P1 Flashcards
What is rigidity and what does it imply?
Rigidity is momentum per unit charge. A high rigidity means it is more difficult for a magnetic field to bend its path, resulting in a longer radius of curvature
How are X-rays detected?
Focussing of X-rays can be done with grazing incidence reflections. Photons are most commonly recorded by a CCD array to give both energy and spatial information or a grating of etched metal disperses the X-rays from bright sources into a 1D spectrum.
How are gamma rays detected? (List techniques)
Compton telescope
Pair conversion detector
Cherenkov telescope
How does a Compton telescope work?
It uses scattering to determine the direction of the source photons. The sum of the energy of the scattered electron and the energy deposited in layer 2 gives the gamma ray energy. Combining the energies, the scattering angles can be found to give the incident direction.
How does a pair conversion detector work?
Layers of silicon are interleaved with layers of tungsten/tantalum causing electron-positron pair production. Using the particle trajectories through silicon layers, the arrival direction can be found. Below layers of silicon, detectors measure energies of the electrons/positrons to determine the energy of the gamma ray.
How does a Cherenkov telescope work?
An incoming gamma ray interacts with particles in the atmosphere and produces a high energy electron-positron pair. A particle emits Cherenkov radiation if it moves into a medium in which its speed is faster than the speed of light in that medium (c/n).
Why are hard-spectrum X-ray sources visible to larger distances than soft-spectrum sources?
X-rays suffer photoelectric absorption from neutral gas in our galaxy.
What does the X-ray sky consist of?
-Local Stars
- Distant AGN
- Galaxy cluster gas
- Nearby normal galaxies
- X-ray binary systems
- Supernova remnants
What does the Gamma ray sky consist of?
- Isotropically-distributed diffuse emission with localized radio-loud AGN and gamma ray burst- High-contrast emission following the milky way. That is diffuse emission and less than 20% discrete sources (pulsars and binary systems).
What is synchrotron radiation?
Electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity.
What is cyclotron radiation?
If a non-relativistic electron is moving in a circular orbit, the radiation is not beamed and the observer sees emission of radiation that varies sinusoidally. the spectrum of this radiation is a Fourier transform of the time variation of this emission giving a delta function at the gyrofrequency.
What is the energy spectrum from synchrotron emission determined by?
The width of the peak (not the interval between peaks).
What determines the width of the peak in a synchrotron spectrum?
- Beaming effect
- Shortening of pulse
What is synchrotron polarisation?
The electric vector of an accelerating charge is a maximum anti-parallel to the direction of the acceleration, which is perpendicular to the direction of motion for a charged particle in a B field. Since radiation is strongly beamed along the direction of particle motion, most photons that are observed are from electrons with velocities directly toward observer so have closely aligned electric vectors so have high linear polarisation.
Where is a high degree of magnetic polarisation expected? Why might this not be the case?
At the source but this may not be observed if:
- The magnetic field at the source is tangled preventing any coherent polarisation
- The radiation passes through plasma on the way to the observer. If the plasma is non-uniform,
Faraday rotation occurs which washes out polarisation
What is brightness temperature?
The temperature of a black body with the same intensity as the source in question at a particular frequency.
How does brightness temperature change as the frequency decreases?
It rapidly increases
What is the effective temperature of emitting electrons?
the temperature equivalent of their relativistic kinetic energy
What prevents the effective temperature of electrons, exceeding the brightness temperature?
Electrons become opaque to their own synchrotron emission, absorbing the photons and obeying the condition Tb = Te. The source is described as self-absorbed.
What is Bremsstrahlung radiation?
Electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus.
What does the Gaunt factor correct for?
Quantum mechanical effects and the effects of distant interactions
What is inverse Compton scattering?
When relativistic electrons lose energy by upscattering photons.
What are the two types of supernovae and what defines each?
Type II show hydrogen lines in their spectra and Type I do not.
What distinguishes Type 1a, Type 1b and Type 1c supernovae from eachother?
Type Ias show certain silicon lines. Type Ibs show helium lines and Ics show neither.