Download Gamma-Ray Astroparticle Physics

Astroparticle physics
with high-energy photons
I – The physics
Alessandro de Angelis
Lisboa 2003
http://wwwinfo.cern.ch/~deangeli
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The starting point
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Physics constructs models explaining Nature (or better our
observations of Nature, or better observations of our
interactions with Nature)
We know Nature mostly through our eyes, which are
sensitive to a narrow band of wavelengths centered on the
emission wavelength of the Sun
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We see only partly what surrounds us
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We see only a narrow band of colors, from red to purple in
the rainbow
Also the colors we don’t see have names familiar to us: we
listen to the radio, we heat food in the microwave, we take
pictures of our bones through X-rays…
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What about the rest ?
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What could happen if we would see only, say, green color?
The universe we
don’t see
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When we take a picture we
capture light
(a telescope image comes as
well from visible light)
In the same way we can map
into false colors the image
from a “X-ray telescope”
Elaborating the information is
crucial
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We know there is something
important we don’t see
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velocity v
radius r
Gravity:
G M(r) / r2 = v2 / r
enclosed mass:
M(r) = v2 r / G
Luminous stars only small fraction of mass of galaxy
Many sources radiate over
a wide range of wavelengths
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The high-energy spectrum
Eg > 30 keV (l ~ 0.4 A, n ~ 7 109 GHz)
Although arbitrary, this limit reflects astrophysical and
experimental facts:
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Thermal emission -> nonthermal emission
Problems to concentrate photons (-> telescopes radically
different from larger wavelengths)
Large background from cosmic particles
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And that things can look different
The subject of these lectures…
(definition of terms)
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Detection of high-energy photons from space
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High-E X/g: probably the most interesting part of the spectrum for
astroparticle
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What are X and gamma rays ? Arbitrary !
(Weekles 1988)
X
X/low E g
1 keV-1 MeV
1 MeV-10 MeV
medium
10-30 MeV
HE
30 MeV-30 GeV
VHE
30 GeV-30 TeV
UHE
30 TeV-30 PeV
EHE
above 30 PeV
No upper limit, apart from low flux (at 30 PeV, we expect ~ 1 g/km2/day)
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Outline of these lectures
0) Introduction & definition of terms
1) Motivations for the study high-energy photons
2) Historical milestones
3) X/g detection and some of the present & past detectors
4) Future detectors
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1) Motivations for the study of X/g
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Probe the most energetic phenomena occurring in nature
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Nonthermal
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Nuclear de-excitation/disintegration
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Electron interactions w/ matter, magnetic & photon fields
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Matter/antimatter ann.
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Decay of unstable
particles

Clear signatures
from new physics
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Motivations (cont’d)
Penetrating
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No deflection from magnetic fields, point ~ to the sources
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Magnetic field in the galaxy: ~ 1mG
R (pc) = 0.01p (TeV) / B (mG)
=> for p of 300 PeV @ GC the directional information is lost
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Large mean free path
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Regions otherwise opaque can be transparent to X/g
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Good detection efficiency
Large mean free path…
Transparency of the Universe
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Astronomy Scales
Nearest Stars
Nearest Galaxies
Nearest Galaxy Clusters
4.5 pc
450 kpc
150 Mpc
1 pc= 3 light years
‘GZK cutoff’
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HE cosmic rays
Interaction with background g
( infrared and 2.7K CMBR)
p g  N
Sources uniform
in universe
100 Mpc
10 Mpc
HE gamma rays
Mrk 501 120Mpc
g g  e+ e
Milky Way
Mrk 421 120Mpc
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Transparency of the atmosphere
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PHYSICS GOALS
Pulsars
GRBs
AGNs
VHM
particles
Anomalous
events
Cold Dark
Matter
SNRs
New
g-ray
Photon
propagationInvariance of c
Backg.
