Post on 19-May-2018
What are X-rays?What are X-rays?" X-rays are photons light (not "solid"X rays are photons light (not solid
particles)X h t l th (hi h )" X-rays are shorter wavelength (higher energy) than visible light photons
" Typical energies are 1 keV to 500 keV(Vs 1 5 eV to 3 eV for visible light)" (Vs. ~1.5 eV to ~3 eV for visible light)
" Typical wavelengths ~10Å to ~0.02Å
What makes X-rays?What makes X-rays?" Very HOT stuff produces X-rays viaVery HOT stuff produces X rays via
blackbody (thermal) radiation
What makes X-rays (II)y ( )" Electrical "collisions" between electrons and
t / l i ("b t hl " di ti )protons/nuclei ("bremstrahlung" radiation)
What makes X-rays (III)y ( )" Motion of relativistic (v~c) particles in a
ti fi ld (" h t " di ti )magnetic field ("synchrotron" radiation)
X-ray Binariesy" Star orbits a
compact objectcompact object (black hole or neutron star)neutron star)
" Gravity of CO pulls star matter offMatter orbits in" Matter orbits in accretion disk due to angular momentumangular momentum (like water in sink drain)drain)
X-ray Binaries - IIy" Friction transports
angular momentumangular momentum, allowing in-spiral of mattermatter
" Friction also heats matter to HIGHtempsp
" At T~108K, X-raysrays
Supernova remnantsp" Very energetic
particles fromparticles from supernova explosion
" Impact local interstellar medium "shock" frontX-ray" X-ray bremstrahlung
Pulsars" Rapidly rotating
neutron stars (more on this later)HUGE magnetic" HUGE magnetic fields (~1012 Gauss or higher)higher)
" Rotating magnet accelerates particles to v~c
" Relativistic particles in B-field in B-field synchrotron X-rays
Pulsar-powered supernova remnantsremnants
" Pulsar-acceleratedPulsar accelerated particles impact residual B-field ofresidual B-field of exploded star
" This gives more X-rays
" Crab Nebula SNR inner X-rayinner X ray synchrotron nebula
X-ray Burstersy" Matter accumulates
on surface ofon surface of neutron star
" More matter increases temp until thermonuclear flash occurs on NS surface
" Creates a "burst" of (mostly thermal) X(mostly thermal) X-rays
Summaryy" X-rays come from material in extreme
conditions (hot high velocity large magneticconditions (hot, high velocity, large magnetic fields)
" X-ray sources are often exotic systems: Black hole or neutron star binaries Black hole or neutron star binaries Supernova remnants
P l Pulsars Etc.
How do we observe X-rays?How do we observe X-rays?" Not from the groundNot from the ground" Atmosphere absorbs X-
rays" Can do a mediocre job j
of observing hard X-rays from balloonsays o ba oo s
" All X-ray astronomy b t d f t llitbest done from satellites
How do we detect X-rays?How do we detect X-rays?" X-rays are very penetrating, but do stop givenX rays are very penetrating, but do stop given
enough materialX d t t t" X-ray detector types:
Scintillators Proportional counters CCDs CCDs Many others
X-ray scintillator detectorsy" Scintillators = crystal or
l ti hi h itplastic which emits a flash of light when hit b Xby X-rays
" Many similar to table ysaltVisible light detector" Visible light detector "reads" scintillator
" Brightness of flash gives energy of X-ray spectroscopy
X-ray scintillator detectors - IIy" Scintillators have good time resolution
(t i ll 1 )(typically ~ 1 s)" Mediocre spatial resolutionp
" Mediocre energy resolution (E/E ~ 10)" Very resistant to radiation damage in orbit
X-ray Proportional Countersy p" X-ray strikes detector,
ti i i ticreating an ionization "cloud" of electrons
" Grid of wires under voltage carries charge g gto amplifiersAmplitude of" Amplitude of electrical impulse gi es energ of X ragives energy of X-ray spectroscopy
X-ray proportional counters - IIy p p" Proportional counters have good time
l ti (t i ll 1 )resolution (typically ~ 1 s)" Good spatial resolutionp
" Mediocre energy resolution (E/E ~ 10)" Very resistent to radiation damage in orbit
" Slowly leak gas limited lifetime in orbitSlowly leak gas limited lifetime in orbit
X-ray CCDsy" X-ray strikes CCD pixel
and deposits chargeand deposits charge
" Ionization energy ~1 eV
" 1 keV X-ray ~1000 electrons
" Read noise ~10 e-, so we have strong signal
" Number of e- gives X-ray energyy gy
X-ray CCDs - IIy" CCDs have poor time resolution (typically a few
seconds) due to clocking timeseconds) due to clocking time" Excellent spatial resolution
" Excellent spectral resolution (E/E ~ 60)S ibl di i d i bi" Susceptible to radiation damage in orbit (electrons/protons damage semiconductor crystal structure)
" Good spectral/spatial properties make these theGood spectral/spatial properties make these the detectors of choice for many current missions.
