Prsentation PowerPoint

1
Hanoi, August 2005
Observations of Cosmic Rays Lecture 2 Detection
methods and experiments gamma rays Tiina
Suomijärvi Institut de Physique
Nucléaire Université Paris XI-Orsay,
IN2P3/CNRS France
2
Experimental techniques

• Gamma rays encompass a vide range of energies,
  from below MeV to 1020 eV -gt several experimental
  techniques are required
• Satellite experiments at MeV to GeV energies
• Cherenkov telescopes between 50 GeV and 50 TeV
• Air shower arrays above 10 TeV

3
Experiments at different energies
X-rays
Low-E g-rays
High-E g -rays

Very-high-E g -rays
keV
GeV
MeV
TeV
Focusing instruments
Coded masks
Tracker (ee-) Calorimeter
Cherenkov telescopes
e.g. INTEGRAL
e.g. HESS, VERITAS, MAGIC
e.g. Chandra, XMM-Newton
e.g. EGRET AGILE,GLAST
Collimators e.g. OSSE
COMPTON telescopes e.g. COMPTEL
SATELLITES
GROUND
4
From radio-waves to TeV gamma-rays

• Spectral energy distribution
  of PKS2155-304

19 orders of magnitude in energy (10-6 to 1013 eV)
E2 d ? /dE E d ? /d ln(E) ? N(photons) /m2/s
Spectral energy distribution -gt
5
Main experimental challenges

• Flux sensitivity
• ? g (gtE) K E-1
• Typical fluxes (e.g. Crab nebula)
• E gt MeV 4 x 10-3 cm-2 s-1
• E gt GeV 3 x 10-8 cm-2 s-1
• E gt TeV 2 x 10-11 cm-2 s-1
• Angular resolution
• Presently up to 1 degree
• Spectral resolution ?E/E
• 10 to 15 (GeV) -gt15-20 (TeV)
• Compare to 0.1 with Ge detectors (100
  keV-1MeV)
• Sensitivity to unexpected transient phenomena
• Field of view
• Alert network for subsequent follow-up of the
  source
• Temporal resolution ? Flux sensitivity

6
Progress in sensitivityNumber of sources in
catalogues vs. year
7
Satellite experimentsA brief historical
background

• SAS-2 (NASA) 1972-1973
• worked during 6 months only
• discovered the diffuse g -ray background and
• 3 point-like sources Crab nebula, Vela ,
  Geminga
• COS-B (ESA) 1975-1982
• Catalogue of 25 sources, all galactic but one
  the quasar 3C273
• EGRET (NASA, within Compton Gamma-Ray Observatory
  or CGRO) 1991-2000
• Discovery of the extragalactic g -ray sky (about
  60 sources)
• 3rd EGRET catalogue about 300 sources
• BATSE All-sky monitor for CGRO (GRBs)

8
ECRET
Energetic Gamma Ray Experiment Telescope, CGRO

• Detects high energy gamma rays via their
  conversion to electron-positron pairs.
• Spark chamber layers for tracking
  electron-positron pairs.
• Total absorbtion calorimeter made of NaI to
  measure track energies.
• Anticoincidence scintillator dome that covers the
  detector to remove charged particles.
• Time-of-flight system to provide a trigger.

Energy range 20 MeV - 30 GeV Angular resolution
3.5 at 100 MeV, improves to 0.35 at 10 GeV
9
Analysis and results EGRET data analysis

• Charged cosmic rays are eliminated
• Anticoincidence detector
• Gap required in tracker prior to ?-ray conversion
• Separate point-like sources from diffuse emission
  
• Model for diffuse emission needed

10
Detecting point-like sources with EGRET

• Main problem separate the contribution of
  point-like sources from that of the diffuse
  emission
• Use maximum likelihood method based on
• Knowledge of the point-spread function (PSF)
• Model of diffuse emission
• Systematics due to the diffuse emission model
  used -gt require 5s to include candidate into the
  catalogue
• Lower sensitivity in the Galactic Plane
• Low statistics

11
Modeling the galactic diffuse emissionin
high-energy g-rays

• Proton or ion collisions with interstellar matter
  -gt p0 -gt g g (main contribution at high energy)
• Matter distribution needed
• Cosmic-ray spectrum within the Galaxy needed
• Bremsstrahlung of energetic electrons
• Inverse Compton scattering of energetic electrons
  on radiation fields

12
Diffuse galactic flux (EGRET measurement vs.model)

• p 0 -gt gg
• Electron Bremsstrahlung
• Inverse Compton scattering
• Extragalactic diffuse component
• Data show excess with respect to model above 1
  GeV (Hunter et al. 1997)

13
(No Transcript)
14
BATSE
Burst And Transient Source Experiment, CGRO
Search for Gamma ray Bursts
Detector 8 detectors at the corners of the
satellite Large Area Detector and smaller
spectroscopy detector, both made of NaI
crystals Large Area Detector collect photons in
the range between 25 keV and 2 MeV The direction
of photons is determined from the relative pulse
heights recorded in PMTs viewing the each
detector -gt Uncertainty of a few degrees
15
Gamma-ray bursts

