Black Hole Candidates

It would be very difficult to observe a black hole directly. Therefore, we must identify black holes indirectly by their effect on the matter surrounding them. There are two general classes of objects that we expect may be strong candidates for black holes: the collapsed cores of massive stars, and the centers of large galaxies where supermassive black holes may have formed. In this section we shall be concerned with star-sized black holes. In Chapter 25 we shall consider supermassive black holes.
Matter and Black Holes
If matter falls into a black hole we may expect it to produce a detectable signal. For example, it should emit X-rays strongly as it is accelerated in the gravitational field of the black hole. We cannot see the radiation emitted from inside the event horizon, but we may expect to be able to detect this radiation emitted by the matter before it passes through the event horizon.
Close Binary Systems with Compact Objects
If a black hole forms in a close binary system where the companion star is a more normal star, we have seen (Chapter 19) that matter may accrete from the normal star onto the black hole. As the gas is accelerated near the black hole it becomes very hot (millions of K) because of violent collisions between the gas particles. Such a hot gas emits X-rays (see this animation concerning the production of X-rays in hot gases), and since the black hole and the star are in orbit around each other we may expect periodic variations in the X-ray emission.

We see many examples in our galaxy and in others of radiation sources that may be associated with black holes in binary star systems. The top right figure shows a Chandra X-ray Telescope image of X-rays from the elliptical galaxy NGC 4697. Superposed on a diffuse glow of X-rays are many point sources of strong X-rays. These are accreting X-ray binary systems, and each of these points of X-ray light probably represents the location of either a neutron star or a stellar-mass black hole in NGC 4697. The question is how to tell which ones are black holes and which are neutron stars.

Stellar Black Hole Candidates
X-Ray Source Mass of Companion Mass of
Black Hole
Cygnus X-1 24 - 42 11 - 21
V404 Cygni ~0.6 10 - 15
GS 2000+25 ~0.7 6 - 14
H 1705-250 0.3 - 0.6 6.4 - 6.9
GRO J1655-40 2.34 7.02
A 0620-00 0.2 - 0.7 5 - 10
GS 1124-T68 0.5 - 0.8 4.2 - 6.5
GRO J042+32 ~0.3 6 - 14
4U 1543-47 ~2.5 2.7 - 7.5
All masses in solar masses

Distinguishing Black Holes from Neutron Stars
Just because we see fluctuating X-ray binary sources where one component is a more normal star (usually inferred from its spectrum), and the other is a compact object that cannot be seen directly, does not mean that the unseen object is a black hole. If the black hole were instead a neutron star, the system would exhibit X-ray emission with many of the same characteristics.

To make a strong case that the unseen companion is a black hole, we must rule out the possibility that it is a neutron star. The surest way to do this is to measure the mass of the unseen companion and demonstrate that it is higher than the upper limit for a neutron star, leaving a black hole as the only plausible explanation. In principle, it is possible to use Kepler's third law to determine the mass of the unseen companion by making careful measurements on the binary system. In practice, we have seen that such determinations are difficult, and only in some favorable cases can they be done reliably. Nevertheless, an impressive list of binary systems where there is an unseen companion likely to be a black hole has been accumulated. The preceding table lists the strongest candidates in our galaxy.

A more detailed discussion of how the mass of the unseen component is estimated is given in the box at the bottom of this page. A step-by-step description of how this approach was used to infer that the X-ray source Cygnus X-1 is a binary star system containing a black hole is given in the following section.

Identification of Cygnus X-1
The first strong case found, and most famous stellar black hole candidate, is called Cygnus X-1 (which means the first X-ray source discovered in the constellation Cygnus). Let us summarize briefly the steps used to conclude that Cygnus X-1 harbors a black hole.

1. In the early 1970s an X-ray source was discovered in Cygnus and designated Cygnus X-1.
2. In 1972, a radio source was found in the same general area and it was identified optically with a blue supergiant star called HDE226868. Correlations in the radio activity of HDE226868 and the X-ray activity of Cygnus X-1 implied that the two were related, probably as parts of a binary system.
3. Measurements of the radial velocity of HDE226868 using the Doppler shift confirmed that it was a member of a binary with a period of 5.6 days. The details of the orbital geometry were further confirmed by observations of periodic brightening of HDE226868 and a periodic decrease in X-ray intensity from Cygnus X-1 correlated with the same 5.6-day period.
4. Detailed analysis of the X-ray spectrum showed that the X-ray source was fluctuating in intensity on timescales as short as 1/1000 of a second. Since signals passing through the object controlling the fluctuation are limited by the speed of light, this implies that the X-ray source must be very compact, probably no more than hundreds of kilometers in diameter (we shall discuss this connection between period of variability and size further when we consider active galaxies and quasars in Chapter 25). From the compact size and the orbital perturbation on HDE226868 and the strong X-ray emission, it was apparent that Cygnus X-1 was a compact object (white dwarf, neutron star, or black hole).
5. The mass of the blue supergiant HDE226868 was estimated from known properties of such stars (it is a spectrum and luminosity class O9.7Iab star). This, coupled with Kepler's third law and assumptions about the geometry of the binary, can be used to estimate the mass of the unseen, compact companion. These estimates are uncertain because the geometry (tilt of the binary orbit) can only partially be inferred by using information such as the presence or absence of eclipses. However, all such estimates place a lower limit of about 5-6 solar masses on the unseen companion, and more likely indicate a mass near 10 solar masses.
6. Since we know of no conditions that would permit a neutron star to exist above about 3-4 solar masses (or a white dwarf above about 1.4 solar masses), we conclude that the unseen companion must be a black hole.

Although this chain of reasoning is indirect, it builds a very strong case that Cygnus X-1 contains a black hole. Some remain skeptical, but most astronomers believe the black hole explanation to be the most consistent one for this system.

Technically Speaking: Lower Limits on the Masses of
Unseen Companions in X-ray Binaries

As noted above, we have strong reason to believe that in binary star systems that are strong sources of X-rays at least one of the components of the binary is a compact object (a white dwarf, neutron star, or black hole). The general way in which we try to establish that the compact object is a black hole is to estimate the mass of the unseen component and to rule out the possibility that it is a white dwarf or neutron star because of the upper limits on the mass for such objects.

The starting point is a radial velocity curve for the binary. The following diagram shows the measured radial velocity curve for the X-ray binary system A 0620 - 00.

(The heliocentric radial velocity is the radial velocity referenced to the Sun, which means that the velocity associated with the revolution of the Earth around the Sun has been removed.)

The Mass Function
Such radial velocity curves can be used to infer the mass (or more often limits on the mass) of the unseen companion. Using methods based on Kepler's laws that were described already in Chapter 19, the mass function f(M) may be defined as shown in the adjacent right box. The mass function allows one to determine the mass of the unseen companion if P and K are measured (see above diagram) and the tilt angle i and mass of the visible companion are known.

Fixing the Parameters
We can often infer the approximate mass of the visible companion from its spectrum and stellar systematics. The tilt angle i is generally unknown except in special circumstances like eclipses. If i is unknown, the mass function yields only a lower limit on the mass, not the mass directly. However, if this permits us to say that the lower limit on the mass of the unseen component is much larger than the theoretical upper limit for the mass of a neutron star, we have obtained rather solid evidence that the unseen component is a black hole. The black hole candidates in the table above have been inferred in this general manner.