Transcript Central Black Hole as a Source of Gamma-Rays and the Knee Cosmic Rays Collaborators : K S Cheng The University of Hong Kong D.

Central Black Hole as a Source of Gamma-Rays and
the Knee Cosmic Rays
Collaborators :
K S Cheng
The University of Hong Kong
D. O. Chernyshov
Lebedev Institute, Russia
W H Ip, C M Ko
NCU, Taiwan
V A Dogiel
I.E.Tamm Theoretical Physics Division of
P.N.Lebedev Institute of Physics, Russia
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
Recent discovery of a pair of giant Fermi
Bubbles in the Galactic center (GC) is
one of the most remarkable events in
astrophysics, which may change our
view on the origin of cosmic rays.
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Fermi Lab
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Fermi instruments
Large Area Telescope (LAT):
• 20 MeV - >300 GeV (including
unexplored region 10-100 GeV)
• 2.4 sr FoV (scans entire sky every
~3hrs)
Gamma-ray Burst Monitor (GBM)
• 8 keV - 40 MeV
• views entire unocculted sky
• Large leap in all key capabilities, transforming our knowledge
of the gamma-ray universe. Great discovery potential.
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First Fermi-LAT Catalog
1,451 sources
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The >1 GeV Sky
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Gas Distribution in the Galactic Plane
Atomic
hydrogen
Molecular hydrogen
IR
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Gamma-Radiation of Protons
I  (E  , l ,  ) ~
E dr n g (r ) (E p , E  )N p (E p , r )
p
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Background Photons
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Gamma-Radiation of Electrons
Inverse Compton Radiation
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Diffuse Emission at Low to High Galactic
Latitudes as Calculated by Strong et al
Pion-decay and
inverse Compton
emission are two
dominant
components –
Low latitudes
allow us to probe
the average CR
proton and
electron spectra
along the line of
sight
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High latitudes
Abdo+’10
Mid-latitudes
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II. Subtracting from the total galactic gamma-ray emission contributions of
point-like sources and the component proportional to the gas column density (
) Dobler
et al. (2010) and then Su et al. (2010 ) found two giant gamma-rays
0
bubbles
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Семинар Отделения теоретической физики
13
The Residual Gamma-Ray Map of Su et al. (2010)
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

They identified two large gamma-ray bubbles at 1<E <50
GeV which had approximately uniform surface
brightness with sharp edges.
The gamma-ray emission associated with these bubbles
has a significantly harder spectrum (dN/dE ∼ E−2) than
the inverse Compton emission from electrons in the
Galactic disk
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A giant structure in gamma-rays in GC (Su et al. 2010)
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Fermi Bubbles in Gamma, Radio and X-ray Ranges
The total energy in the bubbles W~1054-1055erg
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Black Hole at the center of Milky Way –
Mass = 3.7x106 solar masses!
MPE / R. Genzel et al.
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Past GC Activity
indirect evidences only
 Electron/positron
 6.4 KeV iron line
pair annihilation
 Thermal
emission from hot gas
 Fermi Bubbles
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Stellar Capture. Energy Release Stage
RX J 1242.6-1119A (Komossa et al. 2004)
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Star capture (Diener et al. 1997, Ayal et al. 2000,
Alexander 2005).
• Passing the pericenter, a star is tidally disrupted into a very long
and dilute gas stream.
A half of the star matter (i.e. ~ 1057 protons when a one solar
mass star is captured) escapes with a subrelativistic
velocity.
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Energy Release from Tidal Disruption
(see e.g. Alexander, 2005)
2
Wout
1
1/3
 M    R   m / M  
52
4  10 erg 
 
 

6
M
R
10


 
 
 b 
 0.1 


2
rp
b
rt
r p  radius of periastron
r p  radius of tidal disruption
Eesc  68 (b / 0.1)2 MeV/n
•
A total tidal disruption of a star occurs when the penetration parameter b<1.
The tidal disruption rate  can be approximated to within an order of magnitude
from an analysis of star dynamics near a black hole via the Fokker-Planck
equation. For the parameters of the GC it gives the rate  ~10-4 years-1 (see the
review of Alexander, 2005).
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Expansion of the bunch. Hydrodynamic Stage I
Particle mean free path.
For the time 105 -106 s the plasma becomes
collisionless
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Kinetic stage I of the bunch evolution (Ginzburg et al.
2004)
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Electron and proton momentum
distribution
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Average energy release F ~Wout  1040 - 1041 erg/s.
Kinetic stage II – Plasma Heating
protons
Coulomb losses
Bremsstrahlug
losses
For Ep=100 MeV
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Gas heating.
Thermal Xrays
Hard X-ray
emission
Ex<70 keV
GC Thermal Emission
(Koyama et al. 1996-2009, Muno et al. 2004)






