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.
What causes the existence of Type II supernovae?
Type IIs are the result of the collapse of a massive (> 8M) star. Core-collapse supernova. It happens like this:
-The core collapses due to the core being too massive for degenerate pressure to support it
- Nuclear reactions produce large amounts of neutrinos which carry away a significant amount of the total energy
- Collapse stops when the repulsive part of the nuclear force prevents further collapse
- Outer layers bounce back causing a shock wave and further fusion
What do Type II supernovae often leave behind?
A compact object such as a neutron star or a black hole of a few solar masses.
What causes the existence of Type Ib supernovae?
This is a core-collapse supernovae in which the star has already shed its outer layer of H before the core collapses.
What causes the existence of Type Ic supernovae?
This is a core-collapse supernovae in which the outer layers of H and He have been shed before collapse
What causes the existence of Type Ia supernovae?
Type Ia supernovae is a subject of active research, but most models involve a white dwarf star and a companion.
What are white dwarf stars?
The remnants of less massive stars (< 8M) that have shed their outer layers leaving behind a core supported by degenerate electron pressure.
What is a light curve?
A graph of light intensity of a celestial object or region as a function of time.
What characterises the light curves of Type 1a supernovae?
- They rise quickly to a similar peak luminosity and then drop in brightness quickly before slowing after about fifty days to remain detectable for a few hundred days
- They have similar peak luminosities, which means they are very good standard candles; if their redshifts are measured they can be used to construct a Hubble diagram to measure the expansion history of the Universe
What does the Phillips relation give?
The peak luminosity of Type Ia supernovae
What are the two categories of supernova remenants?
- Crab-like/plerions: Filled with synchrotron emission from radio to X-rays and have a central radio source (a pulsar)
- Shell-like: Radio, optical and X-ray emission are seen from the outer shell, but no emission from inside the shell
For filled Crab-like SNR, where is the observed radiation from?
Synchrotron radiation from electrons accelerated by the supernova shock. However the observed X-ray emission implies highly energetic electrons.
What type of supernova are Crab and shell like SNR associated with respectively?
Crab: Type II
Shell: Type Ia
What are neutron stars?
The collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses.
What is a pulsar?
A highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles
What does the short period of pulsars imply?
It is a compact object since causality suggests that the period of a variation cannot be shorter than the light crossing time of the object.
How does the magnetic dipole model, model pulsars?
A simple, rigid, magnetised sphere that is rotating in a vacuum.
How were pulsars first detected?
From radio sources, believed to be generated by curvature radiation.
What are X-ray binaries?
stars in binaries where the binary companion is a
compact object (WD, NS, BH), making them strong sources of X-rays.
What is an accretion disk?
a rotating disc of matter formed by accretion around a massive body (such as a black hole) under the influence of gravitation.
How do XRBs form?
If the stars in a binary orbit are close enough, material from the normal star flows into a compact object and forms an accretion disk, a boundary layer and hot spots. All of which emit X-rays.
If matter fell directly into a black hole, would heat be released? Can this even happen?
No heat is released. This can’t happen as conservation of angular momentum prevents direct infill
What causes an accretion disk to form?
Conservation of angular momentum allows collapse along rotation axis of in-falling material, and a disk forms. Viscous force, transfer angular momentum outwards allowing inner material to fall inwards and provide frictional, dissipative forces to heat material which will then radiate.
Why do viscous forces arise in an accretion disk?
Due to material at different radii moving at different speeds
What is the most efficient energy source in the universe?
A Kerr Black Hole
What happens if luminosity of bodies are too great?
Radiation pressure blows away in-falling material, by now an ionized plasma of equal numbers of electrons & protons.
What is the Eddington Limiting Luminosity
The maximum luminosity a body (such as a star) can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward
What are the two classes of X-ray binary? What defines each?
- Low-mass XRB (LMXRB) where the donor star is slowly evolving low-mass star
- High-mass XRB (HMXRB) where the donor is a young massive star with strong winds
How do HMXRBs form?
Two stars with masses large enough to eventually go supernova successively go to supernova.
