Black Holes Exam 2

Neutron stars and pulsars, Accreting black holes and X-ray binaries, Galaxies and galactic nuclei, Center of the Milky Way, Active Galaxies, Jets, Gamma-Ray Bursts

| What supports a neutron star against gravity?

neutron degeneracy pressure (arises from the pauli-exclusion principle)

| Why do pulsars pulse? What is the energy source for these pulses?

Pulsars (spinning neutron stars)produce a “lighthouse beam” of radiation that swings through our line of sight once every revolution. The source for these pulses is the strong magnetic fields

| Why is there a maximum mass for Neutron Stars? Roughly, what is that mass?

~3 solar masses. This is because neutron degeneracy pressure can no longer support the star against gravity when it exceeds 3 solar masses.

| What are the proposed methods for detecting black holes? Explain each

Gravitational Lensing: uses the bending/distortion of light by a strong gravitational field to detect massive objects

Direct Imaging: imaging of the black hole’s shadow against the background glow

Energy released by Accretion: detects X-ray emissions from matter accreting/falling into the black hole

| Roche Lobe:

“zone of gravitational influence” - the region around a star in a binary system within which orbiting material is gravitationally bound to that star. Mass transfer happens when the star grows beyond its roche lobe

| How does ‘mass transfer’ in binaries affect the evolution of stars and lead to a binary containing a black hole and a normal star?

In a binary system – star A (more massive) evolves first and expands beyond its Roche lobe, and then transfers mass to its companion Star B. This changes the mass and evolution of both stars.

If Star A is massive enough, it might eventually collapse into a black hole, leaving a black hole–normal star binary.

| What is the Algol Paradox and what is its resolution?

Algol = eclipsing binary (small blue star, large red star)

Paradox: The younger blue star is somehow more massive. Massive stars have shorter life times, so why would the more massive star still be on the main sequence while the less massive companion is already becoming a red giant?

Resolution: Mass Transfer! → The red star was originally more massive but transferred a lot of its mass to the younger blue star. Now the blue star is more massive, even though it is younger.

| Why doesn't matter fall straight into a black hole in a binary system?

Because of angular momentum! Matter spirals inward, forming an accretion disk as angular momentum is transported outward.

| What is an accretion disk and how does it produce X-rays?

A flattened, rotating disk of accreting matter that forms around a black hole.

They produce x-rays as matter spirals inward and converts energy:

Gravitational → Kinetic (orbital) → Thermal → X-ray radiation

| Why is “friction” (viscosity) crucial for accretion disks?

because it allows gas in an accretion disk to transfer angular momentum outward, which causes the inner gas to lose angular momentum and spiral inward toward the black hole instead of remaining in orbit.

• (bonus) viscosity is thought to be due to tangled magnetic fields within the disk

| How does the bending of light combined with the Doppler effect and the aberration of light affect the appearance of an accretion disk close to a black hole?

The accretion disk overall appears warped and asymmetrically bright, instead of flat. Light bending warps the disk's appearance and reveals its far side. The Doppler effect blueshifts approaching material and redshifts receding material. Relativistic aberration concentrates radiation forward and makes the approaching side brighter.

| What arguments are used to determine whether an accreting compact X-ray source is a black hole or a neutron star?

If a compact X-ray source has a mass greater than 3 solar masses, it must be a black hole because neutron stars cannot support themselves beyond that.

If you can’t determine Mass:
Neutron stars have “surface” effects (x-ray pulsations, thermonuclear bursts) and Black Holes don’t.

| What are the major types of galaxies and their structural components?

Spiral (Disk) Galaxies: flattened, rotating disks of stars/gas/dust around a center bulge. Stars/gas follow approximately circular orbits around the center. Typically ~100 billion stars, gas amounts to ~10% of stellar mass.

Elliptical (bulge) Galaxies: “red and dead” - mostly older stars, no disk, random stellar motion/orbits, minimal gas. Typically contains ~100 milion — 1 trillion stars

| How would you characterize the Motions of Stars in Disk components compared to Bulge components?