X-ray Telescopesy p" Telescopes focus light onto detectors" Hard to focus X-rays (they go through things
like metal)
" Why? ~ 1 ÅS i b l 1 Å li" Spacing between metal atoms ~1 Å slip through the "cracks"
" General principle: good mirrors must be smooth on size scales <<smooth on size scales
X-ray Telescopes - IIy p" Grazing incidence
tilt mirror so thattilt mirror so that projected distance between atoms is <<1between atoms is <<1 Å
" This only allows weak focusing of X-rays g y(small angle change per bounce)p )
" Use multiple bounces to focus onto detectorto focus onto detector
X-ray Telescopes - IIIy p" Tilt gives very small
projected area ofprojected area of mirror (small collecting area)collecting area)
" Multiple nested mirrors greatly increases collecting gareaXMM uses 58 nested" XMM uses 58 nested mirrors, for example
X-ray Observationsy" Telescopes + detectors combine to provide:
X-ray images X-ray spectra X-ray time series
The Gamma Ray Skyy yThe Electromagnetic Spectrum
Gamma Rays:
Wavelength Eph ≥ 100 keV ≥ 1019 Hz
Frequency
≥ 10 Hz ≤ 0.1 nm
The Atmosphere is
Need satellites to
High flying air
Atmosphere is opaque to
gamma-raysNeed satellites to observe planes or
satellites
g y
The Sky in Different Wavelength Bands
Radio Wavesi ibl li hRadio WavesInfraredVisible light
X-rays-rays
The Gamma-Ray Sky3C279 ( )Plane of the Milky Way 3C279 (quasar)e o e y W y
(diffuse emission)Geminga (pulsar)
Crab (SNR)
PSR 1951+32 (pulsar)
(SNR)
PKS 0528+134 EGRET, E > 100 MeV
(p )Vela (pulsar) (quasar)3C454.3 (quasar)
More than half of all gamma-ray sources are still unidentified!
The Problem of Identifying -ray Sources
EGRET error contours
Pulsar
Black Hole XBlack Hole X-Ray Binary
What’s the source ofWhat s the source of the -ray emission?
Need more information (b db d e t ;(broadband spectrum;
variability)
The Detection of Gamma Rays from SpaceGamma Rays are deeply penetrating and do not
ionize material (hardly at all!)
→ Need to convert the -ray’s energy to kinetic energy of an electron, and detect / gy ,
track the electron
Interactions of gamma-rays with matter
1 Ph l i Ab i (E 300 k V)1. Photoelectric Absorption (E ≤ 300 keV)
hthr = ion
abs ~ 3 ~ -3 abs
Interactions of gamma-rays with matter (cont.)
= 6 65x10-25 cm2
2. Compton Scattering (300 keV ≤ E ≤ 8 MeV)
T 6.65x10 cm
C
hKN≈ 511 keV
3. Electron-Positron Pair Production (E ≥ 8 MeV)
hKN 511 keV→
pe+ e-
p→
hthr= 2 mec2 (1 + me/mp) ≈ 1022 keV
Problems for the Detection of Gamma Rays from Space
1) Low number fluxes
Typical fluxes of the brightest -ray sources in the sky:
FE ~ 10-2 – 10-3 -rays cm-2 s-1 MeV-1
2) High background from cosmic rays
Background of high-energy particles (protons and electrons) constantly irradiating the detector) y g
Problems for the Detection of Gamma Rays from Space (cont )Rays from Space (cont.)
=> Need long integration times to measure a significant signal
Sensitivity limit for detection of a source at a confidence level of n (i.e., an excess of n times the standard deviation of the
background -ray flux dFB/dE):
Fmin = n √ (dFB/dE) E Aeff (E, ) Tobs
whereeff ( ) obs
lid l hi h fl i i i i = solid angle over which -ray flux is impinging on the detector
A ff = effective detector area = A*cos*PdAeff effective detector area A cos PdetTobs = Integration (observing) time
Problems for the Detection of Gamma Rays from Space (cont )Rays from Space (cont.)
=> Problem, in particular for variable sources: , p
Low duty cycle: Source signal may extend onlysignal may extend only over a small fraction of the integration time!
M d fl iMeasured flux is only an average over the integration
Integration time
gtime.
