Black Holes in Binary Systems

After collapse to the neutron star stage, stars with masses less than 2-3 solar masses should remain neutron stars, gradually radiating away their energy, because there is no known mechanism for further combination, and forces between neutrons prevent further collapse. But this neutron force is the last stand, and our best calculations indicate that this repulsion which prevents collapse cannot withstand the gravity force of masses greater than 2 to 3 solar masses. Such neutron stars would collapse toward zero spatial extent - toward a "singularity". Once they collapsed past a certain radius, the "event horizon", then even light could not escape: black hole.

Since black holes by their very definition cannot be directly observed, proving their existence is difficult. The strongest evidence for black holes comes from binary systems in which a visible star can be shown to be orbiting a massive but unseen companion. The indirect evidence for the black hole Cygnus X-1 is a good example of the search for black holes. Another excellent candidate in an object which was discovered in one of the Magellanic Clouds. Some astronomers think the binary system V404 Cygni is the strongest candidate yet.

Cygnus X-1

Doppler studies of this blue supergiant in Cygnus indicate a period of 5.6 days in orbit around an unseen companion. The B-type blue supergiant (HDE226868) is projected to have a mass of about 25 solar masses. The mass of the companion is calculated to be 8-10 solar masses, much too large to be a neutron star.

1. An x-ray source was discovered in the constellation Cygnus in 1972 (Cygnus X-1). X-ray sources are candidates for black holes because matter streaming into black holes will be ionized and greatly accelerated, producing x-rays.

2. A blue supergiant star, about 25 times the mass of the sun, was found which is apparently orbiting about the x-ray source. So something massive but non-luminous is there (neutron star or black hole).

3. Doppler studies of the blue supergiant indicate a revolution period of 5.6 days about the dark object. Using the period plus spectral measurements of the visible companion's orbital speed leads to a calculated system mass of about 35 solar masses. The calculated mass of the dark object is 8-10 solar masses; much too massive to be a neutron star which has a limit of about 3 solar masses - hence black hole.

This is of course not a proof of a black hole -- but it convinces most astronomers.

Further evidence that strengthens the case for the unseen object being a black hole is the emission of X-rays from its location, an indication of temperatures in the millions of Kelvins. This X-ray source exhibits rapid variations, with time scales on the order of a millisecond. This suggests a source not larger than a light-millisecond or 300 km, so it is very compact. The only possibilities that we know that would place that much matter in such a small volume are black holes and neutron stars, and the consensus is that neutron stars can't be more massive than about 3 solar masses.

Magellanic Cloud Black Hole

An object called LMC X-3 discovered in the Large Magellanic Cloud in 1982 had all the earmarks of a black hole. How can "black hole" status be verified?

Steps of investigation 1. Observe the orbital period of the binary system 2. Observe the spectrum of the visual star, compute the orbital speed from Doppler effect, calculate orbit radius 3. Compute sum of masses from velocity and period 4. Estimate the mass of the visual from spectral type 5. Deduct mass of visual to find mass of dark object The conventional criterion is that if the mass of the dark object is >3 solar masses, then it must be a black hole.

Result 41 hour period Orbit radius 7 million miles 14-18 solar masses total Visual about 6 solar masses 8-12 solar masses in dark object

Black Hole in V404 Cygni

The variable star V404 Cygni had been observed to go through several nova stages, or temporary brightening episodes. Then a strong X-ray outburst was observed by the Ginga satellite in 1989 associated with the star. Observations with the William Hershel Telescope led to a characterization of the star as a G or K star (and therefore a low mass star) which was orbiting about a companion with a period of 6.473 days. Analysis of the orbit led to a mass of 8-15.5 solar masses for the unseen companion, considered to be much too massive to be a neutron star.

The picture that emerges from the data is that a star about two-thirds as massive as the Sun is in close orbit around a black hole of mass about 12 solar masses. The black hole's gravity would distort the star and steal mass from it. The gas flow is apparently not uniform, but when enough collects in an acretion disc around the black hole, then an outburst can occur, causing the brightness to increase to hundreds of times the normal brightness. Gas spiraling into the black hole can accelerate to the point that it produces a burst of X-rays, as this system did in 1989. Associated with that burst, the visible output increased by about 200 times.

All the data to date are consistent with a black hole, so this is considered to be one of the best candidates for a black hole.

Sources
A 6.5-day periodicity in the recurrent nova V404 Cygni implying the presence of a black hole.
V404 Cygni