PPT

advertisement
Relativistic Jets in the Universe
Yosuke Mizuno
Institute of Astronomy
National Tsing-Hua University
Contents
• Introduction: Relativistic Jets (observations)
• Jet formation / acceleration mechanism
– Magnetohydrodynamic (MHD) process
• Jet collimation mechanism
– Jet is self-collimated?
• Dissipation of jets
– How magnetic energy converts to jet kinetic energy
• Summary
Astrophysical Jets
• Astrophysical jets are a
tremendous, elongated and
collimated outflows of plasma
• Astrophysical jets can be
observed in a huge spatial and
energetic scale reaching from
stellar size to galaxy size
• There are many sources for jets
• Jets (Outflows) are common
feature in the universe
• The astrophysical jets seen in
AGNs, BHBs, and GRBs have a
relativistic speed = relativistic
jets
M87 jets (AGNs, optical)
Radio observation
M87 = Virgo A
• Nearby: D ~ 16
Mpc (1 mas = 0.08
pc)
• AGNs: FR I,
Misaligned BL Lac
(q ~ 14 deg)
• SMBH mass: 6.6
x 109 Msun
• VLBA resolution:
20 rs at 43 GHz
Frequency: 43GHz
Sources of Jets
Stellar
Objects
Physical Systems
Young Stellar Objects Protostars accreting from disks
jet with
relativistic
speed
HMXBs/LMXBs
(microquasars)
NS/BH accreting from disks
Pulsars
Rotating Neutron Star
Gamma-Ray Bursts
Merging NSs or BH forming
inside collapsing star
Active Galactic
Nuclei
Accreting supermassive BH in
the core of galaxies
Extragalactic
Relativistic jets - properties
• Highly relativistic jets are lunching from accreting compact objects
(BHs/NSs, “central engine”)
• Jets transport energy and angular momentum from central engine
to remote locations and provide feedback to ISM/IGM
• They are fast: Lorentz factors ~ a few to a few tens in AGNs and
microquasars (BHBs), and >100 in GRBs
• High Lorentz factors mean that special relativistic effects are
important (general relativistic effects is also important in the
vicinity of compact objects).
• Jet phenomenon bridges many orders of magnitude in size: forming
in size scale of rg and extending up to 109-10 rg
Jet Similarity
Similar Morphologies But …
Jets from a protostar
• Size: Few-light across
• Speed: few 100 km/s
• Optical: atomic line emission
Jets from a quasars
• Size: ~ million light-years across
• Speed: ~ c (light speed)
• Radio emission: synchrotron
emission (non-thermal)
Cygnus A
Jet Speeds
• Sub-relativistic: protostars, v/c ~ 10-3
• Mildly-relativistic: SS433, XRBs (v/c = 0.26)
– Doppler-shifted emission lines
• Highly-relativistic: X-ray binaries (microquasars), ~10% of
AGNs (G ~ 2-30)
– Doppler beaming (One-sideness)
– Illusion of superluminal motion
– Gamma-ray flares (to avoid gg-pair production)
• Hyper-relativistic: Gamma-Ray Bursts (G ~ 100 - 1000)
– Gamma-ray variability
• Ultra-relativistic: Pulsar jets/winds (G ~ 106)
– Modeling of radiation and pulsar nebulae
Superluminal Motions in
Relativistic Jets
Apparent superluminal motion
(Rees 1967)
towards observer
q
c
v
 x  ct sin q
 t  t 1  ( v / c ) cos q 
v apparent 
x
t

sin q
1  ( v / c ) cos q
c
Relativistic Jets in AGNs
• Jet launched from vicinity of a
supper-massive BH (105-9 Msun)
• ~ 10 % of AGN are radio-loud i.e.,
have prominent jets
• AGN jets can extend from ~ 10-4 pc
to several hundred kpc in size
• Jet composition is unknown (normal
or pair plasma)
• Jet powers : 1043-47 erg/s
•Jet power integrated over radio
source lifetime: 1057-62 erg
Urry & Padovani (1995)
Blazars
• AGN with relativistic jets seen
almost pole-on
• Two sub-category: Flatspectrum radio quasars and BL
Lac objects
• Jet emission enhanced due to
relativistic effects by a factor
of thousands
• Broad-band SED dominated
by non-thermal emission from
jets (Synchrotron + InverseCompton )
• Emission from radio up to
TeV gamma-rays
Average SED of blazars (Fossati et al. 98)
Relativistic Jets in
Observed
Microquasars in radio
Superluminal motion
in microquasar GRS 1915+105; Vapp =1.5c
Microquasar is a scaled down (by
a factor of 106) version of active
galactic nuclei
Relativistic Jets in Universe
Mirabel & Rodoriguez 1998
Fundamental Problems in
Relativistic jets
• How are the jets formed near the compact objects (BHs or NSs)?
