Diffuse TeV gamma-rays
from the Galactic centerand Sgr A*
Yutaka Fujita (Osaka)
Shigeo S. Kimura (Tohoku)
Kohta Murase (Penn State)
Physical Review D 92, 023001 (2015)	

Contents	
• Cosmic-ray (CR) acceleration in Low-luminosity
active galactic nuclei (LLAGNs)– Radiatively inefficient accretion flows (RIAFs)– Sagittarius A* (Sgr A*) and Central Molecular Zone(CMZ)• Sgr A* is an LLAGN• Diffusion of CR protons in the CMZ• Diffuse TeV gamma rays from the Galacticcenter• Sgr A* as a PeVatron (preliminary)• Summary	CR acceleration in a RIAF	
• Accretion flows for LLAGNs are thought
to be RIAFs– Hot and tenuous– Coulomb collision is inefficient (collisionless)• Insufficient thermalization → particle acceleration
Wind, Jet	Hot, Tenuous Plasma	
BH	CR acceleration in RIAF	
• Protons are stochastically accelerated
in turbulence in a RIAF
– pγ and pp-interaction in the RIAFs may be
responsible for production of the neutrinos
detected by IceCube (Kimura, Murase, &Toma 2015)	
ν	γ	p	p	

BH	γ	γ	ν	p	

ν	p	γ	

γ	 p	 p	γ	ν	IceCube Observations	• Uniform distribution of neutrinos– Neutrinos have extragalactic origin	

IceCube home page	5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 !	
Sgr A*	CMZ	CMZ	RIAF	~200 pc	

CR protons in CMZ	• Some of the protons accelerated in theRIAF should plunge into the CMZ– They produce γ-rays through pp-interaction– We calculate the diffusion of CR protons inthe CMZ and the γ-ray emissionγ	γ	p	Sgr A*	p+p	CMZ	p+p	CMZ	p+p	p+p	γ	

γ	p	Diffuse γ-rays from the CMZ	• HESS observation around the Galactic center(GC)– CMZ radiates γ-rays• CRs have diffused from the GC (HESS collaboration 2015)– We investigate this TeV γ-ray emission 	

GC	

Color: γ-ray
Contour: molecular gas(Aharonian+ 06)	

Model	• We solve a diffusion equation– Spherically symmetric• We ignore the outflow perpendicular to the Galactic disc

– Diffusion coefficient (κ)• Typical Galactic value (e.g. 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 Injected at Sgr A*	
• The typical energy of CRs accelerated in a
RIAF is determined by the condition oftacc = tdiff,RIAF• Functional form
– 2nd Fermi acceleration (Becker+ 06)
Kν: Bessel func.	

• Normalization

Spectrum of CRs Injected at Sgr A*	
• RIAF parameters determine the CR
spectrum– Accretion disc size, magnetic fields,turbulence ….
– We chose these RIAF parameters so thatthey are consistent with the IceCube
observations (flux, spectrum)• IceCube neutrinos are coming from numerous
LLAGNs in the Universe– Kimura et al. (2015)

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	HESS	Spectrum
Red: γ-ray
Blue: neutrino	Past activity of Sgr A*	
• Observations have indicated that Sgr A*
was much more active in the past– Sunyaev+1993, Koyama+96, Murakami+00, Totani+06, Ryu+13.	

• m ~ 0.001(! 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
models withdifferent
parameters	History of CR Acceleration	
• Current γ-ray spectrum of Sgr A* does
not have to coincide with that of theCMZ– γ-ray spectrum of the CMZ reflects the past
activity of Sgr A*PeV cosmic rays	
• Supernova remnants (SNRs) are often
thought to be the source of CRs for E <1015.5 eV– PeV=1015 eV• However, observational evidence isscarce for E!!1014 eV– Cutoff at E !1014 eV	Cutoff	

Kappes et al. (2007)	

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 Central
Molecular Zone and generate γ-rays• We solved diffusion equation of the CRsand calculated γ-ray spectra– The results are consistent with TeV γ-rayobservations if Sgr A* was active in the past	Summary	• Sgr A* may be providing PeV CRs
observed at the Earth– The problem of anisotropy must be solved