Astroparticle physicswith high-energy photons

I – The physics

Alessandro de AngelisLisboa 2003

http://wwwinfo.cern.ch/~deangeli

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The starting point 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

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 ?

What could happen if we would see only, say, green color?

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The universe we don’t see

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

Gravity:G M(r) / r2 = v2 / renclosed mass: M(r) = v2 r / G

velocity vradius r

Luminous stars only small fraction of mass of galaxy

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Many sources radiate over a wide range of wavelengths

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The high-energy spectrum

E > 30 keV ( ~ 0.4 A, ~ 7 109 GHz)

Although arbitrary, this limit reflects astrophysical and experimental facts:

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

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The subject of these lectures…(definition of terms)

Detection of high-energy photons from space High-E X: probably the most interesting part of the spectrum for

astroparticle

What are X and gamma rays ? Arbitrary !

(Weekles 1988)

X 1 keV-1 MeVX/low E 1 MeV-10 MeV

medium 10-30 MeVHE 30 MeV-30 GeVVHE 30 GeV-30 TeVUHE 30 TeV-30 PeVEHE above 30 PeVNo upper limit, apart from low flux (at 30 PeV, we expect ~ 1 /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/ detection and some of the present & past detectors

4) Future detectors

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1) Motivations for the study of X/

Probe the most energetic phenomena occurring in nature

Nonthermal

Nuclear de-excitation/disintegration

Electron interactions w/ matter, magnetic & photon fields

Matter/antimatter ann.

Decay of unstable

particles

Clear signatures

from new physics

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Motivations (cont’d)

Penetrating

No deflection from magnetic fields, point ~ to the sources

Magnetic field in the galaxy: ~ 1GR (pc) = 0.01p (TeV) / B (G)=> for p of 300 PeV @ GC the directional information is lost

Large mean free path

Regions otherwise opaque can be transparent to X/

Good detection efficiency

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Large mean free path…Transparency of the Universe

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Astronomy Scales

4.5 pc 450 kpc 150 Mpc

Nearest Stars Nearest Galaxies Nearest Galaxy Clusters

1 pc= 3 light years

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‘GZK cutoff’ HE cosmic rays

HE gamma rays

Mrk 501 120Mpc

Mrk 421 120Mpc

Sources uniform in universe

100 Mpc

10 Mpc

e+ e

p N

Interaction with background ( infrared and 2.7K CMBR)

Milky Way

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Transparency of the atmosphere

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PHYSICS GOALS

AGNsAGNs SNRsSNRs

-ray Backg.-ray Backg.Cold Cold Dark Dark MatterMatter

PulsarsPulsars GRBsGRBs

Photon Photon propagation- propagation- Invariance of Invariance of

cc

AnomalouAnomalous eventss events

New VHM New VHM particlesparticles

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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 -> (& the give a clear signature)

Acceleration mechanisms and the origin of cosmic rays

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Active galaxies

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 spectra

But warning : ~300 sources @ the GeV scale, only 15 @ the TeV

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Pulsars

Rapidly rotating neutron stars with

T between ~1ms and ~1s Strong magnetic fields (~100 MT) Mass ~ 3 solar masses R ~ 10 Km (densest stable object

known)

For the pulsars emitting TeV gammas, such an emission is unpulsed

Crab pulsar

X-ray image (Chandra)

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-ray bursts (History, I)

An intriguing puzzle of today’s astronomy… A brief history

Beginning of the ‘60s: Soviets are ahead in the space war

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

Equipped with NaI (Tl) scintillators

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-ray bursts (History, II)

1967 : an anomalous emission of X and rays is observed. For a few seconds, it outshines all the sources in the Universe put together. Then it disappears completely. Another in 1969...

