GammaRay Astrophysics

1
Gamma-Ray Astrophysics

• Nicola Omodei
• INFN Pisa

2
A brief history...

• ... of the gamma-ray astronomy (in space...)
• OSO-3 (1967-1969) 621 photons detected over 50
  MeV, first full sky survey.
• SAS-2 (1972-1973) first detailed information on
  the gamma-ray sky, diffuse background few point
  sources.
• COS-B (1975-1982) very successful mission,
  performance comparable with SAS-2.
• EGRET (1991-1996) first detailed full sky
  survey, hundreds of point sources discovered.
• GLAST - to be coming soon(er or later)!
• Keep in mind...
• CGRO (Compton Gamma Ray Observatory), including
  EGRET (Energetic Gamma Ray Experiment Telescope,
  10 MeV10 GeV) and BATSE (Burst And Transient
  Source Experiment, 10 keV10 MeV) onboard.
• GLAST (Gamma Ray Large Area Space Telescope).

3
Outline

• Introduction to gamma astrophysics
• Experimental technique
• Ground vs. space.
• Gamma-ray astronomy in space.
• The heritage of the previous missions.
• The design of a gamma-ray telescope basic ideas.
• GLAST design and performance.
• Zoology of the gamma-ray sky (EGRET vs. GLAST!)
• Point sources vs. diffuse background.
• Active Galactic Nuclei.
• Gamma Ray Bursts.
• Pulsars

4

• Part I

5
Introduzione ordini di grandezza
Il risultato delle interazioni che avvengono
all'interno dei lontani corpi celesti è
l'emissione di radiazione luminosa. Più un
fenomeno è violento maggiore è la quantità di
energia che viene rilasciata sotto forma di luce.
Il tipo di luce che viene emessa non è solo
visibile ma può avere energie anche molto
superiori. Lo studio della luce altamente
energetica (radiazione gamma) permette di avere
informazioni sui processi fisici che riguardano
oggetti molto energetici e molto lontani da noi.
Le grandezze studiate da questo ramo
dell'astrofisica arrivano fino a 1052 erg al
secondo ovvero potenze paragonabili a 1019 soli
(circa dieci miliardi di miliardi di soli), le
distanze sono dell'ordine dei milioni di anni
Luce (MLY) ovvero 1018 centimetri (un miliardo di
miliardi di centimetri).
6
Absorptions propagation

• A photon produced in the deep universe may be
  absorbed by
• Diffuse background (gamma-gamma interaction)
• Earth Atmosphere (gamma-Nuclei interaction)
• The photons point directly to the source!
• They are not deviated by the Magnetic Fields
  (GMF)
• Cosmic Rays (charged particles) are deviated by
  the magnetic fields and loose information on the
  production site.
• High Energy Cosmic Rays (HECR) need strong
  acceleration processes.
• The CR emission is related to emission of High
  Energy Gamma-Rays due to the Radiation-Matter
  Interaction (Synchrotron, Compton)
• The only way to understand where CR are produced
  is to observe gamma-rays!

7
Diffuse background absorption
Pair production
Optical depth, tau gt 1 gt Opaque tau lt 1 gt
Transparent
Mannheim, Hartmann Funk, 1996
8
Atmospheric absorption
Transparent
Balloon-rockets satellites
Transparent
Balloon-rockets satellites
O2 O3
Oxygen and Nitrogen
H2O CO2 O3
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The photon cross section
g
e-
g
e-
g
g
e-
e-

• Energy lt 100 keVPhotoeletric effect gt X-Ray
  astronomy
• Energy lt 10 MeV Compton Scattering gt Soft
  gamma-Ray astronomy
• Energy gt 10 MeV Pair Production gt gamma-ray
  astronomy

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Gamma-ray astronomy space based...

• Detection technique
• Pair conversion is the dominating interaction
  process for gamma-rays.
• Electron/positron pair provides the information
  about the gamma direction.
• Electron/positron pair provides a clear signature
  for background rejection.

• Basic structure of a pair conversion telescope
• Tracker/converter (detection planes high Z
  foils) photon conversion and reconstruction of
  the electron/positron tracks.
• Calorimeter energy measurement.
• Anti-coincidence shield backgound rejection
  (cosmic rays flux 104 higher than the gamma
  flux).

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... and ground based

• Detection technique
• Ultra high energy gamma-rays interact with the
  atmosphere and generate showers.
• Cherenkov light provides spectral and directional
  information.
• Gamma and cosmic rays induced showers have
  different shapes high beckground rejection power.

