High energy astrophysics Flashcards
1
Q
What approximations can we make for highly relativistic particles?
A
beta as one and energy as kinetic energy, which is approximately momentum multiplied by the speed of light (because KE is much larger than rest mass energy)
2
Q
What is the motion of a charged particle with a pitch angle in a magnetic field and what is this driven by?
A
It has a corkscrew motion driven by the Lorentz force, it orbits/gyrates around the magnetic field
3
Q
what two forces are equated for a charged particle in a magnetic field?
A
Lorentz force (relativistic so multiplied by gamma) and the centripetal force
4
Q
What is the radius of a charged particle’s path in a magnetic field including rigidity?
A
R (for rigidity) multiplied by sin(theta) over B multiplied by the speed of light
5
Q
What is the equation for rigidity and its units?
A
Momentum times the speed of light divided by the charge number multiplied by the electron’s charge. Units of joules per coulomb or volts
6
Q
What does it mean for a particle with high rigidity?
A
It is more difficult for a magnetic field to bend its path so it has a larger radius of curvature
7
Q
Particles with the same rigidity have the same what for a given magnetic field?
A
Radius of curvature
8
Q
How can we focus x-rays to be able to detect them and why does it need to be done this way?
A
Grazing incidence reflections to change the direction of the x-rays with angled mirrors because we can’t do head on reflections or else it will pass straight through it
9
Q
What is the common set up for grazing incidence reflections to be able to detect x-rays?
A
Two successive reflections and the mirrors are nested inside each other to increase the collecting area and they are polished precisely
10
Q
How are x-ray photons commonly recorded?
A
By a CCD array to give energy and spatial information or a grating of etched metal dispersing the x-rays from bright sources into a 1D spectrum
11
Q
Are gamma rays able to be focussed and why?
A
No because their energies are too high
12
Q
What do we use detectors for with gamma rays?
A
Recording arrival direction
13
Q
What energy ranges matches with what interaction is used to detect gamma rays?
A
1-30MeV for Compton scattering, 30MeV-30GeV for electron/positron pair production and 100 GeV-100TeV for Cherenkov emission
14
Q
What is a Compton telescope that is used to detect gamma photons and what can it detect?
A
It uses Compton scattering to determine the scattering angle of source photons and the energies of the scattered electrons can be added to find the energy of the energy of the incident photon
15
Q
What is the pair conversion detector used to detect gamma photons?
A
layers of silicon interleaved with a heavy metal that trigger gamma to electron/positron pair production
16
Q
How do pair conversion detectors get information about the gamma photons?
A
They follow the detector of the produced electron/positron pair through silicon layers and reconstruct the arrival direction and below the silicon layers calorimeters measure the energies of the electron/positron to determine the gamma energy
17
Q
What are the principles behind Cherenkov telescopes to detect gamma photons?
A
Incoming photons interact with particles in the atmosphere to produce high energy electron/positron pairs and 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 and ground Cherenkov telescopes measure light from atmosphere-induced particle showers and an array of telescopes can localise sources from triangulation
18
Q
Why are hard-spectrum sources (high energy X-rays) visible to larger distances than soft-spectrum sources?
A
Lower energy x-ray suffer photoelectric absorption from neutral gas in our galaxy
19
Q
What are the two main parts of the x-ray sky?
A
Isotropically-distributed discrete sources and discrete sources mapping the milky way
20
Q
What are the main examples of isotropically-distributed discrete sources of x-rays in the sky?
A
Local sky, distant active galactic nuclei (AGN), galaxy cluster gas and nearby normal galaxies
21
Q
What are the main examples of discrete sources mapping the milky way that show the sky in x-rays?
A
X-ray binary systems and supernova remnants
22
Q
What are the main parts of the gamma ray sky?
A
Isotropically-distributed diffuse emission with localised radio-loud AGN and gamma ray bursts. Also, high-contrast emission following the Milky Way, including diffuse emission and some discrete sources (less than 20%) like pulsars and binary systems
23
Q
What does synchrotron radiation come from?
A
Highly-relativistic charged particles (usually electrons) gyrating about magnetic field lines
24
Q
For the rate of synchrotron rate of energy loss, is only the velocity component parallel or perpendicular important and what in the equation shows this?
A
Perpendicular because in the equation they use the sine of the pitch angle that the particle is travelling in relative to the field lines
25
What is the Thomson cross section and why does it tell us that the synchrotron energy loss rate is negligible for protons in comparison to electrosn?
It is an interaction cross section of the scattering of EM radiation from a charged particle and the energy loss rate for electrons is so much higher than that of protons because the mass of electrons is tiny so this cross section is huge
26
For a population of electrons each moving at randomly distributed angles, how do we use the equation for the energy loss rate of a single electron (synchrotron radiation) to give a total direction-averaged energy loss rate per electron?
Average the single electron equation over pitch angle using P(theta)d(theta) and integrate over the pitch angles 0 to pi.
27
When direction-averaging over pitch angles, what does P(theta) d(theta) equal?
One half sin(theta) d(theta)
28
What does the beam shape of the synchrotron radiation look like in the lab frame of the electron?
It is mostly beamed in the forward direction (in the direction of the electron's motion)
29
What does the beam shape of the synchrotron radiation look like in the rest frame of the electron?
Looks like a dipole along the axis perpendicular to the acceleration direction (ie along its direction of motion)
30
What is the non-relativistic gyrofrequency?
The frequency of an electrons orbit in non-relativistic cases is equal to the electrons charge times the magnetic field divided by 2 pi times the electrons mass
31
What is cyclotron radiation from electrons and what does the observer see?
Radiation from non-relativistic electron and its not beamed and the observer sees emission of radiation that varies sinusoidally with period one over the gyrofrequency
32
What does the Fourier transform of the emission of the radiation for cyclotron radiation from electrons give?
The frequency distribution with the delta function at the gyrofrequency
33
What is the opening angle of the narrow cone that a relativistic electron emits (via synchrotron radiation)?
1 over gamma
34
What does the observer see in synchrotron radiation as the electron orbits?