(formation mechanism)
• How are the jets accelerated up to relativistic speed? (acceleration
mechanism)
• How the collimated jet structure make? (collimation mechanism)
• How the jets remain stable over large distance? (Jet stability
mechanism)
• How the jets influence ISM and IGM (feedback)?
• How to emit the radiation in the relativistic jets? (Radiation
mechanism and particle acceleration mechanism)
Jet formation/acceleration
mechanism
• Jet is formed near the central compact objects (BHs/NSs).
• Some accreting matter is getting some force to make jetlike outflows.
• Ingredients: rotation, accretion disk, magnetic fields
• Jet base: rotating disk or compact objects (BHs/NSs)
• The jet formation/acceleration mechanism is still under
debate but …
• The most promising mechanism is the
acceleration/formation by rotating, twisting magnetic
fields (magnetohydrodynamic (MHD) process)
• Other possibility: gas pressure, radiation pressure, …
Jet formation/acceleration
mechanism (cont.)
• Gas or radiation pressure (Blandford & Rees 1974, O’Dell 1984)
– push accretion matter to make and accelerate outflows by pressure gradient
• Expansion of magnetic tower (Lynden-Bell & Boily 1994)
– Mainly toroidal field from start
– Acceleration by magnetic pressure
• Magnetocentrifugal acceleration (Blandford & Payne 1982)
– Mainly poloidal field anchored to disk or rotating objects
– Disk or ergosphere of BH acts like crank
– Torque transmitted though poloidal field powers jet
• Blandford-Znajek process (Blandford & Znajek 1977)
– Directly extract the BH rotating energy and convert to outward Poynting flux
– Consider force-free limit (MHD Penrose process is similar mechanism, MHD
version )
Acceleration & Collimation in MHD
model
Magnetic
field line
• Assume: in ideal MHD, plasma is
attached with magnetic field
• Acceleration
Accretion disk
Centrifugal
force
– Magneto-centrifugal force
– Magnetic pressure
• Like expansion of spring
• Collimation
– Magnetic pinch force
outflow (jet)
• Like shrink lubber band
Jets
accretion
Magnetic
field lines Magnetic
field lines
Jet formation/acceleration
mechanism (cont.)
• In ideal MHD limit (infinite conductivity), plasma flow (motion) is
connected with magnetic field
• The rotation of accretion disks or compact objects (BHs / NSs)
twisted up the magnetic field into toroidal components
Collapsing, magnetized
supernova core (GRBs)
Pulsar
magnetosphere
Magnetospheres of
rotating black holes
Magnetized
accretion disks
around neutron stars
and black holes
Courtesy to David Meier
Relativistic Jets Formation from
GRMHD Simulations
• Many GRMHD simulations of jet formation (e.g., Hawley & Krolik 2006, McKinney 2006,
Hardee et al. 2007) suggest that
• a jet spine (Poynting-flux jet) driven by the magnetic fields threading the
ergosphere via MHD process or Blandford-Znajek process
• may be surrounded by a broad sheath wind driven by the magnetic fields anchored
in the accretion disk (mildly-relativistic wind).
