Role of large scale magnetic fields in AGN jets

26 Pages·2004·7.27 MB·English

by  Maxim Lyutikov

Role of large scale magnetic
fields in AGN jets
Maxim Lyutikov(UBC)
in collaboration with
V. Pariev (Rochester)
D. Gabuzda (Cork)
R. Blandford (Stanford)
What carries bulk of energy in
ultra-relativistic jets?
Prime mover
Ultra-relativistic jets
(cid:190) Ions
(cid:190) Pulsars, Γ~106
(cid:190) Pairs
(cid:190)AGNs, Γ~ 10-30
(cid:190) Large scale B-field
(cid:190) GRBs , Γ~100-1000
1 1 1
B ~ ⇒ B 2 ~ ; ρc 2 ~
(cid:190) Microquasars , Γ~2
ϕ r ϕ r 2 r 2
2
(cid:190) X-ray binaries , Γ~10 B
ϕ
σ = ~ const
8π Γρc 2
(cid:190)What is σin AGN jets?
(cid:190)Πof parsec scale AGN jets imply that σis
non-negligible, σ≥1, and may be >> 1
Jet launching: large scale B-field
Koide et al. 2002,
(cid:190)Jets are launched and
collimated electromagnetically
(Lovelace et al., Blandford & Znajek,
Blandford & Payne, Camenzind,
Fendt; Koide, Shibata and others).
(cid:190) Numerical simulations: McKinney & McKinney & Gammie 04
Gammie: “low density polar regions of
the numerical models agree well with the
Blandford-Znajek model “.
Similar results by Hawley &Krolik
Π in AGN jets
(cid:190) Π at pc scale is produced at internal shocks by compression of
random B-field (?) (Lang 80)
(cid:190)Is Πat pc scale consistent with large scale B-field?
(cid:131) in radio: synchrotron
(cid:131) consider optically thin regions
Synchrotron emission by relativistic
sources: Lorentz transformation of Π
(cid:190) In plasma frame (cid:190) In laboratory frame
B'
e' n× q
e =
2 2
q − (n• q)
n'
∧ ∧
q = B+ n× (v ×B)
e' B', n'
┴
p +1
n ~ γ−p,Π = ∧ ∧
max (e • B) = (e × n)( B× v) ≠ 0
p + 7 / 3
Both B-field and velocity field are important for Π
(Blandford & Konigl 79; Lyutikov,Pariev,Blandford 03)
Π from relativistically moving shell with
helical B-field
Γ=2, ψ’=π/4, θ =π/3
ob
B
'
B B
1 1
' ϕ ϕ
tanψ = = tanψ =
'
B Γ Γ B
z z
Π from relativistically moving source
(cid:190)Bnot orthogonal to e
(cid:190) Observers: always plot e, not “inferred” B-field
(cid:190) One needs to know v-field to infer B-field from e-field
(cid:190)symmetry of the system(e.g. axial) and assumptions
aboutv-field (e.g. sheared cylindrical motion with v (r)) may
z
still provide information about intrinsic B-field
Π from cylindrical shell
(cid:190) Π depends on p
Π along the jet
(cid:190) Even co-spatial populations
with different p may give
different Π (eg radio & optical)
'
B
tanψ' = ϕ
Π across the jet
'
B
z
Γ=10, p=1, different rest frame (emissivity –averaged) pitch angles
Large scale B-field in AGNs
(cid:190) Bimodial distribution of PA (cid:190) For cylindrical jet U=0,
average Π along or across the
axis
(Aller et al)
(cid:190) PA follows the jet as it bends
(cid:190) For fixed ψ, Π mostly keeps
BL Lac 1749+701
its sign
(Gabuzda 03)
(cid:190) Sometimes a change does
(cid:190) Sometimes a bend gives 90
occur
change of PA
Resolved jets: B-field, E-field, v-field and
emissivity profiles
(cid:190) Relativistic jet structures: cylindrical force-free equilibria
E × B
(cid:190) E-field may be important: rotation: Bφ, Bz, Er: Ω = r z
2 2
r(B + B )
1
z φ
2 2 2 2
B = ∂ (B + B − E )
z r z ϕ r
2r
(cid:190) Need to specify force-free equilibria, β (r)
z
(cid:131) Diffuse pinch
(cid:131) Zero poloidal flux pinch
(cid:131) Multiple reversal, B~J(ar)
(cid:190) Emissivity κ(r)~ j2, B’2