A short pulse once per orbit as the beam sweeps by
35
In the relativistic case for electrons emitting synchrotron radiation, what form does the emission have in the electron's rest frame? (in observer frame, they are short pulses separated by a time period)
It looks like that of a cyclotron radiation with a smooth sine wave
36
What is the period related to the reduced frequency of the period between the sharp pulses of synchrotron emission in the observers frame?
The gyrofrequency over gamma
37
What is the energy spectrum of the synchrotron radiation determined by and what is it not determined by?
It is determined by the width of the peak and not determined from the interval between peaks
38
What is the Fourier transform of the sharp peak of synchrotron radiation (ie the frequency distribution)?
Continuous distribution, resulting in a continuous energy spectrum
39
What are the two factors that determines the width of the synchrotron peak?
Beaming and shortening of pulse
40
What is the beaming effect in synchrotron radiation?
Emission is only seen for a small amount of time, where the electron has moved through the opening angle of the emission
41
What is the shortening of pulse in synchrotron radiation?
The pulse is shortened because the electron is travelling close to the speed of light so it almost keeps up with emission from one point as it travels to the next point
42
Is the frequency of synchrotron radiation larger than the non-relativistic gyro-frequency and why?
Yes, many orders of magnitude higher because of the beaming and pulse shortening effects
43
What is the frequency of the synchrotron radiation proportional to when the electrons have the same energy?
The energy squared multiplied by the magnetic field
44
What is the critical frequency of synchrotron radiation?
The frequency at which the spectrum (power per unit frequency) peaks
45
What approximation can you make about the synchrotron radiation frequency and power for electrons of the same energy?
That all the power is emitted at the critical frequency
46
What timescale is the lifetime of the electron population for synchrotron radiation referring to?
The time for them to lose half their energy
47
What does the synchrotron spectrum (luminosity per unit frequency) look like when using a power-law distribution of electron energies (instead of all with the same energy)?
Its another power law with slope alpha, the spectral index (slope = index of power law)
48
Where is the electric vector a maximum for an accelerating charge? (related to synchrotron polarisation)
Anti-parallel to the direction of the acceleration, which is perpendicular to the direction of motion for a charged particle in a magnetic field
49
Why is there high linear polarisation for synchrotron radiation?
Radiation is strongly beamed along the direction of the particles motion, so the observed photons come from electrons with velocities directed towards to observed and therefore, electric field vectors that are closely aligned
50
At what viewing angle is the synchrotron emission 100% polarised?
Looking edge on so in the plane of the electron's orbit
51
Why might there not be an observed high degree of polarisation at the source?
The magnetic field at the source is tangled preventing any coherent polarisation or the radiation passes through plasma on the way to the observer
52
What is 'detailed balance' regarding emission and absorption?
Every emission mechanism has a corresponding absorption spectrum
53
What is synchrotron self-absorption?
When a synchrotron photon is absorbed by one of the synchrotron electrons
54
When does the self-absorption cross section for synchrotron electrons increase and become important?
For low frequency photons
55
What is the brightness temperature of radiation?
The temperature of a black body with the same intensity as the source in question at a particular frequency
56
Why do we talk about black bodies when studying synchrotron radiation?
Electrons can't emit more efficiently than a black body at the same temperature
57
Does a source of higher intensity at a given frequency need a hotter or colder black body to match it and does this mean the brightness temperature is higher or lower?
Hotter and higher
58
Does the brightness temperature increase or decrease rapidly as the frequency decreases for the synchrotron source?
Increases
59
What is the effective temperature for emitting synchrotron electrons?
The temperature equivalent of their relativistic kinetic energy
60
Does the effective temperature increase or decrease as the frequency decreases for the synchrotron source?
Decrease
61
Why can't the brightness temperature be larger than the effective temperature for synchrotron electrons? (if self-absorption didn't occur this would be a problem for low frequencies)
It would mean that the electrons radiate more efficiently than a black body at their effective temperature, which is impossible
62
The brightness temperature cannot exceed the effective temperature, what resolves this?
Electrons become opaque to their own synchrotron emission and absorb photons (self-absorption) to satisfy the condition T_e = T_b
63
The absorbed spectrum for low frequencies of synchrotron radiation only is proportional to what power of the frequency?
2.5
64
When does the frequency (denoted with subscript m) at which the self-absorption condition is just met (T_e = T_b) increase?
When the source size decreases or the magnetic field increases
65
When does the absorption spectrum kick in for synchrotron radiation?
At the frequency denoted by m, determined by magnetic field and source size
66
Do the most energetic electrons that emit the highest frequency synchrotron radiation lose energy most quickly or slowly?
Most quickly
67
What is free-free emission?
Emission that goes from a free state to a free state
68
What is bremsstrahlung emission from?
From charge acceleration from Coulomb interactions between particles in a plasma and it is free-free emission
69
Why are electrons accelerated and losing energy due to Bremsstrahlung instead of ions?
Because they are so much lighter
70
What is the gaunt factor in the Bremsstrahlung emission equation?
It corrects for quantum mechanical effects and the effects of distant interaction and is a slowly varying function of frequency and temperature of order unity
71
How does the bremsstrahlung luminosity per unit volume depend on the density of the plasma?
The luminosity per unit volume is proportional to the square of the density
72
What is inverse-Compton scattering?
Relativistic electrons losing energy by upscattering photons
73
How is inverse-compton scattering analogous to synchrotron radiation?
Electrons lose energy to a radiation field (inverse-Compton) rather than a magnetic field (synchrotron)
74
What situation can we simply model inverse-Compton scattering as for calculations?
An electron absorbing a low energy photon and emitting a high energy photon
75
In inverse-Compton scattering, what is the increase in energy of the photon before the scattering (the one that gets absorbed in the model) and the one after (the one that gets emitted in the model)?
Increased by the square of the Lorentz factor of the scattering electron
76
Is the spectral form for inverse-Compton scattering the same or different from that of synchrotron radiation?
The same
77
Is the scattered radiation for inverse-Compton scattering more or less polarised than that from synchrotron radiation?