• Short and intense pulses
• of low-energy g-rays
• Spectrum peaks
• around 1 MeV
• or below

16
Gamma-ray burst detectors

• Associate wide-field instrument (alert) to
  accurate pointing instruments (X-ray/optical)

BATSE
Compton GRO
WFC

Beppo-SAX
GRBM
FREGATE
HETE-2
WFX
SXC
INTEGRAL
XRT
BAT
SWIFT
GLAST GRB Monitor
10 MeV
10 keV
100 keV
1 MeV
1 keV
17
The  INTEGRAL  mission(INTErnational Gamma-Ray
Astrophysics Laboratory)

• 15 keV 10 MeV
• Launched in November 2002
• Combine imaging and spectrometry
• Two instruments with both capabilities, with
  emphasis on one of them
• SPI spectrometer emphasis on spectroscopy
• 19 high-purity Germanium detectors (85 K)
• IBIS imager emphasis on imaging
• Coded mask pixel detectors (2 layers)
• ISGRI (Cd Te semi-condr.) and PICsIT (CsI
  scintillator)
• Simultaneous multi-wavelength analysis
• X-ray monitor JEM-X (3-35 keV, 4.8 field of
  view)
• Localize source at lt 20 
• Optical monitor OMC (5 field of view)
• Localize source at lt 8

18
Coded mask instruments on board INTEGRAL
IBIS Imager (INTEGRAL) Angular resolution
12 Source location lt 1'
SPI spectrometer ?E/E 0.2 _at_ 1 MeV
19
INTEGRAL Summary
20
The Galactic Centre region as seen by
INTEGRAL-IBIS (Nov. 2003)

• Energy region
• 20-40 keV
• Field of view 2x 2
• Pixel size 5'
• Isosignificance
• contours from 4s to 15s

21
High energy region (30 MeV-100 GeV)

• g -ray conversion into ee- pair
• Tracker
• Converting material
• detection planes
• -gt direction measurement
• Calorimeter
• -gt energy measurement
• Anticoincidence dome
• -gt remove charged particles

The GLAST Large Area Telescope (to be launched in
2007)
22
GLASTGamma-ray Large Area Space Telescope
23
GLAST vs. EGRET
24
Old and new detectors
25
GLAST performance
Angular resolution (compared to EGRET)
Expected number of extragalactic sources
26
Gamma-ray bursts with GLAST

• EGRET was not adapted to the detection of g -ray
  bursts, due its long deadtime (0.1 s)
• GLAST

27
Cherenkov telescopesVery high energies, above 50
GeV

• Very low fluxes
• e.g. Crab nebula flux( E gt 1 TeV ) 2 x
  10-11 cm-2 s-1
• Large effective detection areas (gt30 000
  m2) needed
• -gt Back to the ground
• Use the atmosphere as a
• huge calorimeter and
• detect g-ray-induced
• atmospheric showers
• through Cherenkov light

28
Experimental challenges

• Reduce the energy threshold as much as possible
• Try to get some overlap region with space
  observations
• Increase flux sensitivity
• Remove the huge background of showers induced by
  charged particles (cosmic ray protons, ions and
  electrons)

29
A gamma-ray induced electromagnetic shower
A proton-induced hadronic shower
On average rotational symmetry
Larger tranverse momenta Presence of muons from
meson decays (in red on the figure)
Small transverse momenta (Almost) no
muons Essentially e e- and secondary g-rays
30
Atmospheric Cherenkov techniques

• Only working by clear moonless nights
• -gt Duty cycle 10 or less
• Detection area size of the Cherenkov light pool
  on the ground
• Cherenkov angle 1 at ground level
• Light pool diameter 300 m at 2000 m a.s.l.
• Cherenkov light peaks at short wavelength
  (blue/UV)
• Very brief flash of Cherenkov light (a few
  nanoseconds) -gt need fast photodetectors
• Limited field of view (a few degrees) -gt tracking
  instrument

31
Two different approaches

• Cherenkov Imaging Cherenkov Sampling

Shower image in the focal plane of a telescope
Arrival times amplitudes on many stations
32
Towards lower thresholds
Night sky background light 1012 photons m-2
sr-1 s-1

• Increase photon collection area reflector area
  Acol
• Increase photon detection efficiency ?
  (reflectivity, light collectors, phototube
  quantum efficiency)
• Coincidence time ?t should not be much greater
  than the time spread ? of Cherenkov photons -gt
  isochronous mirror,
• fast trigger
• Solid angle on which photons are summed up ??
  should not be much greater than the angular size
  of the shower ?s
• -gtsmall pixels, trigger based on sectors of
  the field of view

33
Imaging Atmospheric Cherenkov Telescopes

• Shower image in focal
• plane
• Gamma vs. Hadron
• discrimination based on
• Image shape
• Image direction
• (for point-like sources)
• Cherenkov light profile -gt impact distance and
  primary energy