Disk/spiral = almost circular orbits, Bulge = random orbits

| What is the evidence for ‘Dark Matter’?

Its existence is inferred from its gravitational effects on visible matter:

Gas in spiral galaxies orbits faster than expected, galaxies in clusters move too quickly to remain bound by visible matter alone.

These observations suggest additional unseen mass provides the gravitational pull needed to explain their motions.

| What processes might account for the dense concentrations of stars and gas in galactic nuclei? Describe how processes occurring in these regions might lead to the formation of a supermassive black hole.

When galaxies collide, the gas usually drifts toward the center of the combined galaxies, which can create and feed black holes.

| What are the observational methods used to study the center of the Milky Way? What wavelengths are used and how has technology improved our ability to measure stellar motions in our Galaxy’s center?

Infrared and Radio telescopes allow us to see through the dense dust and gas blocking our view (bc of the longer wavelengths).

Adaptive optics have improved our ability to measure stellar motions by correcting the distortion of stars caused by the Earth's atmosphere.

| How do we know there is a supermassive black hole at the center of our Milky Way, and how do we measure its mass?

We infer the presence of a supermassive black hole through its gravitational affects: we observe densely packed clusters of stars orbiting around the same, very massive object at the center. We can measure its mass through the Newtonian law of orbital velocities around a point mass.

| What do we actually observe at the position of the Milky Way’s central black hole?  What are possible explanations for its apparently very low accretion rate?

Stars that orbit a central object. Low accretion rate can likely be attributed to the lack of gas and dust surrounding the black hole.

| What are the major observational signs of “activity” in the nuclei of galaxies?

[1] Nucleus is very bright and may outshine entire host galaxy. 

[2] Rapid variability - brightness can change in a very short amount of time

[3] Always produce substantial radiation over a very wide range of wavelengths (unlike normal stars)

[4] Gas moving at extremely high speeds - sometimes approaching speed of light

[5] JETS (radio galaxies)!

| How is the accretion rate of AGNs related to the energy output (luminosity)? How do we know the energy is coming from a small region?

AGNs are powered by high/intense accretion. The viscosity converts energy into radiation. Higher accretion rate = higher luminosity.

Rapid variability in luminosity indicates the emitting region is very small.

| How do we estimate the minimum mass that must be associated with an active galactic  nucleus (AGN)?

Multiplying the AGN's luminosity by its estimated lifetime to determine its total energy output. Then use E=mc2 and an assumed 10% efficiency to calculate the mass required to produce that energy.

| What are the different types of AGNs?

Seyfert Galaxy: especially luminous at optical/UV/X-ray wavelengths - gas rich, 

Quasar (QSO): Extremely bright, outshine their host galaxies

Radio Galaxy: powerful radio-emitting jets viewed away from the jet axis

Blazars: AGNs with jets pointed toward Earth → intense, rapidly variable radiation

| What is the role of obscuration, viewing direction, and the  presence or absence of a jet in determining the appearance of an AGN?

Obscuration: Gas/dust can block view of the nucleus, making it look dimmer or visible only through reflected radiation

Viewing direction: Face-on views reveal the nucleus, edge-on views may hide it

Jets: Jets pointed toward Earth appear extremely bright and variable (blazars), jets viewed from the side appear as radio galaxies

| How do estimates of sizes and masses of AGNs support the conclusion that a black hole  must be responsible for the activity?

estimates tell us they must be incredibly massive (luminosity x lifetime) with a very small emitting region (rapid variability). The combination of very high luminosity, enormous mass, and small size strongly suggests the presence of a supermassive black hole powering it through accretion.

| What features characterize the spectrum of radiation emitted by an AGN?  What is meant by a “nonthermal” spectrum?

They emit substantial radiation across a wide range of wavelengths (from radio - gamma rays), which is different from normal gas/stars which emit across a more narrow band of wavelengths.

