At M.I.T. we soon focused a major part of our research on the new field of X-ray astronomy. I figured that cosmic photons with energies greater than 15 keV (thousand eV) could penetrate to altitudes accessible by a helium-filled balloon, and decided to try a balloon experiment as a potentially less costly means than rockets for exploratory X-ray astronomy. I prepared an X-ray telescope with a scintillation detector in a payload for a giant “skyhook” balloon. I had planned for a night flight from an airfield near Palestine, Texas to scan over Sco X-1 whose nature was still a mystery. Then I heard news of a rocket experiment by NRL, which demonstrated that the Crab Nebula, known to be an extended source of visible “synchrotron light” emitted by high-energy electrons gyrating in a magnetic field, is also an extended source of 3 keV X-rays. It seemed likely that such a spectrum could extend beyond 15 keV and be detected at balloon altitudes. So I switched my plan to a day flight and tilted my detector for a scan of the Crab Nebula. It worked, and I reported the first observation of cosmic X-rays above 15 keV. Walter Lewin soon joined me in X-ray balloon observations, one of which recorded a ten-minute X-ray flare from Sco X-1, the first sudden change in the brightness of an X-ray source.
M.I.T. and AS&E collaborated in a joint development of a rocket experiment to measure the position of Sco X-1 with sufficient accuracy to enable identification of its optical counterpart. Essential for the experiment was a “modulation collimator,” which made possible precise position determinations before the advent of focusing X-ray telescopes. It was invented by Minoru Oda, a frequent visitor to M.I.T. from the University of Tokyo. Giacconi’s group carried out the experiment in collaboration with Oda, Hale Bradt and others at MIT. It located Sco X-1 to within an area of one-thousandth of a square degree. Within that area the optical counterpart of Sco X-1 was found - a faint flickering blue star of the 13th magnitude. Its X-ray luminosity is more than a thousand times the total luminosity of the Sun in all wavelengths.
The explanation of Sco X-1 came from Iosif Shklovsky, a Russian theoretician. He proposed that it is a binary system consisting of an ordinary star and a neutron star in an orbit so close that it draws matter from its companion in a process called accretion. Neutron stars have the mass of a typical star like the Sun, but crushed by gravitational collapse into a sphere about 10 kilometers in radius. When accreting matter falls onto the surface of a neutron star, the kinetic energy it gained in the fall is converted to heat at 100 million degrees and radiated as X-rays at a power level limited only by the rate of accretion. We know now that all the hundred or so bright X-ray stars in our Milky Way Galaxy are "accretion-powered X-ray binaries".