If the compact object in HMXRB is a neutron star, what will happen?
It can influence accretion due to its strong B-field.
What is Alfven radius?
The distance where the magnetic energy density is equal to the kinetic energy density.
What is the ram pressure at the Alfven radius?
It is balanced by the magnetic pressure.
What happens if the vicinity of a star in a HMXRB is within the Alfven radius?
The vicinity of the star is magnetically dominated and accreting material is funnelled along field lines. Improved calculations for accretion via a disk give similar results; the accretion disk is influenced by the B-field out to significant radii
Explain how an X-ray pulsar is created by a star vicinity within the Alfven radius.
Infalling material forms X-ray emitting hot-spots at magnetic poles. Since in general the magnetic axis of the NS is not aligned with the rotation axis of the binary, observer sees pulsed X-ray emission. This is an X-ray pulsar.
How does the accretion occur in an LMXRBs?
It occurs gradually over a long period of time and if the companion is a neutron star, accretion can impart significant angular momentum, spinning up the NS. In this way, an old pulsar which has spun down due to its magnetic dipole emission can be revived by accretion and spun up to
very short periods.
What happens if at the Alfven radius of a LMXRB, the disk is rotating faster than the star?
The accretion disk exerts a torque leading to spin up.
How can XRBs be used to find evidence for stellar mass black holes?
The last stable orbit around a Schwarzschild BH is similar to a NS we expect X-ray emission.
If X-rays are produced from an X-ray binary with a similar orbit to a NS, what would bright and pulsed X-rays imply?
There is an X-ray pulsar
If X-rays are produced from an X-ray binary with a similar orbit to a NS, what would bright but not pulsed X-rays imply?
There could be a stellar mass black hole
What are ultra-luminous X-ray sources?
An astronomical source of X-rays that is less luminous than an active galactic nucleus but is more consistently luminous than any known stellar process, assuming that it radiates isotopically
Why are ultra-luminous X-rays of interest?
Their luminosities exceed that of the Eddington luminosity of neutron stars and stellar black holes.
What is an active galaxy?
One in which a significant fraction of its emission is non-thermal so not originating from the stars or ISM.
What is an active galactic nucleus?
The central few parsecs of an active galaxy. It is characterised by emission of large amounts of energy and often jets of relativistic material.
What are the four categories of AGN?
- Quasars
- Radio Galaxies
- Seyfert Galaxies
- Blazars
What are Quasars?
Bright, compact centres outshining the rest of their galaxy by factors of 10 to 100,000. They emit a nearly featureless spectrum from radio to X-rays but with broad emission lines in the optical.
What are radio galaxies?
Galaxies that look like normal elliptical galaxies in the optical but are very luminous in radio. The radio emission comes from the nucleus and/or pair of roughly symmetric lobes extending each side of the nucleus.
What are the two Fanaroff-Riley (FR) types of radio galaxies?
FR I: The brightest in the centre but less luminous than FR IIs. Jets are usually present but less collimated than FR IIs.
FR II: Have powerful collimated jets, often terminating in bright hot spots and are brightest in the lobes.
What are Seyfert Galaxies?
Spiral galaxies with very bright unresolved cores. Show strong optical emission lines from excitation and ionisation states too high to be produced by stars. Weak or no radio emission.
The brightest Seyferts are as bright as faint quasars. Seyfert galaxies are more often involved in
mergers with other galaxies compared to normal galaxies.
What are Blazars?
Extremely luminous and variable sources dominated by synchrotron emission. Weak emission lines, swamped by the synchrotron continuum. As bright as quasars.
What distinguished radio-loud and radio-quiet active galaxies?
It depends on the ratio of their radio to optical luminosity. Radio loud AGN are at least 10 times brighter in the radio than in the optical. Radio-loud AGN almost exclusively occur in elliptical galaxies.
What factors indicate the presence of a very large mass occupying a small region in the centre of a galaxy?
- Kinematics of water-like structures
- Kinematics of water masers
- Gravitational redshifts in X-ray Fe lines
What is likely at the centre of all galaxies?
