Transcript High Energy Astrophysics

Cosmic Rays
High Energy Astrophysics
[email protected]
http://www.mssl.ucl.ac.uk/
5. Cosmic rays: Primary and secondary Cosmic
Rays; Chemical composition; Energy spectrum;
Isotropy; Origin of CR, Primary Gamma-rays
[2]
2
Cosmic Radiation
Includes –
• Particles (2% electrons, 98% protons and atomic nuclei)
• Photons
• High energies ( 109 eV  E  1020 eV)
• Gamma-ray photons from high energy particle collisions
• Surprisingly there are many unanswered questions
3
Astrophysical Significance of
Cosmic Radiation
• Where do CR particles come from?
• What produces them and how?
• What can they tell us about conditions along
the flight path?
• ‘Primary’ CR can only be detected above
the Earth’s atmosphere.
4
Primary and Secondary CR
• Magnetic fields of Earth and Sun deflect primary
cosmic rays (especially at low energies).
• Only secondary particles reach the ground - and
they can spread over a wide area of ~ km2
• Extensive air showers can deposit up to 10
particles/km2 - good because high energy primary
particles are rare!
10
5
Development of Cosmic Ray Extensive Air Showers
• Incoming primary cosmic ray particle, proton or
heavier nucleus, interacts with an atmospheric nucleus
• Disintegration products are:
→ Neutrons and protons that cause a
nucleonic cascade at the core
→ p mesons that cause an outer electromagnetic cascade
• Primary gamma-rays undergo pair production
to cause an electromagnetic cascade only
• Secondary particles spread over a wide area
with ~ 1010 particles/km2
• Largest array, the Pierre Auger system in
Argentina, will have 1600 Cerenkov detectors
on an area of 3000 km2
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Detecting Cosmic Rays
• Scintillation counters
• Cerenkov detectors
• Spark chambers
• Large detector arrays are constructed
on the ground to detect extensive air
showers.
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Cosmic rays (cont.)
• Features of interest are:
- Chemical composition
- Energy spectra
- Isotropy
- Origin
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Chemical Composition
• Cosmic abundances of the elements in the CR and the local
values plotted against nuclear charge number
• 70 < E < 280 Mev/Nuc
° 1 < E < 2 GeV/Nuc
◊ Solar system
a) Relative to Si at 100
b) Relative to H at 1012
Solar System
CR
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Light element abundance
• Overabundance of Li, Be and B due to spallation medium (C, N, O) nuclei fragment in nuclear collisions;
remains are almost always Li, Be or B.
• Quantitative analysis is complicated; requires collision Xsections for various processes and relative abundances
seem to change with energy.
• However:
Abundance – weighted
formation probability (mbarn)
Li
Be
B
24
16.4
35
Measured CR abundance
(Si = 100)
136
67
233
- while mean path that medium elements must pass through
to create observed (Li, Be, B) abundances is ~ 48 kg/m2
10
which is similar to the galactic mean free path
Cosmic Ray lifetime in Galaxy
• CR mean free path through galaxy is
~ 50kg / m
- however all high-mass particles break up.
• Assuming particles of v ~ c traverse a path of :
50kg / m   .c.t
2
n  10 m
6
3
 27
m p  1.67 10
kg
t  10 s  3 10 yrs
14
6
in disc.
11
2
Escape from the Milky Way
• Lifetime could be 10 or 100x larger in the
Galactic halo where the density is lower.
• Note - galactic disk thickness ~1kpc,
=> 3000 years for particles to escape at ~ c
• BUT the magnetic field would trap them
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Energy spectra of particles
Log Particle flux
m-2 s -1 ster-1 eV-1
3 Z 5
M: 6  Z  9
H: Z  10
L:
-6
H
-12
P
a
M
L
-18
6
- this is a differential
spectrum N(E) dE = kE-x dE
- sometimes use integral
spectrum N(>E) = kE-x
9
12
Log Energy (eV per nucleon)
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Integral spectrum of primary CR
Integral spectrum:N ( E)  kE
x
Log N(>E)
m-2 s-1 ster-1
0
-4
N ( E )  E
12
1.5
E
-8
-12
-16
N(>E) is
number of
particles with
energy > E.
2 . 5
E
1 . 5
??
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16
18
20
Log E (eV)
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Cosmic Ray Isotropy
Anisotropies are often quoted in terms of the
parameter d:
I max  I min
d
100%
I max  I min
where I max and I min are the minimum and
maximum intensities measured in all directions.
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Isotropy (cont.)
