astronomical observational techniques and instrumentation

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Astronomical Observational Techniques and Instrumentation

RIT Course Number 1060-771 Professor Don Figer

Other wavebands: gamma-ray, FIR, submm, mm

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Gamma Ray Imaging

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Introduction

• Gamma rays are photons with energies above 105 eV

• Gamma rays can be detected directly in space or as secondary showers on the ground.

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Gamma Ray Astronomy

• There are ~100 known gamma ray sources.

• Crab is the standard candle (2.1 × 10−11 photons cm−2 s−1 above 1 TeV).

• Today’s technology gives sensitivity limit of ~1% Crabs.

• Typical objects– supernova remnants– pulsar wind nebulae– broad-lined active galactic nuclei (AGN with jet aligned along the line

of sight)– pulsars– binaries– a few starburst galaxies– a few radio galaxies

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Typical Sensitivity (VERITAS)

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Gamma Ray Results: Pulsar Wind Nebula

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Gamma Ray Detectors

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Gamma Ray Detectors

• Scintillators

• Solid State Detectors

• Compton Scattering Detectors

• Pair Production Detectors

• Cherenkov Radiation Detectors

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Scintillators

• A scintillator is a material that emits low-energy (usually in the visible range) photons when struck by a high-energy charged particle.

• The scintillator emits photons that are then detected by photomultiplier tubes (PMTs).

• The most common scintillators are sodium iodide (NaI) or cesium iodide (CsI).

• Scintillators have been used as gamma-ray detectors aboard many space-based missions to observe sources of cosmic gamma-radiation. These missions include: the Compton Gamma-Ray Observatory (CGRO), the first High Energy Astrophysical Observatory (HEAO-1), and the Rossi X-Ray Timing Explorer (RXTE).

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Solid State Semiconductor Detectors

• Solid state detectors use semiconductors, e.g. germanium or cadmium zinc telluride (CdZnTe).

• The gamma ray deposits energy in the semiconductor, thus promoting many electron/hole pairs into the conduction band.

• These materials often use coded aperture masks or Compton scatter type configurations.

• Most of the advanced materials being considered for future missions have the problem that the arrays are small.

• High energy resolutions, E/dE of 500, can be achieved with germanium detectors. Other materials can routinely match or improve on the resolution available with scintillators.

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Compton Scattering

• Compton scattering occurs when a photon "hits" an electron with some of the photon energy being transferred to the charged particle. The Compton Scatter Telescope uses this interaction as the basis of its detection scheme.

• Compton scatter telescopes are typically two-level instruments. In the top level, the cosmic gamma-ray Compton scatters off an electron in a scintillator. The scattered photon then travels down into a second level of scintillator material which completely absorbs the scattered photon. Phototubes viewing the two levels can approximately determine the interaction points at the two layers and the amount of energy deposited in each layer.

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Pair Production

• A high energy photon (like a gamma ray photon) can create an electron/positron pair (especially in the presence of a “converting” nucleus).

• The standard instrument design is to have a layered telescope, with converter layers interleaved with tracking material.

• The converter is typically a high Z material (ex. heavy metal such as lead) which provides the target for creating the initial pair while the tracking material detects the pair.

• Once the electron/positron pair has been created in one of the converter layers, they traverse the chamber, ionizing the gas. Triggering the detector causes the wires to be electrified, attracting the free electrons which provides the detected signal. The trail of sparks essentially provides a three dimensional picture of the the e+/e- paths.

• By reconstructing the tracks of the charged pair as it passes through the vertical series of trackers, the gamma-ray direction and therefore its origin on the sky are calculated.

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Gamma Ray Detection Airshowers

• It is possible to detect gamma rays by the presence of their by-products produced in Earth’s atmosphere.

• Ground-based gamma ray telescopes actually detect Cherenkov radiation emitted by high energy particles produced through the interaction of the gamma rays and atmospheric particles.

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Cherenkov Technique

• A hadronic cosmic ray or high energy γ-ray incident on the Earth’s atmosphere triggers a particle cascade, or air shower.

• Relativistic charged particles in the cascade produce Cherenkov radiation, which is emitted along the shower direction.

• Cherenkov light is produced throughout the cascade development, with the maximum emission occurring when the number of particles in the cascade is largest, at an altitude of ~10 km for primary energies of 100 GeV–1 TeV.

