1 Introduction

• X-ray and γ-ray astronomy share common astronomical sources and also detecting devices
• High-energy photons trace energetic phenomena (shocks, particle annihilation, magnetic reconnection), possibly transient, and hot gas (10⁵ K and above), through continuum and lines;
• Yet, because they cover different energy ranges, they must be treated separately.
• X-ray astronomy deals with energy ∼ 0.1-100 keV; above ∼ 15 keV, hard X-ray
• γ-ray: from 0.1 MeV to more than 100 EeV (10²⁰ eV)
  • Medium Energy (ME): 0.1 (or 1 MeV) to 30 MeV
  • High Eenergy (HE): ∼ 30 MeV to 100 GeV
  • Very High Energy (VHE): 100 GeV to 100 TeV
• Overall, little X-ray and γ-ray photons are emitted when compared to lower energy windows; detect photons (individually !) and derive direction, energy, arrival timing, polarization
• Atmosphere is opaque to X-rays and γ-rays: space astronomy
  • In the 60s and 70s: balloons and rockets (alt. ∼ 40 km for observing times ∼ few minutes)
  • Today, satellites
• Ground-based γ-ray astronomy: very high-energy

1.1 The x-ray sky

• "The rocket reached a maximum altitude of 225 km and was above 80km for a total of 350 seconds."
  • Aerobee rocket with three large area Geiger counters
• Discovery of the first extra-solar X-ray source, Sco X-1, Giacconi et al 1962 (Nobel Prize 2002); Sco X-1 is a neutron star in a binary system (accretion disk)
• Also discovered was a diffuse emission, the Cosmic x-ray Background;

The X-ray sky: discrete sources

• Discrete X-ray sources (700) from the NRL (Naval Research Laboratory) Sky Survey Experiment on HEAO-1

X-ray astronomy: From solar system to high-z sources

• From left to right: Jupiter, 30 Doradus (LMC), Antenna Galaxies
• Synchrotron emission and inverse Compton scattering
• Accretion disks, solar flares

Star Formation in the Antennae Collision

• Optical (wide field), Chandra (X-ray), Spitzer (IR), HST (VIS)
• Central regions of two galaxies in collision; bright spots=gas infalling onto BH and NS; superbubbles of hot (10⁶ K) gas due to SN explosions; collision of GMCs lead to star formation (~20 Myr); collision started ~100 Myr ago.

Supernova remnants

• Rayleigh-Taylor instability

The cosmic X-ray background (XRB)

• Left: ROSAT PSPC/HRI false color image of the Lockman Hole region, in the 0.1-2.0 keV energy bande. FoV ∼30×30 arcmin². Right: Color composite image of the deep-field XMM-Newton image (∼800 ks), combining the 0.5-2, 2-4.5, and, 4.5-10 keV (red, green, and blue) energy bands. FoV=43×30 arcmin².
• A disputed origin: discrete source vs thermal plasma emission
• ROSAT obs. of the Lockman Hole: ∼ 70–80% of the X-ray background has been resolved into discrete sources at a flux limit of ∼10^-15 erg cm^-2 s^-1 in the 0.5-2.0 keV energy band
• Best candidates: AGNs

1.2 The γ-ray sky

• The emission correlates well with interstellar hydrogen column density in this band.
• Cosmic ray interactions with matter and photons in the interstellar medium.
• More on EGRET (an instrument onboard CGRO, launched in 1991)

The γ-ray sky: full sky survey

The γ-ray sky: discrete sources with CGRO/EGRET

The γ-ray sky: discrete sources with FERMI/LAT

LAT Full Sky Survey

γ-ray spectroscopy

• γ-ray spectroscopy provides constraints to stellar evolution models (e.g. explosive nucleosynthesis) through observation of radioactive elements (e.g. 26-Al at 1809 keV)
• what is the source of the 511 keV e^--e⁺ annihilation line from the Galactic Center ? Actually the brightest γ-ray line; firm evidence with Ge detectors (1978); annihilation rate 10⁴³ s^-1.

