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7. Astronomia Gamma
I- Introduzione e satelliti
(Cap. 8 Libro)
Corso “Astrofisica delle particelle”
Prof. Maurizio Spurio
Università di Bologna a.a. 2014/15
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Outline
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Il cielo visto da EGRET
FERMI-LAT
Il fondo diffuso di raggi gamma
Noto ed ignoto nel cielo Gamma
I Gamma Ray Bursts
Osservazioni sperimentali di acceleratori astrofisici nella
Galassia: astronomia g con telescopi Cherenkov
g dal piano galattico come indizio dei RC
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Thermal
radiation:
Black Body
Spectrum
CMBR:
2.7 K
A Galactic
gas cloud
60 K
Dim star in the
Orion Nebula:
600 K
The Sun:
6000 K
Cluster of very
bright stars,
Omega Centauri:
60,000 K
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High Energy g rays: non-thermal Universe
• Particles accelerated in extreme environments interact with medium
– Gas and dust; Radiation fields – Radio, IR, Optical, …;
Intergalactic Magnetic Fields, …
• Gamma rays traveling to us!
– HE: 30 MeV to 30 GeV
– VHE: 30 GeV to 30 TeV
• No deflection from magnetic fields, gammas point ~ to the sources
– Magnetic field in the galaxy: ~ 3mG
Gamma rays can trace cosmic rays at energies ~10x
• Large mean free path
– Regions otherwise opaque can be transparent to X/g
Studying
Gamma Rays allows us to see different aspects of the Universe
Examples of known extreme environments
SuperNova
GRB
Remnants
Pulsars
Accretion
Disk 3- 10 rs Black Hole Diameter = 2rs ~ 4 AU
Active
Galactic
Nuclei
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Gamma rays interact with the atmosphere
energy
 GeV detection requires satellites; TeV (VHE) can be done at ground
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Precision
Detectors
Si-strip Tracker (TKR)
18 XY tracking planes
Single-sided silicon strip detectors 228
mm pitch, 8.8 105 channels
Measure the photon direction
• Satellites (AGILE, Fermi)
– Silicon tracker (+calorimeter)
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+
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• Cherenkov telescopes
(HESS, MAGIC, VERITAS)
• Extensive Air Shower det.
(ARGO): RPC, scintillators
HEP detectors!
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Satelliti
Fino a qualche centinaio
di GeV
100 GeV – decine di TeV
Telescopi Cherenkov al suolo
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Compton Gamma Ray Observatory
• Second NASA telescope mission after Hubble
• Launched using the Space Shuttle in April 1991 and operated
successfully until it was de-orbited on June 4, 2000
• The CGRO carried four instruments for g-ray astronomy, each
with its own energy range, detection technique, and scientific
goals, covering energies from less than 15 keV to more than 30
GeV
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1. The Burst and Transient Source Experiment (BATSE) was the smallest
of the CGRO instruments, consisting of eight modules located one on
each corner of the spacecraft, consisting of a large flat NaI(Tl)
scintillator and a smaller thicker scintillator for spectral measurements,
combined to cover an energy range from 15 keV to over 1 MeV.
2. Oriented Scintillation Spectrometer Experiment (OSSE). It used four
large, collimated scintillator detectors to study g-rays in the range from
60 keV to 10 MeV.
3. The Compton Telescope (COMPTEL) detected, for medium energy
g-rays between 0.8 MeV and 30 MeV, used a Compton scattering
technique.
4. The Energetic Gamma Ray Experiment Telescope (EGRET) was the
high-energy instrument on CGRO, covering the energy range from 20
MeV to 30 GeV.
http://imagine.gsfc.nasa.gov/docs/sats_n_data/missions/cgro.html
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Il cielo visto dal satellite EGRET
20 MeV <Eg<30 GeV
Piano galattico
Fondo + sorgenti
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Energetic Gamma Ray Experiment
Telescope (EGRET)
•EGRET ha rivelato g-rais tra 20 MeV-30 GeV .
• Aveva un campo di vista molto largo, circa 80° in diametro.
• Area: 1000 cm2 tra 100 MeV e 3 GeV
• Precisione angolare dipendente dall’energia: 5.5° a100 MeV, sino a 0.5° a 5 GeV;
• le sorgenti brillanti di Rg potevano essere localizzate entro approssimativamente 10' .
