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Alessandro De Angelis, INFN/INAF Trieste, U. Udine & IST/LIP Lisboa 09

Gamma astrophysics

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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 rainbowAlso 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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Many sources radiate over a wide range of wavelengths

Basic physics of the emission: the SSC (but one might have additional mechanisms on top, e.g. hadronic interactions)

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And they can look different

γ (MAGIC)

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We think there’s 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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The high-energy spectrum

Eγ > 30 keV (λ ~ 0.4 A, ν ~ 7 109 GHz)

Although arbitrary, this limit reflects astrophysical and experimental facts:

Thermal emission -> nonthermal emissionProblems to concentrate photons (-> telescopes radically different from larger wavelengths)Large background from cosmic particles

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Definition of terms

High-E X/γ: 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 γ 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 γ/km2/day)

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Motivations for the study of X/γ

Probe the most energetic phenomena occurring in nature

Nonthermal

Nuclear de-excitation/disintegration

Electron interactions w/ matter, magnetic & photon fields

Matter/antimatter annihilation

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: ~ 1μGR (pc) = 0.01p (TeV) / B (μG)=> for p of 300 PeV @ GC the directional information is lost

Large mean free path

Regions otherwise opaque can be transparent to X/γ

Good detection efficiency

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Large mean free path…Transparency of the Universe

4.5 pc

450 kpc

150 Mpc

Nearest Stars

Nearest Galaxies

Nearest Galaxy Clusters

Milky Way

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‘GZK cutoff’ HE cosmic rays

HE gamma raysMrk 501 120Mpc

Mrk 421 120Mpc

Sources uniformin universe

100 Mpc

10 Mpc

γ γ → e+ e−

p γ → π N

Interaction with background γ( infrared and 2.7K CMBR)

Milky Way

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Transparency of the atmosphere

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Consequences on the techniques

The fluxes of h.e. γ are low and decrease rapidly with energyVela, the strongest γ source in the sky, has a flux above 100 MeV of 1.3 10-5

photons/(cm2s), falling with E-1.89 => a 1m2 detector would detect only 1 photon/2h above 10 GeV

=> with the present space technology, VHE and UHE gammas can be detected only from atmospheric showers

Earth-based detectors, atmospheric shower satellites

The flux from high energy cosmic rays is much larger

The earth atmosphere (28 X0 at sea level) is opaque to X/γ => only sat-based detectors can detect primary X/γ

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Satellite-based and atmospheric: complementary, w/ moving boundaries

Flux of diffuse extra-galactic photons

Atmospheric

Sat

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Ground-based vs Satellite

Satellite:primary detectionsmall effective area ~1m2

lower sensitivitylarge duty-cyclelarge costlower energylow bkg

Ground basedsecondary detection huge effective area ~104 m2

Higher sensitivitysmall angular openingsmall duty-cyclelow costhigh energyhigh bkg

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Satellite-based detectors:figures of merit

Effective area, or equivalent area for the detection of γAeff(E) = A x eff.

Angular resolution, important for identifying the γsources and for reducing the diffuse background

Energy resolution

Time resolution

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EGRET

High Energy γ detector 20 MeV-10 GeV

on the CGRO (1991-2000)

thin tantalium plates with wire chambersScintillators for trigger

Energy measured by a NaI (Tl) calorimeter 8 X0 thickEffective area ~ 0.15 m2 @ 1 GeVAngular resolution ~ 1.2 deg @ 1 GeVEnergy resolution ~20% @ 1 GeV

Scientific successIncreased number of identified sources, AGN, GRB, sun flares...

