Topic 6 The Extreme Universe Flashcards
What are active galaxies?
These are galaxies that emit electromagnetic radiation that can not be attributed to the normal constituents of galaxies such as stars, gas and dust.
Only a small fraction of galaxies are active.
What are active galactic nuclei?
The central region of an active galaxy, abbreviated AGN. This is the region of the active galaxy that differs most strongly from a normal galaxy. They often exhibit strong optical and X-ray emission and show bright emission lines.
The activity is powered through accretion.
What are Seyfert galaxies?
A type of active galaxy chacterised by optical emission from an active nucleus that is less luminous than a quasar.
What is the difference between Seyfert 1 galaxies and Seyfert 2 galaxies?
These Seyfert galaxies exhibit both broad and narrow emission lines, Wheras Seyfert 2 galaxies have no broad spectrum emission lines. Seyfert 2 galaxies are seen at a large angle to the line of sight so broad line emission is obscured by a dusty torus.
What is a quasar
An active galaxy that is characterised by a point like appearance in optical images and a very large redshift. Quasars spectra exhibit strong optical emission lines, and they may be further classified as radio loud or radio quiet depending on the strength of their radio emissions.
What is a lobe?
A extended feature lying at the end of jet emanating from an active galactic nucleus, seen in many radio galaxies and quasars.
What is a jet?
A long narrow linear feature emanating from an active galactic nucleus, seen in many radio galaxies and quasars, most often via its radio synchrotron emission but sometimes also at optical and X-ray wavelengths. Jets are also present in a small number of X-ray binaries and believed to be present in gamma-ray bursts.
What is a radio galaxy?
An active galaxy that shows (usually) two regions of diffuse radio emission on either side of it, i.e. radio lobes. The radio emission is powered by an active galactic nucleus (AGN).
What is a blazar?
A galaxy which is a member of the class of active galaxies which encompasses the BL Lac objects and the OVV quasars. The common property that unites blazars is strongly variable radio- and optical emission, which is thought to be due to a relativistically moving jet seen at a small angle to the line of sight.
What is a central engine?
The energy source lying at the centre of an active galaxy. Usually considered to be an accreting supermassive black hole.
What is an accretion disc?
A flow of matter that is largely confined to a plane and that is spiralling in towards a central object.
What is advection dominated accretion flow (ADAF)?
A process whereby energy is transported without significant energy loss, as a result of bulk motion of material. An advection-dominated accretion flow will occur if the cooling time of the material exceeds the time to fall onto the compact object. Often abbreviated to ADAF.
Flux density equation
Fν = Aν^-α
Fν is flux density, ν is frequency, A is a constant and α is a value that typically lies between 0-2.
What is a big blue bump?
A feature in the spectral energy distribution of some active galactic nuclei (AGN) that is due to thermal emission from an accretion disc.
What is an accretion disc corona?
A cloud of hot gas that may exist above and below an accretion disc and is the source of X-ray emission, arising from inverse Compton scattering, in some active galaxies and some X-ray binaries. Often abbreviated to ADC.
What is inverse Compton scattering?
Electromagnetic radiation with a continuous, non-thermal spectrum, emitted when relativistic electrons collide with and scatter photons. This removes energy from the electrons and increases the energy of the photons, which is equivalent to the emission of radiation.
What is a broad-line radio galaxy?
A radio galaxy that displays both broad and narrow emission lines in its optical spectrum.
What is a narrow-line radio galaxy?
A radio galaxy that displays only narrow emission lines in its optical spectrum.
What is the schwarzschild radius? (equation)
The characteristic radius (Rs) of an isolated non-rotating spherically symmetric black hole of mass M. The schwarzschild radius of a one solar mass black hole is about 3 km.
Rs = 2GM / c^2
The schwarzschild radius proportionality to mass
The radius is directly proportional to mass, so can be written
Rs = 3 (M /M solar mass) km
Gravitational potential energy equation
ΔEg = GMm/R
What is accretion rate?