Much less
78
What are examples of photon fields of known energy density? (to use for inverse-Compton scattering for example)
Cosmic microwave background radiation and electrons scattering their own synchrotron emission (called synchrotron self-Compton)
79
What is the effect of inverse-Compton scattering in terms of photons and electrons?
It produces high energy photons and reduces the energy of relativistic electrons passing through radiation fields
80
How are supernovae classified?
Their spectra around their peak output
81
What is a supernova?
It is the end of stellar evolution and are characterised a sudden brightening (explosion), followed by a gradual fading.
82
What is the graph of luminosity against time called and what is a good for?
Light curve and good for objects that change their brightness over time, like supernovae
83
What characterises type 2 supernovae?
They show hydrogen lines in their spectra
84
What characterises type 1 supernovae?
They do not have hydrogen lines.
85
What makes type 1 supernovae either type 1a, 1b or 1c?
1a: they show certain silicon lines. 1b: No silicon lines but have helium lines. 1c: No silicon or helium lines
86
Type 2 supernovae are the result of what size stars and what is the name for this?
Over 8 solar masses and core-collapse supernova
87
What is the first step of how and why core-collapse supernova happen?
Iron core grows too massive for degenerate electron pressure support to counter gravitational compression (Chandrasekhar limit) and the core collapses
88
What is the Chandrasekhar limit?
The maximum mass of a core is 1.4 solar masses, beyond which the electron degeneracy cannot counter collapse because electron capture occurs
89
Why does degenerate electron pressure occur?
There are electrons in the same state, which is not allowed and resists collapse
90
After the core collapses in type 2 supernovae, what does the collapse and further nuclear reactions produce?
Large amounts of neutrinos that carry away a significant amount of the total energy
91
When does the collapse stop in the creation of type 2 supernovae?
The repulsive part of the nuclear force and degenerate neutron pressure prevents further collapse
92
What happens in the creation of type 2 supernovae after the core collapse stops?
The outer layers bounce back causing shock waves and compression and heating occurs, causing further fusion of elements heavier than iron.
93
What do type 2 supernovae usually leave behind?
A compact object, like a neutron star or black hole
94
What is a progenitor star or just progenitor?
The star that exploded into a supernova
95
Above and below what weights does the progenitor star need to be to create a black hole or neutron star after a type 2 supernovae?
Less than 25 solar masses for a neutron star and more than 25 solar masses for a black hole
96
How does the light curve of type 2 supernovae compare to type 1a?
The peak is typically fainter and they show a lot of variation
97
Are type 1b and 1c supernovae closer to type 1a or type 2 supernovae and why?
Type 2 because they are also core-collapse supernovae. (This does depend on the evolution of the progenitor star though)
98
Why do we not see hydrogen lines in type 1b supernovae spectra and why do we not see helium or hydrogen lines in type 1c supernovae spectra?
For type 1b, the star already shed its outer layer of hydrogen before the core collapses and for type 1c, the outer layers of hydrogen and helium shed before collapse.
99
Whilst the exact mechanism of type 1a is active research, what do most models involve?
A white dwarf star and a companion
100
What are white dwarfs?
Remnants of less massive stars (less than 8 solar masses) that have shed their outer layers and leaving behind a core that is support by electron degeneracy
101
What is the historical view of how type 1a supernova were ignited?
The white dwarf gains mass through accretion until is passes the Chandrasekhar limit and then collapses
102
Why is the historical view of how type 1a supernova were ignited assumed to be incorrect now?
An accreting white dwarf will only get within 1% of the Chandrasekhar limit (not over) so it is not the failure of degenerate electron pressure that makes the supernova start
103
Instead of degenerate electron pressure causing type 1a supernovae, what does?
The white dwarf still accretes from a companion star but the increasing density and temp leads to runaway carbon fusion in its corer, which ignites the supernova (in single degenerate model I think)
104
What is the single-degenerate model for type 1a supernova formation? (now a disfavoured model)
The white dwarf accretes material from a companion star in a binary star until it has enough mass for the supernova to ignite. This destroys the white dwarf, leaving nothing behind, whilst the companion star survives
105
Why is the single-degenerate model for type 1a supernovae formation now disfavoured?
There hasn't been evidence from observations for the companion stars in type 1a systems
106
What percent of type 1a supernova are predicted to come from the single-degenerate mechanism?
No more than 20%
107
What is the double-degenerate model for type 1a supernovae? (currently believed model)
The merger of two white dwarfs in a binary system and the resulting object exceeds the Chandrasekhar limit and collapses to ignite
108
Is there a lot of similarity or differences in type 1a supernovae light curves and spectra?
Extremely similar
109
What does the light curve of type 1a supernovae look like?
They rise quickly to a similar peak luminosity and then drop in brightness quickly at first before slowing after about 50n days to remain detectable for a few hundred days
110
Why are type 1a supernovae good standard candles?
They have similar peak luminosities
111
What has been a purpose of using type 1a supernovae as standard candles in the past?
Using their redshifts to construct a Hubble diagram to measure the Universe's expansion. It previously showed the accelerating expansion, providing evidence for dark energy
112
Are the peak luminosities of type 1a supernovae identical?
No. Observations have shown that brighter supernovae have a slower decline in brightness than fainter ones
113
How can we measure the peak luminosity of a type 1a supernovae?
Using the Philips relation, which depends on the fluxes measured at the peak of the light curve and 15 days later
114
Does the single-degenerate or the double-degenerate model explain the similarity in light curves in type 1a supernovae and why?
Single-degenerate because they have the same mass when carbon fusion ignites, whilst in the double-degenerate model the merging white dwarfs can have different masses
115
What happens after a supernova explodes?
The ejected material from the star expands into the surround interstellar medium that drives heat and sweeps up the ISM in a roughly spherical region, which is the supernova remnenat
116
What EM regions does the supernova remnant produce?
Radio to x-rays
117
What are the two categories of supernova remnant?
Crab-like (or plerions) or shell-like
118
What are crab-like (or plerions) supernova remnants?
They are filled with synchrotron emission (radio to x-rays) and have a central radio source (pulsar)
119
What percent of supernova remnants are crab-like and what type of supernovae do they come from?
