X-rays from outside the solar system were first detected in 1962. A few years later one of the strongest sources, Cygnus X1, was found to be varying on very short timescales. These were interesting observations, but by the end of the 1960s one had only identified twenty or so X-ray sources. Most of these were located in the galactic plane. It was very difficult to conduct a search for X-ray sources from the ground, since the bulk of the incoming X-rays was absorbed by the Earth's atmosphere. The era of X-ray astronomy truly began with the launch of the Uhuru (which means ``Freedom'' in Swahili) satellite off of Kenya's coast on December 12 1970. The satellite was devoted to X-ray observations in the 2-20 keV band. By the time of its demise in March 1973, Uhuru had discovered more than 300 discrete sources. This data enabled scientists to make positive identification of X-rays from binary systems. From the Uhuru observations it was inferred that most of the galactic X-ray sources ought to be compact objects either neutron stars or black holes) accreting matter from a binary companion. These conclusions followed from i) the fact that the sources varied on short time scales, which indicates a small emitting region, ii) the confirmation that some sources were in binary systems, i.e. there were cases when optical companion could be observed, and iii) the efficiency with which accretion onto a compact object can convert energy into X-rays.
 |
X-rays are emitted as material from the binary companion falls onto the compact object in an X-ray binary. The individual masses can be inferred from detailed observation of the orbital velocities. If the deduced mass is above (roughly) three solar-masses the compact object is likely a black hole. Otherwise it is may be a neutron star. |
Since the early 1970s, many other satellites have confirmed the Uhuru data and provided us with further details. With the continued observations the evidence for the existence of solar mass black holes is mounting. Nevertheless the oldest candidate, Cygnus X1, still presents one of the strongest cases. The case that Cygnus X1 contains a black hole of perhaps more than ten solar masses has been argued since the first observations of this peculiar source.
 |
In December 1974 Stephen Hawking and Kip Thorne made their famous bet on whether or not Cygnus X1 is a black hole. |
The method for infering the presence of a black hole in an X-ray binary is based on original idea of Reverend John Michell - one can `weigh' the dark object in a binary system by observing the orbit of the visible companion. If the mass deduced in this way is significantly above the theoretically suggested upper mass limit of neutron stars (roughly three solar masses) one has evidence for the presence of a black hole. Of course, this method is quite uncertain and it is only in cases when one deduces a mass of (say) ten solar masses for the unseen companion that the evidence can be considered strong. Current data indicate the existence of a handful of black holes in X-ray binaries. Most systems seem to contain neutron stars.
It is, in fact, much easier to infer the presence of a neutron star than a black hole. Particularly clear evidence comes from systems that show occasional X-ray bursts. The natural explanation for such bursts is that they indicate matter hitting the surface of a neutron star and triggering local explosions. Recent observations (mainly with the Rossi X-ray Timing Explorer) have provided clear indications that the neutron stars in Low-Mass X-ray binaries (where the companion is typically a white dwarf) spin very fast (with a rotation period in the range 2-5 ms). This provides strong support for the long held belief that these systems are the progenitors of the so-called millisecond pulsars (the fastest rotating neutron stars known). For tens of millions of years the neutron star accretes matter from its companion. As this material falls onto the star it transfers angular momentum to the neutron star which spins up as a result.
 |
 |
|