1.3 Historical elements

X-ray astronomy

• Discovery of the 1^st extra-solar X-ray source: Scorpius-X1, 1962, Giacconi et al (he later became the 1st director of the Space Telescope Science Institute)
• Challenges: reduction of background noise → focussing telescopes
• 1970s: Uhuru space mission
  • collimated gas proportional counter; full-sky scanning;
  • X-ray binary with neutron star (Cen X-3) or with black-hole (Cyg X-1)
  • accretion energy source (AGN)
  • discovery of million K plasmas in galaxy clusters
• Einstein (1978): X-ray imaging and unprecedented sensitivity
• ROSAT (1990): all sky survey with imaging; ∼ 10⁵ sources
• 1999: NASA/Chandra (1.2 m telescope), ESA/XMM; CCDs
• Spectroscopy: Einstein, Chandra, XMM
• Hard X-ray imaging achieved with coded-masks (ESA/Integral)

γ-ray astronomy

• γ-ray astronomy is deeply linked to the discovery of cosmic-rays by V. Hess through a series of balloon ascents; 1912, decisive balloon flight, up to 5300m during a near-total solar eclipse; Nobel prize in 1936
• "The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above."
• Cosmic-rays are indeed particle, deflected by magnetic fields; γ-rays are photons that propagate in straight lines; study CRP with γ-rays
• 1960s: P. Morrison (MIT) predicts that CRP interacting with interstellar matter will produce γ-ray emission;

Cosmic-ray astrophysics

• γ-ray was the last electromagnetic window on the Universe to be opened
• Birth of 100 Mev γ-ray astronomy: Morrison (1957);
• Fermi, Cocconi, Moscow group (Shklovski and Ginzburg), Hoyle, Burbidge
• 1968: first detection of cosmic γ-rays, above 70 MeV, concentrated to the galactic plane (MIT group) with NASA/OSO-3 satellite

• Birth of TeV astronomy: prediction of detectable γ-rays from the Crab Nebual by Cocconi (1959) (intensity overestimated by factor 1000)
• Very high-energy γ-ray window (above 100 GeV) was opened in the 80s, with the Cerenkov technique

γ-ray bursts

• GRB: discovered in the late 60s by VELA military satellites, flashes of γ-rays not linked to the Earth or the Sun
• GRB light curve exhibit two peaks
• Compton Gamma-Ray Observatory (CGRO)/BATSE (Bursts And Transient Source Experiment): ∼ 3000 GRBs; ∼ 1 GRB/day, isotropic; two groups according to time duration and spectral hardness:
  • long-soft (>2s up to min): death of massive stars
  • and short-hard (<2s, down to ms): extragalactic, not associated to death of massive stars;
• Extragalactic origin demonstrated by BeppoSAX (1997): GRB at X-ray wavelengths; emitted at cosmological distances up to 13 Gly (NASA's Swift satellite, 2008)
• Shortests GRBs imply physical source size ∼ 100 km

2 Light-matter interactions at high-energy

Basic principles

• Cross-sections takes three contributions: photoelectric effect, Compton effect, pair-production
  • X-ray, up to hard x-ray, low energy γ-ray: photoelectric effect
  • γ-ray: Compton effect and pair production
    • Interact readily with matter; within a few mm of lead or a few km of air: scattered, absorbed, or converted into particles.
    • Because of these destructive interactions with matter, telescopes do not focus γ rays; look for signatures of their passage through the telescope.
    • ME: 0.1 (or 1 MeV) to 30 MeV: Compton scattering, satellite-borne Compton telescopes
    • HE: 30 MeV to 100 GeV: electron-positron pair in detectors onboard balloons or satellites
    • VHE: 100 GeV to 100 TeV: cascade developping in Earth's atmosphere, with ground-based detectors.
• Cross-sections for each processus depend on Z of detector and E of incoming photon
  • σ_PE ∼ Zⁿ E^8/3, n∈[4:5]: high-Z materials absord more efficiently
  • σ_Compton ∼ Z/E
  • σ_pp ∼ Z²

2.1 Overview

• Materials used in X-ray and γ-ray detectors are characterized by the wavelength-dependent attenuation length which is related to the mean-free path of the photons in the given material
• The thickness of the various elements of a detector is dictated by the attenuation length.
• Interaction = absorption and scattering (elastic, or inelastic)

This lecture

• Detectors: how do we detect high-energy photons ?
• Imaging: how can we focus X-ray light ?
• Spectroscopy: how do we measure the energy of X-ray and γ-ray photons ?

