Magnetic ﬁelds at the base of AGN jets:
the case of M87
AGNジェット最深部の磁場：M87の場合

紀 基樹(Kogakuin U/NAOJ)
2017/12/21

星形成と銀河構造に置ける磁場の役割@ Kagoshima U

Contentsl Introductionl Black hole and jetl BZ processl Closest look of “central engine”

l Energetics of “central engine” in one-zone
framework(MK+ 15)l Beyond one-zone(Kawashima, MK in prep)

IntroductionBlack Hole (BH)lBH as a vacuum solution ofEinstein equation is well-deﬁned.
lBH as as an astronomical object
is full of mystery.

BH: big issues
lDo BHs really have event horizon
and spin?
l 「事象の地平面」はまだ直接観測されていない。
l 「スピン」についてもクリアな制限がついていない。

lHow BHs produce jets/outﬂows?
l central engineの仕組みはまだ理解されていない。宇宙物理学屈指の難問。
l ブラックホール噴流が星銀河形成へ本質的影響を与えていると目される(AGNフィードバック)が、素過程は分かっていないことが多い。

Jet engine

https://ja.wikipedia.org/wiki/ジェットエンジン

空気を吸い込み、熱を加え、動力を取り出す。
well-known heat cycle

BH jet engine?!

?

Cygnus A(c) NRAO

BHが本質的な関与していると目される。
しかし、駆動エンジンの仕組みがよく分かっていない.

Blandford & Znajek (1977) proposed
the idea of spinning BH can drive a jet via
magnetic-ﬁeld.

BZ processl EM extraction of BH-spin energy

Ω_F
B-ﬁeldΩ_BH > Ω_FBH

Ω_BH

Is the BZ77 really in action?
l ~2005年以降のGRMHD数値実験
では、一見もっともらしいBZ駆動のジェット噴流を形成しているようにみえる。
l しかし、実際の天体の観測と比較してBZ機構をテスト(初期条件/境界条件)するという視点の研究はまだほとんど行われていない。

Best example: M87
lThe angular size of BH in M87 islargest among all of AGN jets!
n Schwarzschild radius: Rs = 2GM/c2 = 1.9×1015 cm
n Angular size of Schwarzschild radius: θs = 7 μas

lDirect observation of “centralengine” is possible w/ VLBI!

Jet base image of M87
140 μas ~ 20 Rs

radio core
τssa= 1

Hada, MK, Doi et al. (2013), ApJ

Closest look of“central engine”

Kino et al. 2015, ApJ, 803, 30

Energetics of “central engine”
w/ one-zone frameworkOutstanding questionl Need to clarify energy source of“central engine”
l B-ﬁelds?, particles?, radiation?, BH-spin?

l Observed synchrotron emission[B-ﬁeld strength] ×[particle density].
l How to resolve the degeneracy?

?

eeB+γ p e-Idea: usage of SSA-thick radio core!

radio core

τssa= size*αν_ssa = 1
Co-eﬃcient for Synchrotron Self Absorption (SSA)Kino+ 14, 15, ApJ

We can uniquely determine B and Ue/UB
radio core

θ obs: observed angular size of the radio core <= VLBI!
S_νssa : observed ﬂux density of the radio core
νssa: SSA turnover frequency (here νssa= νobs, see next)

Hada, Doi, MK+ (2011) Nature

Striking evidence of SSA-thick core!
i.e., core shift

843GHz 2415

BH is here!
EHT emission region
θFWHM ~ 40 μas

One-zone (θFWHM=40μas, 1Jy) estimate leads toBtot~ 300 gauss i.e., too large L_poy…
If the ﬁeld strength is,

θ_FWHM * 1.8 =72 μas?

The 1.8 factor by Marscher (1983)

then the Poynting power below exceeds L_jet, max ~ 5*1044 erg/sSolution: Partially SSA-thick (two-zone)
EHT region

SSAthickSSAthin• The idea of partial-SSA-thick region can avoid
too-large-L_poy problem because B νssa ^5.
• BH-shadow may be hidden by SSA-thick region.