A supermassive black hole.
What are X-rays observed from AGN believed to arise from?
Hot corona around the accretion disk that are generated by inverse Compton scattering disk photons.
Where does the radio emission from AGN originate?
It comes from synchrotron emission from the jets and lobes. Jets travel at very high speeds and can exhibit superluminal motion.
Why when jets are angled close to our line of sight is only one side visible?
Due to Doppler boosting.
What do the jets from AGN consist of and where do they terminate?
Relativistic electrons (and probably protons or positrons) and terminate in hot spots at the end of lobes. Electrons may then be accelerated in shocks but spread out to fill the lobes.
How does the slope of the electron spectrum of jets from AGN vary with time?
The high energy electrons age fastest so we expect the slope of the spectrum to steepen with time.
What is the minimum energy B-field from AGN synchrotron radiation roughly the same as?
The equipartition
What is the broad line region of the optical spectra of quasars and seyferts?
They indicate the velocities of emitting regions. Line widths are so large that thermal broadening is impossible so there would be no emission lines. Gas clouds with these velocities must be close to a very massive object if their motion is caused by a gravitational field.
What is the narrow line region of the optical spectra of AGN?
They correspond to slower bulk velocities. The region emitting these narrow lines has been resolved in some seyfert galaxies and is thought to come from gas clouds further out than the broad line region which move more slowly but are still ionised by the disk emission.
What is the molecular torus on the optical spectra of quasars?
The emission is consistent with black body emission from dust at 80 K and is believed to originate from a dusty torus that absorbs the emission from the nucleus and reemits it in the IR.
What do unified models of AGN attempt to do?
To explain the range of types and properties of AGN that are seen. AGN in this model consist of:
- Central black hole
- Accretion disk
- X-ray emitting corona around disk
- broad line region
- Narrow line region
- Molecular torus
Consider a radio loud AGN. Viewed approaching 90 degrees to the viewing axis, what would be seen on the optical spectrum?
The direction emission from the accretion disk and broad line region are obscured by the torus, while the emission from the narrow line region, jet and lobes are visible. This would appear as a radio galaxy with a narrow but not broad emission lines.
Consider a radio loud AGN. Viewed between 90 and 0 degrees to the viewing axis, what would be seen on the optical spectrum?
The inner regions are no longer obscured and so will be a radio galaxy with broad and narrow emission lines. As the angle approaches the jet axis, Doppler boosting effects become important and the jet will appear more asymmetric.
Consider a radio loud AGN. Viewed approaching 0 degrees to the viewing axis what would be seen on the optical spectrum?
The beaming of the jet emission becomes stronger and the source becomes more compact. The observer sees the emission from the disk and broad narrow line regions, plus the synchrotron from the jet and the source appears as radio-loud quasar.
Consider a radio loud AGN. Viewed at 0 degrees to the viewing axis what would be seen on the optical spectrum?
The same source viewed directly down the jet would appear as a blazar. The Doppler boosted jet emission drowns out the emission lines, and small variations in jet speed or angle give large variations in the boosting factor, explaining the variability.
Consider a radio quiet AGN. Viewed approaching 90 degrees to the viewing axis, what would be seen on the optical spectrum?
This would appear as a Seyfert galaxy with broad and narrow emission lines.
Consider a radio quiet AGN. Viewed approaching 0 to 90 degrees to the viewing axis, what would be seen on the optical spectrum?
A Seyfert galaxy with with broad and narrow emission lines if low luminosity or a radio quiet quasar if high luminosity.
What are galaxy clusters?
The largest gravitationally bound structures in the universe, that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity
What is the intra-cluster medium?
Primordial gas that falls into the potential well of a galaxy cluster, that gains bulk KE, converted into internal energy by shocks with the gas already in the cluster, heating the gas to high temperatures. The gas in the cluster is the intra-cluster medium.
What does the ICM being in virial equilibrium once in the cluster mean?
The virial theorem applies.
How do clusters appear from X-ray sources on spectra and what is the emission mechanism?