34
Historical background

• 1986 Whipple Observatory (Arizona)
• 75 m2 reflector
• First imaging camera (37 pixels)
• 1989 Whipple Observatory discovery of the Crab
  nebula TeV signal (T.C. Weekes et al.)
• 1995 HEGRA experiment (Canary Islands)
• First stereoscopic system (5 tel. x 8.5 m2)
• 1996 CAT (French Pyrenees) fast electronics
• high-definition camera (600 pixels)-gt 250
  GeV
• threshold with 18 m2 telescope

35
Present imaging atmospheric telescopes
36
VERITAS
CANGAROO III
MAGIC
HESS I
37
Mirrors and cameras

• Mirrors have parabolic or spherical shape,
  varying in size from 2 tto more than 10 m.
• Mirror segments are front surfaced with aluminum
  for maximum reflectivity of light between 250 and
  550 m.
• PMTs are traditionally used as photon detectors
  because of their speed, high gain, good linearity
  and reasonable cost.
• PMT quantum efficiency typically about 20.

38
Imaging telescopes the cameras
39
High-definition cameras
VERITAS
MAGIC
40
High-definition cameras(H.E.S.S.)

• 960 phototubes
• equipped with
• light collectors
• (Winston cones).
• Trigger electronics
• within the camera
• (overlapping sectors
• majority logic).
• Readout from
• analogue memories
• (1 GHz sampling) within the camera.
• Analogue signal integrated over 12 ns -gt ADC

41
Calibration

• Light collection efficiency
• Use muons falling onto the
• mirror -gt ring-like image
• (cf. Ring Imaging Cherenkov)
• Incoming light yield known
• -gt overall efficiency f (time)
• Atmosphere
• Radiometer, LIDAR
• Trigger rate, corrected for
• the effect of zenith angle
• Effective detection area
• from simulations

42
Hardonic rejection

• Image shape
• Electromagnetic showers
• elongated, quasi-elliptic shape
• Hadronic showers
• more irregular shape
• Image direction
• Electromagnetic showers
• point to the source (the center of the field
  of view)
• Hadronic showers
• randomly oriented in the focal plane
• Image light profiles
• (longitudinal and transverse)
• help finding the source position

43
Stereoscopic measurement (e.g. HEGRA, H.E.S.S.)

• Direct measurement of the g-ray origin in the
  field of view (important for extended sources)
• Direct measurement of the impact on the ground
  (important for energy measurement)
• Better hadronic rejection
• Much better angular resolution

44
Single telescope analysis

• Hillas analysis (Whipple Obs.)
• Fit the light distribution as a bi dimensional
  Gaussian -gt first and second moments
• Cuts on width, length, angular distance depending
  on image  size  to strongly reduce the
  hadronic backround.
• For a given  size , an electromagnetic shower
  yields a thinner and more regular image than a
  hadronic shower.
• Distribution of the pointing angle a
• Signal should show up at small angles over a
  rather uniform background.

45
Distributions of the pointing angle

• Crab nebula

MAGIC
VERITAS
46
Background monitoring and subtraction(point-like
source)

• Aim at a location in the sky shifted by e.g. 0.5
  in right ascension or declination with respect to
  the source
• Use the symmetrical position in the field-of-view
  as an  anti-source  selected events pointing
  to the anti-source monitor the remaining hadronic
  background (after shape cuts)-gt  OFF -source
  distributions.

47
HESS results New pulsar wind nebulae

• MSH15-52 powered by pulsar PSR B1509-58
• Elongated (two arms) similar to X-ray morphology

48
HESS results The galactic central region
White lines are radio contours

• Central region of the Galaxy showing the HESS
  signal
• (compatible with SgrA)
• and G0.90.1 (north)
• Black hole SgrA
• M 2.6 x106 Msol
• Supernova remnant
• SgrA East

49
The galactic centre energy spectrum

• Location within 51stat20syst from SgrA (if
  point-like)
• Power-law spectrum from 25 GeV to 20 TeV
  Differential spectral index 2.290.05stat0.1syst
  
• No evidence for a cutoff
• Ecut gt 6 TeV (95CL)
• Flux (Egt1TeV) (1.80.1stat0.3syst)x10-12
  cm-2s-1

50
Bounds on Dark Matter at the Galactic Centre

• Neutralinos ? (supersymmetric Majorana particles)
  are dark matter candidates
• Accumulation of dark matter in the vicinity of
  the Galactic Centre -gt ? ? annihilation -gt
  gs
• J(??) ?? astrophysical factor integral
  over line of sight of squared dark matter density

51
Bounds on Dark Matter at the Galactic Centre

• Energy spectrum cannot be reconciled with a pure
  dark matter spectrum with neutralino mass below
• 12 TeV (90 CL)

-gt no real constraints on SUSY models
52
TeV Blazars

• TeV blazars have rather low redshift compared to
  EGRET blazars (e.g. 3C279 has z0.54)
• Presently detected TeV blazars

53
Conclusions

• Three years ago, only a handful of TeV sources,
  most of them extragalactic
• Now, more than 20 sources above 120 GeV, both
  galactic and extragalactic.
• In 2007, GLAST should also completely change the
  GeV landscape with a catalogue of more than 3000
  sources.