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The GLAST/Fermi observatory and the LAT

Large Area Telescope (LAT)Gamma Ray Burst Monitor (GBM)

Spacecraft

Rocket Delta II Launch base Kennedy Space CenterLaunch date June 2008Orbit 575 km (T ~ 95 min)

LAT Mass 3000 KgPower 650 W

Heart of the instrument is the LAT, detecting camma conversions γ

e+ e–

TRACKER

CAL

ACD

International collaboration USA-Italy-France-Japan-Sweden

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LAT overview

γ

e+ e-

Si-strip Tracker (TKR) 18 planes XY ~ 1.7 x 1.7 m2 w/ converter Single-sided Si strips 228 μm pitch, ~106

channelsMeasurement of the gamma direction

Calorimeter (CAL)Array of 1536 CsI(Tl) crystals in 8 layers

Measurement of the electron energy

AntiCoincidence Detector (ACD)89 scintillator tiles around the TKR

Reduction of the background from charged particles

Astroparticle groups INFN/University Bari, Padova, Perugia, Pisa, Roma2, Udine/Trieste

The Silicon tracker is mainly built in Italy

Italy is also responsible for the detector simulation, event display and GRB physics

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The GLAST LAT outperforms EGRET by two orders of magnitude

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2525

Lancio di GLAST/Fermi (Cape Canaveral, giugno 2008)

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Esempio di rivelazione di un raggio gamma

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AGILE: the GLAST precursor

IASF-INAF Milano

IASF-INAF Bologna

IASF-INAF Roma

IASF-INAF MilanoINFN Roma2

Gruppi INFN di Trieste, Roma1, Roma2 e istituti IASF-INAF di Milano, Roma, Bologna

Anticoincidenza di scintillatore plastico

• 12 piani di tracciatore di silicio, 10 con convertitore di tungsteno (0.07 X0)

• Mini-Calorimetro (15+15 barre di CsI ~1.5 X0)

• Rivelatore X (15-40 keV) a maschera codificata

L’uso della tecnologia del silicio per rivelare i gamma nasce con un rivelatore spaziale tutto italiano, AGILE

Prima piccola missione scientifica dell’ASILancio previsto per aprile 2007 (vettore indiano)

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Ground-based telescopesstill needed for VHE…

Peak eff. area of Fermi: 0.8 m2

From strongest flare ever recorded of very high energy (VHE) γ-rays:

1 photon / m2 in 8 h above 200 GeV(PKS 2155, July 2006)

The strongest steady sources are > 1 order of magnitude weaker!

Besides: calorimeter depth ≤ 10 X0

⇒ VHE astrophysics (in the energy

region above 100 GeV) can be

done only at ground

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Ground detectors: EAS and IACT

• EAS (Extensive Air Shower): detection of the charged particles in the shower

• Cherenkov detectors: (IACT): detection of the Cherenkov light from charged particles in the atmospheric showers

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Extensive Air Shower Particle Detector Arrays

Built to detect UHE gammas small flux => need for large surfaces, ~ 104

m2

But: 100 TeV => 50,000 electrons & 250,000 photons at mountain altitudes, and sampling is possible

Typical detectors are arrays of 50-1000 scintillators of ~1m2/each (fraction of sensitive area < 1%)

Possibly a μ detector for hadron rejection

Direction from the arrival times, δθ can be ~ 1 deg

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EAS Particle Detector ArraysPrinciple

Each module reports:Time of hit (10 ns accuracy)Number of particles crossing detector moduleTime sequence of hit detectors -> shower direction

Radial distribution of particles -> distance LTotal number of particles -> energy

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Cherenkov (Č) detectorsCherenkov light from γ showers

Č light is produced by particles faster than light in airLimiting angle cos θc ~ 1/n

θc ~ 1º Threshold @ sea level : 21 MeV for e, 44 GeV for μ

Maximum of a 1 TeV γ shower ~ 8 Km asl200 photons/m2 in the visibleDuration ~ a few nsAngular spread ~ 0.5º

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Cherenkov detectors: principles of operation

Cherenkov light detected using mirrors which concentrate the photons into fast PMs

In the beginning, heliostats Problem: night sky background

Signal ∝ Afluctuations ~ (AτΩ)1/2

=> S/B1/2 ∝ (A/τΩ)1/2

Now, a 3rd generationSmaller integration times ~nsImproved PMsLarge areas => Low threshold Multi-telescopeClose the gap ~ 100 GeV between satellite-based & ground-based instruments