Attenuation coefficients, cross section, and mean free path

• Beam of photon with intensity I₀ incident through thickness dx of absorbing material:

  dI/dx = -I(x) Nσ_tot = -I(x) μ₀

  • Probability of interaction: dW = σ N dx
  • Total cross section: σ_tot (cm²) (1 barn = 10^-24 cm²)
  • N: the density of colliding particles (cm^-3)
  • total linear-attenuation coefficient (cm^-1): μ₀ = Nσ_tot
• Characteristics quantities:
  • Attenuation length (mean free path): ℓ = 1/μ₀ = (Nσ_tot)^-1
  • Mass-attenuation coefficient (cm²/g): μ₀/ρ
  • Mass-attenuation length (g/cm²): ρ/μ₀

Mass attenuation length

• choice of the material conditions the size of the detector for a given energy

Attenuation coefficient for photons in air

• dashed: Compton scattering
• dot-dashed: pair production

2.2 Details on the processes

Basic processes

• Photoelectric absorption: incident photon is absorbed and generate a photoelectron that carries kinetic energy; dominates below ∼0.1 MeV
• Compton scattering, MeV, up to few 100 Mev depending on material: photon scatters off a free-electron (loosely bound, or valence, electron); conversely moving electron can increase the photon energy (inverse Compton);
• Electron-positron pair production (above 100 MeV): electron-positron pair production with threshold at 2m₀ c²=2x511 keV, carrying E₀-2m₀ c²
• Detectors in X-ray and γ-ray astronomy are based on either process, depending on the specified observed energy range.

2.2.1 Photoelectric absorption

• Incident X-ray photon ionizes high-Z gas atom; E₀ = E_K-shell + kinetic energy ≈ <w> + E_kin
• Energy to create an ion-electron pair: <w> ≈ 28 eV
• Secondary electrons are created until E_kin < <w>
• Number of charged created N_c ≈ E₀/<w>
• Edges: electronic transitions:
  • lower level n=1 K series (α, β, etc)
  • lower level n=2: L series
• σ_PE ∼ Zⁿ E^-8/3, n∈[4:5]: high-Z materials absorb more efficiently; photo-absorption decreases rapidly with E

Photoelectric absorption cross-section

2.2.2 Compton scattering

• hν' = hν [1+α(1-cosθ)]^-1, α=E/m₀ c²
• relativistic description from Klein and Nishina 1928
• σ = scattering + absorption (see Evans 1955; Rybicki & Lightman)
• α << 1: scattering dominates
  • σ ≈ scattering = σ_Thomson [1-2α+O(α²)]
  • σ_T = 8/3 π r₀² = 6.653 x 10^-29 m²/electron (=2/3 barn/e-)
• α >> 1: absorption dominates
  • σ ≈ σ_abs ∼ 3/8 σ_T α^-1 (1/2 + ln 2α) cm²/electron
  • total absorption cross-section ∼ Z/E

2.2.3 Pair production

• Requires the presence of an e-m field (nucleus, electron, etc) or third body, carrying some of the momentum;
• Dominant energy loss for HE γ-ray photons
• Generates cascades in the atmosphere
• Choice of material is made in terms of so-called radiation length X₀; X₀=37 g/cm² for air, 8.6 g/cm² for \(^{74}\)W); mean free path λ_pp ≈ 9/7 X₀
• Note: next threshold at 2×105=210 MeV for muon pair creation
• Photon-photon collision can also produce electron pair (e.g. VHE 400 TeV + CMB 0.6 meV)
• Cross section rises sharply above threshold, σ_pp ∼ Z²

Cross-section for pair production

• For E>1022 keV: photon is completely absorbed; electron-positron pair with total energy = hν
• Total cross-section: σ_pp = σ₀ Z² <P> ∼ Z²
  • σ₀ = 1/137 (e²/m₀ c²)² = 5.80 x 10^-28 cm²/nucleus
  • P-factor depends on Z and hν ∈ [0:20]; σ₀ <P> (columns 3 and 4 in Table) weakly dependent on α
• Corrections include screening effects (above ∼ 20 MeV)

3 X-ray astronomy

• Thermal emission from very high temp.
• Non-thermal synchrotron radiation from very high-energy particles in magnetic fields
• Inverse Compton scattering of low-energy photons into X-ray
• Primary detection method: individual photon detection
  • arrival direction (images)
  • arrival time
  • energy (spectroscopy)
  • polarization angle

Evolution of the Sensitivity

• N: number of sources/degree² with a flux larger than S
• 10⁹ increase in sensitivity over 50 years
• Lowest flux: ≈ 1 photon/day !
• Best sensitivity ≈ 3×10^-17 erg/s/cm²