Clusters appear as extended centrally peaked X-ray sources and are among the most luminous sources in the sky. The primary emission mechanism is thermal bremsstrahlung.
What would we expect from the emission spectra if ICM were primordial gas? What do we actually find?
We would expect it to be fully ionised pure H and He i.e. no emission lines but instead we find strong emission lines (notably Fe)
What do measurements of detailed metal abundances in ICM tell us?
Records on the history of star formation. For example the relative numbers of type II and type Ia SN.
What is a cooling flow in ICM?
The denser cool regions of the ICM cool most rapidly and so contract increasing its density and hence moving faster. This runaway process is called a cooling flow and should lead to massive stars formation in the central galaxy in the cluster as cold gas accumulates on it. Since this is not observed, something must be balancing the cooling.
What is believed to be responsible for balancing the cooling of jets of AGN in the central galaxy?
Energy output of the jets. Indeed AGN jets are seen inflating huge bubbles in the ICM providing clear evidence for interaction.
What is the Sunyaev-Zel’dovich effect?
The spectral distortion of the cosmic microwave background through inverse Compton scattering by high-energy electrons in galaxy clusters. The low-energy CMB photons receive an average energy boost during collision with the high energy cluster electrons. Observed distortions of the CMB spectrum are used to detect the disturbance of density in the universe.
What are cosmic rays?
High-energy particles or clusters of particles that move through space at nearly the speed of light.
What does the lattice effect mean in relation to cosmic rays?
The number of cosmic rays is larger at the Earth’s poles than at the equator. This is because CRs are charged particles that spiral along magnetic field lines.
What charge are cosmic rays? How do we know this?
The direction the particles bent in the Earth’s magnetic field was used to determine that most CRs have positive charge
What is the East-west effect?
if CRs were positive, then low-energy, and hence low rigidity
particles arriving from the East would be blocked by the Earth, while higher energy particles with a larger radius of curvature would not be blocked.
How can low energy cosmic rays be measured?
In small detectors on satellites or balloons in the upper atmosphere. Detectors measure the ionisation caused by a CR along its path through the detector and the range of track in detector material and detect the Cherenkov radiation from when particles move into medium in which its peed is faster than the speed of light.
What type of cosmic rays are detected in the upper atmosphere?
Primary rays
How can high energy cosmic rays be measured?
Due to high energy CRs being hard to stop, the Earth can be used as a large detector. A high-energy CR entering the atmosphere will produce nucleonic cascades and electromagnetic
showers. Typically, a CR proton collides with a nucleus in the atmosphere, ejecting a nucleon and producing pions. The primary CR and the secondary particles go on to interact with further nucleons in the first nucleus and in additional nuclei. Pions decay through various paths to give γ-rays, electrons and neutrinos etc. which can be detected.
How can intermediate energy cosmic rays be measured?
At intermediate energies we can detect the Cherenkov light produced in the Earth’s atmosphere from the primaries and secondaries, using photo-multiplier tubes in combination with mirrors or lenses.
How can the highest energy cosmic rays be measured?
We use “Extensive Air-Shower Arrays” of detectors (over many sq km). E.g. Pierre Auger observatory in Argentina uses detectors which each contain 12 tons of purified water, and photomultiplier tubes detect the Cherenkov radiation of incoming secondary particles from >∼ 1018 eV primaries as they pass through the water. It is then possible to reconstruct the energy and arrival direction of the primary CR from the shower of secondaries that was detected.
What is spallation in relation to cosmic rays?
CRs show more Li, Be, B and elements just below
Fe, Pb. This is a signature of spallation. CR-ISM interactions chip away at nuclei (fragmentation) - elements just below the most common ones are enhanced.
What trend do cosmic ray spectra follow?
A power law
How do primary cosmic rays move at low energies?
Primary CRs will have low rigidity and travel in tight circles around Galactic magnetic-field lines. Anisotropy in arrival directions should be low regardless of CR origin.
How do primary cosmic rays move at high energies?
their paths are less bent by passage through the Galaxy and they should begin to point to their origin. Since we are in the disk of our Galaxy and not at its centre, we should be able to tell if these higher-energy CRs have a Galactic origin by looking for anisotropies.