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Ground-based vs. Space-based

• Ground-based detectors
• Advantages
• Huge effective area (10000 m2).
• Excellent angular resolution (0.02).
• Good energy resolution (10-15 ).
• Wide energy range (50 GeV-100 TeV).
• Disadvantages
• Small field of view (few degrees). Pointing
  device.
• Not sensitive below few tens of GeV (not enough
  Cherenkov light).
• Short duty cycle (10)

• Space-based detectors
• Disadvantages
• Small effective area (1 m2).
• Not sensitive above few hundreds of GeV (lack of
  photons).
• Advantages
• Good angular resolution (0.1-1).
• Good energy resolution (10-15 ).
• Wide energy range (10 MeV-300 GeV).
• Huge field of view (2.5 sr). Scanning device.
• Long duty cycle (100)

Sometimes words have two meanings... (Led
Zeppelin, 1972)
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Ground and space

• Complementarity between ground and space
• GLAST will match the sensitivity of the new
  generation of ground-based experiments.
• Crucial for multi-wavelenght campaigns.
• Overlap for the brighter sources
  cross-calibration of the absolute energy scale
  and alerts.

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Instruments for space-based gamma-ray astronomy

• SAS-2 (1972 - 1973)
• Energy range 30 MeV 1GeV
• Energy resolution 100
• Peak effective area 100 cm2
• Field of view 0.25 sr

• EGRET (1991 - 1996)
• Energy range 20 MeV 30GeV
• Energy resolution 15
• Peak effective area 1500 cm2
• Field of view 0.5 sr

1970
1980
1990
2000
Time

• COS-B (1975 - 1982)
• Energy range 30 MeV 5GeV
• Energy resolution 40
• Effective area 70 cm2
• Field of view 0.25 sr

Balloon flights, Small satellites ( 621 photons
above 50 MeV detected by OSO-3!)
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Basic definitions the IRFs

• The Instrument Response Function
• Allow to evaluate the detector response (in terms
  of the measured quantities E and W) to a know
  flux F (function of the true quantities E and
  W).
• Effective area Aeff.
• Energy response DE.
• Point Spread Function (PSF) angular resolution.
• Usually NEEDED to go the other direction from
  the measured counting rate to the real flux!
• Excellent knowledge of the IRF crucial for the
  overall normalization of the flux. The IRF depend
  not only on the instrument design, but also on
  the reconstruction algorithms!

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Basic definitions the effective area

• The effective area
• Determines the REAL RATE of detecting a signal,
  given a source flux, after ALL detector,
  reconstruction and background rejection effects
• It can be written as the product of geometric
  area, conversion probability, efficiency of the
  detector and of the reconstruction algorithms.

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Basic definitions the FOV

• The Field Of View
• It is defined as the integral of the Effective
  area over the solid angle divided by the peak
  effective area.
• If the angular response does NOT depend on the
  angle, the FOV is 4p (the whole sky).
• In the case of a planar detector (Aeff(q)
  A0cosq), the FOV is p (1/4 of the sky).
• The FOV basically depends on the Aspect Ratio of
  the instrument (height/width).

TKR
Low aspect ratio Large FOV
High aspect ratio Small FOV
TKR
CAL
CAL
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Basic definitions the PSF

• The Point Spread Function
• It is the effective angular resolution after all
  detector, reconstruction and background rejection
  effect.
• For a purely gaussian response, 2D 68
  containment angle is 1.41 times the 1D error and
  the 95 containment angle is 1.6 times the 68
  containment angle.
• The response is typically NOT gaussian and the
  crucial parameter is the PSF95/PSF68.

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Flown instruments SAS-2

• Basic design
• Tracking system based on spark chambers (upper
  set, interleaved with conversion foils, and lower
  set).
• Triggered by external scintillators.
• No calorimeter (energy information derived by the
  measurement of the multiple scattering).
• Monolithic anticoincidence shield.
• Last Solid State Recorder failed after 6 months
  of mission...

• SAS-2 (1972 - 1973)
• Energy range 30 MeV 1GeV
• Energy resolution 100
• Peak effective area 100 cm2
• Field of view 0.25 sr

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Flown instruments COS-B

• Basic design
• Tracking system based on spark chambers. Sensible
  degradation in performance after few years.
• Triggered by external scintillators.
• No calorimeter (energy information derived by the
  measurement of the multiple scattering).
• Monolithic anticoincidence shield.