Fom detection to imaging and spectroscopy

• First detectors: proportional and scintillation counters
  • Effective area ∼ few 100 cm²
  • Directivity by limiting the FoV to ∼ few sq. degrees (by the use of collimators)
  • Caveat: background radiation from diffuse X-ray sky and charged cosmic-ray particles;
• Major step: imaging
  • noise reduction (cosmic-ray induced events and soft x-ray background reduction)
  • Einstein Observatory, first satellite carrying a focusing and imaging X-ray optics (Wolter telescope), combined with imaging focal plane detectors
  • ROSAT: first all-sky survey (125000 sources)
• Today
  • actively cooled CCDs (Chandra and XMM-Newton)
  • high-resolution grating spectrometers (Einstein, Chandra, XMM-Newton)

Perspectives

• Focusing telescopes at energy higher than 10 keV
• Polarimetry
• mas spatial resolution

3.1 Imaging X-ray focussing telescopes

Why focusing telescopes

• To obtain imaging capability (angular resolution and morphology)
• Increase the collecting area: better sensitivity
• Reduction of background noise and of cosmic-ray induced events (background counts are uniformly distributed on the detector area)
• Up to 79 keV (NuSTAR)
• Concentrate light into a small area: highly dispersive spectrometer hence spectral resolution

Major issue

• X-ray focussing telescopes must be operated at grazing incidence
• Thus, the collecting area ≫ detector area

3.1.1 Grazing telescopes

Refractive index

• X-rays at normal incidence are efficiently absorbed by all atoms: grazing incidence
• But refractive index: n≈ 1: refractive telescope barely deflect x-ray; prohibitively large focal lengths
• Complex index of refraction: n(λ)=1-δ(λ)-iβ(λ)
  • δ(λ) ∝ Z λ² << 1, ∼ Z/E²
  • β(λ): absorption

Grazing telescopes

• Propagation from n=1 (in vaccum) upon n=1-δ<1
• Total reflection for grazing angle α<α_t=arccos(1-δ)
• Note: because β≠0, reflection is not total;
• High-frequency: δ∝ Z/E² and, for δ << 1, α_t ≈ (2δ)^1/2

• Numerically, for heavy elements, this leads to

  α_t = 56 ρ^1/2 λ_nm = 69 ρ^1/2/E_keV, (ρ in g/cm³)

• For δ < 10^-4, the grazing angle is α_t < 1°

Grazing incidence: reflectance and critical angle

• X-ray telescope based on total reflection: E<10 keV
• α_t ∝ Z^1/2 / E

Refractive index (details)

• Complex index of refraction: n(λ)=1-δ(λ)-iβ(λ)
  • δ(λ) ∝ Z λ² << 1: phase change
  • β(λ): absorption

• General expression (Drude model):

  \[ n(\omega) = \left[ 1-\frac{e^2n_a}{\epsilon_0 m} \sum_s \frac{g_s}{(\omega-\omega_s)^2+i\gamma\omega} \right]^{1/2} \]

  • High frequency limit (ω≫ω_s):

    \[ n(\omega) \approx 1-\frac{e^2n_a}{2\epsilon_0 m} \sum_s \frac{g_s}{(\omega-\omega_s)^2+i\gamma\omega} \]

  • Important: at high frequency, δ ∼ Z/E²

Optical design of grazing telescopes

• Wolters I type of confocal X-ray telescope
• Other types (II, III) but I offers the largest aperture-to-focal length ratio: maximizes collecting area while keeping the telescope compact
• focal plane is curved: but imperfections (alignement of mirror shells) so, in the end, flat imaging detector (a bit forward the theoretical focus)

Angular resolution

• Angular resolution: determined by off-axis angle (FoV), moderately by energy
• Chandra: 0.5"
• X-ray telescopes are not diffraction limited: λ/D<1 mas

Performances of X-ray telescopes

• small area: most cosmic sources are photon starved and low sensitivity
• If the HPBW is greater than ~10": source confusion in deep exposures.

Effective area

• ESA/XMM
• NASA/NuSTAR

3.1.2 Gallery of grazing telescopes

Chandra

XMM-Newton: nested mirrors

58 nested mirrors

The Nuclear spectroscopic telescope array: NuSTARR

• Launch date: June 13, 2012; Low-Earth Orbit: 650 km x 610 km, 6 degree inclination; still operating
• Instrumentation:
  • 133 nested shells approximately in Wolter-I geometry
  • CCDs 4x(32x32) 12.5" pixels: arrival time, energy and position of interaction of each incident X-ray.
  • Energy Band: 3-79 keV; Angular Resolution: 58" (HPD), 18" (FWHM)
• Focal Plane Size: 12' x 12'; Focal length: 10m
• Energy Resolution: 0.4 keV at 6 keV, 0.9 keV at 60 keV (FWHM)

