Physics of Relativistic Jets - PowerPoint Presentation

Slide1

Physics of Relativistic Jets

Yuri Lyubarsky

Ben-Gurion University

Beer-

Sheva

, Israel Slide2

Universality of

relativistic jets

M 87

M 87

Crab in X-rays

GRBs

t

ime, s

t

ime, s

PKS 2155-304 Slide3

Pulsar magnetosphere

Collapsing, magnetized supernova core

Magnetized accretion disks around neutron stars and black holes

Magnetospheres of Kerr black

holes

A

rapidly spinning central

body

twists up the magnetic field into a

toroidal

component and

the plasma is ejected by the magnetic tension

.

Relativistic flow can be produced by having a very

strong

rotating magnetic field, B

2

>>4

c

2

.

Courtesy to

David Meier

All

these sources likely share

a common

basic mechanism, in which relativistic outflows are launched hydromagnetically

Rotational energy Poynting ?Slide4

Beyond the light cylinder, each revolution of the source adds to the wind one more magnetic loop

.

In expanding flows, Bf becomes dominating

Magnetic field lines rotate rigidly at the rate .

Plasma moves along the rotating field lines.Rotation twists up the field into toroidal component, slowing rotation.

At

W

r~c, the field gets wound up, Bp~B

f

- light cylinder radius

Basic picture of

relativistic

magnetohydrodynamic

outflowsSlide5

.

In

highly relativistic flows, the Lorentz

and electric forces nearly cancel each other.

Acceleration and collimation are only due to a small residual force. Without external confinement, the flow remains nearly radial and

Poynting

dominated (no collimation, no acceleration).

In relativistic flows, the electric force is important

In the far zone of the outflow, v c and E B.Slide6

In accreting systems, the relativistic outflows from the black hole and the internal part of the accretion disc could be confined by a

slow

wind from the outer parts of the disk.

In

long GRBs, a relativistic jet from the collapsing core bores its way through the envelope of the progenitor stare.Externally confined jetsSlide7

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?Poynting dominated jets.What do we want to know?

How and where does s decrease from >>1 to <<1?Slide8

cylindrical

equilibrium at

any z

B

p

is negligible; purely azimuthal field 2. Non-equilibrium regime: Qg

>1

1. Equilibrium regime

:

signal crossing time is less than the expansion time (strong causal connection),

Qg

<1.

Collimation vs acceleration: two flow regimes

The

flow

is

accelerated when expands

Weak causal connection:

No causal connection:

equilibrium

non-equilibrium

Z=r

2

/R

L

Weakly causally connected flows are slowly accelerated until and then stop accelerating Slide9

MHD jet confined by the external pressure

B

p

B

f

E

v

p

ext

The spatial distribution of the confining pressure determines the shape of the

flow

and the acceleration rateSlide10

MHD jet confined by the external pressure (cont)

Equipartition

,

g~g

max

,

is achieved at

- equilibrium regime

Beyond the equipartition:

g/g

in

sSlide11

MHD jet confined by the external pressure (cont)

- non-equilibrium

Jet asymptotically approaches conical shape

r

=

Q

zSlide12

MHD jet confined by the external pressure (cont)

2.

A special case;

k=2

If b<1/4, the flow is acceleratedtill s~

1 and then collapses. Slide13

GRBs:

g~

10

2 - 103; minimal z0~1011 cm – marginally OK. But achromatic breaks in the afterglow light curves and statistics imply gQ>>1, which is fulfilled only if the flow remains Poynting dominated. Magnetic dissipation is necessary.

Some implications

AGNs: g~10 implies the size of the confining zone z

0>100Rg~10

16cm. The condition of efficient acceleration (Qg

<1) may be fulfilled: <Qg

>=0.26 (Pushkarev

et al ‘09).

But according to spectral

fitting of

blazars

,

jets are already matter dominated at ~1000

R

g

(

Ghisellini

et al

‘10

). Violent dissipation somewhere around 1000Rg? Slide14

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 field

lines could come close to each other?

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

Alternating magnetic field could be present in the flow from the very beginning.Slide15

But: The necessary condition for the instability – strong causal

connection, gQ<1. Not fulfilled

in GRBs; may be fulfilled in AGNs. The growth rate is small in relativistic case. Evidences for saturation of the instability.

Mizuno et al ‘12

MHD instabilities The most dangerous is the kink instabilitySlide16

In an expanding flow, B becomes predominantly toroidal; current sheets are stretched. Local structure: plane current sheet separating oppositely directed fields.

Let alternating fields preexist in the jet

Striped jets?Slide17

Rayleigh-Taylor instability of currents sheets in accelerating

flows

j

D

In an accelerating

flow, effective gravity force arises

Dissipation rate

Instability time-scale

Magnetic dissipation in striped jetsSlide18

Interplay between acceleration and dissipation;

a self-consistent

picture

AGNs

GRBs

Complete dissipation:

In accreting systems,

l~R

gSlide19

Magnetic

fields are the most likely means of extracting the

rotational energy

of the source and of producing

relativistic outflows from compact astronomical objects.

3. An extended acceleration region is a distinguishing characteristic of the Poyntyng dominated outflows. Within the scope of ideal MHD, acceleration up to g~g

max is possible only in highly collimated flows (

).

2

.

External confinement is crucial for efficient

collimation of

Poynting dominated outflows.

Conclusions

4

. Even

though

an externally confined jets are accelerated by

magnetic tensions, conditions for efficient

transformation

of

the

Poynting into the kinetic energy are rather restrictive. Dissipation (reconnection) is necessary in order to utilize the EM energy of the outflow.

5

. If alternating field preexisted in the flow, they are efficiently dissipated via the Rayleigh-Taylor instability.