomenon3 .Gamma-ray bursts (cont)

Pulsar magnetosphere Collapsing, magnetized supernova core

Magnetized accretion disks around neutron stars and black holes

Magnetospheres of rotating black holes

Rotation twists up field into toroidal component, slowing rotation

Courtesy to David Meier

All relativistic cosmic jet sources may be connected by a common basic mechanism: acceleration by rotating, twisted magnetic fields

Energy balance in magnetized outflows

0BvE

0E

c1 frame lab In the

' frame plasmaproper In the

v4

flux Poynting2

4 Bc BES

Relativistic flow can be produced by having a very strong rotating magnetic field such that B2>>4c2 low “mass loading” of the field lines

The flow starts as Poynting dominated. How the EM energy transformed into the plasma energy?

By definition, ultrarelativistic flow implies Ekinetic>> c2

Rotational energy Poynting?

Formation of jet during collapse of a rotating massive star into a rotating BH (Barkov&Komissarov 08)

At RL: EBB p ~~

The characteristic size of the magnetosphere, RL=c/For rapidly rotating BH, RL~Rg.

222 ~4

~ gLEM cRBRcEB

L

22 ~

gR

cMB

Equipartition:

2cMLEM

Force balance in Poynting dominated flows

Magnetic hoop stress

BjF c1

Plasma pressure and inertia<< Lorentz force .Does it mean that huge tension ofwound up magnetic field (hoop stress)

compresses the flow towards the axis?

NO!

In the current closure region,the force is decollimating.

An external confinement is necessary!

In accreting systems, the relativistic outflows from the black hole and the internal part of the accretion disc could be confined by the (generally magnetized) wind from the outer parts of the disk.

Externally confined jets

In GRBs, a relativistic jet from the collapsing core pushes its way through the stellar envelope.

.

Total electromagnetic force

0c1 BvE E 4

1e

BjEF ce1

In the far zone, v c and E B.

In highly relativistic flows, the electric force nearly cancels the Lorentz force.

Acceleration is very slow!

Force balance in Poynting dominated flows (cont)

As the jet moves out, acceleration converts the EM energy into the matter kinetic energy. How efficient is this conversion?

What are the conditions for acceleration and collimation?

What is the final collimation angle?

Where and how the EM energy is released? Conversion to the kinetic energy via gradual acceleration? Or to the thermal and radiation energy via dissipation?

Externally confined jets.What do we want to know?

How and where decreases from >>1 to <<1?

fluxenergy plasma

flux Poynting

When the flow becomes matter dominated?

Jump conditions at relativistic shock.f- fraction of energy transferred to the plasma. Compression ratio = c/v2

The flow could be considered as matter dominated if most of the energy could be released at the termination shock. This condition isfulfilled only if <0.1-0.2.

Axisymmetric flows are nested magnetic surfaces of constant magnetic flux .These surfaces are equipotential .

Plasma flows along these surfaces.

At RL: EBB p ~~

In the far zone:

rrprcrI EBB 1112 ; ; 2

E, Bp

Structure of relativistic MHD jet

Simple example: cylindrical configuration.

B

Bp

1 .v=0; E=0

02

2

2 )(1 dr

rBd

rdr

dBp

pBB ~

2 .v c; B’p= Bp; B’ E’=(v/c)B

Bp

B

E

vpBBE '''

Relativistic MHD in the far zone, r>>c

Without external confinement, the flow is nearly radial;the acceleration stops at an early stage (Tomimatsu94; Beskin et al 98)

3/1max

3/1maxmax

3/1max

;/ln

;/

cr

cr

max>>1 - the Lorentz factor achieved when and if the Poynting flux is completely transformed into the kinetic energy

Jet confined by the external pressure: the spatial distribution of the confining pressure determines the shape of the flow boundaryand the acceleration rate (Tchekhovskoy et al 08,09; Komissarov et al 09; Lyubarsky 09,10(.

Intimate connection between collimation and acceleration

Collimation vs acceleration: two flow regimes

cr /~

Z=r2/c

equilibrium

non-equilibrium

cylindrical equilibrium

at any z

angle opening - ;1

1'

:Causality2

dz

dr

r

zc

r

z

r

ct

purely azimuthal field, Bp=0

1

1. r2<<cz – equilibrium regime

2. r2>>cz – non-equilibrium regime

MHD jet confined by the external pressure

Bp

B

E

v

pext 0ext

z

cpp

MHD jet confined by the external pressure (cont)

2 .1

Equipartition, max, at /4max0 ~/ cz

4/ zr

4/zr

czcr /~/~ ;2at on acceleratifastest The

equilibrium regime ,4/zr

0ext

z

cpp

2max0 ~/ ,2At cz

Beyond the equipartition:

0/ln

1~

zz

in

1

2 .3

zr shape conical

approachesally asymptoticJet

20

082)]-1/[2()2(

211 ;

B

p

4/zr

zr

3at /56.0

2.5at /2.0

2.2at /01.0 5.2

MHD jet confined by the external pressure (cont)

0ext

z

cpp

1 ;~

1 ;~ until grows

maxmaxt

max

3/1

2max

t

non-equilibrium

equilibrium

GRBs: few 102; minimal z0~1011 cm – marginally OK. But achromatic breaks in the afterglow light curves and statistics imply >>1, which is fulfilled only if the flow remains Poynting dominated. Magnetic dissipation is necessary.

Some implications

AGNs: impliesthe size of the confining zone z0>100Rg~1016cm.The condition of efficient acceleration is fulfilled: <Pushkarev etal 09)

The observed opening angles vs Lorentz factor (Pushkarev etal 09)

The flow could be accelerated till provided it is confinedby an external pressure, which decreases with distance butnot too fast.

The acceleration zone spans a large range of scales so that one has to ensure that the conditions for acceleration are fulfilled all along the way. Transition to the matter dominated stage, could occur only at an unreasonably large scale.

To summarize the dissipationless MHD acceleration

These conditions are rather restrictive. It seems that in real systems, some sort of dissipation (reconnection) is necessary in order to utilize the electromagnetic energy completely.

Some evidence for slow acceleration and collimation of Poynting dominated jets.

Gracia et al 2005

2 .The absence of bulk-Comptonization spectral signatures in blazars implies that Lorentz factors >10 must be attained at least on the scale1000Rg ~ 1017 cm (Sikora et al. 05).

1 .M87: a large opening angle near the base of the jet and tighter collimation on larger scales

Walker et al 2008

But according to spectral fitting, jets are already matter dominated at ~1000 Rg (Ghisellini et al 10). Violent dissipation somewhere around1000Rg?

Beyond the ideal MHD: magnetic dissipation in Poynting dominated outflows

current sheet

The magnetic energy could be extracted via anomalous dissipation in narrow current sheets.

How differently oriented magnetic fieldlines could come close to each other?

1. Global MHD instabilities could disrupt the regular structure of the magnetic field thus liberating the magnetic energy.

2. Alternating magnetic field could be present in the flow from the very beginning.

Moll et al 10

The most dangerous is the kink instability (Lyubarsky 92, 99; Eichler 93; Spruit et al. 97; Begelman 98; Giannios&Spruit 06).

But: The necessary condition for the instability – causal connection, Not fulfilled in GRBs; may be fulfilled in AGNs. The growth rate is small in relativistic case (Istomin&Pariev 94,96; Lyubarsky 99). Some evidence for saturation of the instability (McKinney&Blandford 09)

Mizuno et al 09

Global MHD instabilities

Bj

l~Rg

Dissipation of alternating magnetic fields in the jet (Levinson & van Putten 97; Levinson 98; Heinz & Begelman 00;Drenkhahn 02; Drenkhahn & Spruit 02; Giannios & Spruit 05;07; Giannios 06;08)

If the reconnection rate is vrecc, the energy release scale is

cm 10

cm 10~~ 2

3001.012

2

101.017

21

gRr

AGNs

GRBs

Advantage of the magnetic dissipation models

1. Dissipation easily provides large radiative efficiencies and strong variability inferred in GRBs.

2. Reconnection at high produces relativistic “jets in a jet”; this could account for the fast TeV variability of blazars (Giannios et al 10; also talk by Giannios)

Caveat: why the fast reconnection occur in jets?

8

2BnT current sheet

enB

eBT

Br 8

Anomalous resistivity implies vcurrent=j/en~c or rB=kT/eB~

In AGNs and GRBs, rB<<l~Rg

resistivity

inflow

outflow

Fast reconnection at rB<<l

We still do not understand why such a configuration is maintained within the jet

1 .Magnetic fields are the most likely means of extracting the rotational energy of the source and of producing relativistic,

outflows from compact objects (pulsars, GRBs, AGNs, galactic X-ray binaries(

3 .An extended acceleration region is a distinguishing characteristic of the Poyntyng dominated outflows .

2 .External confinement is crucial for efficient collimation and acceleration of Poynting dominated outflows.

Conclusions

4 .Efficient acceleration (up to max) is possible only in causally connected flows, 1

5. Dissipation (reconnection) is necessary in order to utilize the EM energy of the outflow.