ESA/ATHENA (launch 2031)

• Focal length of 12 m and an effective area of ~1.4 m² at 1 keV
• Wolter I design with new reflecting surfaces (see here);
• Two instruments:
  • X-ray Integral Field Unit (X-IFU) microcalorimeter (TES, @100mK) spectrometer for high-spectral resolution imaging
  • Wide Field Imager (WFI) semi-conducting detector for high count rate, moderate resolution spectroscopy over a large field of view

3.2 Imaging with non-focussing X-ray telescopes

Non-focusing telescopes

• Require special techniques
  • Mechanical collimator
  • Multiwire imaging proportional counters
  • Aperture modulation
• Old telescopes; some details follow for your records

Mechanical Collimator

• Telescopes for hard X-ray (and low energy γ-ray)
• Tubular collimator; opening angle ∼ angular resolution ≈ d/h
• In combination with proportional counters or scintillators; primarily multi-wire PC divided in cells
• Require large area detectors with high background rejection capabilities
• Detection limit for point source: diffuse X-ray background + cosmic-ray background

Position sensitivity and background rejection

• Avalanche close to the wire: precise position (0.1 mm) can be obtained (see also multi-wire chambers and time-projection chambers in particle physics)
• Background rejection: X-ray and cosmic-ray background dominate; high-energy ionizing particle can generate fluroescent X-ray
  • geometric shape of ionized cloud: point-like (X-ray event) vs track-like (ionizing particle)
  • detector with surrounding anti-coincidence detectors

Collimator

Example: RXTE PCA

• Operated 1995-2012
• Three instruments: PCA (2-60 keV), HEXTE (15-250 keV), All-Sky Monitor (2-12 keV)
• PCA: three layers + propane veto layer; better bg rejection (high-energy particles travel through several layers ≠ X-rays);

Large area, position sensitive, PC for X-ray astronomy

Imaging proportional counters

• first anode: energy of the X-ray
• orthogonal cathods measure the two coordinates
• measure the position of the charge cloud; avalanche amplification 10⁴ - 10⁵
• second anode: anticoincidence filter
• position accuracy limited by charge cloud size

Multiwire proportional counters: readout methods

1. Einstein satellite: rise time at both ends (1mm FWHM)
2. ROSAT: charge signal on individual stripes; 250mic FWHM at 0.93 keV

Other methods

• aperture modulation telescopes
  • temporal modulation
  • spatial modulation: coded-aperture or coded-mask telescopes

Spatial aperture modulation: INTEGRAL

• coded-mask telescope: IBIS mask on ESA/INTEGRAL; cell size: 11.2mm x 11.2mm
• D = S * M + B
• mask pattern: pattern shift ≡ source position shift
• θ∼ d/D, (d: width of the holes, D: mask-detector dist.)
• Recovering the image: inversion problem (degenerate result)
• Difficulties: Poisson statistics due to background (sources and diffuse)

3.3 Detectors for X-ray astronomy

Types of detectors: overview

• Semiconductors (including CCDs)
• Supraconductors: microcalorimeters (Athena) using Transition Edge Supraconductor (TES); δ E ∼ few eV @ 6 keV
• Detectors using high-Z gas and avalanche followed by a detecting setup (wires) to locate the electrons produced
  • Microchannel plates (high spatial resolution but low energy res.); NASA/Chandra
  • Proportional counters: incident radiation produces photoelectrons by ionizing a high-Z (noble) gas contained in a closed chamber; electrons are accelerated, and detected on a wire;
  • Scintillation counters (above 50 keV): incident radiation energy is converted into visible photons which are detected by a light sensor having high absorption cross section and short time response; hard X-rays from the Crab (1964) with scintillation onboard a balloon

Characteristics of X-ray detectors

• Sensitivity
• Quantum efficiency
• Energy resolution
• Time resolution
• Angular resolution

Specific requirements

• Space borne: robust, reliable, work in vacuum
• Capabilities to discriminate between X-ray and, optical, IR photons, or energetic particles
• Detectors body usually absorb celestial X-rays: calibration needed to convert the measured counting rates into celestial photon fluxes

