Question 1

Matter in an accretion disk is in orbit around a black hole, but friction within the disk causes the matter to gradually spiral into the black hole. What will change in terms of the observable properties of the black hole as this process continues?

Accepted Answer

As matter from the accretion disk spirals into a black hole, both the mass and the angular momentum of the black hole increase. This is because the matter falling into the black hole contributes both mass and angular momentum to the black hole.

Question 2

If a black hole is truly black and the escape velocity associated with it is greater than the speed of light such that no light can escape it, where do the X-rays from the black hole candidates so far identified originate?

Accepted Answer

The X-rays detected from black hole candidates originate from the accretion disk of matter surrounding the black hole. This matter is heated to extremely high temperatures due to gravitational forces as it spirals into the black hole, emitting X-rays before crossing the event horizon.

Question 3

What is believed to be the solar mass of the black hole candidate Cygnus X-1?

Accepted Answer

Cygnus X-1 is believed to have a solar mass of about 15. This is based on observations and measurements of the system it is part of, including the mass function and the properties of its companion star.

Question 4

How has the diameter of the black hole candidate Cygnus X-1 been estimated?

Accepted Answer

The diameter of the black hole candidate Cygnus X-1 has been estimated from the time scale of the flickering of the X-rays it emits, which provides information about the size of the region where the X-rays are produced.

Question 5

The accretion disk surrounding a black hole is characterized by the emission of

Accepted Answer

The accretion disk surrounding a black hole is extremely hot due to the intense gravitational forces, causing it to emit high-energy radiation, primarily in the form of X-rays.

Question 6

The X-ray source Cygnus X-1 is a black hole candidate located in a binary star system. The X-ray source is believed to occupy a volume smaller than Earth. This size is deduced from

Accepted Answer

The rapid flickering in the X-ray brightness of Cygnus X-1 suggests that the emitting region is very small. This is because the time it takes for the brightness to change is related to the size of the emitting region; shorter times imply a smaller size.

Question 7

Why is Cygnus X-1 thought to be a black hole?

Accepted Answer

The answer of Why is Cygnus X-1 thought to be...

Question 8

How has the mass of the black hole candidate Cygnus X-1 been estimated?

Accepted Answer

The mass of Cygnus X-1 has been estimated by observing the periodic wobble in the spectral lines of a normal companion star, indicating the gravitational influence of Cygnus X-1 as they orbit their common center of mass.

Question 9

A compact body pulls material from a binary companion. This material plunges toward the compact body and emits X-rays. Which of these phenomena would this description NOT fit?

Accepted Answer

Novae involve a thermonuclear explosion on the surface of a white dwarf in a binary system, primarily emitting visible light, not X-rays caused by material plunging toward a compact body.

Question 10

Supposedly nothing can escape from a black hole, yet astronomers are locating black hole candidates by the X-rays they emit. How can X-rays be coming from a black hole?

Accepted Answer

The X-rays detected by astronomers originate from the material in the accretion disk around the black hole, not from within the black hole itself. This material heats up as it spirals inwards, before crossing the event horizon, emitting X-rays in the process.

Question 11

How was the black hole candidate Cygnus X-1 first discovered?

Accepted Answer

Cygnus X-1 was first discovered as a strong X-ray source by the Uhuru satellite, which was the first satellite launched specifically for the purpose of X-ray astronomy in 1970. The detection of X-rays indicated the presence of extremely high-energy processes, which led to the identification of Cygnus X-1 as a black hole candidate.

Question 12

Gas can be pulled off a binary companion by a black hole. The black hole's event horizon is too small to allow all the gas to enter perpendicularly, so it orbits in a ring around the black hole or a neutron star. What is this ring called?

Accepted Answer

The ring of gas that orbits around a black hole or a neutron star as it is being pulled off a binary companion is called an accretion disk. This structure forms because the material spirals in towards the compact object, heating up and emitting radiation as it does so.

Question 13

Which effects have been useful (and successful) in the search for and identification of black holes in the universe?

Accepted Answer

The gravitational influence of black holes on nearby matter, especially companion stars, has been a key method in identifying black holes, as it allows astronomers to infer the presence of a black hole by observing the motion of nearby objects.

Question 14

In principle, if astronomers were in the vicinity of a Schwarzschild black hole, how could they measure its mass?

Accepted Answer

A satellite's orbital period around a massive object, according to Kepler's laws and general relativity, can be used to calculate the mass of the object it orbits. This principle applies to any massive body, including black holes.

Question 15

One object that is believed to be a black hole in the Milky Way Galaxy is the

Accepted Answer

Cygnus X-1 is widely accepted as a strong candidate for a black hole in the Milky Way Galaxy due to its powerful X-ray emissions and the observed properties of its binary system, which suggest the presence of an extremely dense and massive object that cannot be explained by other known types of celestial bodies.

Question 16

The intense X-rays emitted by a suspected black hole are generated by what physical mechanism?

Accepted Answer

The intense X-rays emitted by a suspected black hole are primarily generated by the compressional heating of material as it moves into the black hole. This process occurs in the accretion disk surrounding the black hole, where matter falling towards the black hole gets heated to extremely high temperatures due to gravitational compression, emitting X-rays.

Question 17

What method is used by astronomers to infer the existence in space of a dark object of about 5 solar masses, such as a black hole?

Accepted Answer

Astronomers infer the existence of a dark object, such as a black hole, by observing its gravitational effects on a companion object in a binary system. This method allows them to deduce the presence of the black hole due to its significant mass influencing the orbit of the visible companion star.

Question 18

What mechanism in the vicinity of a star gives astronomers a hint of the presence of a black hole as a companion to the star?

Accepted Answer

The intense and rapidly fluctuating flux of X-rays is a key indicator of a black hole's presence. This phenomenon occurs when gas from a companion star is drawn towards the black hole, heating up to very high temperatures and emitting X-rays as it is compressed and accelerated.

Question 19

Which of these techniques has been successful in identifying good candidates for a black hole in the Milky Way Galaxy?

Accepted Answer

B) The detection of X-rays from a binary star system, where one star is transferring mass to its companion, is a method that has been successful in identifying black hole candidates. This process often involves a black hole accreting matter from a companion star, which releases a significant amount of X-rays. By studying these systems and measuring the masses of the components, astronomers can identify when one of the objects has a mass that is consistent with that of a black hole.

Question 20

X-rays that come from the vicinity of a black hole actually originate from

Accepted Answer

X-rays from the vicinity of a black hole originate from the hot gas in the accretion disk just outside the event horizon, not from inside the event horizon or the singularity itself, where no information can escape.

Quiz 15: Black Holes: Matters of Gravity