UV, X-ray, and gamma ray astronomy

ASTR 3010

Lecture 22

Not from Textbook

UV, X-ray, Gamma-ray

Wavelength Energy

UV 10nm – 400nm 124eV – 3.1eV

X-ray 1pm - 10nm 124eV – 1MeV

Gamma ray < 1pm > 1MeV

Index of Refraction

• n (real part) : phase speed• β (imag part) : amount of absorption

For high energy photons,

Material become transparent to high energy photons!!

If high energy photons are transparent to all material, how can we focus them to form an image?

Snell’s Law and Total internal reflection

For high energy photons, n<1 and nvac=1This means that incident angle should be close to 90 degrees!For heavy metal atoms w.r.t. hard X-ray, n~0.995 angle~88 degrees

Design of X-ray Telescopes

Effective light collecting area is very small!

Design of X-ray Telescopes : confocal mirrors

Focal length is large! long telescope tube!

Wolter type telescope

XMM Newton

Chandra

Next generation• Multiple mirror assembly to increase the light collecting area!

FOXSI (for Focusing Optics X-ray Solar Imager)

Telescope designs : Coded Apperture• Single, small aperture very low efficiency

Telescope designs : Coded Apperture• Multiple apertures (of known pattern) interference pattern

• interference pattern is deconvolved by a computer to form an image

• Disadvantage : a single noise on the detector can propagate into multiple pixels of the final image!

Detectors : Proportional counter• using electron cascade. Similar to Geiger counters. Using gas proportional

counter (Ne or Ar) which has a typical ionization potential of 25eV. So, if an incoming X-ray photon ionizes one gas atom produces a photoelectron, this electron then continue to ionize other atoms until most of the input energy is used to create N number of 25eV electrons.

By counting the produced primary electrons, one can trace the energy of the original input X-ray photon energy. E.g., for 4keV photon 4000/25 = 160 electrons. Sqrt(160)= noise.

Detectors : Scintillators• at higher E (hard X-ray and gamma ray). Using Compton scattering or pair

production generated high E electron. Using high Z material, these high E electron lose energy via bremsstrahlung and ionization eventually in excited electrons which fluoresce. Measure the fluorescent light E of incident photon.

UV astronomy• Most stars are too cold to emit UV

photonso Young massive starso Old white dwarfs

• Interstellar medium is opaque to UV (ionizing Hydrogen!)

• Galaxy Evolution Explorer (GALEX)M81

Mira

X-ray Astronomy• Thermal radiation requires temperature above millions of degrees (solar

corona)• Bremsstrahlung X-rays :

• Supernova remnants• Active galactic nuclei• Cataclysmic binaries (WDs, Neutron star + Blackhole)

Gamma ray astronomy• Cosmic rays interact with ISM gas• Supernova explosions• Relativistic electrons interact with magnetic field synchrotron radiation • Gamma-ray bursts

Gamma-ray bubbles in our Galaxy! (by Fermi)

In summary…

Important Concepts• Telescope designs for high energy

photons• Detectors for high energy photons

• UV astronomy• X-ray astronomy• Gamma ray astronomy

Important Terms

Chapter/sections covered in this lecture :