• High magnetized flow accelerates G >>1, but most of energy remains in B field.
Non-rotating BH
Spine Sheath
Fast-rotating BH
Total
velocity
Disk
Density distribution
(McKinney 2006)
Disk Jet/Wind
BH Jet Disk Jet/Wind
(Hardee, Mizuno & Nishikawa 2007)
Jet Energetics
Gravity, Rotational energy (BH or accretion disk)
Efficient conversion to EM energy
Poynting flux (magnetic energy)
Easy to get ~ equipartition,
hard to get full conversion
Jet kinetic energy
Magnetic field is a medium for a transmission not a source
Jet Collimation
Magnetic hoop stress
• Jet is produced by MHD process near
the central objects and magnetic field
is tightly tied (toroidal field is
dominated)
• Lorentz force >> plasma pressure &
inertia
 Huge tension force of wound up
magnetic field (hoop stress)
compress the flow towards the axis
(self-collimation)?
 Answer: No!
• In the current closure region, the
force acts to de-collimation
• Need external confinement
F
1
c
j B
External
Confinement
• In BH - accretion disk systems,
the relativistic outflows from the
black hole and the internal part of
the accretion disk could be
confined by the mildlyrelativistic magnetized wind from
the outer parts of the disk.
• In GRBs, a relativistic jet from
the collapsing core pushes its
way through the stellar envelope
(confinement).
Collimation vs Acceleration
• For jet collimation, external confinement is necessary
• Without external confinement, the flow is near radial and
acceleration stops at an early stage (Tomimatsu 1994; Beskin et al. 1998)
• The gas pressure profile of external confinement medium is the
important parameter
• The spatial distribution of confining gas pressure determines the
shape of the jet flow boundary, magnetic field configuration and
acceleration rate (Tchekovskoy et al. 2009, 2010; Komissarov et al. 2009;
Lyubarsky 2009,2010).
• Optimal collimation  pressure decrease slowly along jets
• Optimal acceleration  pressure decrease rapidly along jets
=> Collimation and acceleration of jet are related (poloidal) magnetic
field configuration
Effects of external confinement
2D RMHD simulations (Komissarov et al. 2009)
Parabolic (z ∝ r2)
Acceleration: slow, collimation: OK
Conical (z ∝ r):
Acceleration: fast, collimation: X
Lorentz factor
• Some
part of jets can convert Poynting flux to Kinetic Energy but most
can’t.
• Energy conversion is too slow to become kinetic energy dominated, it
is unreasonably long distance = inconsistent of observations.
• We need to consider some sort of dissipation (rapid energy conversion)
Observed Jet structure
Global structure of M87 jet (Asada
& Nakamura 2012)
• The parabolic structure
(z ∝ r1.7) maintains over
105 rs, external
confinement is worked.
• The transition of
streamlines presumably
occurs beyond the
gravitational influence
of the SMBH (= Bondi
radius)
• Stationary feature
HST-1 is a consequence
of the jet recollimation
due to the pressure
imbalance at the
transition
• In far region, jet stream
line is conical (z ∝ r)
Parabolic streamline
(confined by ISM?)
Conical
streamline
(unconfined,
free
expansion)
Over-collimation at
HST-1 stationary knot
(recollimation
shock?)
HST-1 region
Observed Jet structure (cont.)
• In M87 jet, the asymptotic
acceleration from nonrelativistic (0.01c) to relativistic
speed (0.99c) occurs over 102-5 rs
• This is very slow acceleration
= consistent with theoretical
results?
• The absence of bulkComptonization spectral
signatures in blazars implies
that Lorentz factors >10 must
be attained at least ~1000 rg
(Sikora et al. 05).
• But according to spectral
fitting, jets are already matterdominated at ~1000 rg
(Ghisellini et al 10).