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Incoming γ-ray

~ 10 kmParticleshower

~ 1o

~ 120 m

−+→+ eepγγ→+ −+ ee

Image intensityShower energy

Image orientationShower direction

Image shapePrimary particle

Observational Technique

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Better bkgd reduction Better angular resolutionBetter energy resolution

Systems of Cherenkov telescopes

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VERITAS: 4 telescopes operational since 2006

H.E.S.S.: 4 telescopes operational since 2003

The VHE connection…

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MAGIC

Refl. surface:236 m2, F/1, 17 m ∅

– Lasers+mechanisms for AMC

Camera:~ 1 m ∅, 3.5°FoV

AnalogicalTransmission(optical fiber)

TriggerDAQ > 1 GHzEvent rate ~300 Hz

Rapid pointing– Carbon fiber structure– Active Mirror Control

⇒ 20÷30 seconds

Very fast movement (< 30 s)

MAGIC

2

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A summary (oversimplified…)

Fermi IACTs EAS’Energy 20MeV – 200GeV 100GeV – 50TeV 400GeV–100TeVEnergy res. 5-10% 15-25% (*) ~50%Duty Cycle 80% 15% >90%FoV 4π/5 5deg x 5deg 4π/6PSF 0.1 deg 0.07 deg 0.5 degSensitivity (**) 1% Crab (1 GeV) 1% Crab (500 GeV) 0.5 Crab (5 TeV)

(*) Decreases to 15% after cross-calibration with Fermi(**) Computed over one year for Fermi and the EAS, over 50 hours for the IACTs

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A summary (oversimplified…)

10-14

10-13

10-12

10-11

10 100 1000 104 105

E [GeV]

Crab

10% Crab

1% Crab

Fermi

Magic2

Sen

sitiv

ity [

TeV

/cm

2 s ]

Hess/Veritas

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Origin of γ rays from gravitational collapsesSSC: a (minimal) standard model

(0.1-1)% of energy into photons…SSC explains most observations, not necessarily the most interesting…

energy E

IC

e- (TeV) Synchrotronγ (eV-keV)

γ (TeV) Inverse Comptonγ (eV)

B

leptonic acceleration

π0decay

π-

π0

π+

γγ (TeV)

p+ (>>TeV)

matter

hadronic acceleration

VHEIn the VHE region,dN/dE ~ E−Γ (Γ: spectral index)

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Dec 2009: release of the 1st Fermi catalog above 100 MeV (1451 sources, more than half extragalactic: EGRETx5)

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Day-by-day variability (> 1/3 extrag)

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>100 GeV: 93sources (as of Dec 09)

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Impressive analysis of the Milky Way by H.E.S.S.

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Cosmic rays: evidence for the emission of VHE hadrons by SNR

Existence of possible mechanismsConsistent w/ energetics

MorphologySeveral regions /SSCStatistics of PWNPower law consistent with powering by protons with Γ ~ 2

Up to 100 TeV → CR at O(1 PeV)

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Standard Model of galactic Cosmic Rays

Galaxy is Leaky BoxEnergy Dependent Escape of CR from the GalaxyCR source spectra must be dN/dE = E-2.1 to -2.4 to match E-2.7 CR spectrum measured at Earth

Supernova Remnants accelerate cosmic raysAcceleration of CR in shock produced with external medium that lasts ~1000 years SN rate of 1/30 years means ~30 SNR are needed to maintain cosmic ray fluxGamma fluxes consistent with an energetics of 1053 erg -> OK for the detection of galactic neutrinosSN must convert few % of energy of the ejecta into CR

Model explains most observations, and is consistent with many details

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Evidence for the emission of EHE hadrons by AGN

The “direct” measurement by AUGER (E > 60 EeV)

Orphan flares in TeV band (?)The production region of gammas from flares in M87 is very close to the BH, where there is abundance of protons

If SNRs O(10 SM) can explain CR at O(1 PeV), BH O(109 SM) are likely to explain CR up to O(1023 eV)

27 events as of November 2007 58 events now (with Swift-BAT AGN density map)

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Dark Matter

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- other γ-ray sources in the FoV=> competing plausible scenarios- halo core radius: extended vs point-like

BUT:

Highest DM density candidate:Galactic Center? Close by (7.5 kpc)Not extended

DM search(Majorana WIMPs) )(Z

nqqγγχχ

γχχ→

×→→

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γ-ray detection from the Galactic Center

Chandra GC surveyNASA/UMass/D.Wang et al.