Acceleration mechanisms and
the origin of cosmic rays
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Energetic protons and electrons in the vicinity of
astrophysical objects might produce gammas
Synchrotron radiation by electrons in magnetic fields could
be boosted to TeV energies by inverse Compton scattering
If acceleration mechanisms involve hadronic interactions,
there are many 0 -> gg (& the g give a clear signature)
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Active galaxies
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Many sources, mostly classified
according to observational criteria
Unified AGN model (Begelman et
al. 1984): 10% of the accreted
mass is transformed into radiation
Different models predict
different g spectra
But warning : ~300 sources
@ the GeV scale, only 15 @ the TeV
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Pulsars
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Rapidly rotating neutron stars
with
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T between ~1ms and ~1s
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Strong magnetic fields (~100 MT)
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Mass ~ 3 solar masses
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Crab pulsar
R ~ 10 Km (densest stable object
known)
For the pulsars emitting TeV
gammas, such an emission is
unpulsed
X-ray image (Chandra)
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g-ray bursts (History, I)
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An intriguing puzzle of today’s
astronomy… A brief history
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Beginning of the ‘60s: Soviets
are ahead in the space war
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1959: USSR sends a satellite to
impact on the moon
1961: USSR sends in space the
27-years old Yuri Gagarin
1963: the US Air Force launches
the 2 Vela satellites to spy if
the Soviets are doing nuclear
tests in space or on the moon
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Equipped with NaI (Tl)
scintillators
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g-ray bursts (History, II)
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1967 : an anomalous emission of X and g
rays is observed. For a few seconds, it
outshines all the g sources in the
Universe put together. Then it
disappears completely. Another in 1969...
After careful studies (!), origination
from Soviet experiments is ruled out
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The bursts don’t come from the vicinity
of the Earth
1973 (!) : The observation is reported to
the world
Now we have seen hundreds of gamma
ray bursts...
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g-ray bursts: why they are important
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They might represent
objects near the edge of
the observable Universe
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The energy could be 1015
times larger than the
energy from a supernova
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E ~ 1045 J
They could be a new
observational tool for
cosmologist
g-ray bursts: what we know
and what we’d like to know
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They come from every
direction in the sky
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Frequently no optical emission
(BeppoSAX 1997)
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Far away from the galaxy
A puzzle…
Time duration is wildly
variable
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Mostly extragalactic
Afterglows after > 1h…
Several mechanisms
proposed, enormous energies:
a great chance that they’re
so far...
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Importance of the multiwavelength
approach
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A recent consensus
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Many sources can be related to
SN remnants
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Mechanism accounting for repeated
shocks (Dar, De Rujula)
Matter of precise poninting:
Work for GLAST
Synergy with gravitational wave
detectors
Work for LIGO
But: Maybe different kinds of
bursts…
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Probability of bursts
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Present estimate: 1
GRB/100My/Milky Way
Galaxy
=> Already ~ 100 GRB in our
galaxy
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Energy ~ 1045 J
According to Dar, it is not
unlikely that a GRB has
already interacted with the
atmosphere…
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Diffuse background radiation
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Is it really diffuse (<- produced at a very early epoch) or a
flux from unresolved sources ?
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Angular resolution is the key
Physics in extreme
conditions: photon propagation
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Due to gg -> e+e-, CMB and visible
light absorb g at the PeV and at
the TeV
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At the GKZ cutoff (1020 eV) the
Universe regains transparency to g
The transparency of the Universe
gives insights on the infrared/
optical diffuse background
Quantum gravity (Amelino-Camelia
et al., Ellis et al.)
V = c (1 - e E/EQG)
Effects on GRB could be O(100 ms)
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=> Intergalactic g absorption
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Photons interact with the IR
background => relationship
source distance / maximum
observed photon energy
Measurement from the
distortion of AGN spectra
Data in the range 50 GeV - 300
GeV would be crucial
And an important byproduct:
the best constraints on Lorentz
violation, photon oscillations etc.
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Particle physics at high energies
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Today’s accelerator
physics limited & many
early discoveries in
particle physics came
from the study of
cosmic rays
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Motivation for particle
physicists to join
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Particle Physics
 Particle Astrophysics
Energy of accelerated particles
Active Galactic Nuclei
Binary Systems
SuperNova
Remnant
LHC CERN, Geneva, 2007
Cyclotron Berkeley 1937
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DM Candidates
M > ~ 40 GeV
if SUSY (LEP)
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Probing dark matter: WIMPs
Some dark matter candidates (e.g.