What are the origins of cosmic rays?
Most CRs ae of galactic origin but ultra high energy CRs are of extragalactic origin.
When are direct observations of cosmic ray electrons possible? Why?
Direct observations of CR electrons only possible at > 10 GeV since at lower energies they lose flux to the magnetosphere and solar wind B-field, so cannot be measured reliably.
By what processes can cosmic ray electrons lose energy?
- Ionisation
- Bremsstrahlung
- Synchrotron
- Inverse Compton
What is ionisation?
Electrons kick electrons out of atoms by electrostatic repulsion. As for protons, there is
a weak dependence on energy.
What is Bremsstrahlung?
Coulomb-force acceleration causing EM radiation. Stronger if atoms ionised
What is synchrotron?
B-field-force acceleration causing EM radiation
What is inverse Compton?
Photons in radiation field of energy density urad up scattered at expense of electron
energy
What is the energy loss rate proportional to for each energy loss process for cosmic rays?
- Ionisation: constant with energy
- Bremsstrahlung: Proportional to E
- Synchrotron: Proportional to E squared
- Inverse Compton: Proportional to E squared
When do ionisation losses dominate for Cosmic rays?
At low energies.
When do synchrotron and Inverse Compton losses dominate?
At high energies
Why can high energy electrons from cosmic rays not have comological origin?
Energy loss to inverse scattering on the Cosmic Microwave Background (CMB)
What is the source term for modelling cosmic rays?
The number of new particles created per unit energy per unit time.
What does the measured cosmic ray electron spectrum show?
Th process that dominates the energy losses at different energies. this is ionisation, then escape, then synchrotron and inverse Compton.
For what p value is there abroad agreement in the measured cosmic ray electron spectrum?
p = 2.4, with the breaks in the spectrum due to electrons escaping the galaxy in 10^7 years.
If the residence time is not 10^7 years in the measured cosmic ray electron spectrum, what would the spectrum look like?
The spectral breaks would not be where they are observed.
What does the positron spectrum of cosmic ray electrons and positrons show?
The positron spectrum is slightly steeper than the electron spectrum, consistent with the positrons being secondary particles. In other words, the numbers of high-energy positrons drops off more quickly with increasing energy than the numbers of electrons. This makes sense if positrons are secondary since secondary particles must be produced by an even higher energy primary so it is hard to produce very high energy secondaries.
When can cosmic ray proton spectra be observed directly?
When E > 1 GeV. Below this they suffer solar
modulation.
How do cosmic ray protons lose energy?
CR protons lose energy through strong interactions – inelastic collisions with protons in the ISM which produce mainly pions. The charged pions decay to give secondary CR electrons.
Why must cosmic ray protons be galactic in origin?
If CR protons were extragalactic, we would them to be uniformly distributed through
the Galaxy, but this assumption gives a prediction for the distribution of γ-rays produced by those CR
protons that is too smooth compared with observations. Instead, if the CR protons are produced in the
Galaxy at locations that trace the ISM, then the predicted γ-ray distribution is a much better match to
observations.
What does the leaky box model for cosmic ray heavies assume?
CR sources are uniformly distributed in space, in a finite confinement volume (i.e. the Galaxy), with diffusion giving a distribution of path lengths before the CRs escape from the Galaxy. Also assume a steady state and new production and diffusion with leakage are allowed.
What is leakage lifetime?
The mean lifetime of cosmic ray heavies before escape.
How do spallation secondary spectra compare to primaries?
Spallation secondaries have steeper energy spectra than primaries. This is understood within the leaky-box model if higher-energy CRs escape more easily from the Galaxy, so spallation is reduced.
Is it it is feasible that SN explosions and their remnants have enough energy in high-energy particles to explain the population of CRs that originates in our Galaxy?
Yes
How does fermi acceleration work?
Fast particles bounce off magnetic field irregularities in moving clouds. There will be more head-on
than ‘following’ collisions ⇒ net energy gain for particles
