X Ray Sources - Scorpious, Hercules and Cygnus X-1 – by Albert Lim (2000)

Scorpious X-1 is the first cosmic X-ray source discovered in the constellation of Scorpious and hence the number 1. It is recognised as one of the brightest persistent X-ray source in the sky. Over 150 discrete X-ray sources are now listed in the X-ray source catalogs derived from the all-sky surveys conducted by the Uhuru, Ariel V, the HEAO-1 and other satellites. ROSAT has extended this list significantly by providing accurate X-ray positions enabling many of their optical counterpart to be identified.
The majority of the bright X-ray sources are binary star systems, where  a neutron star or black hole accretes material from it’s companion. X-ray binaries are generally classified into 2 types - low mass X-ray binaries (LMXBs) in which the companion stars are smaller than about 2 solar mass and high mass X-ray binaries (HMXBs) where the companion stars are above 10 solar masses. Companion stars between 2 to 10 solar masses are very rare. Others X-ray sources include supernovae remnant, cataclysmic variables, stellar corona emitters and active galactic nuclei (AGNs). Examples of 2 prominent X-ray sources like the Scorpious X-1 are Hercules X-1 and Cygnus X-1. All 3 objects are X-ray binary star systems. We will discuss Hercules X-1 first.

Fig 01 - Artist impression of the Hercules X-1 binary star system. HZ Herculis is assumed to fills its Roche lobe and is transferring material into the accretion disk of the neutron star which is a pulsar at right. The neutron star is thought to have an incredibly small diameter of only 10 km.

Hercules X-1 was discovered by Harvey Tananbaum in the early 1970s by the UHURU X-ray satellite. It is located at Right Ascension 16h 57m 50s and Declination +35d 20m 33s based on 2000 co-ordinates. This X-ray binary has a 2 solar mass companion star called HZ Herculis orbiting a pulsar with a spin period of 1.24 seconds. HZ Herculis itself orbits the pulsar in 1.7 day periods. In 1985, Hutchings calculated the radial velocity of HZ Herculis to determine the mass of the spinning neutron star as being less than 1 solar mass - below the 1.4 solar mass of Chandrasekhar’s limit. This controversy was later resolved by astronomers using spectrophotometry on the 2.5 meters Issac Newton Telescope in July 95 to accurately re-measure HX Herculis’s radial velocities.

When these velocities are corrected for the severe heating on the companion surface to the neutron star due to X-ray irradiation, they found that the mass of neutron star to be more appropriately 1.6 +/- 0.3 solar mass - thus resolving the earlier controversy.

Another example is Cygnus X-1 which is quite different. Like Hercules X-1, Cygnus X-1 was also the first X-ray source to be discovered in the early 1970s in the constellation of Cygnus. In spring of 1972, an unexplained strong radio source appeared in the vicinity of Cygnus X-1. This radio object was positively identified as a blue supergiant star HDE226868. At the same time, astronomers noticed that Cygnus X-1 also flared up in X-rays in tune with the radio flares. Therefore, the logical conclusion was that Cygnus X-1 and HDE226868 must somehow be related - they are indeed an X-ray binary pair ! It took sometime however for astronomers to figure that out as outlined below.

Fig 02 -Cygnus X-1, stellar wind from blue supergiant HDE226868 accreting material onto an accretion disk surrounding a black hole. The infalling gases are heated to extremely high temperatures as they spiral into the black hole thus emitting vast quantities of X-rays.

Astronomers later analysed and found that the X-ray emissions from Cygnus X-1 varies on extremely small timescales of thousandths of a second. Einstein’s special theory of relativity suggest that the object must necessarily therefore be no more than a few hundred kilometers in size. Astronomers concluded at this point that they must be dealing with an extremely small and compact object. At this time, astronomers knew that both neutron star as well as a black hole can account for the X-rays observed. It was not yet conclusive though that Cygnus X-1 is a black hole. Suspicion was however strong for a black hole because emissions from Cygnus X-1 showed no regularity whereas specific pulses are visible from neutron star binaries such as Hercules X-1. It was later that astronomers finally established the lower mass limit of Cygnus X-1 at 3 solar masses (above Chandrasekhar limit of 1.4 solar mass). This provided the first overwhelming evidence that Cygnus X-1 must indeed be a black hole.