Less than 10% and core-collapse supernovae
120
For crab-like supernova remnants, why do we know the electrons must be continually replenished from the central pulsar?
The lifetime for x-ray emitting electrons is less than the known age of supernova remnants
121
What are shell-like supernova remnants?
Radio-optical and x-ray emission seen from the outer shell but no emission from inside the shell
122
What type of supernovae do shell-like supernovae remnants come from?
Type 1a
123
Since electrons replenished with a central pulsar in a crab-like supernova remnants, what type of supernova is it usually associated with?
Type 2
124
For crab-like supernovae remnants, what causes the synchrotron emission?
The supernova shock accelerates electrons
125
For crab-like supernova remnants, why does the source decrease in volume from radio to x-rays?
High energy x-ray emitter electrons can't travel as far before they radiate away their energy
126
For crab-like supernova remnant, there is a break in the synchrotron spectrum at a frequency that correspond with what?
It corresponds to when the lifetime of the electrons emitting the frequency are similar to the age of the source
127
Why is there a break in the synchrotron spectrum at a particular frequency for crab-like supernova?
Lower-frequency-emitting electrons not affected by energy loss but high-frequency-emitting electrons are, which steepens the electron and synchrotron spectra. In the middle, the electron lifetime is similar to the age of the source
128
What is a neutron star?
A compact object consisting almost entirely of neutrons and supported by degenerate neutron pressure
129
When was it predicted that core-collapse supernova could leave a neutron star?
In the 1930's
130
What are pulsars and when were they discovered?
Regularly pulsating radio source and 1960's
131
Pulsars are interpreted as being rapidly rotating, highly magnetised what?
Neutron stars
132
Where are pulsars mainly found?
In the disk of the galaxy
133
Why can pulsars have velocities of up to 100km per second after a supernova?
Asymmetries in the supernova explosion impacts a kick to a neutron star and angular momentum conservation means collapse of star leads to rapid rotation
134
The masses of neutron stars in binaries are what mass and why is this consistent with type 2 supernovae?
Over 1.4 solar masses, which is consistent with the collapse of the iron core at the Chandrasekhar limit
135
With pulsars, the magnetic and rotation axes are offset, which means the radiation beamed along the magnetic axis will do what and what will an observer see?
It will sweep the sky (like a lighthouse) and an observer will see pulses
136
What are the periods of pulsars?
A few tenths to a few seconds (separate class for millisecond pulsars)
137
Does the rotation for pulsars slow down quickly or slowly?
Slowly (period increases at 10 to the minus 15 seconds per second)
138
Why does causality mean the short period of pulsars implies a compact object?
Causality suggest the period of a rotation can't be shorter than the light crossing time of the object
139
Where does a pulsar emit beams of EM radiation from?
Its magnetic poles
140
What is the magnetic inclination angle? (referring to pulsars)
The angle between the rotation and magnetic axis
141
When we model a pulsar as a simple rigid magnetised sphere rotating in a vacuum, what do we assume radiates?
A time-varying magnetic dipole
142
Why is the magnetic field of a collapsing star amplified by a factor proportional to the ratio of its initial to final surface area?
The magnetic field lines will be closer together when collapsed
143
Why is the magnetic dipole emission from pulsars to responsible for the observed pulses?
Dipole radiation has a very low frequency and gets absorbed in the ionised ISM and powers the nebula surrounding the pulsar so not observed
144
Where does the luminosity of the magnetic dipole emission come from and is it a lot? (this gets absorbed by the ISM so not observed by us)
The slowing down of the pulsar spin and yes
145
What part of the EM spectrum is the pulse emission from pulsars detected in?
Mostly radio but sometimes across the whole EM spectrum
146
What is believed to be the mechanism by which the pulsed emission from pulsars occurs?
Curvature radiation as charged particles (plasma) stream away on curved magnetic open field lines at the poles along the rotation axis
147
Why is there open field lines that plasma can stream away on for pulse emission on pulsars?
Close to the pulsar, the field lines and plasma rotate with the pulsar, but further away from the pulsar, the rotation would exceed light speed so field lines cannot close past a certain radius
148
What is an x-ray binary?
A binary system containing a normal star and compact object, like white dwarfs, neutron stars and black holes. They are strong x-ray sources from accretion from the donor to the accretor
149
What is accretion of material?
Material falling down onto compact objects
150
About what fraction of all stars are in binaries?
About half
151
When do x-ray binaries occur?
The binary orbit is close enough
152
What happens when x-ray binaries from?
Material from the normal star flows onto the compact object, forming an accretion disk, boundary layer and hot spots (all emit x-rays)
153
What is the most efficient power source in the universe?
Accretion of material onto compact objects
154
The potential energy of infalling material in accretion gets converted to what?
Kinetic energy, which changes into heat and then radiation
155
Is accretion an efficient method of converting potential energy to radiation if all the PE goes into radiation?
Yes in comparison to other methods (especially for neutron stars) but only in the highly idealistic case of all PE going into radiation
156
Because it is idealistic to assume we can extract all of the potential of the material right down to the objects surface with accretion, what do we use instead?
The potential energy of the last stable orbit of matter around the compact object
157
Why is there no direct infall in accretion and how does it fall instead?
Conservation of angular momentum prevents this, so there must be some rotation instead
158
When is there no heat released in accretion onto a compact object and does this situation occur?
If matter fell in directly, and no this doesn't happen
159
How and why does an accretion disk form around a compact object in an x-ray binary?
Conservation of angular momentum allows collapse along rotation axis of in-falling material, forming a disk
160
How does viscous forces (frictional) arise in the accretion disk and what does this cause?
Material at different radii move at different speeds, so it causes instability that cause orbiting material to spiral inwards
161
When comparing the accretion efficiency of converting potential energy to radiation of direct infall (not accurate) to rotating infall, how much does the efficiency change?
It halves when you consider no direct infall
162
For spherically symmetric (Schwarzschild) black holes, what is the radius of the last stable orbit around a black hole? (needed for accretion)
3 times the Schwarzschild radius
163
What is Eddington limiting luminosity?