• COS-B (1975 - 1982)
• Energy range 30 MeV 5GeV
• Energy resolution 40
• Effective area 70 cm2
• Field of view 0.25 sr

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Flown instruments EGRET

• Basic design
• Tracking system based on spark chambers (upper
  set, interleaved with conversion foils, and lower
  set). 100 ms dead time per event.
• Triggered by external scintillators. Time Of
  Flight (TOF) improve background rejection
  capabilities.
• Monolithic calorimeter (8 radiation lenghts!).
• Monolithic anticoincidence shield (high energy
  effective area drops down because of the self
  veto due to albedo electrons from the
  calorimeter).

• EGRET (1991 - 1996)
• Energy range 20 MeV 30GeV
• Energy resolution 15
• Peak effective area 1500 cm2
• Field of view 0.5 sr

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How should a conversion telescope look like?

• Basic features
• Large effective area (but remember the
  ground-based detectors..)
• Large Field Of View.
• Good angular resolution.
• Good energy resolution over a wide energy range.
• Short instrumental dead time (i.e. NO spark
  chambers).
• No consumables (again NO spark chambers).
• Operation in space requires
• Modular, robust, redundant design.
• Mass budget.
• Power budget.

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Overall design drivers

• Mission divers
• Allocated space on the launcher
• Forces the maximum possible lateral dimension
  (and geometric area).
• Power budget
• Restricts the number of readout channels in the
  tracker (i.e. strip pitch, number of layers).
• Mass budget
• Basically bounds the total depth of the
  calorimeter.

• Science drivers
• Background rejection
• Drives the ACD design.
• Also impact on TKR/CAL design.
• Effective area and PSF
• Drive the converter thicknesses and layout.
• PSF also drives sensor performance, layers
  spacing and overall tracker design
• Energy range/resolution
• Drive the thickness/design of the calorimeter.
• Field of view
• Basically sets the aspect ratio (width/height).

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Design drivers the tracker

• Tracker/converter design determines the PSF
• Low energy PSF completely dominated by multiple
  scattering effects ( 1/E).
• High energy PSF set by hit resolution/lever arm.
• Converter foils layout/detectors design/layers
  spacing determine the rollover energy and the
  asymptotic value of the PSF _at_ high energy.

• Complex trade off
• Converter foils thickness detection efficiency
  vs. multiple scattering degrading the PSF _at_ low
  energy.
• Spacing between conversion foils multiple
  scattering vs. aspect ratio.
• Spacing between tracking planes high energy
  angular resolution (big lever arm) vs. aspect
  ratio.
• Pitch of the readout hit resolution vs. number
  of channels (i.e. power consumption).

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The GLAST design

• Overall modular design
• 4x4 array of identical towers - each one
  including a Tracker, a Calorimeter and an
  Electronics Module.
• 3000 kg, 650 W.
• Surrounded by an Anti-Coincidence shield.

• Anti-Coincidence (ACD)
• Segmented (89 tiles).
• Self-veto _at_ high energy limited.
• 0.9997 detection efficiency (overall).

• Tracker/Converter (TKR)
• Silicon strip detectors.
• W conversion foils.
• 80 m2 of silicon (total).
• 106 electronics chans.
• High precision tracking, small dead time.

• Calorimeter (CAL)
• 1536 CsI crystals.
• 8.5 radiation lengths.
• Hodoscopic.
• Shower profile reconstruction (leakage
  correction)

Tower DAQ (TEM)
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Trigger and onboard data processing

• Level 1 trigger
• Hardware trigger, single-tower level.
• Three_in_a_row three consecutive tracker x-y
  planes in a row fired. Workhorse g trigger.
• Cosmic rays in the L1T! 13 kHz peak rate.
• Upon a L1T the LAT is read out within 20 ms.

• On-board processing
• Identify g candidates and reduce the data
  volume.
• Full instrument information available to the
  on-board processor.
• Use simple and robust quantities.
• Hierarchical process (first make the simple
  selections requiring little CPU and data
  unpacking).

x
x
x

• Level 3 trigger
• Final L3T rate 30 Hz on average.
• Expected average g rate few Hz
• (g rate cosmic rays rate 1 few).
• On-board science analysis (flares, bursts).
• Data transfer to the spacecraft.

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GLAST vs. EGRET

• 1After background rejection.
• 2Single photon, 68 containment, on axis.
• 31s, on axis.
• 41s radius, high latitude source with 10-7
  cm-2s-1 integral flux above 100 MeV.
• 51 year sky survey, high latitude, above 100 MeV.

