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7. Astronomia GammaI- Introduzione e satelliti

(Cap. 8 Libro)

Corso “Astrofisica delle particelle”

Prof. Maurizio SpurioUniversità di Bologna a.a.

2014/15

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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

Outline

Thermal radiation: Black Body Spectrum

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

CMBR: 2.7 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

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~ parsecs

Accretion Disk 3- 10 rs Black Hole Diameter = 2rs ~ 4 AU

GRB SuperNova Remnants

Pulsars

Active Galactic Nuclei

Gamma rays interact with the atmosphere

GeV detection requires satellites; TeV (VHE) can be done at ground

energy

Detectors

• Satellites (AGILE, Fermi)– Silicon tracker (+calorimeter)

• Cherenkov telescopes (HESS, MAGIC, VERITAS)

• Extensive Air Shower det.(ARGO): RPC, scintillators

HEP detectors! 7

Precision Si-strip Tracker (TKR) 18 XY tracking planes

Single-sided silicon strip detectors 228 mm pitch, 8.8 105 channels

Measure the photon direction

e+

e-

g

Satelliti

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Fino a qualche centinaiodi GeV

Telescopi Cherenkov al suolo

100 GeV – decine di TeV

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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 R g 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/Fermi-LAT 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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• 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?

Noto ed ignoto da EGRET: segnale- fondo = sorgenti

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 γ-ray 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 counterparts

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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):

ep

ep

dE

dN

dE

dN

,

,

g-rays production@sources

e- (TeV) Synchrotron (eV-keV)

B

leptonic processesE2 d

N/d

E

energy E

IC

(TeV) Inverse Compton (eV)

-

0

+

(TeV)

p+ (≫TeV)

matter

hadronic process

To distinguish between hadronic/leptonic origin the Spectral Energy Distribution (SED) must be studied with different experimental techniques

Radio Optical X-ray GeV TeV

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n F

(n)

p0

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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 p0 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 po 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)

Possiamo stimare la luminosità attesa di fotoni dal piano galattico:

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Bremmstrahlung

Compton Inverso

Decadimento po

eeldNuclearFie

highlow ee ' 0

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Stima del flusso da p0

– spp=sezione d’urto inelastica= 40 mb

– n = densità del mezzo ISM = 1p/cm3

– rCRg =densità dei RC che produce g-ray: 10% of rCR. Dovuto al fatto che 10% dell’energia viene trasferita ai p

– c = velocità della luce = 3 1010 cm/s– Energia trasferita ai pioni neutri: 1/3 del totale 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)

The estimated average value is nD 1020 cm-2 and:

Confronta con la Fig.

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Sorgenti= segnale – fondo diffuso

GeV g-rays sky (April, 15rd 2013)

32arXiv :1304.4153

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Fermi LAT Second Source Catalog (2y)

 arXiv:1108.1435v2

LAT 2-year Point Source Catalog (2FGL)

Maurizio Spurio - Frascati Workshop '13

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Energy spectral index of 2FGL

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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!

Maurizio Spurio - Frascati Workshop '13

FERMI Hard sources >10 GeV sky

259

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71

11

9

55

6

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36arXiv :1304.4153

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Sorgenti Galattiche

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What sort of compact object?

How are the particles accelerated?

Are there different types of such high-mass binary systems?

Esempi di sorgenti galattiche: microquasars• 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.

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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?

AGN

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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?

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 R g e l’emissione radio vista dai VLBI

Immagine da VLBI. Vedihttp://web.whittier.edu/gpiner/research/index.htm

Leptonic vs. hadronic models

?Multifrequency/multimessenger observation required

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La SED (Spectral Energy Distribution)

• 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.

GLAST LAT AGILE

TeV

INTEGRAL GLAST GBM Swift

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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)

Origine Extragalattica. Possibili candidati di meccanismi di accelerazione per i RC di energia estrema.

BATSEon CGRO

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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 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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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 different physical properties

A sampling of the large variety of GRB time profiles, as detected from the CGRO satellite

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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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 times the SN).

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The Italian BeppoSAX satellite 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

BeppoSAX was equipped with both a -g ray and an X-ray

detector. It spotted the X-ray afterglow signature associated with the GRB on February 28, 1997

Discovery of the extragalactic origin of GRBs

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 and its extremely short variability time