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Diffuse Galactic g-Ray Emission (DGgRE)
• L’emissione diffusa di raggi Gamma dal piano galattico è dominante
nella rivelazione di raggi gamma di energia > 100 MeV.
• Prime misure di EGRET, confermate da Fermi-LAT (2009).
• La DGgRE è prodotta dalle interazioni di protoni ed elettroni dei RC,
che interagiscono col ISM durante la propagazione.
• La distribuzione spaziale della DGgRE osservata da EGRET/FermiLAT può essere interpretata in termini della distribuzione di gas
atomici e molecolari nel mezzo interstellare della nostra Galassia,
utilizzando il modello di confinamento dei RC Galattici.
• Tuttavia, lo spettro della DGgRE non è completamente spiegato in
termini del solo modello di interazione dei RC col mezzo interstellare
galattico: sono evidenti punti di accumulo (sorgenti).
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SORGENTI di Rg (e di RC ?)
3o Catalogo EGRET : 270
sorgenti (93 blazars, 170
non identificate).
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Noto ed ignoto da EGRET: segnalefondo = sorgenti
• Circa il 50% delle
sorgenti scoperte da
EGRET sono state
identificate (osservate
anche in precedenza in
altre lunghezze d’onda).
• Metà sono non
identificate.
• Quali oggetti
producono raggi gamma
di alta energia, ed
emettono anche nel
radio?
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Segnale ottenuto dopo la sottrazione del fondo galattico diffuso
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Fermi-LAT (a: 11/6/2008)
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Il rivelatore FERMI-LAT
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The LAT is a pair-production telescope.
 The tracking section has 36 layers of silicon microstrip detectors,
with 16 layers of tungsten foil (12 thin layers, 0.03 X0, at the top or front
of the instrument, followed by 4 thick layers, 0.18 X0, in the back section
for γ-ray pair conversion.
 The tracker is followed by an array of CsI crystals to determine the γray energy and is surrounded by segmented charged-particle detectors
(plastic scintillators with PMTs) to reject cosmic-ray backgrounds.
 The LAT’s improved sensitivity compared to EGRET stems from:
 a large peak effective area (∼8000 cm2, or ∼6 × EGRET’s),
 large field of view (∼2.4 sr, or nearly 5 × EGRET’s),
 good background rejection,
 superior angular resolution (68% containment angle ∼ 0.6◦ at 1
GeV for the front section and about a factor of 2 larger for the back
section), improved observing efficiency
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FERMI-LAT sky map
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Sky
map of the LAT data for the first 3 months, Aitoff projection in Galactic
coordinates. g-ray intensity for E>300 MeV, in units of photons m−2 s−1 sr−1.
 The list of sources was obtained after three steps which were applied in sequence:
detection, localization, significance estimate.
 Source characteristics (flux in two energy bands, time variability) and possible
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counterparts
Aristotele sbagliava !
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Modelli di emissione
I raggi gamma possono essere emessi in prossimità di una sorgente
che accelera elettroni (modello di emissione leptonica). E’ necessaria
la presenza di un campo magnetico;
 Anche interazione di adroni (tramite il decadimento dei mesoni
neutri) possono produrre g-rays (modello di emissione adronica).
 Per l’emissione adronica, in prossimità delle sorgenti, deve essere
presente della materia che funge da bersaglio con densità maggiore della
densità della ISM (1p/cm3), o campi di radiazione.
In entrambi i casi, lo spettro energetico di emissione dei fotoni
secondari è legato allo spettro energetico alle sorgenti delle particelle
che li originano (elettroni e/o protoni):
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dNg
dEg

dN p ,e
dE p ,e
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g-rays production@sources
leptonic processes
hadronic process
Synchrotron
g (eV-keV)
e- (TeV)
p+ (≫TeV)
B
g (eV)
gg (TeV)
0
matter
g (TeV)
Inverse Compton
IC
0
energy E
Radio Optical X-ray GeV
TeV
+
To distinguish between
hadronic/leptonic origin
the Spectral Energy
Distribution (SED) must be
studied with different
experimental techniques 25
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Spectral
energy distribution of photons produced in leptonic/hadronic models.
