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2
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
3
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…
5
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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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
10
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
12
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
16
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
17
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
19
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
20
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...
21
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
27
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)
28
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
29
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
30
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
31
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
32
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
33
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
34
Incoming γ-ray
~ 10 kmParticleshower
~ 1o
~ 120 m
−+→+ eepγγ→+ −+ ee
Image intensityShower energy
Image orientationShower direction
Image shapePrimary particle
Observational Technique
35
Better bkgd reduction Better angular resolutionBetter energy resolution
Systems of Cherenkov telescopes
36
VERITAS: 4 telescopes operational since 2006
H.E.S.S.: 4 telescopes operational since 2003
The VHE connection…
37
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
39
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
40
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
41
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)
42
Dec 2009: release of the 1st Fermi catalog above 100 MeV (1451 sources, more than half extragalactic: EGRETx5)
46
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)
47
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
48
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)
50
- 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→
×→→
51
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
52
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
52
53
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)
53
54
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
54
5555
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
57
Can Cherenkov distinguish positrons?
The task might be possible for multiple IACTsMoon shadow + geomagnetic field
Very hard, anyway worth trying…
TIBET AS-gamma
59
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
60
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]
64
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
65
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-
66
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
67
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?
68
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…