Transition of Sub- to super-luminal
motion in M87 jet
Asada & Nakamura (2014)
Dissipation in the Jet
• Time-dependent energy injection to jet
=> Internal shocks in jets
• Sudden change of confined external medium spatial profile
=> Recollimation shock/ rarefaction acceleration
• Magnetic field reversal or deformation of ordered magnetic field
=> Magnetic reconnection
• MHD Instabilities in jets
– Kelvin-Helmholtz instability at jet boundary
– Current-Driven Kink instability at jet interior
=> Turbulence in the jets and/or magnetic reconnection?
Dissipation in the Jet: Energetics
• Tapping kinetic energy
– Internal shock
– Recollimation shock
– Kelvin-Helmholtz instability
• Tapping magnetic energy
– Rarefaction acceleration
– CD kink instability
– Magnetic reconnection
Prefer dissipation
mechanism for Poyntingdominated jet (conversion
from Poynting flux to
Kinetic energy)
Rarefaction Acceleration
• If the external confined media is
suddenly disappeared and jet becomes
free expansion (parabolic => conical),
the rarefaction wave is formed by
overpressure of jet and propagates in
the jets during the transition from jet
boundary.
• Rarefaction wave converts jet thermal
& magnetic energies to jet kinetic
energy efficiently (e.g., Aloy & Rezzolla
2006; Mizuno et al. 2008; Komissarov et al.
2010), so-called rarefaction acceleration
• The mechanism is favor for GRBs and
Komissarov et al. 2010
Rarefaction wave
propagates from
the jet boundary
Change channel shape from
parabolic to conical
may be possible for AGNs.
– stationary feature by recollimation shock?
Lorentz factor
CD Kink Instability
• Well-known instability in laboratory
plasma (TOKAMAK), astrophysical
plasma (Sun, jet, pulsar etc).
• In configurations with strong toroidal
magnetic fields, current-driven (CD)
kink mode (m=1) is unstable.
• This instability excites large-scale
helical motions that can be strongly
distort or even disrupt the system
• For static cylindrical force-free
equilibria, well known KruskalShafranov (KS) criterion
Schematic picture of CD kink instability
– Unstable wavelengths:
l > |Bp/Bf |2pR
• However, rotation and shear motion could
significant affect the instability criterion
• Distorted magnetic field structure may
3D RMHD simulation of CD kink
trigger of magnetic reconnection.
instability in PWNe (Mizuno et al. 2011)
CD Kink Instability in Jets
• Helical structure is developed by CD kink instability.
• Growth rate of CD kink instability is small
• Magnetic energy in the jets converts thermal and
kinetic energies by development of instability (via
turbulent structure)
• Jet structure is strongly deformed but may be not
disrupted entirely (depends on magnetic pitch, density,
and flow profiles) (Mizuno et al. 09, 11, 12, 14).
Mizuno et al. (2009)
Density
+ B-field
Mizuno et al. (2014)
• Regular helical
magnetic field is strongly
deformed via CD kink
instability => may be
triggered magnetic
reconnection
jet
Density
+ B-field
Magnetic reconnection
• Ideal MHD gives frozen in
magnetic fields.
• Resistive MHD (non-ideal MHD)
allows diffusion of fields.
• Magnetic reconnection occurs
through diffusion in narrow current
sheets.
• Magnetic reconnection is the
process of a rapid rearrangement of
magnetic field topology.
• Magnetic energy could be
converted to thermal and kinetic
energies
• Problem: how differently oriented
magnetic field lines could come close
each other?
– Global MHD instability
– Alternating B-field formed at jet
begging.
Magnetic Reconnection (cont.)
Dissipation of alternating magnetic fields in the jet (e.g.,
Giannios & Spruit 05,07; Giannios 06,08; McKinney & Uzdensky
2012)
j
g
Alternating
polarity by
misaligned
dipole field
multipolar
field (local
closed field)
Ordered field
broken by
instability
Mckinney & Uzdensky (2012)
Ultra-Fast TeV Flare in Blazars
• Ultra-Fast TeV flares are observed in
some Blazars (AGN jets).