• So far, experimental results indicate only small
amounts of anisotropy at low energies, with d
increasing with E.
• Below E ~1010 eV, solar modulation hides the
original directions.
• For higher energies, direction of maximum
excess is close to that of the Local Supercluster
of Galaxies.
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Isotropy Table
Log E (eV)
12
14
16
18
19-20
d(%)
~0.05
~0.1
~0.6
~2
~20+
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Isotropy and magnetic fields
At low energies, magnetic fields smear original
directions of particles, e.g. 1014 eV protons in an
interstellar magnetic field of
(m)v 2
 evB
r
and
11
10
Tesla:
v~c
(r = radius of curvature)
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Direction of low-E Cosmic Rays
m c
2
E
r = radius of
r

curvature
eBc eBc
14
19
10 1.6 10

m
19
11
8
1.6 10 10  3 10
16
 3 10 m
= 1pc or << distance to Crab Nebula
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Thus ‘information’ about the original
direction would be totally lost.
At higher energies, particles should retain
more of their original direction (r increases
with E), but their (number) fluxes are lower so
no discrete source has been observed yet.
At 1020 eV, r = 1Mpc:
- these particles cannot be confined to the Galaxy,
hence they must be extragalactic.
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The Origin of Cosmic Rays
• Galactic
Ordinary stars (produce ~1028 J/s)
Magnetic stars (produce up to 1032 J/s)
Supernovae (produce ~3x1032 J/s)
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Novae (produce ~ 3x10 J/s)
• Extragalactic
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Origin of Galactic Cosmic Rays
• Energy output required:
assume Galaxy is sphere of radius 30kpc,
63 3
21
= 10 m, => volume = 10 m
-13
6
-3
• Energy density CR ~ 10 J m (10 eV m-3) Thus
50
total energy of CR in Galaxy ~ 10 J.
17
10
• Age of Galaxy ~10 years, ~ 3x10 sec
hence average CR production rate ~ 3x1032 J s-1
• Possible sources must match this figure
• Particles shortlived => continuous acceleration
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Cosmic Rays from stars
• Ordinary stars
Too low!!!
Sun emits CR during flares but these have low-E
(up to 1010-1011 eV); rate only ~1017 J/s, total 1028
J/s (1011 stars in Galaxy)
• Magnetic stars
Optimistic!!!
Magnetic field about a million times higher than
the Sun so output a million times higher, but only
1% magnetic (and low-E); ~1032 J/s
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Supernovae
• Supernovae
- a likely source
Synchrotron radiation observed from SN so we know
high energy particles are involved.
42
• Total particle energy estimated at ~10 J per SN (taking
B from synchrotron formula and arguing that
UB ~ UParticles though this is uncertain due to magnetic
field and volume estimates).
• Taking 1 SN every 100 years,
32
=> 3x10 J/s (also, SN produce heavy elements)
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And from Novae
• Novae
also promising…
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Assuming ~10 J per nova and a rate of
about 100 per year, we obtain a CR
production rate of 3x10 32 J/s.
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Extragalactic Cosmic Rays
1020eV protons (r~1Mpc) cannot be contained
in the Galaxy long enough to remove original
direction
=> travel in straight lines from outside Galaxy
What conditions/geometry required to
produce energy density of cosmic rays
observed at these energies?
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• ‘Limited’ extragalactic region, r = 300Mpc
estimate 1000 radio galaxies in that region,
53
55
emitting 10 -10 J in their lifetime, 106 yrs.
• Volume of region – the local supercluster, is
75 3
27
V~10 m
• Total energy release over life of Universe =
10 4 x 103 x 10 55 J ~ 10 62 J (1000 radio galaxies)
- the radio galaxies must be replaced
10,000 times
-13
-3
• Energy density ~ 10 J m – this is the order of
the energy density required for the Local Group
volume if the value measured at Earth is
universal
• Quasars are another possible source of CR
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Electron sources of Cosmic Rays
• Electron mass small compared to protons and
heavy nuclei, => lose energy more rapidly
• Lifetimes are short, => electron sources are
Galactic.
3
• Observed energy density ~ 4x10 eV m-3
6
(total for cosmic rays ~ 10 eV m-3 )
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Pulsars as cosmic ray sources
• Assuming Crab pulsar-like sources…
can Galactic pulsars source CR electrons?
Need first to calculate how many electrons
produced by the Crab nebula.
