Loading astro130..

Can we “see” a black hole in the sky?

The black hole would have to be VERY CLOSE to us. At a distance of 10 Schwarzschild radii (hereafter R_sch), the black hole would appear as in the right panel of the figure; at a distance of 1000 R_sch it would appear as in the left panel of the figure.

Click the question to check your answer:

Detections are possible with this method, but the mass of the lensing object is difficult to estimate from such observations. (Stellar-mass black holes are the ones formed by the gravitational collapse of dead stars that are too massive to become neutron stars or white dwarfs. They are different from the supermassive black holes found in centers of galaxies.)

Based on what you know about black holes, how would you search for one?

As you saw in the first section of Ch. 3, this is not an easy question to answer and took many iterations of scientific thought to get to the two methods.

1. search for binary star systems with X-ray emission. If one of the stars in the binary system is a black hole and the stars were close enough together, material from the visible star could fall onto the black hole, creating an X-ray source. Difficulty with this technique is that X-rays are absorbed strongly by the Earth’s atmosphere (and by interstellar gas and dust), so we must make the observations from space (X-ray space telescopes).
2. Orbital motion of companion stars –use Newton’s laws to work out themass of an unseen companion. If the compact companion star has a mass > 3 M_sunthen it is most likely a black hole. (3 M_sunis the mass limit on neutron degeneracy pressure, just like the electron degeneracy mass limit for white dwarfs is 1.4 M_sun). When we observe X-ray radiation coming frombinary star systems, they tend to flicker rapidly on time scales of milliseconds. This is an indication that the radiation must come from a compact region with the dimensions of a black hole or a neutron star.

What is an X-ray binary? Why are these objects important?

X-ray binaries are a class of binary stars that are luminous in X-ray wavelengths. X-rays are produced by matter falling from one star (usually a regular star*) to the other compact star, which is typically a white dwarf, a neutron star of a black hole. Detecting X-rays from these binary systems is the best technique for finding black holes.

*[“Regularstar” is a star supported by nuclear fusion in its core]. High-energy light should be emitted by material falling onto black holes; this, in fact, should be one of the principal signatures of a black hole because it is hard for ordinary astronomical objects to emit X rays.

A few more terms with “X-ray”in them:

A system is called anX-ray pulsarif the source of the X-rays is from aspinning neutron star, i.e.pulsar. [p.56]

When enough matter accumulates on the surface of a neutron star it can start nuclear fusion for a short amount of time producing X-rays. We call these eventsX-ray bursts.

Example of an Evolutionary Path from a Binary to an X-Ray Binary

We start with a system of two stars in a binary. The most massive star is the quickest to evolve, burn its Hydrogen supply, and become a red giant. It then transfers mass to the other star and eventually blows up as a supernova. For a period of time the system will consist of a compact object (neutron star or a black hole) and a massive companion with a strong mass outflow (stellar wind).