The amount of matter accreted per unit time interval. Typically measured in kg /s or solar masses per year.
How can a black hole be luminous?
As matter falls into an object (accreted) gravitational potential energy is converted into electromagnetic radiation heating up accreting material via frictional forces.
Accretion luminosity equation
L = GM M / R
Where M is the rate at which the central body is gaining mass. And M and R are the central bodies mass and radius respectively.
What is the Eddington limit?
An upper limit the luminosity that can be generated in a system powered by accretion.
The limit arises because the photons emitted by the accreting body except a force on the elections in the accreting material. At the Eddington limit the force exerted by the photons is sufficient to overcome the inward gravitational attraction.
What is the Eddington luminosity? (Equation)
The accretion luminosity corresponding to the situation where an object is accreting matter at the Eddington limit. This is an approximate upper limit on the accretion luminosity since radiation pressure prevents accretion at a higher rate.
Ledd = 4π G M mp c / σT
Where M is the mass of the accreting object, G is the gravitational constant, mp is the mass of a proton (1.673x10^-27 kg), c is the speed of light (3x10^8 m/s), and σT is the Thomson cross section (6.652x10^-29 m^2)
Or the equation
Ledd = 1.26x10^31 (M/M solar masses) W
What is reverberation mapping?
A method of determining the distance of the broad line region from the active galactic nucleus by correlating variations in the broad lines and the active galactic nucleus AGN continuum, measuring, for example, the time delay between a brightening of the continuum and the corresponding brightening of the emission lines.
What is a supermassive black hole?
A black hole with a mass in excess of about 10^6 M⊙.
What is the broad line region?
This is a system of rapidly moving clouds of gas that lie close to the supermassive black hole, either side of the accretion disc, and within the dusty Taurus.
What is the dusty Taurus?
This is a region of dust and gas that surrounds the central engine. It is used to explain the emission of infrared radiation as x-ray and ultraviolet radiation is absorbed by the dust and remitted as infrared at the same rate.
What is the sublimation radius? (Equation)
The minimum distance from an active galactic nucleus or other luminous source at which dust grains can exist without sublimating (i.e. changing from the solid to gaseous state).
r =sqrt(L / 16π σΤ^4)
Where σ is the stefan Boltzmann constant (5.67x10^-8).
what is the narrow line region?
This is a system of clouds of gas moving less rapidly than those in the broad line region which lies outside the dusty Taurus.
Why is the reverberation mapping of the narrow line region in AGN unlikely to be possible?
The size of the NLR implies that the light travel time across it is of order several thousand years. Time lags between the flux of narrow line emission and the flux of continuum emission are therefore not possible to observe.
What are forbidden lines?
An emission line that occurs as a result of a transition between atomic energy levels that is very unlikely to occur spontaneously. In laboratory, atoms in excited states tend to strike other atoms, or the walls of the container, and so de-excite before they can emit a photon. This means that these spectral lines are very unlikely to be seen.
They can be seen in narrow line regions due to the very low cloud density.
What are Fanaroff-Riley types I and II?
Two classes into which the radio galaxy population can be divided. Objects that are brightest towards the centre are classified as FRI while those that are brightest at the edges are classified as FRII.
What role do active galaxies play in influencing galaxy evolution?
One effect may be to heat up and remove some fraction of the interstellar medium from the central regions of the galaxy. This would likely suppress star formation by removing the raw ingredients needed for the process. This supports the idea of negative feedback.
Another effect of the outflow may be to induce staff formation as a result of the compression of molecular clouds encountered by the outflowing material. This is supported by the observation that the luminosity of some active galaxies is correlated with their observed nuclear star formation rate. This supports the idea of positive feedback.
What is Fanaroff-Riley type I (FRI) radio galaxy?
This is a radio galaxy that exhibits decreasing radio luminosity in moving away from the central core.
What is Fanaroff-Riley type II (FRII) radio galaxy?