• Observed synchrotron X-rays from SNR,
18
36
2
nm~10 Hz = 4 x 10 E B Hz
assume B SNR = 10-8 Tesla
-6
=> E e- = 5 x 10 J
13
= 3 x 10 eV
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Power radiated per electron
• Pe- = 2.4 x 1012 E 2 B 2 J/s
12
-11
-16
= 2.4 x 10 x 2.5 x 10 x 10 J/s
= 6 x 10 -15 J/s
• Observed flux = 1.6 x 10-10 J m -2 sec -1 keV-1
• Distance = 1kpc = 3 x 1019 m
• Total luminosity, L = 1.6 x 10 -10x 4pd 2 J/s
-10
2
38
= 1.6 x 10 x 10 x 10 J/s
= 1.6 x 10 30 J/s
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• Number of electrons = luminosity/power per e30
-15
44
= 1.6 x 10 / 6 x 10 = 2.6 x 10
• Synchrotron lifetime, t syn =5 x 10-13 B -2 E-1 s
= 30 years
Thus in 900yrs since SN explosion, must be 30
replenishments of electrons and these must be
produced by the pulsar.
44
• Total no. electrons = 2.6 x 10 x 30
45
~ 8 x 10
-6
each with Ee- = 5 x 10 J
32
• Total energy is thus 4 x 10 40J
10
Assume 1 SN every 100 years for 10 years
=> total energy due to pulsars :
4 x 1040 x 10 8 J = 4 x 1048 J
-3
63
in a volume of ~10 m (ie. the Galaxy)
• => energy density of electrons produced by
63
48
pulsars : =4 x 10 / 10 J m -3
-15
= 4 x 10 J m-3
= 4 x 10-15/ 1.6 x 10 -19 eV m -3
4
= 2.5 x 10 eV m -3
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3
• Observed e energy density is ~ 4 x 10 eV
40
• Total energy is thus 4 x 10 J
10
Assume 1 SN every 100 years for 10 years
=> total energy due to pulsars:
40
48
8
4 x 10 x 10 J = 4 x 10 J
63 -3
in a volume of ~10 m (i.e. the Galaxy)
• => energy density of electrons produced by
48
63
pulsars : =4 x 10 / 10 J m-3
= 4 x 10 -15 J m-3
-15
-19
-3
= 4 x 10 / 1.6 x 10 eV m
4
-3
= 2.5 x 10 eV m
and observed e- energy density is ~ 4 x 103 ev/m3
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Resolved Image of a TeV Gamma-ray Source Southern Hemisphere SNR RXJ 1713.7 - 3946
• An array of Cerenkov telescopes located in Namibia, imaged the SNR
in the range 0.8 – 10.0 TeV
• Each telescope has a 13m segmented parabolic collector that reflects
light onto a 960-photomultiplier focal-plane array
• Incoming gamma-ray photons creates a shower of electrons and
positrons by pair production – particles are highly relativistic
• Cerenkov radiation, like a
sonic shock wave, occurs
when a particle travels at
v > c/n in a medium of
refractive index n
• Wave angled to the
particle direction such that
cos q = c/nv
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Image and Spectrum of RXJ 1713.7 – 3946 (0.8 – 10.0 TeV)
• SNR image shows that TeV gamma-rays originate from the outer shell
i.e. from the shock as do the keV X-rays, and not from centre!
• Spectrum for both gammas and X-rays indicates non-thermal emission;
for X-rays almost certainly by synchrotron process
• Gamma-ray spectrum dNn/dE = k E-2.19±0.2 photons m-2 s-1 TeV-1
• Gamma-ray production by:
- Inverse Compton scattering by relativistic electrons or
- Decay of neutral pions following collision of TeV protons with
nuclei in an interstellar cloud
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Cosmic Ray Problems to be Further Studied
• Summary of problems from Longair, Vol 1, p 296:
- Acceleration of particles to very high energy, E ≥ 1020 eV
- Nature of acceleration processes that generate power-law
particle energy spectra – particularly in SNR
- Origin of high light element abundances (Li, Be, B) and (Sc,
Ti, V) in CR as compared to Solar System values
- Overall preservation of universal element abundances
throughout the periodic table
- Origin of anisotropies in the distribution of CR
- Astrophysical sources of the CR and their propagation
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COSMIC RAYS
END OF TOPIC
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Energy spectra of particles
Log Particle flux
m2 s-1 ster-1 eV-1
L: 3 ≤ Z ≤ 5, M: 6 ≤ Z ≤ 9
H: Z ≥ 10
-6
H
-12
P
a
M
L
-18
6
9
12
Log Energy (eV per nucleon)
39