CCD for X-rays

• Introduced in X-ray astronomy in 1987 to observe SN 1987A, with a rocket flight
• ASCA (Japan), launched in 1993: first X-ray satellite with CCD
• Now on all large missions: ACIS, Chandra, EPIC, RGS, XMM-Newton, Suzaku; future ESA/ATHENA
• Usually based on silicon
• Attenuation length in Si: ℓ is large enough, but not too large, (1-100 micr) for hν in the range ∼ 0.5 to 10 keV
• Number of electron-hole pairs is N_e = hν/<w> ∼ 10-10³, <w>=3.7 eV in Si.
• Operation mode of X-ray CCD:
  • photon-counting mode: position and energy of each photon is measured, contrary to lower energy photons
  • accurate spectroscopy requires detection of single photons: readout rate

• from 100 eV to 15 keV
• size of the depleted region, sensitive thickness
• increase up to 500micr: increase up to 30 keV

Performances: quantum efficiency

• Two contributions
  • transmission T through dead layers (dashed line); T=e^{-μ d}, linear absorption μ and thickness d
  • detection through absorption A in semi-conductor (Si) (dotted line); A=1-e^{-μ_Si d_Si}, μ_Si=absorption coefficient for silicon, d_Si is the thickness of the depletion region.
  • QE = η = e^-μd (1-e^{-μ_Si d_Si})
  • At high energy, QE is sensitive to d_Si, and to d at low-E

3.3.1 Gaseous detectors: overview (may be skipped)

• Basic idea: capaciter filled with gas; incoming radiation generates electrons and ions which are collected at electrodes; energy is extracted from the capacitor by charges accelerated in the electric field;
• Electric field strength
  • moderate: ionization-chamber mode: e- and q+ are simply collected by the electrodes
  • higher: scintillation proportional counter; accelerated e- excite (noble gas) atoms which emit UV light
  • strong: proportional counter; accelerated e- ionize gas atoms leading to charge multiplication
  • stronger: Geiger counter; multiplication no longer proportional to incoming ionizing radiation flux; avalanche into the entire detector;
  • strongest: spark chamber (γ-ray astronomy)

Proportional counter

• Measure a voltage created by e-/ions pairs created by incoming photons
• Gas within a chamber: absorption properties of window and gas define the spectral range;
• Number of e- proportional to energy of incoming photon

Note

• Difference between Geiger counter and proportional counter: voltage!
  • Geiger: avalanche saturation; pulse height no longer ∝ incident photon energy
  • Proportional counter: voltage tuned to guarantee linearity

Proportional counters (details)

1. Generation of a primary photoelectron
2. Charges drifts towards the high-voltage anode wires (1500 V, diameter ∼ 20mic) and are accelerated:
   • ionize gas atoms, electric field increases, avalanche, leading to charge pulse; nb of electrons ≈ 1000 times the original nb of e-/ion pairs
3. Charge pulse is detected and generates a current before digitization
   • electric field close to anode ≈ 10⁵ V/cm
   • multiplication takes place ∼ few mfp from the wire
   • waveform: two components; short rise time, due to the e- (m_p/m_e=1836); rise time of 0.1 ms due to ions

Proportional counters (details): example

• Incoming photon E₀=hν ionizes gas atom
• Ar (IP(K)=3.2 keV), photoelectron kinetic energy is E₁=E₀-3.2 keV
• Primary photoelectron energy: ionization, ion/e- pairs measured at the anode
• e- in Ar gas loses ≈25 eV: typically E₁/25 pairs are created
• What about the remaing 3.2 keV ? Restored through relaxation as:
  • fluorescence (Kα): new ionizations, etc
  • ejected photoelectron from higher shells (Auger effect)
  • Both processes lead to ionization of Ar atoms (IP=25 eV)
• Eventually, ≈ E₀/25 eV e-/ion pairs created
• A 1 keV X-ray photon creates ≈ 40 e-/ion pairs

Quantum efficiency of proportional counter

• Probability of event detection has two contributions
  • photon must not be absorbed by the window
  • photon must be absorbed by the gas
  • P = e^-τ_w (1-e^-τ_g)
    • opacity = mass density (g/cm³) x mass attenuation coefficient (cm²/g) x thickness (cm)
    • τ_w = ρ_w μ_w(E) d_w
    • τ_g = ρ_g μ_g(E) d_g
    • μ(E) = cross-section per gram of material = σ(E)/m; Data are available from the Photon cross sections databases (NIST).

