Black Hole or Neutron Star

How the Mass Tells Us What an Astronomical X-Ray Source Is

Aug 26, 2007

Measuring the mass of the unseen companion in a binary X-ray source, such as Cygnus X-1, tells us if it is a black hole or a neutron star.

Black Hole or Neutron Star?

Astronomers observe many X-ray sources in binary stars. A visible star mutually orbits an unseen companion.

The first such example studied is Cygnus X-1. The X-rays from Cygnus X-1 and the fact that the optical counterpart, HDE 226868 is orbiting an unseen companion smaller than 300 km, suggest that Cygnus X-1 is a black hole while leaving open the possibility that it may be a less exotic neutron star.

Are such unseen companions black holes or Neutron stars? Mass is the key. A neutron star can not have more than about 2 to 3 times the mass of the Sun. If it is more massive, it collapses into a black hole.

Mass of the Unseen Companion in Cygnus X-1

To decide if Cygnus X-1 contains a black hole, astronomers measure the mass of the unseen companion. If it is above the upper mass limit for a neutron star, it is likely to be a black hole.

The mass of orbiting stars determines the gravitational force and hence the orbital properties. Astronomers can therefore estimate the mass of all binary stars by studying their orbits and applying Newton's and Kepler's laws. They measure the orbital period and separation of two stars in a binary system to find the masses of the two components.

The situation is a bit more complicated for Cygnus X-1 because we can only see HDE 226868 and not the smaller companion. We can therefore only directly measure the ratio and the sum of the masses of the two stars in the system. We can not measure each mass individually.

First Mass Measurements of Cygnus X-1

These mass estimates were first published in 1972. Working at the Royal Greenwich Observatory, Louise Webster and Paul Murdin took a series of spectra of HDE 226868 from August to October 1971. They found the spectral lines alternated between red (moving away from us) and blue (moving towards us) shifts with a 5.6 day period. They detected no spectral lines from the companion. Hence HD 226868 is orbiting an unseen companion with a 5.6 day period.

Shortly thereafter, Charles Bolton combined new spectra from David Dunlap Observatory with the previous spectra to refine the mass estimates. To estimate the mass of the unseen companion, Bolton assumed that HDE 226868 has a mass of 30 solar masses. This assumption was based on the spectral type of HDE 226868 and the average measured mass of stars of that spectral type.

The mass of HDE 226868 and the ratio of the masses in the system allowed Bolton to estimate the mass of the unseen companion. He estimated about 14 solar masses, with a range of possible values between 10 and 20 solar masses.

The modern estimates for the most likely values are about 33 solar masses for HDE 226868 and 16 solar masses for the companion. These masses have a considerable uncertainty, but given the current data it would be extremely difficult for the mass of the unseen companion to be less than 3 solar masses. It is therefore very likely to be a black hole.

Cygnus X-1 a Black Hole

Despite this mass estimate, it remained, for a while, possible to construct scenarios that were consistent with the observational data, but did not have a black hole as an unseen companion. These scenarios were somewhat contrived and perhaps comparable to scenarios a sly defense lawyer might contrive to suggest that an obviously guilty client might actually be innocent. They were however possible even if very unlikely. Since the mid 1970s, astronomers have gradually closed these loopholes. The case that Cygnus X-1 contains a black hole is now virtually certain.