The maximum luminosity a body can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward (hydrostatic equilibrium)
164
Other than the luminosity, what else does the Eddington limit also limit and why?
It limits the accretion flow, as radiation pressure blows away in-falling material if the luminosity is too high
165
When considering the Eddington limit with accretion, the ionised plasma is made of equal parts electrons and protons. Radiation pressure only acts on electrons but how does this affect the protons too?
For electron-protons, it gets communicated to the proton by electrostatic forces
166
What is the radiation force? (considering Eddington limit and accretion)
The momentum transfer per unit time from the photons (radiation from accretion) to the electrons ( part of the infalling material)
167
What interaction cross section do we need to consider for electrons that infalling material in accretion?
Thomson cross section
168
If the luminosity exceeds the Eddington limit, what occurs?
Material is pushed away and accretion stops, which decreases the luminosity below the limit and accretion starts again (approx equilibrium around limit)
169
Assuming isotropic emission, the Eddington limit is the maximum luminosity for what type of source?
Accretion-powered source
170
If we equate the blackbody luminosity with the Eddington limit luminosity, what can we find out?
The temperature of the source and the energy so we can work out what type of EM wave it is
171
What is the equation for blackbody luminosity?
Surface area (4 pi r^2) times (Stefan Boltzmann equation) sigma times temperature ^4
172
What are the two classes of x-ray binaries (XRB)?
Low-mass XRB (LMXRB) and high-mass XRB (HMXRB)
173
What defines a low-mass XRB and what do some of them form?
The donor star is slowly evolving (long-lived) with a low mass (less than 1.4 solar masses) and some form millisecond pulsars
174
What defines a high-mass XRB and what do some of them form?
The donor star is a young massive (more than 10 solar masses) star with strong winds, and some form x-ray pulsars
175
Where do we expect to find high-mass XRBs and low-mass XRBs?
High-mass XRBs in star-forming regions and low-mass XRBs may have moved from birthplace
176
What does high-mass XRBs formation require and what happens?
2 stars with large enough masses to go supernova. A HMXRB is created after the first supernova and after the 2nd supernova, a binary consisting of 2 compact objects will be formed
177
If there is a neutron star as the compact object in a high-mass XRB, what will it have?
A strong magnetic field that can influence accretion
178
What is ram pressure when considering accretion? (I think just HMXRBs)
The pressure from inflowing material, caused by bulk motion
179
In accretion at the Alfven radius, the ram pressure inwards is balanced by what for HMXRBs?
The magnetic pressure outwards from the field lines resisting being pushed together
180
At the Alfven radius, accretion flow is dominated by what for HMXRBs?
The magnetic field and accreting material is funnelled along field lines
181
Infalling material forms what at the magnetic poles in HMXRBs?
X-ray emitting hot spots
182
For HMXRBs, the magnetic axis of the neutron star is not aligned with the rotation axis of the binary, what does the observed see and what does this mean?
Pulsed z-ray emission so this is an x-ray pulsar
183
What fraction of HMXRBs show x-ray pulsar behaviour?
About half
184
How does accretion occur in LMXRBs?
Gradually over time
185
If the companion is a neutron star or white dwarf in LMXRBs, what can accretion do to the compact object?
It can impact significant angular momentum, which spins it up
186
Are millisecond pulsars old or young?
Old pulsars that have been spun down due to its magnetic dipole emission
187
How does the spin-up mechanism happen for millisecond pulsars?
At the Alfven radius, if the disk is rotating faster than the star, then the accretion disk exerts a torque, leading to a spin-up
188
What is a Kepler orbit?
The motion of one body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional orbital plane. It considers only the point-like gravitational attraction of two bodies
189
When will a pulsar reach a shorter period of rotation and what stops it getting a shorter period?
If it has a higher accretion rate it makes a shorter period and if it has a high magnetic field, it stops it getting a short period
190
In comparison to young pulsars that have periods of milliseconds, do millisecond pulsars have higher or lower magnetic fields and what does this lead to?
Lower so it has much slower decays of their period (high magnetic fields of young pulsars with short periods mean their period decay rapidly)
191
What do XRBs also provide evidence for?
Stellar mass black holes
192
How do we distinguish between x-ray pulsars and stellar mass black holes, as they both emit x-rays?
X-ray pulsars emit x-rays that are bright and pulsed, whilst stellar mass black holes emit x-rays that are bright but not pulsed
193
Why does random flickering from a galactic source suggest a black hole?
It is not regular like a pulsar and it is consistent with accretion onto black holes
194
What is the maximum theoretical limit of the mass of neutron stars?
3 solar masses
195
The luminosity of an ultra-luminous x-ray source is between what two sources?
Less than an active galactic nucleus but more than any known stellar process (exceeding Eddington luminosity)
196
Although it is not certain what powers ultra-luminous x-ray sources, what do the models include as possibilities?
Beamed emission of stellar mass objects, accreting intermediate-mass black holes and super-Eddington emission
197
What is an active galaxy?
A galaxy with a significant fraction of its emission is non-thermal, so the emission doesn't originate from the stars or ISM (3% of galaxies). It contains an active galactic nucleus
198
What is an active galactic nucleus (AGN)?
The central few parsecs of an active galaxy
199
What characterises an active galactic nucleus region?
Emission of enormous amounts of energy (very luminous) and often jets of relativistic material
200
What are the types that AGN are classified into?
Quasars, radio galaxies, Seyfert galaxies and blazars
201
What defines a quasar and what is its other name?
Bright compact centres that outshine their galaxy and emit a nearly featureless spectrum from radio to x-rays, but broad emission lines in the optical. Quasi-stellar object
202
What defines a radio galaxy?
Very luminous in radio, look like elliptical galaxy in optical. Radio emission comes from nucleus and/or pair of lobes on each side of nucleus
203
What are the two types of radio galaxies?
Fanaroff-Riley (FR) 1 and 2
204
What distinguishes between the radio galaxy types of Fanaroff-Riley (FR) 1 and 2?