CANGAROO (80%)

Whipple(95%)

H.E.S.S.

from W.Hofmann, Heidelberg 2004

detection of γ-rays from GC by Cangaroo, Whipple, HESS, MAGIC

σsource < 3’ ( < 7 pc at GC)

hard E-2.21±0.09 spectrumfit to χ-annihilation continuumspectrum leads to: Mχ > 14 TeV

other interpretations possible (probable)Galactic Center: very crowded sky region, strong

exp. evidence against cuspy profile Milky Way satellites Sagittarius, Draco, Segue, Willman1, Perseus, …

proximity (< 100 kpc)low baryonic content,

no central BH (which may change the DM cusp)

large M/L ratioNo signal for now…

no real indication of DM…

The spectrum is featurless!!!

…and satellite galaxies

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Search for spectral linessmoking gun signal of dark matter

Search for lines in the 30-200 GeV energy rangeSearch region |b|>10 deg and > 20 deg around GCNo line detected, 95% limits placedFor each energy (WIMP mass) the flux ULs are combined with the density function to extract UL on the annihilation cross section <σv>Limits on <σv> are too weak (by O(1) or more) to constrain a typical thermal WIMP

Some models predict large σ into lines: exg. Wino LSP (Kane 2009): γZ line, but they were disfavored by a factor of 2-5 depending on the halo profile

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Study of dSph

Select most promising dSph based on proximity, stellar kinematic data: <180 kpc from the Sun, more than 30o

from the galactic plane14 dSph have been selected for this analysis. More promising targets could be discoveredVery large M/L ratio: 10 ~> 1000 (~10 for Milky Way)

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Flux UL are combined with the DM density inferred by the stellar data for a subset of 8 dSph => constraints on <σv> vs WIMP mass for specific DM modelsExclusion regions already cutting into interesting parameter space for some (extreme) WIMP models

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γ detection from satellite galaxiesFermi vs. MAGIC/HESS/VERITAS

Willman-I [ApJ 697 (2009) 1299]

Profumo et al:

at m~0.5-1 TeV, comparable sensitivities for Fermi vs IACTsat m>1 TeV, IACTs outperform Fermi

potential for discovery even in the Fermi era

Maybe Profumo is pessimistic about MAGIC

S. Profumo: presented @TeVPA 2009, SLAC

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The ATIC electron excess

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Can Cherenkov distinguish positrons?

The task might be possible for multiple IACTsMoon shadow + geomagnetic field

Very hard, anyway worth trying…

TIBET AS-gamma

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Going far away…

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Variability: Mkn 421, Mkn501

Two very well studied sources, highly variable

Monitoring from Whipple, Magic…TeV-X Correlation

No orphan flares…See neutrino detectors

Mkn421 TeV-X-ray-correlation

Mkn421

However, recently Fermi/HESSsaw no correlation in PKS 2155

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Rapid variability

HESS PKS 2155z = 0.116

July 2006Peak flux ~15 x Crab

~50 x averageDoubling times 1-3 min

RBH/c ~ 1...2.104 s

H.E.S.S.arXiV:0706.0797

MAGIC, Mkn 501Doubling time ~ 2 min

MAGIC08

HESS08

6161

Violation of the Lorentz Invariance?Light dispersion expected in some QG models, but interesting “per-se”

0.15-0.25 TeV

0.25-0.6 TeV

0.6-1.2 TeV

1.2-10 TeV 4 min lag

MAGIC Mkn 501, PLB08Es1 ~ 0.03 MPEs1 > 0.02 MP

HESS PKS 2155, PRL08Es1 > 0.06 MP

anyway in most scenariosΔt ~ (E/Esα)α, α>1

VHE gamma rays are the probeMrk 501: Es2 > 3.10-9 MP , α=2

1st order

V = c [1 +- ξ (E/Es1) – ξ2 (E/Es2)2 +- …]

> 1 GeV

< 5 MeV

Es1

6262

LIV in Fermi vs. MAGIC+HESS

GRB080916C at z~4.2 : 13.2 GeV photon detected by Fermi 16.5 s after GBM trigger.