Dead time

• Dead times and strong fluxes from bright sources
  • event taking place while the previous is not finised (digitall processing not finished, analogic electronic signal not relaxed)
  • charge pulse may overlap (deformation)
  • dead time: time interval between end of event and detector reset
  • live time = exposure time - dead time (ratio is duty cycle)

3.4 Spectroscopy

• Recent X-ray telescopes: imaging + low-spectral resolution
  • Chandra, XMM: R = E/Δ E ∼ 10-50
  • Past observatories (Einstein, ROSAT): R∼ 1-10
• Using gratings (e.g. XMM) and transmission gratings (Einstein, EXOSAT, Chandra): R∼ 100-1000

Spectroscopy using CCD

• resolution ∼ 100 eV: set by the capacity to detect single photons
• readout rate and noise: ACIS on Chandra: 1024x1024, 4 readout nodes per CCD, 100kHz readout ⇒ 3s frame time
• spectral resolution (in eV): FWHM limited by two factors

  • σ_e: variance on charge, which is smaller than Poisson (free electrons produced by the single X-ray photon are not independent); reduction factor F is called Fano factor:

    σ_e² = F × N_e = F hν/<w>

  • σ_read²: the readout noise, typically smaller than 5e^-
  • FWHM[eV] = 2.35 <w> (σ_e² + σ_read²)^1/2, typically 50-150 between 1 to 8 keV

3.5 Sensitivity

• Detected photons: source + cosmic bg (diffuse bg + galactic and heliospheric emission) + instrumental bg (false detections due to particles)
• Background and foreground dominate
  • Extragalactic bg, 0.1-10 keV: unresolved AGN
  • Galactic foreground: high latitude and disk
    • Local Hot Bubble (LHB): irregular, mostly ionized, bubble around the Sun, radius ∼ 100-200 pc, T∼ 10⁶ K; seen unabsorbed in any direction
    • Galactic Halo: show absorption and not isotropic; 1-3 million K;
• Fluctuations in number of photons
  • <ΔN²> = (N₁ + AΩ N₂) Δt
  • Δt: integration time
  • AΩ: detector+telescope throughput (effective area x solid angle)
  • N₁: particle background ≈ dark current: background recorded by the instrument when it is not exposed to cosmic X-rays.
  • N₂: cosmic background photons

4 Gamma-ray astronomy

Sources

• γ-rays are emitted by decaying radioactive nuclei, nuclear transitions, or particle annihilation
• γ-rays are emitted during the pion decay: tracers of cosmic-rays; sources of acceleration: black holes, shockwaves, rotating neutron stars
• Sources: steady and transient (e.g. GRBs)
• Broad characteristics of γ-ray sources
  • variety of techniques
  • high-sensitivity, good resolution, timing and fast response, broad energy coverage, fast mapping
• Photon fluxes are low: strongest γ-ray source (Vela pulsar) has a flux of 1 ph/min.

γ-ray energy: from 0.1 MeV to more than 100 EeV (10²⁰ eV)

• Medium Energy (ME): 0.1 (or 1 MeV) to 30 MeV; space-borne telescopes
• High Eenergy (HE): ∼ 30 MeV to 100 GeV; space-borne telescopes
• Very High Energy (VHE): 100 GeV to 100 TeV; ground-based telescopes

Foreword

• γ-ray astronomy can be misleading because γ-rays are produced by the interaction of high-energy particles (cosmic-rays) with baryonic matter; both cosmic-ray observatories (more like particle detectors) and γ-ray photon observatories (more like telescopes) deal with γ-rays.
• To add to the confusion: some detector particles (e.g. Water Cenrenkov Telescopes) also detect the Cerenkov light produced within the detector by the passage of high-energy particles (e-, e+, μ, photons);
• γ-ray astrophysics aims at observing photons and cosmic-ray particles
  • the most energetic photons (up to ∼10 TeV) are associated with SNR, AGN, γ-ray bursts
  • cosmic-rays: charged particles (protons, electrons, heavy nuclei), with E up to 10²⁰ eV (100 EeV) (Linsey PRL 1963)
  • both produce Extensive Air Showers when interacting with matter (atmosphere or detector)

ME and HE Telescopes: peculiarities

• γ-rays are extremely penetrating: can not be focussed through reflection/refraction: detector = telescope (but not for VHE telescopes)
• However, background noise is important: small effective area, usually less than 1 m²
  • CGRO: ME and HE telescopes had 1600 and 5 cm² resp.
  • In addition: low photon flux
  • Background: dominated by CRP and false detections (use of anticoincidence shields); secondary interactions within the detector
• Space borne: mass attenuation length, ℓ(air) ≈ 40 g/cm² (48 at TeV) ≪ vertical thickness of atmosphere, 1030 g/cm² at sea level; γ rays convert at altitudes high above sea level, typically above 10 km.