FR1 galaxies brightest in the centre but less luminous than FR2. In FR1, jets usually present but less collimated than FR2, which have powerful, collimated jets that often terminate in bright hot-spots, and are brightest in the lobes.
205
What is a Seyfert galaxy?
They are spiral galaxies with very bright unresolved cores. Strong optical emission lines from excitation and ionisation states too high to be produced from stars. Low radio emission
206
What are blazars?
Extremely luminous and variable sources dominated by synchrotron emission. Weak emission lines. As bright as quasars
207
What distinguishes an active galactic nucleus between being radio-loud or radio-quiet?
The ratio of their radio to optical luminosity. Radio-loud are at least 10x brighter in the radio (usually in elliptical galaxies) and have jets and lobes, whilst radio-quiet ones do not
208
List the main components of active galactic nuclei:
Central black hole, accretion disk, hot corona around disk, broad line region, narrow lines region, molecular torus
209
What mass is a super-massive black hole?
10^7-10^8 solar masses
210
What about a supermassive black hole could provide the energy to fuel an AGN if harnessed?
Its gravitational potential
211
What is some of the evidence that suggest that all galaxies host a super massive black hole?
Images of stars moving as if there is a large compact object in the galactic centre. Gravitational redshifts in x-ray Fe lines. Kinematics of water masers and disk-like structures
212
Studies from nearby galaxies have found a strong correlation between the mass of the central black hole and what?
The mass of its host galaxy
213
If we assume there is a supermassive black hole in the AGN, How large would the accretion rate need to be to power the AGN?
Not large
214
Assuming there is a supermassive black hole near the AGN, the accretion disk emits mostly in ultraviolet. What could explain the x-rays observed from AGN?
They are believed to come from a hot corona around the accretion disk, generated by inverse- Compton scattering disk photons
215
Where does the radio emission from AGN come from?
Synchrotron emission from the jets and lobes (lobes created by the jets)
216
What are the speeds of jets from AGN and what does it mean tat they can exhibit superluminal motion?
Beta of 0.999 and it means that its speed can appear to be faster than the speed of light
217
What is doppler boosting with jets in AGN?
When jets are angled close to our line of sight, only one side may be visible, leading to asymmetry where one jet is a lot brighter than the other
218
The specific intensity (energy flux per unit solid angle and frequency) divided by the frequency cubed is what?
Lorentz invariant
219
How is the observed specific intensity of a jet travelling towards an observer related to the source specific intensity?
They are linked by a relativistic Doppler formula
220
What do the jets in AGN consist of and where do they terminate?
Relativistic electrons and probably positrons or protons and terminate in hot spots at the ends of the lobes
221
After electrons in jets in AGN have reaches the hotspots in the lobes, what do they do?
They could be accelerated in shocks but then spread out to fill the lobes and since they are no longer being accelerated, they will lose energy to synchrotron emission (ageing)
222
Where does the hotspot position change on the lobes and how does this effect the electrons?
The hotspots move outwards so newer (recently accelerated) electrons are further out while older electrons are closer to the centre of the AGN
223
Since the highest energy electrons age the fastest, what do we expect of the slope of the electron spectrum?
It steepens with time
224
What gives the direction of the jets in AGN?
The rotation axis of the accretion disk
225
The total energy of a synchrotron source comes from what?
The particles and magnetic field
226
For a synchrotron source, when we use the magnetic field corresponding to the minimum total source energy, how does the energy in the magnetic field relate to the energy in the particles and what does this mean?
The energy in the particles is three quarters the energy in the magnetic field. Roughly the same as the equipartition magnetic field, where there is equal energy in particles and field
227
What do the broad emission lines in optical spectra of quasars and Seyferts (and other AGN) indicate?
Bulk velocities of emitting regions of 10^5 km per second
228
What causes the broad line region in AGN?
Gas clouds with high velocities moving very close to a massive object (eg 1 pc from central BH) illuminated (ionised) by photons from disk
229
What does the narrow line region in most AGN spectra indicate?
Bulk velocities of 100-1000km per second from gas clouds further out than the broad line region and move more slowly but are still ionised by the disk emission
230
What is the molecular torus in AGN?
Doughnut-shaped structure of cool molecular gas that is dusty and absorbs emission from the nucleus and re-emits in the IR
231
What made scientists consider the presence of the molecular torus?
In the quasar spectra there was a bump in emission in the IR that is not from the disk or jets and the emission is consistent with BB emission frim dust
232
Where and how is the molecular torus position around the AGN?
It extends from 1 pc to about 100 pc beyond the accretion disk but is in the same plane
233
How does the unified model of AGN explain the different classes of AGN?
They consist of the same set of things and different classes correspond to different viewing angles of the same type of object. Viewing angle affects obscuration by torus and beaming along jets
234
The appearance of the AGN depends on the viewing angle relative to what axis?
The jet axis
235
What does a radio-loud AGN look like when viewed near 90 degrees from the jet axis and why?
Radio galaxy with narrow but not broad emission lines. Torus obscures the direct emission from the accretion disk and broad line region, whilst emission from narrow line region and jets and lobes are seen
236
What does a radio-loud AGN look like when viewed at an intermediate angle to the jet axis (not near 0 or 90 but between) and why?
Radio galaxy with broad and narrow emission lines. Inner regions no longer obscured. As angle approaches jet axis, Doppler boosting effects make jets appear more asymmetric
237
What does a radio-loud AGN look like when viewed approaching 0 degrees from the jet axis and why?
A radio-loud quasar, as the beaming of the jet synchrotron emission becomes stronger and the source becomes more compact. Observer sees emission from disk and broad and narrow line regions
238
What does a radio-loud AGN look like when viewed along the jet axis (0 degrees) and why?
A blazar. The Doppler boosted jet emission drowns out the emission lines and small variation in jet speed or angle give large variation in boosting factor
239
What does a radio-quiet AGN look like when viewed near 90 degrees from the jet axis?
Seyfert galaxy with narrow but not broad emission lines. Called Seyfert2 galaxies
240
What does a radio-quiet AGN look like when viewed at either an intermediate angle or along the jet axis?