Measured flux
Problems for the Detection of Gamma Rays from Space (cont.)
3) Collimation / Source Localization
G hi hl iGamma-rays are highly penetrating Can not be focused, e.g., by mirrors or lenses!
Solutions:
A) Passive Collimation
Shi ld D f di i f h k dShield Detector from most directions of the sky, except a narrow cone;
virtually no directional information about sources D
Shie
ld Shield
virtually no directional information about sources within the cone (typically ~ few o).
Detector
Collimation / Source Localization (cont.)
B) Orientation Effects
A is proportionalAeff is proportional to cos
BATSE (Burst and
Detector
BATSE (Burst and Transient Source Experiment) on the C G R ShieldCompton Gamma-Ray Observatory
Collimation / Source Localization (cont.) Fl
ux( )C) Occultation Techniques
F
Fsource
For example: Earth Occultation Technique
t
Source
Collimation / Source Localization (cont.)
D) Coded Masks
C d d k tCoded mask casts a shadow pattern on the detector, whichthe detector, which can be unfolded to
calculate the distribution of
sources in the field of viewof view.
DDetector
Collimation / Source Localization (cont.)
E) Tracking the trajectory of secondary particles
For example: In pair Anti-Coincidence For example: In pair conversion telescopes
Scintillation Dome
p
Trajectory of
Pair conversion layers + closely
spaced spark secondary
electron/positron pair is tracked by imaging e+ e-
p pchambers
Widely spaced sparkis tracked by imaging (optical readout) or
spark chamber
Widely spaced spark chambers; Time-of-flight coincidence
systemptechnique
system
Detection Techniquesq
1) Scintillation Techniques) q
Gamma-ray produces electron-hole pair;
recombination produces a (often UV) photon;
registered with optical readout
2) S lid St t D t t2) Solid State Detectors
Gamma-ray produces multiple electron-hole pairs i d d i din doped semiconductors;
recombination produces an optical photon;
registered with optical readout
Detection Techniques (cont.)q ( )3) Compton Telescopes
For -rays with energies of ~ 1 – 10 MeV, direct scintillation or solid state detection becomes inefficient. Photons interact with matter mainly
through Compton scattering
EHave the -ray undergo Compton scattering
event in an upper detector layer (1);
determine direction of motion and energy of the down-scattered photon in a second, lower
detector layer (2).
E’ =E
1 + (E
y ( )
L1
E’1 + (Emec– cos
Need to also measure energy and direction of the recoil electron in layer 1 to uniquely
determine -ray direction. L2
Detection Techniques (cont.)q ( )
4) Spark Chambers4) Spark Chambers
Gamma-ray produces electron-positron pair;Gamma ray produces electron positron pair;
pair trajectory is traced by spark chamber technique
5) Drift Chambers5) Drift Chambers
Gamma-ray produces electron-positron pair;
pair trajectory is traced by drift chamber technique
Detection Techniques (cont.)q ( )6) Imaging Atmospheric Cherenkov Telescopes
High-energy -rays (GeV – TeV energies) produce air showers in the atmosphereproduce air showers in the atmosphere.
Relativistic particles with energy Photons ElectroRelativistic particles with energy
Ethr = mec2 (n/√n2 – 1 – 1)
Photons ElectroPositrons
(n = index of refraction)
produce nano-second flashes ofproduce nano second flashes of Cherenkov radiation.
Imaging the shape and extent of Cherenkov light pattern gives energy and arrival direction of primary -ray.
Detection Techniques (cont.)q ( )7) Secondary particle detector arrays; wave
front samplingfront sampling
High-energy -rays (GeV – TeV energies) produce air showers in the Photons Electro
atmosphere.Photons Electro
Positrons
Measure the time evolution of the wave front of secondary particles y p
(electrons and positrons) to determine primary -ray’s energy (E > 1 – 10
T V) d di iTeV) and direction.
Brief History of Gamma Ray Astronomy2008 F i (GLAST) All k it i f th M V G V k 3 h
2004 Swift: Dedicated -ray burst mission: prompt X-ray/optical follow-up;
2008 Fermi (GLAST): All-sky monitoring of the MeV – GeV sky every 3 hr with sensitivity a factor of > 10 better than EGRET.