Synchrotron radiation is produced by relativistic electrons accelerated in a magnetic
field. The produced photons represent also the target for inverse Compton
scattering of the parent electrons.
When hadrons interact with matter, a distribution of g-rays from 0 decays as
indicated by the green curve could be produced. Superimposition of g-rays
produced in leptonic and hadronic mechanisms is assumed in case of mixed models
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Osservazione di g diffusi (DGgRE) dal piano
Galattico (EGRET, Fermi-LAT)
• La componente diffusa nel piano galattico è dovuta alla
presenza di protoni ed elettroni nei RC
• Se I RC permeano la Galassia, le collisioni con il materiale
IG attraversato (5 g cm-2) produrranno sciami EM, in cui il
decadimento dei o produrranno fotoni di alta energia.
• Altre sorgenti di g nel piano galattico sono:
– la bremmstrahlung di elettroni di alta energia
– Compton inverso di e di alta energia su fotoni (luce stellare)
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Possiamo stimare la luminosità attesa di fotoni dal piano galattico:
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Bremmstrahlung e  eg
NuclearField
Compton Inverso eg low  e'g high
Decadimento o  0  gg
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Stima del flusso da 0
– spp=sezione d’urto inelastica= 40 mb
– n = densità del mezzo ISM = 1p/cm3
– rCRg =densità dei RC che produce g-ray: 10% of rCR.
Dovuto al fatto che 10% dell’energia viene trasferita ai 
– c = velocità della luce = 3 1010 cm/s
– Energia trasferita ai pioni neutri: 1/3 del totale
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Interaction rate of one relativistic CR with the ISM protons:
energy emitted isotropically as g-rays per unit of solid angle per cubic
centimeter of the Galaxy per second corresponds to:
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Inserting
the numerical values:
The
photon flux at the detector depends on the linear distance D from
which photon can arrive from the galactic plane (nD= column density)
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The estimated average value is nD 1020 cm-2 and:
Confronta
con la Fig.
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Sorgenti= segnale – fondo diffuso
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GeV g-rays sky (April, 15rd 2013)
arXiv :1304.4153
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LAT 2-year Point Source Catalog (2FGL)
Fermi LAT Second
Source Catalog (2y)
 arXiv:1108.1435v2
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Maurizio
Spurio - Frascati Workshop '13
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Energy spectral index of 2FGL
Distributions of the spectral index for the 1FGL (1451 sources, dashed
line) and for the 2FGL (1873 sources, solid line) catalogs
 Fermi mechanism at work!
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FERMI Hard sources >10 GeV sky
259
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6
11
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arXiv :1304.4153
Maurizio
Spurio - Frascati Workshop '13
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Sorgenti Galattiche
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Esempi di sorgenti galattiche: microquasars
What sort of
compact object?
• Le microquasar sono dei corpi celesti simili alle quasar: le
caratteristiche comuni sono: emissioni radio forti e variabili,
spesso in getti, e un disco di accrescimento che circonda un
buco nero.
•Nelle quasar, il buco nero è supermassiccio (>106 masse
solari) mentre nelle microquasar, la massa del buco nero è di
poche masse solari.
• Nelle microquasar, la massa di accrescimento deriva da una
normale stella e il disco di accrescimento è molto luminoso
nello spettro visibile e nei raggi X.
How are the
particles
accelerated?
Are there different
types of such highmass binary
systems?
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AGN
M87: Immagine da HESS
(vedi: ) tramite gamma del
TeV; immagine radio. M87 è
una delle più potenti radio
galassie viste in raggi gamma.
Variabilità di M87 vista nel TeV
Is the gamma-ray variability related to
changes in the jet? In the core?
What about fainter radio galaxies?
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Esempi di sorgenti: Blazars
• Le Blazars sono Galassie nel cui centro è ospitato
un Buco nero supermassivo.
• Le Blazars sono tra le principali sorgenti di Rg
• C’è evidenza di correlazione tra i getti di Rg e
l’emissione radio vista dai VLBI
What do the combined radio/gamma-ray
observations tell us about particle acceleration and
interaction – processes, location?
What can this information reveal about jet
formation and collimation?