SUSY particles) would lead to monoenergetic g lines through annihilation
X
q
X
q
or gg or Zg
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Anomalous events
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Anomalous showers at UHE (> 7 PeV)
from Cygnus X-3 (Samorski & al. 1983):
almost no photons…
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Increasing total photon X-section
due to virtual gluons
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Increasing neutrino X-section
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New particles
Anomalous events (highly penetrating
hadrons)
Normally killed as “irreproducible
results”, but…
Study of exotic objects:
other phenomena
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Top-Down : Decay of massive cosmic strings
(1015 GeV, Kolb & Turner 1990)
Unknown transients
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Time resolution is the key
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2) Historical milestones
1952 Prediction of He X/g high energy emission (Hayakawa)
1957 Sputnik 1
1958 Inventory of cosmic sites expected to radiate in the
X/g (Morrison)
1968 (11 years after the Sputnik): X emission of the galaxy
1972 g from Crab Nebula
1973 First report on gamma ray bursts
1978 Gamma-ray spectroscopy : e+e- annihilations @ the GC
1983 Nuclear processes at the GC
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Some selected results
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X/g Satellites in the ’90s
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GRANAT (SIGMA), 1990/97
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Accreting black holes
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Jets
CGRO, 1991/2000
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BATSE, thousands of GRB
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EGRET, hundreds of GRB in the HE region
BEPPO Sax, 1996/2002
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SN remnants
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Gamma satellites
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EGRET [+BATSE]
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Diffuse g emissions dominate the
g-ray sky. After removing the
identified point sources, ~ mass
distribution
Moreover, isotropic emission at
high latitude going like E-2.07+-0.03
Pulsars, all observed also in the
radio (apart from Geminga)
Most point sources unidentified
Gamma-Ray Bursts, not expected
in any model. No apparent E cutoff, E as high as 18 GeV
The pulsar spectrum depends
on the wavelength =>
Different energies produced
in different regions
Results from
ground-based
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VHE sources
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Observations in the ‘90s confirm earlier
detection of VHE emissions from Crab nebula
and discover new VHE sources in pulsars (PSR
1706-44, Vela)
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No pulsed emission
TeV emission from AGN, with flares
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Mkr 421
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Mkr 501
Models differ in the kind of particles
emitted & E spectrum
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Synchrotron model => 2 humps, one from
synchrotron and one from inverse Compton
Variability over a large range of timescales
Observational hole
upper limit from EGRET
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UHE (and EHE ?)
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No sources of UHE g (only diffuse
emission)
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No signal from established VHE g
sources
No signals from hypothetical new
sources (primordial black holes, black
holes accreting from a nearby star…)
Although the GRB spectrum from
BATSE/EGRET is hard
(E-2), no UHE g seen (and they
would be expected…)
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Absorption in the em field ?
Detection problems ?
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Comment on VHE and UHE gammas
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Ground-based astronomy operates in regimes of large background =>
results are matter of discussions
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VHE emissions from Crab and Vela are accepted as genuine
No episodic emission widely accepted yet
Many astronomical models of AGN suffer from lack of information in
the ~50 GeV region…
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Fill the hole
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No relevant information for particle physics, yet
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Relevant is what should have been observed, but has not
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TeV gammas from SN shocks should have been seen
Correlation between EGRET objects, TeV emissions and SNR ?
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The progress at a glance
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Sensitivity
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Summary
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High energy photons (often traveling through
large distances) are a great probe of physics under extreme conditions
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Observation of X/g rays gives an exciting view of the HE universe
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Many sources, often unknown
Diffuse emission
Gamma Ray Bursts
No clear sources above ~ 30 TeV
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What better than a crash test to break a theory ?
Do they exist or is this just a technological limit ?
We are just starting… Next lecture: many new detectors being built or
planned
Future detectors: have observational capabilities to give SURPRISES !