Seyfert galaxy with broad and narrow emission lines (called Seyfert1 galaxies) if low luminosity or a radio-quiet quasar if high luminosity
241
What are galaxy clusters?
Largest gravitationally bound structures in the universe
242
How are galaxy clusters formed?
High density peaks in the early universe and primordial gas falls into the potential well of a galaxy cluster and this infalling gas builds bulk KE that gets converted to internal energy by shock and heats the gas to high temps (10^8 K)
243
What is the gas in the cluster called and what is it mostly made of in high temperatures?
Intra-cluster medium (ICM) and a fully ionised hydrogen plasma (just electrons and protons)
244
What does it mean that the intra-cluster medium is in virial equilibrium? This leads to the virial theorem, what is this?
Not expanding or contracting, and 2 times the KE is minus the PE
245
What is the primary emission mechanism in galaxy clusters and how do they appear in the sky?
Thermal bremsstrahlung and extended, centrally peak x-ray sources
246
How can we estimate the temperature of relatively cool galaxy clusters (10^7K) and hot clusters (10^8K)?
Cooler ones: from emission lines. Hotter ones: (plasma fully ionised so no lines) Use the frequency of the bremsstrahlung cut-off in the x-ray spectrum and this method can be used with cooler galaxies too
247
What percent of galaxy clusters are predicted to be dark matter?
90%
248
Why are there strong emission lines shown in the intra-cluster medium (eg highly-ionised Fe lines)? There shouldn't be any emission lines in the ICM if it was just primordial gas as that is fully ionised pure hydrogen and helium
The ICM is enriched by metals produced from supernovae in the cluster galaxies which have been mixed in the ICM and accumulated over time
249
What does the measurements of detailed metal abundances in the intra-cluster medium tell us and why?
The relative numbers of type 2 and type 1a supernovae because clusters record history of star information in member galaxies
250
What is the cooling flow process in the intra-cluster medium?
A runaway process where denser core regions in the ICM cool most rapidly and contracts, increasing its density and then cools faster
251
Why does cooling flow happen in the intra-cluster medium happen?
Emissivity of bremsstrahlung radiation is proportional to density squared
252
From the cooling flow in ICM, massive star formation should occur in the central galaxy as cool gas accumulates on it, but this doesn't happen. What do we think is balancing the cooling?
Energy output in the jets of AGN in the central galaxy
253
What is the Sunyaev-Zel'dovich effect?
The spectral distortion of the cosmic background radiation photons through inverse Compton scattering by high-energy electrons in galaxy clusters
254
Why does the Sunyaev-Zel'dovich effect give a method of detecting galaxy clusters?
The CMB photons are scattered to higher energies by the electrons, which are distinguishable from normal CMB photons
255
In the early 20th century, what did they believe the observed ionisation in the atmosphere come from?
Radioactivity in the Earth's crust
256
Who proved that a large source of ionisation in our atmosphere comes from outside the atmosphere and how?
Hess by flying high and found above 1km altitude that the ionisation level increased
257
What is the latitude effect made by Jacob Clay?
The number of cosmic rays was larger at the Earth's poles than at the equator
258
What were cosmic rays first considered to be and what disproved this?
High energy photons and the latitude effect, which only worked with charged particles as they spiral along magnetic field lines so there would be more near the poles
259
What was the effect called that proved cosmic rays were mostly positively charged?
The East-West effect
260
Why did the East-West effect prove most cosmic rays are positive?
If CRs were positive, the low energy, low rigidity particles arriving from the East would be blocked by the Earth, while high energy particles with a more rigidity would not be blocked, but from the West no particles are blocked. Reverse if assuming negative charge. More particles arrived from the West than the East
261
How many electron volts makes a cosmic ray either low or high energy?
Below or above 10^12 eV
262
How can we stop and measure low energy cosmic rays and what is the advantage of this method?
Small detectors on satellites or balloons in the upper atmosphere. Primary cosmic rays are detected rather than secondary particles generated by cosmic ray interactions in the atmosphere
263
How do the low-energy cosmic ray detectors measure things and what information can they find?
They measure ionisation caused by the cosmic ray along its path in the detector and the range of track in the material. If they also find Cherenkov measurements, charge and mass number can be found
264
What do we have to use to detect high energy cosmic rays and why?
We use the Earth's atmosphere to produce nucleonic cascades and electromagnetic showers because they are hard to stop
265
What happens in nucleonic cascades and EM showers which is what occurs when high energy cosmic rays enter the atmosphere?
Cosmic ray protons collide with atmospheric nuclei, ejecting a nucleon and producing pions, which can decay into gamma rays, electrons, positrons or neutrinos. Primary cosmic ray and secondary particles go on to interact with further nucleons.
266
How do we detect the Cherenkov light produced in the Earths atmosphere after high energy cosmic rays enter it?
Using photo-multiplier tubes in combination with mirrors or lenses
267
What do we use to detect the highest energy cosmic rays?
Extensive Air-Shower Arrays of detectors over many sq km
268
What percent of cosmic rays are stripped nuclei and what is the spare percent made of?
98% and electrons (2%)
269
Out of the stripped nuclei that are cosmic rays, what are they specifically and what percentages are they comparatively?
87% protons, 12% helium nuclei and 1% heavier
270
Why is there no neutron cosmic rays?
Free neutrons have a mean lifetime of about 10 mins and high energy ones could exist but none have been detected
271
What does it demonstrate that there are more cosmic rays for some elements like Li, Be, B and elements below Fe, Pb compared to local galactic abundances?
They indicate spallation, where cosmic rays interact with the ISM that causes fragmentation so that elements just below the most common ones are enhanced
272
Why is it hard to find the origin of low energy cosmic rays?
They have low rigidity and travel in tight circles around magnetic field lines so direction of arrival doesn't point back to origin and this makes them uniformly distributed across the sky
273
How can we start to find the origin of cosmic rays?
High energy cosmic rays are less bent as they travel so they should point to their origin and there should be patterns in arrival directions (anisotropies)
274
Most cosmic rays have what origin, whilst ultra-high energy cosmic rays have what origins?