• Following absorption and scattering in the atmosphere, the Cherenkov light at ground level peaks at a wavelength, λ ≈ 300–350 nm.

• The photon density is typically ~100 photons/m2 at 1 TeV, arriving in a brief flash of a few nanoseconds duration.

• This Cherenkov pulse can be detected from any point within the light pool radius by using large reflecting surfaces to focus the Cherenkov light on to fast photon detectors

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Cherenkov Technique

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VHE gamma-ray sources status ICRC 2007

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Current generation Cherenkov telescopes

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Gamma Ray Facilities

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Recent X-/Gamma Ray Facilities

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Recent X-/Gamma Ray Sensitivities

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Fermi Gamma-ray Large Area Space Telescope (GLAST)

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GLAST LAT

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GLAST Science Goals

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Recent X-/Gamma Ray Sensitivities

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Far-infrared/mm Imaging

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Introduction

• Far-infrared/mm covers 30 um to 1 mm wavelengths

• Most of this waveband is only accessible from space/stratosphere, except for partial transmission over small portions of the range at high altitude/dry observing sites (e.g. south pole)

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FIR/mm Detectors

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FIR/mm Detectors

• FIR/mm detectors:– bolometers– transition edge sensors

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Bolometers

• Bolometers convert light into thermal energy (heat).

• They have been used for ~40 years for FIR/mm astronomy.

• Most previous applications used single element detectors, but imaging arrays are now being implemented.

• A signal is detected as a change in voltage due to the change in resistance in the thermometer for a constant bias current.

• Performance is characterized in terms of the Noise Equivalent Power (NEP).

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Transition Edge Sensors

• TES materials have extremely high sensitivity, i.e. change in resistance versus temperature, at the transition between normal and superconducting modes.

This is a picture of an 8 x 8 array of TES microcalorimeters built at the NASA Goddard Space Flight Center. Each Pixel is 250 μm on a side.

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FIR/mm Facilities

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FIR/mm Facilities

• Caltech Submillimeter Observatory (CSO)/Bolocam, SHARC

• James Clerk Maxwell Telescope (JCMT)/SCUBA

• SOFIA

• Herschel

• Planck

• ALMA

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Caltech Submillimeter Observatory (CSO)

• CSO has a 10.4m primary dish.

• SHARCII has 350, 450, 850um passbands, 12x32, 2.6x1amin field.

• Dry nights lead to better sensitivity

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James Clerk Maxwell Telescope (JCMT)

• JCMT has a 15m primary dish.

• SCUBA is the primary instrument, 450/850um with dichroic, 2.3amin field diameter

Frequency Wavelength Beamwidth Atmos.

(GHz) (um) (arcsec) trans.

150 2000 28 0.97

230 1300 21 0.96

345 870 14 0.88

492 610 12 0.43

690 435 8 0.44

870 345 6 0.53

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Stratospheric Observatory for Infrared Astronomy (SOFIA)

• SOFIA has 2.5m mirror.

• It has a variety of instruments (see below) covering optical to FIR.

• HAWK is being upgraded with new detectors and polarimeters.

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Herschel

• The Herschel telescope is a Cassegrain design with a 3.5m primary. The three scientific instruments are: – HIFI (Heterodyne Instrument for the

Far Infrared), a very high resolution heterodyne spectrometer

– PACS (Photodetector Array Camera and Spectrometer) - an imaging photometer and medium resolution grating spectrometer

– SPIRE (Spectral and Photometric Imaging Receiver) - an imaging photometer and an imaging Fourier transform spectrometer

• Covers 60-670 um.

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Herschel: RCW 120 and Triggered Star Formation

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Herschel Compared to Spitzer

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Planck

• The Planck telescope has an off-axis 1.5m primary. The scientific instruments are: – LFI (Low Frequency Instrument),

a High Electron Mobility Transistor based radio receiver.

– HFI (High Frequency Instrument), a bolometer based imaging array

• Covers 300um to 1.2cm.

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ALMA

• The Atacama Large Millimeter/submillimeter Array

• Covers 300um to a few cm

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ALMA

• Science Goals– Detect spectral line emission from CO or CII in a normal galaxy like

the Milky Way at a redshift of z = 3, in less than 24 hours of observation.

– Image the gas kinematics in protostars and in protoplanetary disks around young Sun-like stars at a distance of the nearest star-forming clouds. Provide precise images at an angular resolution of 0.1 arcsec.