T ~ 108 K
L2-10 ~ 2x1036ergs/s
Size ~ 50pc x 30pc
nave ~ 0.1cm-3
npeak ~ 0.4cm-3
Egas ~ 3x1052ergs
• The source of energy with an output ~1041erg/s is required!!!
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Two recent super-Eddington events - Swift 1644+573 and
Swift J2058.4 + 0516
Sw1644+57
M~10^6 solar masses
Sw2058+05
M<10^8 solar
masses
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Shock Generation
Single ( 
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ion
 cap
) or Multi-Shock ( 
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ion
 cap
) Structure.
Solution for Continuous Energy
Injection (Weaver et al. 1977)
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Multi-Shock Structure
Solutions of Kompaneets (1960) for a shock in the
exponential atmosphere
v
v 1 
v

0
t
r  r


v
v
v

2
0
t
r
r
r
 
 


v
p

0
 t

r 


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
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Acceleration by shock waves
U(x)
U1
U2
x
Jump of velocity + spatial diffusion

x
u 1   3 f 
 f

D

u
(
x
)
f


p

 x

2
3 p p  p 


u1
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3u1
u1 u 2
For strong shocks
(M>>1) u1/u2=4
u2
D/u1
f p

f p
D/u2
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4
Electron Acceleration by Shocks
(Cheng et al. unpublished)

x
u 1   3 f 
 f
 1   2 dp f 
D

u
(
x
)
f

p



p

 x
 p 2 p 
2
dt

p
3
p

p

p






dp  dp 
 dp 


dt  dt synch  dt IC
dp
e  
dp
dt
e  D  e
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Spatial distribution of relativistic electrons
accelerated by the GC shocks
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Spatial Distribution of IC
Radiation
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IC Radiation of Accelerated
Electrons
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Cosmic Rays (CRs) in the Galaxy
Cosmic Rays = energetic nuclear particle component, impinging on Earth’s
atmosphere from ~ uniform population in the Milky Way (Electrons ~ 1% )
 Energy spectrum over ~
11 decades
Single power law ∝ E – 2.7 below ~ 3 x 10 15
eV (“knee”). Energy density in Galaxy
beyond knee negligible ( ~ 10 – 3 of total )
 Source spectrum below “knee” ∝ E – 2.0
to E – 2.1, very hard ~ equal energy/decade
 Total energy density Ec ~ 1 eV/cm3
~ (BISM)2 /8 ~ EturbISM ~ EturbISM

Energy input rate into CRs  10 41 erg/s
 Cosmic Rays = nonthermal relativistic
gas of high pressure in Galaxy
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Proton acceleration by SNRs

Bohm diffusion (at shocks)
mc 2 DB (E max )
DB ~ cRL , RL 
,
~ T SN
2
eH
u

Maximum energy of the accelerated particle
In the Bohm approximation Emax = 1014 eV (see e.g.
Berezhko and Voelk, 2000)
 Multi-shock acceleration in OB associations?
(see Bykov and Toptygin, 1993)

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
Processes of particle acceleration by the
bubble shocks in terms of the released
energy, size of the envelope, maximum
energy of accelerated particles, etc. may
differ significantly from those obtained
for SNe.
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Particle Acceleration by Supersonic
turbulence (Bykov, Fleishman, Toptygin)

Parameter of acceleration
Acceleration length  u
uL
, where u  the shock velocity

L  the space between shocks
  the diffusion coefficient of CRs
 
  1
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Diffusion Model of CR propagation in the Galaxy
(Ginzburg and Syrovatskii 1964, Berezinsky et al. 1990)
electrons
protons
f f
1  2 
f  1 
f 

  2
p  B ( p , r )f  A (p , r )   2 r 2 U (r )f  D (r , p )   Q (p ,t )
t T p p
p  r r 
r 

  dp   dp 
 dp  
B (p )  p 2      
  
  dt   dt 
 dt br 
i
synIC

f
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
0
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A giant structure in gamma-rays in GC (Su et al. 2010)
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Proton multi-shock acceleration in the Bubble
(Cheng et al. accepted in ApJ)
 Stochastic acceleration

Diffusion coefficients

Shock separation

Maximum energy before escape
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The CR spectrum near the Earth
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Distributionof the SNR and Bubble cosmic rays in the Galaxy
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The spatial distributions of seed and reaccelerated CRs in the disk are quite
different. In principle, CR distribution can
be derived from gamma-ray data.
 If the diffuse gamma-ray data at E >
1015 eV were available, the gradient test
would be a nice tool to investigate
possible proton sources in this energy
range and might lend support to our
model.

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Conclusion
Stellar capture processes may be
responsible for many high energy
phenomena around GC
 The gamma-ray emission from FB is
due to IC and radio-to-microwave due to
synchrotron radiation
 The spatial distribution of emission is
due to multiple shocks
 The FB’s shocks can accelerate protons
above the ‘knee’(>1015eV)

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