Tomohisa KAWASHIMA（NAOJ）

Yosuke MATSUMOTO（Chiba U.）Ryoji MATSUMOTO (Chiba U.)

A possible time-delayed brightening of the Sgr A*accretion flow

after paricenter passage of G2

Workshop "Challenges of AGN jets” @Mitaka

Introduction of Sgr A*

• Sgr A* should have a massive black hole with ~ 4 million solar mass, according to the observation of the orbit of the stars surrounding the Sgr A*.

• In multi-band observations, Sgr A* is bright, so that it is interpreted that the gas accrete onto the black hole.

• Sgr A* is a good laboratory to study accretion processes, since it has the nearest black hole accretion flow.

Various Types of Accretion Flows

1) hot accretion flow (ADAF, RIAF)• geometrically thick (H/R~1)• optically thin• hot (Te~10^9 K)• advective cooling dominant• gas pressure dominant

2) thin disk (standard disk, Shakura-Sunyaev disk)• geometrically thin (H/R<<1)• optically thick • cool (in stellar-mass BH, Te~10^7 K

and Te ∝ M^{-1/4))• radiative cooling dominant• radiation/gas pressure dominant

in the inner/outer disks3) super-critical accretion flow (slim disk)

• geometrically thick(H/R~0.5)• optically thick • warm (Te > 10^7 K in stellar-mass BHs)• advective cooling dominant • radiation pressure dominant

radiation

advection of gas internal

energyBH

radiationBH

radiation

advection of trapped photons

BH

mas

s ac

cret

ion

rate

and

lum

inos

ity

円盤回転軸

中心天体

降着円盤

円盤内部

磁力線

重力

角速度

遅い

角速度

速い 遠心力

角運動量輸送

rotation axis of the disk

central object

accretion disk

inside the disk

angular momentum transport

slow rotation

fast rotation

gravity magneticfield

centrifigal force

MRI in accretion disks• Magneto-Rotational Instability (MRI) Balbus & Hawley (1991)

• Due to the effects of the differential rotation of the accretion disks with their angular momenta increase outward, the MRI is driven and the B-flelds are amplified.

• The growth timescales of axisymmetric and non-axisymmetric modes of MRI are ~ 1 and 10 orbital period.

Dynamo action driven by MRI is essential for the energetics and dynamics in accretion flows!

Balbus & Hawley (1998)

Sgr A* accretion flow

• A hot accretion flow should exist, since the mass accretion rate should be very low:

while

• Synchrotron/bremsstrahlung emission is the dominant radiative process in the inner/outer accretion disks.

• It is interpreted that bremsstrahlung emission from the inner disk does not dominate X-ray emissions, while the Synchrotron emission in radio-band is in an opposite manner.

• No currently-active-jet has been reported.

• We have not yet well understood the global disk structure (e.g., the inclination angle of accretion disk, position of the inner edge of the accretion flow, etc.) as well as the micro physics of the relativistic plasmas.

X-ray image of Sgr A* in the Bondi radius scale (Roberts et al. 2017 )

G2 passing through the GC

• Gillessen et al. (2012) reported the discovery of G2, which was interpreted as a gas cloud approaching the Galactic center (GC).

• They estimated the mass of G2 to be 3 M_earth and the radius to be ~100AU. They also reported that G2 is expected to approach the pericenter at 2400 Rs from the SMBH in 2014.25 from the analysis of Br-γ line observed by Sinfoni (Gillessen et el. 2013).

• G2 has been attracted the astrophysicists because it may increase the mass accretion rate onto the SMBH and may trigger the flare.

Observation of L’ band (Gillessen et al. 2013)

But it has not yet shown the increase of luminosity…

Many simulations of G2 have been carried out

HD/MHD dimension wi accretion flow and outflow?

with radiative cooling?

Burkert et al. (2012) HD 2 No No

Anninos et al. (2012) HD 3 No No

Saitoh et al. (2013) HD(SPH) 3 No Yes

Sadowski et al. (2013) MHD 3 Yes No

Abarca et al. (2014) HD 3 (partly) Yes No

There is no long term numerical simulation of G2 taking into account the effects of MHD and the radiative cooling simultaneously.

We, therefore, carry out 3D-MHD simulations of the hot accretion flow onto SgrA* interacting with G2, taking into consideration the radiative cooling.(Kawashima et al. submitted to PASJ )

Examples of the G2 simulations

Models of G2(Pfuhl et al. 2015)

• It is still controversial whether or not G2 harbors a star(s) in its center.• Pfuhl et al. (2015) observed G2 in infrared band using SINFONI, and proposed that

G2 is a pure gas clump, which is possibly formed from a gas stream around SgrA*.

• On the other hand, Witzel et al. (2014) observed G2 in L’ band using Keck observatory, and proposed that G2 harbors a star because the size of the dust of G2 does not change during its presenter passage.

• The pv diagram of G2 (Br-γ line) shows that the gas is actually tidally disrupted by the supermassive BH at the Galactic center.