Ground-based vs Spaced-based γ-ray telescopes

• γ-ray photons or cosmic-rays do not reach the ground:
• Space-based telescopes
  • high-altitude ballons, space telescopes
  • Compton effect and/or pair-production within a detector
  • Example: Compton Gamma-Ray Observatory
• Ground-based telescopes
  • Use the pair production interaction within the atmosphere as the pair-production device
  • Two types of detection: relativistic particles and Cerenkov light, both produced in the Extensive Air Shower (EAS), and reaching the ground

4.1 30 MeV-10 GeV: Space-based, pair production, telescopes

• Photoelectric effect and Compton scattering: 0.1 to 30 MeV
  • Early detectors: simple scintillators
    • solid or liquid
    • high energy electrons produce photons in the scintillator, which are detected with photomultiplier tubes (PMT)
    • electrons are produced by primary cosmics rays exciting atoms, or by hard X-rays or \gammar-ray through PE, Compton scattering, or pair production
    • small opening acting as a collimator, improving selectivity and signal-to-noise ratio (angular resolution, lower background)
  • Today: proportional counters, wire and multiwire chambers, spark chamber

• Spark chamber: was the primary detector in the 30 MeV - 10 GeV energy range
• Effective collection area << geometrical cross-section of the telescope

• Four subsystems
  • anticoincidence system (reject charged particles)
  • First set of spark chambers: series of closely spaced parallel metal plates
  • triggering system (which imposes a lower limit on E)
  • calorimeter to measure E_γ
• General setup of Spark chamber telescopes
• Opening angle ≈ 0.8/E_γ

EGRET: the CGRO pair production instrument

Current pair-production satellites

• Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope
  • New concept to follow the track of the e-/p pair: instead of a gas tracker, silicon strip detectors interleaved (produce photon, trigger readout, tracking)
• AGILE

4.2 Ground-based VHE Telescopes: electromagnetic Air Shower

Extensive Air Showers

• Extensive air showers begin with a single photon producing e^± pairs in the atm. over 9X₀/7, X₀=radiation length, ≈ 36.5 g/cm2 in air.
  • e^± pairs produced by TeV energy gamma rays are extremely energetic and do not proceed far from their creation point
  • A TeV photon passes through a depth of about 48 g/cm² of air before undergoing pair production.
  • γ rays convert at altitudes high above sea level, typically above 10 km (ind. of photon energy)
• e^± lose energy through BS over the next radiation length, and emit high-energy photons
  • a BS photon carries about half of the e- (or p): emitted photons can generate another pair, etc
• Characteristic length for pair production and BS are similar
• Shower develops until photon energy drops below pair-production threshold
• Air shower duration ∼ 10^-4s; max num of shower particles at 250-450 g/cm2 for primary γ-rays in 20 GeV to 20 TeV, at 7-12 km altitude.

Directivity

• Information on the primary γ photon encoded in air shower
• Direction of shower ∼ incoming γ (although refraction, terrestrial magnetic fields)
• Narrow plane of the shower front: few ns correspond to 1m thickness, extending over ∼ 200m at 10 TeV; angular error ∼ 0.3°

Confusion with CRP and the γ-ray background

• Cosmic ray particles also penetrate the atmosphere
  • Create π⁰ which decays into 2 γ-ray photons (this is how we trace the mass of the neutral gas in the galaxy), and π^±; background air showers
  • Flux of CRP ≫ γ-ray flux: large background

Examples

• γ-ray telescopes: designed to observe the γ-ray sky
  • cosmic-ray induced showers are a pollution !
  • High-Altitude Water Cerenkov (HAWC) observatory (Mexico)
• cosmic-ray telescopes: designed to observe cosmic-ray particles
  • Pierre Auger Observatory (Argentina)
• Imaging Atmospheric Cherenkov Telescopes (IATC):
  • Detection of the Cherenkov light produced in the atmosphere
  • Detector tank = atmosphere
  • HESS (Namibia)

5 Bibliography

• The universe in X-rays (Trumper et al)
• Bradt (p133)
• Websites: XMM, Chandra
• Gamma-ray
  • Most references include some historical perspectives.
  • What are GRBs ?, Joshua S. Bloom, 2011, Princeton University Press
  • GRBs, G. Vedrenne and J.-L. Atteia, Springer-Praxis, 2009

6 Lockman Hole

• Located in the constellation of Ursa Major, regions like this one are almost completely devoid of objects in our Milky Way galaxy
• Herschel image: cosmic infrared background