Kino et al. 2015, ApJ, 803, 30

Two zone ﬁt to the early EHT data

With phased-ALMA &USA stations, S_thickis testable！

SSA-thick
21μas, 0.27J
yKino+ 15

Kino+ 15

Allowed log (U±/UB), Btot , γ ±,min in the SSA-thick region

U±<<UBA remaining issuel What if General Relativistic (GR)eﬀects signiﬁcantly violate thisone-zone approximation?

Kawashima, MK in prep

Beyond one-zoneEvent Horizon Telescope (EHT)
EHT is a project to assemble a VLBI network of230 GHz wavelength dishes that can resolve GRsignatures near a SMBH with spatial resolutionof ~20 μas!

(c) Nature 2017 Future News

Primal goal of EHT
The primal goal of EHT isimaging BH shadow(~photon ring w/ diameterof ~5 Rs) of Sgr A* and M87.

(c) Akiyama

What is the photon-ring ?
Bardeen 73, Luminet 79

reﬂected light艶消し

courtesy: Luminetreﬂected lightcourtesy: Luminetphoton ring!

courtesy: Luminetphoton-ring (~BH-shadow)
w/o and w/ spin

slide by Pu HYBH shadow + τssa = ?
Kawashima, MK in prep

Arising question:R_photon-ring vs R_ISCOl For higher BH-spin, R_photon-ring (~5 Rs)> R_ISCO realizes. Then, the photon-ringwould be partly smeared out due to SSA.
l Most of previous work seems to focus on
fully SSA-thin case at 230GHz (e.g.,Brodelick & Loeb 09). It may not be thecase for M87 (Kino+15).

?

GR radiative transfer codeby Kawashima-sanl Basic Scheme:l Ray-tracing: based on Schnittman & Krorik 13,
solving r, θ, φ, p_r, p_θ.
l Radiative transfer: based on Dolence+09, Monte-Carlo method for ICl Kerr Metric w/ Boyer-Lindquist coordinate

Test runs are well consistent with previous work
(Bardeen 73, Luminet+77, Chan+12 and Pu+16).

Setting (1/2)
l As a ﬁrst step, we go witha simple disk model
without jet to avoid “jetcontamination” in BHshadow images.
l disk thickness:h = H/R = 0.1(H: scale height, R:cylindrical radius)
We mimic fast cooling.

geometrically-thin disk
w/ cooling
Machida+ (2006)

Setting (2/2)
l ρe and Te r^{-p}
For ρe: p = 1.1
For Te: p = 0.84
(e.g., Pu+2016)l plasma beta = 0.1
l r_out = 500 GM/c^2l high BH-spin: a=0.998

Kawashima, MK in prep

BH shadow in M87 at 86 GHzThis BH-shadow is not “photon-ring”
because the disk’s inner edge(<R_photon_ring) is SSA-thick and it
smears out the photon ring.

Hada, MK, Doi+ (2016) ApJ

Actual VLBA+GBT obs. at 86GHz:
measured core size (θmaj, θmin)

Many thanks to GBT!
Green Bank
Telescope

Kawashima, MK in prepτssa distribution at 86 GHz!

Diameter of tau_ssa=1 (narrow gray region) is ~ 20
Rg ~ 70 μas well agrees the size of radio core at86GHz!

Kawashima, MK in prep

Something at 130 GHz?

something?

Kawashima, MK in prep

New structure at 230 GHz“dark crescent”!

“dark crescent”!

Kawashima, MK in prepτssa distribution at 230 GHz!
photonring

SSA-thickregiondark crescentKawashima, MK in prep

photonring

SSA-thickregiondark crescent

A new manifestation of high BH-spin!

Summary
Clarifying energetic of “central engine” is essential toresolve BH-jet formation mechanism.
l U_B dominance in one-zone SSA-thick region (MK+15).
l We start beyond-one zone description via BH-shadow.
l Inclusion of SSA and GR predict a dark crescent in BHshadow when high BH-spin. (Kawashima, MK in prep)