Galactic origins for most CRs, whilst ultra-high energy CRs have extragalactic origins
275
How do we know ultra-high energy cosmic rays have an extragalactic origin?
A statistical correspondence has been found between arrival direction and positions of nearby Active Galactic Nuclei
276
Why is direct observations of cosmic ray electrons only possible when they are above 10GeV?
At lower energies they lose flux to the magnetosphere and solar wind magnetic field
277
What does solar modulation do to the number of cosmic rays detected?
Less CRs are detected when the sun is more active
278
Cosmic ray electrons of energy 10-10^3GeV have what p value and how are they detected?
3.3 and direct measurements
279
Cosmic ray electrons of energy 0.5-8GeV have what p value and how are they detected?
1.8-2.6 and radio synchrotron
280
Cosmic ray electrons of energy below 0.1GeV have what p value and how are they detected?
1.6 and gamma rays
281
For ionisation, how does the energy-loss rate depend on energy?
It is constant with energy
282
For bremsstrahlung, how does the energy-loss rate depend on energy?
Proportional to energy
283
For synchrotron, how does the energy-loss rate depend on energy?
proportional to energy squared
284
For inverse Compton, how does the energy-loss rate depend on energy?
Proportional to energy squared
285
At low electron energies, what type of energy loss dominates?
Ionisation
286
At high electron energies, what type of energy loss dominates?
Synchrotron and inverse Compton
287
How do you roughly the energy loss timescales for different mechanisms using energy and energy loss?
Energy divided by energy loss (energy loss = -dE over dt)
288
Why can high energy electrons not have cosmological origins?
The energy loss of electrons to inverse-Compton on CMB photons means they would lose all their energy before travelling great distances
289
What is the residence time of electrons?
Amount of time they spend in our galaxy before they escape
290
What is Q(E), the source term when considering the residence time of cosmic ray electrons?
The number of electrons produced by sources per unit energy, volume and time
291
What does it mean in terms of cosmic rays that the system is in steady state?
The number of electrons isn't changing with time. So energy losses are balanced by new particles
292
What is the residence time of elextrons?
10^7 years
293
What determines that the residence time is 10^7 years?
The location and presence of the spectral breaks of cosmic ray electrons and the fact that we don't see the unmodified electron spectrum the whole time
294
Why can't the residence time be shorter than 10^7 years?
None of the energy loss mechanisms would have time to take effect and the electrons would escape so we would observed the unmodified electron spectrum, which is not what we actually see
295
What percent of total numbers of cosmic ray electrons and positrons are primary electrons, secondary electrons and secondary positrons?
80% are primary electrons, 10% are secondary electrons and 10% are secondary positrons
296
Why is the cosmic ray positron spectrum steeper than the electron spectrum? (eg the number of high-energy positrons drops off more quickly with increasing energy than numbers of electrons)
Cosmic ray positrons are only secondary and it is harder to produce high energy secondaries
297
Why can cosmic ray protons spectra only be observed directly for energies above 1 GeV?
Below this they suffer solar modulation
298
Why do we not need to worry about energy losses to synchrotron, inverse-Compton and bremsstrahlung cosmic ray protons?
They scale as 1 over mass squared so this is very small
299
How do cosmic ray protons mostly lose energy?
Strong interactions - inelastic collisions with protons in the inter-stellar medium which produce mainly pions
300
What do charged and neutral pions decay into? (cosmic ray proton section)
Charged pions decay to give electrons or positrons, whilst 99% of neutral pion decays are two gamma photons
301
Neutral pions decay in 2 gamma photons, in the rest frame equal energies go into the photons, why is there a spread in gamma ray energies in the lab frame?
Due to relativistic effects
302
What does the local gamma ray spectrum look like considering cosmic rays?
There is a hump where CR protons make neutral pions that decay into photons and another hump for soft bremsstrahlung emission for low energy CR electrons
303
Since we see more photons (from CR proton interactions) where the ISM is more dense, what does this show? (gamma ray density follows ISM density)
There is more CR protons when the number density of ISM is higher so CR protons are galactic in origin
304
What would the gamma ray and cosmic ray proton distribution look like if cosmic ray protons were extragalactic and do we see this?
It would be uniformly distributed through the galaxy and the distribution would be too smooth in comparison to what we observe
305
What do we need to consider in the model for spallation from cosmic ray heavies?
Diffusion, where there is a change in the number density of particles into and out of a volume
306
Particles diffuse down what type of gradients in the spallation model for CR heavies?
Density
307
Why did we ignore diffusion for electrons?
Energy loss dominated
308
What is the density-weighted distance between collisions?
Density of material between multiplied by physical distance between collisions
309
What assumptions do we make for the leaky box model?
CR sources uniformly distributed in space in a finite volume (Galaxy), with diffusion giving a distribution of path lengths before CRs escape. Steady state. New production (Q is not zero). Diffusion with leakage (diffusion term not zero)
310
What is the leakage lifetime and what is assumed about it in the leaky box model?
Mean life before escape and it is assumed to be the same for all species
311
Why is density weighted distance when considering spallation for cosmic ray heavies?
The same distance travelled through a high density medium will produce more spallation
312
In the leaky box model for cosmic ray heavies, what is the equivalent for t time, tau_L leakage lifetime and tau_j time between collisions in distance terms?
time-total DW distance, tau_L leakage lifetime - x_nought characteristic DW distance before escape out of volume, tau_j - lambda DW distance between collisions
313
What is the units of density weighted distance?
Kilograms per metres squared
314
What x_nought value works for the characteristic DW distance before escape in the leaky box model for cosmic ray heavies?
50 kg per metres squared
315
What elements are usually spallation secondaries?
Li, Be and B
316
Why do spallation secondaries have steeper energy spectra than primaries in the leaky box model?
Higher energy cosmic rays escape more easily from the galaxy so spallation is reduced
317
Why can't cosmic ray heavies be streaming away from their production sites?
There is not enough time to produce the observed spallation
318
What is the leakage lifetime for cosmic rays?
10^7 years
319
Is it feasible that supernovae explosions and their remnants have enough energy in high-energy particles to explain the population of cosmic rays originating in our galaxy?
Yes