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Tracker construction work flow

• SSD procurement and testing

• Ladders assembly

• Towers assembly

18
10,368
342
2592

• Trays assembly and test

648

• Panels fabrication

• Readout electronics fabrication, test and burn-in

29
Assembly of tower 0
30
Assembly of tower 0
31
Test of tower 0
32

• Part II

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The sky above 100 MeV

• The heritage of EGRET
• Diffuse extragalactic background ( 1.5x10-5
  cm-2s-1sr-1 integral flux).
• Much larger (100 times) backgound on the
  galactic plane.
• Few hundreds of point sources (both galactic and
  _at_ high latitude).
• Essential characteristics variability in time.

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The sky map EGRET vs. GLAST

• 3rd EGRET catalog
• 271 point sources, based on 5 years of data.
• GLAST 1 year sky survey (simulated)
• Thousands of sources will be discovered.
• All EGRET sources detected in one day!
• Detailed study of galactic and extragalactic
  background.

Integral Flux (Egt100 MeV) cm-2s-1
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Unidentified sources I

• What do we know about it?
• 170 unidentified sources in the 3rd EGRET
  catalog.
• No counterpart at other wavelenghts.
• High Lg/Lradio, Lg/Loptical, Lg/Lx.
• Some are variable, others are steady, both in the
  galactic plane and at high latitude.

Counting stats not included.
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Unidentified sources II

• What do we need?
• Better angular resolution (smaller error bars).
• Excellent spectral and timing capabilities to
  detect typical signatures (flares, spectral
  features, pulsation) and make the identification
  easier.

Cygnus region 15o x 15o, E gt 1 GeV
Counting stats not included.
GLAST and EGRET 95 containment regions
superimposed on a portion of the 1.4 GHz NRAO VLA
sky survey.
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Diffuse background I

• According to EGRET...
• Apparently isotropic, extragalactic flux of
  gamma-rays (already discovered by SAS-2) above 30
  MeV.
• Removal of point sources (impossible with COS-B
  and SAS-2 data) very difficult due to the large
  EGRET uncertainties.
• What remains after the removal of point sources?
• 1) Nothing the background is nothing but
  composite light of many faint sources. GLAST will
  surely resolve most of the diffuse background
  into point sources.
• 2) Relic radiation from high energy processes in
  the early universe. Exciting perspectives for
  cosmology.

38
Diffuse background II

• Removal of point sources galactic flux
• Analysis conducted with EGRET data.
• Contribution of point sources removed (difficult
  due to the large size of the Point Spread
  Function).
• Contribution of galactic diffuse emission (mainly
  due to the interaction of cosmic rays with the
  interstellar nuclei and photons) also subtracted
  (difficult, as well).
• The remaining flux...
• Well described by a power law with spectral index
  a 2.1 0.3 over the EGRET energy range.
• Apparently isotropic (but with large statistical
  uncertainties!) on a fairly large scale (30).
• Spectral index compatible with the average
  spectral index of EGRET blazars.
• Data somehow support the hypotesis 1), but still
  difficult to infer the fraction of diffuse
  background due to point sources.

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Diffuse backgrund III

• Particle decays in the early universe
• Energy spectrum of this component should be
  different from the AGN contributions.
• Statistically significant detection of this
  contribution impossible with EGRET.

40
Diffuse background IV

• Possibility of lines from dark matter
  annihilation
• Neutralino (c) is a good candidate for the dark
  matter of the universe.
• LSP (Lightest Supersimmetric Particle) it is
  neutral and stable if R-parity is not violated.
• Current viable models with Mc in the 30 GeV 10
  TeV range.
• Possible scenario dark matter made of
  non-relativistic neutralinos gamma-ray lines in
  the
• cc -gt gg (Eg Mc)
• cc -gt gZ (Eg 1mZ2/4Mc2)

41
AGNs

• Basic phenomenology
• Vast amount of energy (1049 erg/s) emitted from a
  compact central volume.
• Energetic, highly collimated, relativistic
  particles jets.
• Significant fraction of the radiation emitted in
  the gamma-rays.
• Blazars
• Constitute most of the EGRET point sources.
• Highly variable (hour timescale).
• Strongly polarized (in the radio band!).
• Prevailing idea
• Accretion onto supermassive black holes.

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Spectra

• Add fv, vfv, e2 fv

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Blazars

• Blazars models
• nFn spectra generally show two distinct
  components.
• Low frequency (peaking between radio and x-rays)
  substantially polarized, believed to be
  synchrotron emission from high energy electrons.
• BTW it would be interesting to perform
  polarimetry in the x/gamma rays!
• High frequency peaking in the gamma-rays.