“Non-thermal” = radiation does not have a defined peak or temperature, so it cannot be described by a black body spectrum.

| How do powerful jets interact with their surroundings?

by colliding with surrounding gas in the intergalactic medium, producing lobes and hot spots. 

| Describe the observed features of jets:

Radio Lobes: the big “blobs” of radio emissions on either side of a jet

Hot Spots: intense spots of radio emissions at the ends of the lobes

Splash Points (‘working surfaces’): the point where jets collide with surrounding gas and deposit energy

Backflow: gas expands backwards rapidly after colliding with surrounding gas in the intergalactic medium

Cocoon: region of backflowed hot “shocked” gas between each lobe

| Can we see traces of the jets themselves?

Yes: radio waves show streams connecting the lobes/hot spots to the nucleus. They also leave trails, bends, and corkscrew patterns as they interact with intergalactic medium.

| How is synchrotron radiation produced?

A lot of electrons moving in the presence of intense/strong magnetic fields at very close to the speed of light. These electrons can’t move in a straight line because the magnetic field is so strong, so they are pushed into a corkscrew motion (a form of acceleration) which emits the radiation. 

| What can we deduce from observations of radio galaxies in which the jets are not straight?

That they are in fact streams of gas → Their shapes can reveal the galaxy's motion through surrounding gas or changes in the direction of the jet source

| What causes the jets to form “radio trails”? “Reflection symmetric” structure?  “Inversion symmetric” structure?

When galaxies are moving through space, it causes the jets to curve backwards. The radio trails are always found where the galaxy is expected to be moving pretty fast.

Inversion symmetric structures correspond to those jets that appears "s-shaped" due to the black hole's axis wobbling or precessing over time.

Reflection Symmetric pertains to the jets aiming in opposite directions (straight, not wobbly looking). 

| How do we know that the jet velocity in SS 433 is 26% of the speed of light?

We know the speed because we are able to measure it using the doppler shift. 

| Why do jets often appear one-sided, or with one jet much brighter than the other?

Relativistic aberration (the side moving away from us is going to be redshifted). Beaming makes the approaching jet appear much brighter, producing apparent one-sidedness

| What arguments lead us to believe that the jets are really two-sided and symmetrical, but that the approaching jet is much brighter?

When moving close to the speed of light most of the energy/light emitted is going to be focused very linearly/concentrated in front. So we won’t see the energy on the back side

| What is “superluminal expansion” and what is believed to cause it?

A jet appears to be moving faster than the speed of light → caused by the jet moving almost but not exactly towards the observer (relativistic effects)

| Why are magnetic fields thought to be responsible for accelerating jets?

The magnetocentrifugal process: rotating accretion disks twist magnetic field lines → forces charged gas particles to rotate and flings them outward in narrow “beams”.

| What are Gamma-Ray Bursts?

Extremely bright, short-lived bursts of high-energy gamma radiation. Rapid variability suggests they originate from very compact regions. We observe a GRB when a jet points toward Earth.

| What are the observational features of gamma-ray bursts?

Duration: typically seconds to minutes (short = ~0.1s, long ~10s)

Frequency: rare, usually 1 detected per day across the sky

Afterglows: some produce fading X-ray, optical, and radio emission

| Why do astronomers think they are located in distant galaxies and are therefore very  luminous?

They are distributed isotropically (uniformly) across the sky rather than concentrated toward the Milky Way band. Fading afterglow observations allowed astronomers to locate their distant host galaxies which confirms they are very far away. Because they still appear bright at enormous distances, they must release huge amounts of energy.

| Why are GRBs thought to contain jets?

If they emitted energy equally in all directions, their estimated energy output would be unrealistically large for a stellar event. With jets, energy is concentrated into beams which reduces the total energy required.

| Why are (at least some) gamma-ray bursts thought to represent the formation of stellar mass black holes?

Because long ones come from collapsing massive stars (core-collapse supernovae), and short ones originate from neutron-star mergers. Both of which can form stellar mass black holes.