We assume that G2 is a pure gas cloud.

(N$, N', Nz) = (256, 128, 320)

Simulation Set-Up (1) Hot Accretion Flow onto Sgr A*

•　Cylindrical Coordinate•　Number of Grid Points：

• Initial Condition：- We set a rotating torus in Equilibrium state, embedded in a hot, non-rotating, and static corona.- Purely toroidal magnetic fields inside the torus are assumed. (The initial corona is non-magnetized.)

We set the G2 in the simulation box after 30 rotation periods at the maximum pressure (i.e., torus center).

i

Simulation Set-Up (2) G2

•　We set G2 at 24,000Rs away from the SMBH.

• We set the mass of G2 to be 1, 2 and 3M_earth.•　The density profile of G2 is assumed to be Gaussian distribution( we set FWHM=3x10^15 cm)•　The pressure is set to be the same as the background gas.• We assume that the pericenter of G2 is on the equatorial plane of the hot accretion flow, and the inclination i=0°, 30°, and 60°.

G2 + accretion flow with radiative cooling(The unit of the length is 3000Rs.)

The magnetic energy amplification

• The magnetic field is amplified due to dynamo ~5 yrs after the pericenter passage.

the region of the lower panels

(The unit of the length is 3000Rs.)

The simulation movie of the model with i=60

i

mass density magnetic energy density

The time evolution of the magnetic energy

• The magnetic energy increase up to ~4 times larger than that w/o G2 collision ~5 and ~10 yrs after the presenter passage for i=0 and i=60°, respectively.

• The increase of the magnetic energy may be observed via the synchrotron emission in the radio band after 5-10 yrs.

pericenter passage

The time evolution of direction of angular momentum

• The accretion disk is disturbed by the G2 impact. For i=π/3, the rotation axis of the disk attains the quasi steady state ~5 yrs after the pericent passage of G2.

• → The B-field amplification is delayed in i =π/3 case.

year5 100

10-7

10-6

The dependency of the time evolution of on the inclination angle

• increase 3-4 times larger than those before the interaction with G2.

• The increase of mass accretion rate may not affect the observation until 5~10 after the pericenter passage of G2.

M

M

pericenter passage

year5 100

↵

10-3

1

10

10-2

10-1

The time evolution of the α-parameter

• We define

• The α for the model i=60° increase after the ~5 yrs of delay. This time delay may be caused by the evanescent disarray of the accretion disk.

↵ ⌘< BrB' > / < Pg >

pericenter passage

Time lag for the increase of the magnetic energy in the inner region

• We assume the amplified B-field at ~1000Rs accretes to inner disk region in the timescale of viscous accretion.

• 1-10 year after the increase of magnetic energy at ~1000Rs (i.e., totally ~10 years after the pericenter passage of G2), the increase of luminosity in the Radio band is expected.

Tvis =R2

⌫=

R2

↵csH

R⇠H=R

↵c(Rs/R)1/2

⇠ 30↵�1r3/2[sec] r ⌘ R/Rs

⇠year(↵⇠0.1, r⇠1000)

• We have carried out the 3D-MHD simulations of the accretion flow onto SgrA* interacting with G2, taking into consideration the effect of the radiative cooling.

• The increase of mass accretion rate during the pericenter passage could be negligible if the mass of G2 is less than ~M_earth, because SgrA* is bright at larger radius in X-ray.

• The B-field is amplified 5-10 yrs after the pericenter passage of G2. The luminosity may increase in the radio band after ~10 yrs. After the amplification of B-field, slightly bright X-ray flares might be observed.

• We have to make it clear how significantly the model of the Sgr A* accretion flow affects the amplification of the magnetic energy after the pericenter passage of G2. More parameter studies are needed!!

Summary

• We hopefully think that the increase of B-field may be observed by the future VLBI observations!

appendix

0 5 10 15year

10-5

10-4

10-3

10-2

Mag

netic

Ene

rgy

0 5 10 1510-5

10-4

10-3

10-2

Mag

netic

Ene

rgy

year

The time evolution of the magnetic energy

• The magnetic energy increase up to ~4 times larger than that w/o G2 collision ~5 and ~10 yrs after the presenter passage for i=0 and i=60°, respectively.

• The increase of the magnetic energy may be observed via the synchrotron emission in the radio band after 5-10 yrs.

pericenter passage pericenter passage

The effects of radiative cooling on the time evolution of

w/o radiative cooling

with radiative cooling

M

Collapse of gas induced by the radiative cooling →cross section of the gas could decreases → the angular momentum transport via the rum pressure becomes less effective → mass accretion rate is lower than the model w/o radiative cooling (This mechanism is the same as Saitoh et al. 2013)

pericenter passage

CANS+ New public MHD code

• HLLD (Miyoshi & Kusano 2005)

• MP5 (Suresh & Huynh 1997)

• 3rd order TVD Runge-Kutta• Divergence Cleaning (Dedner et al. 2002)