Immagine da VLBI. Vedi
http://web.whittier.edu/gpiner/research/index.htm
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Leptonic vs. hadronic models
Multifrequency/multimessenger observation required
La SED (Spectral Energy Distribution)
GLAST LAT
AGILE
TeV
INTEGRAL
GLAST GBM
Swift
• Le sorgenti di Rg sono
non-termiche (ossia, non
emettono uno spettro di
corpo nero)
• I Rg sono tipicamente
prodotti dalle interazioni
di particelle di alta
energia
•Le classi di sorgenti di g
conosciute emettono ( e
sono rivelate) anche in
altre lunghezze d’onda.
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I Gamma Ray Bursts (GRBs)
Scoperta di Sorgenti
Transienti (GRB’s)
(Gamma Ray Bursts)
• Non sappiamo quando e
dove guardare!
• Indicazioni di una
componente secondaria di
alta energia (afterglows)
BATSE
on CGRO
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Origine Extragalattica. Possibili candidati di meccanismi di
accelerazione per i RC di energia estrema.
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The 2704 BATSE
GRBs
Map of the locations of a total of 2704 GRBs recorded with the
BATSE on board NASA's CGRO during the nine-year mission.
 The isotropy of the GRB distribution is evident from this figure.
 The projection is in galactic coordinates; the plane of the Milky Way
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Galaxy is along the horizontal line at the middle of the figure
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VEDI: http://imagine.gsfc.nasa.gov/docs/science/know_l1/bursts.html
GRBs are short-lived bursts of gamma-ray photons. At least some of
them are associated with a special type of supernovae;
 Lasting anywhere from a few milliseconds to several minutes, GRBs
shine hundreds of times brighter than a typical supernova, making them
briefly the brightest source of cosmic gamma-ray photons in the
observable Universe.
 GRBs are detected roughly once per day, from random directions in
the sky by satellite experiments;
 Until recently, GRBs were the biggest mystery in HE astronomy.
They were discovered serendipitously in the late 1960s by U.S. military
satellites looking for Soviet nuclear testing in violation of the
atmospheric nuclear test ban treaty. These satellites carried g-ray
detectors since a nuclear explosion produces g-rays.
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As recently as the early 1990s, astronomers didn't even know if GRBs
originated in our Galaxy or at cosmological distances
BATSE detector catalogued 2,704 GRBs during nine year lifetime
(1991 - 2000). It was not equipped to make afterglow observations.
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A sampling of the large
variety of GRB time
profiles, as detected from
the CGRO satellite
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GRBs are separated into two classes: long- and short-duration bursts.
Long duration ones last more than 2 seconds and short-duration
ones last less than 2 seconds
Long and short duration GRBs are created by fundamentally
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different physical properties
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Long and
short GRBs
Possible candidates for short GRBs are mergers of neutron star binaries
or neutron star - black hole binaries, which lose angular momentum and
undergo a merger
 Possible candidates for long GRBs are core collapse of a special kind of
very massive star. This core collapse occurs while the outer layers of the
star explode in an especially energetic supernova (the “hypernova”, 100
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times the SN).
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The Italian BeppoSAX satellite
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Satellite observations (starting form the Italian satellite Beppo-SAX),
follow-up ground-based observations, and theoretical work have
allowed astronomers to link GRBs to supernovae in distant galaxies
http://bepposax.gsfc.nasa.gov/bepposax/italver.html
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BeppoSAX was equipped
with both a g-ray and an Xray detector. It spotted the
X-ray afterglow signature
associated with the GRB on
February 28, 1997
Discovery of the
extragalactic origin of
GRBs
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X-ray image of the first BEPPO-SaX GRB
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Theoretical models of GRBs
Long GRBs: The explosion originates at the center of these massive
stars. While a black hole forms from the collapsing core, this explosion
sends a blast wave moving through the star at speeds close to the speed
of light. The gamma rays are created when the blast wave collides with
stellar material still inside the star.
Erupting through the star surface, the blast wave of stellar material
sweeps through space, colliding with intervening gas and dust,
producing additional emission of photons. These emissions are
believed responsible for the "afterglow" of progressively less energetic
photons, starting with X rays, visible light and radio waves
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The Fireball
model
The
Fireball model is the most widely used theoretical framework to
describe the physics of the GRBs.
 It originates from considerations on the total energy release of a GRB
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and its extremely short variability time
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