Sgr A*の過去の活動を起源とする銀河系中心からのガンマ線放射

Yutaka Fujita (Osaka)
Shigeo S. Kimura (Tohoku)
Kohta Murase (Penn State)
Physical Review D 92, 023001 (2015)

Contents• Particle acceleration in Low-luminosity
active galactic nuclei (LLAGNs)– Radiatively ineﬃcient accretion ﬂows
(RIAFs)– Sagittarius A* (Sgr A*) and Central
Molecular Zone (CMZ)• Diffusion of cosmic ray (CR) protons inthe CMZ• Gamma rays from the CMZ• SummaryCR acceleration in RIAF
• Accretion flows for LLAGN are thought tobe RIAFs– Hot and tenuous– Coulomb collision is inefficient (collisionless)• Insufficient thermalization → particle acceleration
Wind, Jet

Hot, Tenuous Plasma
BHCR acceleration in RIAF
• Particles are stochastically accelerated
by turbulence in RIAF
– pγ and pp-interactions in the RIAFs may be
responsible for production of the neutrinos
detected by IceCube (Kimura, Murase, &Toma 2015; A16a 講演)
νp

BHγ pγ
νppγ
γ

ν

pγpγ
ν

Sgr A* and CMZ• Sgr A* is the supermassive black holeat the Galactic center– It is a LLAGN and has a RIAF• CRs may be accelerated– Surrounded by molecular gas (CMZ)• Mass of CMZ is ~ 107 M ŸSgr A*CMZCMZRIAF~200 pc

CR protons in CMZ• Some of the protons accelerated in theRIAF should plunge into CMZ– They produce γ-rays through ppinteractions– We calculate the diffusion of CR protons inthe CMZ and the γ-ray emission
γγ

pp+pp+pCMZ

CMZ

p+pp+pγγpγ-rays from the CMZ• HESS observation of the Galacticcenter– CMZ radiates γ-rays

Color: γ-rayContour: molecular gas(Aharonian+ 06)

Model• We solve diffusion equation– Spherically symmetric• We ignore the region perpendicular to the disc

– Diffusion coefficient (κ)• Typical Galactic value (Gabici+ 09)• B ~ mG in CMZ– Source (Q)• CRs accelerated in Sgr A* (Kimura+ 13)• Only a tiny fraction of CRs plunge into the CMZ
λ ～10-3

Spectrum of CRs• The typical energy of CRs accelerated in aRIAF is determined by the condition oftacc = tdiff,RIAF• Functional form
– 2nd Fermi acceleration (Becker+ 06)
Kν: Bessel func.

• Normalization

Results• Current accretion rate on Sgr A* is verysmall. . .
m = M/MEdd = 4.2 × 10-6.
• If m is constant, γ-ray luminosity ismuch smaller than the Fermi and HESS
observations
Fermi
HESSSpectrum
Red: γ-ray
Blue: neutrino

Past activity of Sgr A*• Observations have indicated that Sgr A*
was much more active in the past
– Koyama+96, Murakami+00, Totani+06,Ryu+13.

• m ~ 0.001(t 100 yrs ago)

• TeV γ-ray
observations
can be explained– GeV emission has
another originUncertainty• Since our model is rather simple, there
are uncertainties in parameters– Increase the typical energy of protons
assuming more efficient acceleration– CTA can discriminate
the models

History of CR Acceleration• In our model, γ-rays from the outer CMZ
are radiated by older CRs– It takes a longer time for the CRs to diffuse• Spectral variance of γ-ray spectrum
may reflect the history of CR
acceleration inSgr A*– Change of typicalenergy of CRs

Other galaxies• Centaurus A– Nearby radio galaxy (3.84 Mpc)
– TeV γ-ray emission can be explained if theAGN is well covered by molecular gas• λ ~ 0.02(by ESO)

Summary• LLAGNs may accelerate CRs and may be
the source of neutrinos detected byIceCube– If so, Sgr A* should produce a lot of CR
protons– Some of them should enter the CMZ andgenerate γ-rays• We solved diffusion equation of the CRs
and calculated γ-ray spectra– The results are consistent with TeV γ-ray
observations if Sgr A* was active in the past