• Vary on timescale as sort as
tv~3min << Rs/c ~ 3M9 hour
• For the TeV emission to escape pair
creation Γem>50 is required (Begelman,
PKS2155-304 (Aharonian et al. 2007)
See also Mrk501, PKS1222+21
Fabian & Rees 2008)
• But PKS 2155-304, Mrk 501 show
“moderately” superluminal ejections
(vapp ~several c)
• Emitter must be compact and
extremely fast
•Model for the Fast TeV flaring
• Magnetic Reconnection inside jet
(Giannios et al. 2009)
Giannios et al.(2009)
Advantage of Magnetic
Reconnections
• Magnetic reconnection easily provides large radiative efficiencies
and strong variability inferred in AGNs and GRBs.
• Reconnection at high s produces relativistic “jets in a jet”; this
could account for the fast TeV variability of blazars (Giannios et
al 2010)
• Questions: why fast reconnection occur in jets?
• If we consider anomalous resistivity in small dissipation region,
we get fast reconnection. But we do not know what is anomalous
resistivity.
• This is still unsolved problem in physics.
Mizuno 2013, ApJS
Relativistic Magnetic
Reconnection using
RRMHD Code
• To handle relativistic magnetic
reconnection numerically, we need
to perform resistive relativistic
MHD simulations.
Initial condition
• Consider Pestchek-type magnetic
reconnection
• anti-parallel magnetic field
• Anomalous resistivity for
triggering magnetic reconnection
Results
• B-filed:typical X-type topology
• Density:Plasmoid
• Reconnection outflow: ~0.8c
Five Regions in AGN Jets
Jet Launching
Region
Modified from Graphic
courtesy David Meier
Jet Collimation Region
(10 –100  Launching Region)
Alfven Point
Slow
MS Point
Poynting Flux Dominated
CD Unstable
Magnetic Helicity
Driven Region
Sheath
Modified Fast
Point
High speed spine
Fast MS Point
Collimation
Shock
Combined CD/KH
Unstable Region
Kinetic Energy Flux Dominated
with Tangled (?) Field
KH Unstable Velocity Shear Driven Region
Summary
• Relativistic jet is formed and accelerated by the MHD
process near the compact objects.
• Flow is dominated by the Poynting flux (EM energy)
• No self-collimation: need external confinement
• External confinement is crucial for efficient jet
collimation and acceleration of Poynting dominated
outflows.
• Relativistic jet accelerates gradually => need
dissipation to convert EM energy of jets
• Magnetic reconnection is a key for the dissipation
Frontier of research
• Numerical simulations:
– GRMHD simulations are possible
– Additional physics: Resistivity, radiation, microphysics, …
– Effects of time-dependence, non-asymmetry (3D)
• GR radiation transfer calculation is a key tool to connect MHD
simulations and observations (Need correct radiation process
including particle acceleration)
• Observations: mm & submm-VLBI trying to observe BH
shadow & jet launching region (EHT, GLT, Black Hole Cam
projects)
BH shadow image (no jet)
a=0
Optically
thick
Geometric
ally thin
a=0.5
a=0.999
Takahashi et al. (2004)
Pu et al. (2014)
i=10 deg
i=85 deg
Optically
thin
Geometric
ally thick
i=85 deg
High resolution VLBI observations
Source
SgrA*
M87
Cen A
BH Mass (Msun)
0.045 x 108
66 x 108
0.45 x 108
Distance
0.008 pc
16.7 Mpc
3.4 Mpc
rS
11 mas
8 mas
0.3 mas
Size of shadow
52 mas
40 mas
1.5 mas
• In radio VLBI observation, angular
resolution ∝ l/D,
l: wavelength, D: baseline
• Resolution ~ 50 mas at l ~1mm
(300GHz) & D=4000 km
• In GLT project (with other sub-mm
array), D > 9000 km
=> 20 mas at 345 GHz
Submm-VLBI
GLT
SMA
ALMA
Download
Related flashcards

Astrophysics

46 cards

Astrophysics

65 cards

Create Flashcards