At 1st order

The MAGIC result for Mkn501 at z= 0.034 is Δt = (0.030 ± 0.012) s/GeV; for HESS at z~0.116, according to Ellis et al., Feb 09, Δt = (0.030 ± 0.027) s/GeV

Δt ~ (0.43 ± 0.19) K(z) s/GeV

Extrapolating, you get from Fermi (26 ± 11) s (J. Ellis et al., Feb 2009)

SURPRISINGLY CONSISTENT:

DIFFERENT SOURCE TYPE

DIFFERENT ENERGY RANGE

DIFFERENT DISTANCE

Es1 ~z

6363

z = 1.8 ± 0.4

one of the brightest GRBs

observed by LAT

after prompt phase, power-law emission persists in the LAT data as late as 1 ks post trigger:

highest E photon so far detected: 33.4 GeV, 82 s after GBM trigger

[expected from Ellis & al. (26 ± 13) s]

much weaker constraints on LIV Es

(EBL constraints)

Fermi: GRB 090510 GRB 090902

• z = 0.903 ± 0.003

• prompt spectrum detected,

significant deviation from Band

function at high E

• High energy photon detected:

31 GeV at To + 0.83 s

[expected from Ellis & al. (12 ± 5) s]

• tight constraint on Lorentz

Invariance Violation:

– MQG > x MPlanck [x = O(1)]

[arXiv:0909.247v1]

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Interpretation of the results on rapid variability

The most likely interpretation is that the delay is due to physics at the source

By the way, a puzzle for astrophysicists

HoweverWe are sensitive to effects at the Planck mass scale

More observations of flares will clarify the situation

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Propagation of γ-rays

x

xx

For γ−rays, relevant background component is optical/infrared (EBL)different models for EBL: minimum density given by cosmology/star

formation

Measurement of spectral features permits to constrain EBL models

≈

γVHEγbck → e+e-dominant process for absorption:

maximal for:

Heitler 1960

σ(β) ~

Mean free path

e+

e-

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Are our AGN observationsconsistent with theory?

Selection bias?New physics ?

obse

rved

spe

ctra

l ind

exredshift

De Angelis, Mansutti, Persic, Roncadelli MNRAS 2009

The most distant: MAGIC 3C 279 (z=0.54)

Measured spectra affected by attenuation in the EBL:

~ E-2

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Interaction with a new light neutral boson? (De Angelis, Roncadelli & MAnsutti [DARMA], arXiv:0707.4312, PR D76 (2007) 121301arXiv:0707.2695, PL B659 (2008) 847

Explanations go from the standard ones

very hard emission mechanisms with intrinsic slope < 1.5 (Stecker 2008)Very low EBL

to possible evidence for new physics

Oscillation to a light “axion”? (DA, Roncadelli & MAnsutti [DARMA], PLB2008, PRD2008)

Axion emission (Hooper et al., PRD2008)

Could it be seen?

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Summary

VHE photons (often traveling through large distances) are a powerful probe of fundamental physics under extreme conditions

What better than a crash test to break a theory?

Observation of γ rays gives an exciting view of the VHE universeFermi resultsIACTs (>80 new VHE sources discovered in the last 5 years, and growing…)

Transparency of the Universe: new physics?Rapid variability: new physics?

Just started, a lot of physics… and in 2010/2011:Analysis of 1st Fermi Catalogfactor 2-3 improvement by HESS2, MAGIC2, VERITASFor the future, large Cherenkov (CTA) would allow a leap…