44
Blazar jets electrons vs. adrons

• Leptonic models
• High energy gamma-ray emission due to Compton
  scattering of soft photons on high energy
  electrons and positrons in the jet (ICS, Inverse
  Compton Scattering).
• Where do the soft photons come from?
• SSC (Synchrotron Self Compton) low energy g from
  synchrotron emission of the leptons themselves.
• ECS (External Compton Scattering) photons from
  an accretion disk enter the jet directly.
• Adronic models
• If the jets include hadrons, they can interact
  with surrounding matter/radiation producing
  secondary pions and electrons.
• e/e- and p can in turn initiate electromagnetic
  cascades through Compton scattering and
  synchrotron emission leading to emerging power
  law spectra.

45
Blazar jets

• Things to do...
• Evolution of synchrotron/gamma-ray components
  during flares can discriminate between models.
• Need of time-resolved spectroscopy!

46
GRBs I

• Basic phenomenology
• Flashes of high energy photons in the sky
  (typical duration is few seconds).
• Isotropic distribution in the sky (surprise from
  BATSE).
• Cosmological origin accepted (furthest GRBs
  observed _at_ z5 billions of light-years).
• Not repetitive, as far as we know. Never seen two
  GRBs from the same location (destructive
  phenomenon?).
• Extremely energetic and short (the greatest
  amount of energy released in a short time after
  the big bang).
• Sometimes x-rays and optical radiation observed
  after days/months (afterglows).

47
GRB observations

• First detected...
• ... in early 70 by military satellites.
• Originally connected with Neutron Stars (NSs) in
  the Milky Way.
• Interpreted as powerful but NOT spectacular
  flashes.
• Then CGRO came...
• EGRET (10 MeV-10 GeV) 1 burst per year.
• BATSE (10 keV-10 MeV) 1 burst per day.
• Distribution in the sky found to be isotropics.
• Cosmological origin.
• The afterglow era...
• BeppoSax X-ray afterglows.
• Keck optical afterglow.

48
GRBs distribution in the sky

• No correlation with the galactic plane
• Cosmological origin

49
GRBs timescale

• Prompt emission
• Burst duration 1-100 sec.
• Bimodal distribution (IMPORTANT short bursts vs.
  long bursts).
• Afterglow
• Gamma-ray 1 hour.
• X-rays 1 day.
• Optical 1 month.
• Radio...

50
Light curves

• Variability
• Timescales as short as few ms.
• Shape of the light curves
• Very various phenomenology.
• One smooth peak.
• One short spike.
• Several peaks.
• Intermittent activity.
• NO PERIODICITY observed.

51
GRBs Power Density Spectra

• PDS
• Ligth curves very different, similar PDS (sample
  of 527 light curves of long bursts, T90 gt 100 s).
• PDS are, with good approximation, power laws with
  a spectral index of -5/3.
• Non trivial dependence of spectral index upon the
  energy bin.
• GRBs can be interpreted as random short
  realizations of a standard process.

52
Energy dependence of light curves I

• BATSE data in 4 energy bins
• Harder pulses are shorter.

20-50 KeV
50-100 KeV
100-300 KeV
gt 300 KeV
53
Energy dependence of light curves II
WFWHM of ACF
Burst duration

• Scaling law
• General shape for the ACF.
• WFWHM 3.2E-0.42.

54
High energy spectra

• High energy behavior of GRBs
• Still poorly known.
• Need of more sensitive instrumentation!

55
GRB physics

• The standard fireball model
• Central engine emits shells of matter (e/e-)
  with Lorentz factors up to 1000.
• Inelastic shocks between fast and slow shocks.
• Particles acceleration on the shock front, Self
  Synchrotron production of high energy gamma-rays.
• Afterglows interpreted as interaction with the
  interstellar medium (External Shock).

56
The central engine

• The debate is still opened
• Merging of compact objects (NS-NS, NS-BH, BH-BH).
  Observed within our galaxy.
• Supernovae massive stars collapsing into
  spinning black holes. Identification with SN
  explosion observed at least in one case.

57
Pulsars

• Phenomenology
• First point sources detected in the gamma rays.
• Extraordinary cosmic laboratories.
• Extreme gravitational fields.
• Extreme magnetic fields.
• Pulsars studied in all the regions of the
  electromagnetic spectrum.

58
Polar Cap vs. Outer gap
59
Quantum Gravity

• Lorentz invariance breaking
• Considered within various theoretical frameworks.
• In its simpler form
• E2 m2 p2 f(p2, E, m Mp)
• Mp is a mass scale (1019 GeV).
• Photons propagating in vacuum may exibit a non
  trivial refractive index.

Easiest case NO intrinsic delay (
)
E1
E1
E2
E2
tti
ttf