This is a radio galaxy that shows increasing radio luminosity away from the core and have bright radio lobes at the end of the jets.
What drives galaxy wide outflows of material from AGN?
This is driven by either radio jets or intense electromagnetic radiation and may serve to suppress or enhance star formation.
What is a compact binary star?
A binary star system where at least one of the components is a compact object, usually accreting material from its companion star.
What is the Oppenheimer Volkoff limit?
The upper limit to the mass of a neutron star supported by neutron degeneracy pressure. Equal to about 2.5 solar masses.
What are Roche equipotentials?
Contours of equal potential (energy per unit mass) in a rotating binary star system.
What are potential wells?
A location of minimum potential (energy per unit mass) in a binary star system.
In what direction will matter flow if it has potential energy?
The matter will always flow in the direction that will result in no potential energy.
What is aroche lobe?
The equipotential surface in a binary star system where the potentials of the two stars meet at the inner Lagrangian point.
(Topic 6 section 2.2.1)
What is Roche lobe overflow?
Mass transfer from a Roche lobe filling star via the inner Lagrangian point. The Roche lobe is the maximum volume available to a star in a binary system if a star expands beyond its Roche lobe, it will transfer mass to its companion.
What is a semi-detached binary?
A binary star in which one star fills its Roche lobe.
Why can material not be deposited directly from the accreting star onto the primary star?
The material possesses angular momentum due to the orbital motion of the stars so will therefore create a flattened accretion disc around the primary star. Due to friction the inner material of the accretion disc will eventually settle onto the compact object.
What is a cataclysmic variable?
A compact binary system, consisting of a white dwarf and a low mass main sequence star, in which much of the optical emission comes from an accretion disc around the white dwarf primary.
What is a low mass x-ray binary?
An x-ray binary where the donor star has a mass below about two solar masses. They usually occur via Roche lobe overflow.
How do we approximate the radius of the Roche lobe? (Equation)
R L,2 / a = 0.462(M2 / (M1 + M2))^1/3
Where R L,2 is the radius of the Roche lobe of the secondary (donor) star, a is the separation between the centres of the two stars, and M1 and M2 are the masses of the primary and secondary stars, respectively.
Maximum temperature of an accretion disc equation
Tmax = 0.5 x ((3 G M1 Mdot) / (8π σ (R1)^3))^1/4
Where M1 and R1 are the mass and radius of the compact object and Mdot is the mass accretion rate.
What factors affect the temperature of accretion discs around compact objects?
The smaller the radius of the compact object, the hotter the accretion disc can become as the matter can fall further into the potential well of the object.
As peak wavelength emitted relates to temperature (weins law) we know CV s emit UV and LMXBs emit X-rays .
What are novae?
A member of a particular sub class of cataclysmic variable which undergoes a significant outburst with an amplitude of typically 10 magnitudes. Nova outburst typically last tens of days and may re occur on a time scale as long as 10,000 years. The cause of the phenomena is run away Thermonuclear burning on the surface of a white dwarf.
What are X-ray bursts (nova)?
A short flash of Bright x-rays, lasting between a few seconds and a minute. They are thought to be due to the Thermo nuclear burning of material (hydrogen) onto the surface of a neutron star; this makes them analogous to Novae and requires that the primary object should be a neutron star rather than a black hole which has no solid surface.
What is the cause of Novae and x-ray blast?
As material, primarily hydrogen, is deposited onto the surface of a compact star a runaway nuclear fission occurs. Initially the CNO cycle which converts much of the hydrogen into helium, carbon, nitrogen, oxygen, neon and magnesium. It releases a fast amount of energy which blows away the rest of the accumulated material and releases copious amounts of electro magnetic radiation.
Detected flux equation
F = (R^2σT^4) /d^2
R is radius, T is temperature and d is distance.
What is a dwarf nova?