2002 INTEGRAL: Major advances in high-resolution imaging and spectroscopy of Galactic -ray sources
arcsecond localization of GRBs
1991 CGRO: First all-sky survey of the -ray sky; major discoveries in all
1997 BeppoSAX: First high-precision localization of -ray bursts; cosmological origin of GRBs established
1987 Whipple (ACT): First credible detection of a TeV source (Crab Nebula)1989 SIGMA: First high-resolution images (13’) in hard X-rays / soft -raysareas of -ray astronomy; -ray astronomy becomes an integral part of astronomy
1975 COS B: First detailed ra map of the Milk Wa ith 24 point1979 HEAO-3: Discovery of radioactive 26Al emission in the Milky Way1981 SMM: Studies of solar flare -ray emission; 56Co-lines from SN 1987A
1968 OSO-3: Discovery of > 100 MeV -ray emission from the Milky Way1972 SAS-2: First high-energy -ray images; discovery of Geminga pulsar1975 COS-B: First detailed -ray map of the Milky Way with 24 point sources
1961 EXPLORER-II: First detection of high-energy -rays from space1967 VELA satelllites: Discovery of -ray bursts (not published until 1973)1968 OSO 3: Discovery of > 100 MeV ray emission from the Milky Way
1) The Compton Gamma Ray Observatory (CGRO)
(1991 – 2000)
Oriented Scintillation-Spectrometer Experiment (OSSE): ~ 0 1 10 MeV
Compton Telescope (COMPTEL): ~ 1 30 M V(OSSE): ~ 0.1 – 10 MeV – 30 MeV
Energetic Gamma-Ray Experiment Telescope (EGRET): pair
conversion telescope,
~ 20 MeV – 30 GeV
Burst and Transient Source Experiment (BATSE):p ( )
0.015 – 110 MeV
2) The International Gamma-Ray Astrophysics L b t (INTEGRAL)Laboratory (INTEGRAL)
Launched 2002Launched 2002
Two -ray telescopes:
Imager on Board the INTEGRAL Satellite (IBIS), optimized for high ( ) p g
spatial resolution;
Spectrometer on INTEGRAL (SPI), p ( ),optimized for high spectral
resolution.
E 20 k V 8 M VEnergy range: ~ 20 keV – 8 MeV
Both use coded-mask technique for imaging.
3) AGILE(A i l G I i i LE )(Astro-rivelatore Gamma a Immagini LEggero)
Italian gamma-ray satellite mission;
Similar technology and capabilities as
launched April 23, 2007
EGRET, intended to bridge the gap between EGRET and Fermi (GLAST)
Two instruments:
Gamma RayGamma-Ray Imaging Detector
(GRID): SuperAGILE:( )
30 MeV – 50 GeV
p
18 – 60 keV
4) The Fermi Gamma-Ray Space Telescope (formerly: Gamma Ray Large Area Space Telescope (GLAST)
Launched June 11, 2008Si il t h l EGRET ( iSimilar technology as EGRET (pair
conversion), but much improved sensitivity, large field of view (~ sensitivity, large field of view ( sr), and slightly extended energy range (~ 20 MeV – 300 GeV).
Will operate in constant slewing mode to survey the sky for flaring high energ raflaring high-energy -ray
sources:
O f ll k 3 hOne full-sky scan every 3 hr.
Fermi
i iTwo main science instruments:
• LAT (Large Area• LAT (Large Area Telescope)
GBM (GLAST• GBM (GLAST Burst Monitor)
The Large Area Telescope (LAT)Pair Conversion Telescope
Quantity LAT EGRETQ yEnergy Range 20 MeV – 300 GeV 20 MeV – 30 GeV
Peak Effective Area > 8000 cm2 1500 cm2
Field of View > 2 sr 0.5 sr
Angular Resolution < 3.5o (at 100 MeV) 5.8o (at 100 MeV)< 0.15o (at > 10 GeV)
Point Source Sensitivity < 6*10-9 cm-2 s-1 10-7 cm-2 s-1
The GLAST Burst Monitor (GBM)All-sky Monitor optimized to
detect X-ray / soft -ray flashesflashes
(~ 8 keV – 30 MeV)
Source localization to < 15o
5) Atmospheric Cherenkov Telescopes
Experiment Technique Ethr (TeV) Location
Whipple IACT 0 25 Arizona USAWhipple IACT 0.25 Arizona, USAHEGRA-IACT IACT array 0.50 Canary Islands
CANGAROO-II IACT 0.1 Woomera, AustraliaHEGRA-AIROBICC Wavefront
sampling15 Canary Islands
Themistocle Wavefront 3 Themis, Francesampling
STACEE Solar Tower ACT 0.05 Albuquerque, NM, USA
HESS IACT array 0 04 Gamsberg NamibiaHESS IACT array 0.04 Gamsberg, NamibiaMAGIC IACT 0.01 Canary Islands
VERITAS IACT array 0.05 Arizona, USA