After careful studies (!), origination from Soviet experiments is ruled out

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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-ray bursts: why they are important

They might represent objects near the edge of the observable Universe

The energy could be 1015 times larger than the energy from a supernova

E ~ 1045 J They could be a new

observational tool for cosmologist

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-ray bursts: what we knowand what we’d like to know

They come from every direction in the sky

Mostly extragalactic

Frequently no optical emission (BeppoSAX 1997)

Far away from the galaxy A puzzle…

Time duration is wildly variable 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

Many sources can be related to SN remnants

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

Present estimate: 1 GRB/100My/Milky Way Galaxy=> Already ~ 100 GRB in our

galaxy

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

Is it really diffuse (<- produced at a very early epoch) or a flux from unresolved sources ?

Angular resolution is the key

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Physics in extreme conditions: photon propagation

Due to -> e+e-, CMB and visible light absorb at the PeV and at the TeV

At the GKZ cutoff (1020 eV) the Universe regains transparency to

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/EQG)

Effects on GRB could be O(100 ms)

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=> Intergalactic absorption

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

Today’s accelerator physics limited & many early discoveries in particle physics came from the study of cosmic rays

Motivation for particle physicists to join

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Energy of accelerated particles

Dia

met

er o

f co

llid

er

Cyclotron Berkeley 1937

LHC CERN, Geneva, 2007

Active Galactic Nuclei

Binary Systems

SuperNova Remnant

Particle Physics Particle Astrophysics

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DM CandidatesM > ~ 40 GeV if SUSY (LEP)

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Probing dark matter: WIMPs

Some dark matter candidates (e.g. SUSY particles) would lead to mono-energetic lines through annihilation

X

X

q

qor or Z

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Anomalous events

Anomalous showers at UHE (> 7 PeV)

from Cygnus X-3 (Samorski & al. 1983): almost no photons… Increasing total photon X-section

due to virtual gluons Increasing neutrino X-section New particles

Anomalous events (highly penetrating hadrons)

Normally killed as “irreproducible results”, but…

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Study of exotic objects: other phenomena

Top-Down : Decay of massive cosmic strings (1015 GeV, Kolb & Turner 1990)

Unknown transients

Time resolution is the key

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2) Historical milestones

1952 Prediction of He X/ high energy emission (Hayakawa)

1957 Sputnik 1

1958 Inventory of cosmic sites expected to radiate in the X/(Morrison)

1968 (11 years after the Sputnik): X emission of the galaxy

1972 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/ Satellites in the ’90s

GRANAT (SIGMA), 1990/97 Accreting black holes Jets

CGRO, 1991/2000 BATSE, thousands of GRB EGRET, hundreds of GRB in the HE region

BEPPO Sax, 1996/2002 SN remnants

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Gamma satellites

EGRET [+BATSE] Diffuse emissions dominate

the -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 cut-off, E as high as 18 GeV

The pulsar spectrum depends on the wavelength => Different energies produced in different regions

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Results from ground-based

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VHE sources

Observations in the ‘90s confirm earlier detection of VHE emissions from Crab nebula and discover new VHE sources in pulsars (PSR 1706-44, Vela)

No pulsed emission TeV emission from AGN, with flares

Mkr 421 Mkr 501

Models differ in the kind of particles emitted & E spectrum

Synchrotron model => 2 humps, one from synchrotron and one from inverse Compton

Variability over a large range of timescales

Observational holeupper limit from EGRET

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UHE (and EHE ?)

No sources of UHE (only diffuse emission)

No signal from established VHE 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 seen (and they would be expected…)

Absorption in the em field ?

Detection problems ?

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Comment on VHE and UHE gammas

Ground-based astronomy operates in regimes of large background => results are matter of discussions

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…

Fill the hole

No relevant information for particle physics, yet

Relevant is what should have been observed, but has not 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

High energy photons (often traveling through large distances) are a great probe of physics under extreme conditions

What better than a crash test to break a theory ?

Observation of X/ rays gives an exciting view of the HE universe Many sources, often unknown Diffuse emission Gamma Ray Bursts

No clear sources above ~ 30 TeV Do they exist or is this just a technological limit ?

We are just starting… Next lecture: many new detectors being built or plannedFuture detectors: have observational capabilities to give SURPRISES !