A member of a particular sub class of cataclysmic variable which alternates between Bright states (outburst) and dim states (quiescence) with an amplitude of a few magnitudes. The outbursts typically last for a few days and recur every few weeks. The cause of the phenomenon is an instability in the accretion disc.
What are soft X-ray transients?
A member of a particular sub class of x-ray binary which alternates between Bright state (x-ray outburst) and dim states (Quiescence). The outbursts typically last for a few months and recur on a time scale of decades. The cause of the phenomenon is an instability of the accretion disc surrounding neutron stars and black holes.
What is the difference between local mass accretion rate and global mass accretion rate?
Local mass accretion rate is the rate which material is transferred from an accretion disc onto the star.
Global massacre rate is the rate rich material is added to the accretion disc.
What are the four distinct phases of a light curve of an outburst?
1 - quiescence
2- rise to outburst
3- outburst
4- transition back to quiescence
these can be related to an S curve for the local mass accretion rate and surface density of the accretion disc. It starts on the cool branch and keeps getting bigger until it can jump up to the hot branch which is an outburst. The accretion disc then loses mass as the local mass accretion rate is higher than the global mass accretion rate and moves back down the curve until it reaches the unstable branch where it jumps back down to the cool branch and the process starts again.
What is the magnetospheric radius?
The radius around a magnetic compact object within which the magnetic field can control the accretion flow.
How does a white dwarf magnetic field affect the accretion disc?
Disc material can attach onto the magnetic field lines at the radius where the magnetic forces become strong enough to disrupt the flow. From here the material flows along to curtains towards the magnetic poles of the white dwarf.
Four white dwarfs with strong magnetic fields, they can prevent the creation of an accretion disc by drawing the material directly from the L1 point.
Minimum rotation period equation
Pmin = 2π(R^3 / GM)^1/2
What is an accretion powered x-ray pulsar?
A binary star containing a rapidly rotating neutron star that is accreting material from a companion star.
Why are neutron stars spinning so fast?
Neutron stars in low mass binary systems can exist for billions of years. As material accretes onto the neutron star from the companion star the stars rotation speeds up due to the conservation of angular momentum. This can reduce the stars rotation period to a few milliseconds.
What is a high mass x-ray binary?
An x-ray binary where the donor star has a mass above five solar masses (K or M type). They usually their bondi Hoyle accretion.
What is bondi Hoyle accretion?
accretion onto a compact object (neutron star or black hole) arising from the stellar wind of a high mass star. Seen in high mass X-ray binaries.
What is the binary mass function? (equation)
The minimum mass of a star in a binary system for which the period and motion of the companion star may be measured.
(Mx sin(i))^3 / (Mo + Mx)^2) = (P Ko^3) / (2πG)
Where Mx and Mo are the masses of the compact object and donor star respectively and P is their binary orbital period. Ko is the radial velocity amplitude.
The left hand side is the lower limit for the mass of the compact object.
Calculating the radius of an orbit of two stars from their measured speeds (equation)
a = (P(Ko + Kx) / 2πsin(i)
Where kx and Ko are the velocities of the compact and partner star respectively.
Using Keplers third law to calculate the sum of stellar masses. (Equation)
(Mo + Mx) = (4π^2a^3) / 2πsin(i)
Rate of masses of stars in a binary system (equation)
Mx / Mo = Ko / Kx
Where M and K are mass and radial velocity amplitude of the stars.
How do the recurrence times compare for the three types of x-ray outburst seen in x-ray bin containing neutron stars?
Outburst due to the Thermo nuclear burning recur every few hours.
Outburst due to disc instability (and soft x-ray transients) recur every few decades.
Outburst due to The passing through B stars discs in high mass x-ray binary pulsar recur every few months.
What is a microquasar?
An x-ray binary that emits radio jets. The analogy is that these are scaled down versions of quasars.
What causes outbursts in HMXBs containing Be stars?
Outbursts are caused when the neutron star in an eccentric orbit passes through the disc of the material expelled by the rapidly rotating Be star, increasing the amount of material accreted.
