The expected target was the Moon
Late on 18 June 1962, local time, an Aerobee rocket left White Sands in New Mexico carrying three radiation counters. Riccardo Giacconi, Herbert Gursky, Frank Paolini and Bruno Rossi hoped to detect X-rays produced when sunlight struck the Moon, and to look for other sources in the night sky. The strongest signal came from another direction. The source later called Scorpius X-1 was extraordinarily bright in X-rays, despite having no comparably conspicuous appearance to the naked eye.
That mismatch was the discovery's promise. An optical telescope ranks objects by the visible light they send us. A detector for X-rays, photons with much greater energy, can rank the same sky very differently. The new source suggested that some objects release a large share of their energy through processes that ordinary starlight barely reveals. The observation opened a way to find them.
Earth's air concealed the view
X-rays from space are absorbed by Earth's atmosphere. Putting a better detector beside an optical telescope on the ground would not solve that problem. The instrument had to rise above most of the absorbing gas. The rocket reached about 225 kilometres and spent roughly 350 seconds above 80 kilometres: less than six minutes of high-altitude observing time.
The payload carried Geiger counters, gas-filled detectors that produce an electrical pulse when radiation deposits energy inside. Thin mica windows let X-rays enter the gas. The windows differed in thickness, giving the team a way to compare how easily the incoming radiation was absorbed. Two counters worked properly during the flight. A third produced electrical trouble and was left out of the analysis.
Rotation turned a counter into a rough sky scanner
The rocket spun about twice per second. As it rotated, each counter looked across different directions. Optical sensors recorded the orientation, allowing the researchers to group the radiation counts by pointing direction. A real source could then appear as a rise at the same direction through repeated turns, even though the detector formed no image.
The resulting graph had a strong peak away from the Moon's direction. Both working counters saw it. Radiation remained outside the peak as well, suggesting a broader background. The counters had wide views and no mechanical collimator to make those views narrow, so the discovery did not immediately provide an exact position or identify the optical star. Better localization came later. The early result was already striking: the X-ray sky contained an intense extra-solar source.
A narrow field of view admits light from a smaller patch of sky. The discovery counters instead combined light arriving over a broad range of angles. They could reveal a peak as the rocket turned, but the width of that response limited the position estimate. Locating an optical companion called for a sharper measurement.
The window thickness tested the radiation
A counter can also respond to energetic charged particles. The payload used surrounding detectors to reject many events accompanied by such particles, and the team compared the two functioning X-ray counters. Their different window thicknesses changed the response strongly. Together with the directional peak, that difference supported radiation that the mica could absorb: the relatively low-energy X-rays the experiment was designed to detect. Giacconi and his colleagues published the result in December 1962. The identification rested on the instrument's response as well as the surprising direction in the sky.
Sources: [1]
Longer observations revealed what the sources were doing
A rocket flight gives only a brief look. A satellite can return to a source, watch it vary and survey much more sky. Uhuru, launched in 1970, made that step with proportional counters and restricted fields of view. A proportional counter records an electrical pulse related to the energy deposited in its gas. Repeated observations supplied positions and timing patterns that a short discovery flight could not.
Those observations helped establish a population of binary systems containing compact stars. Gas drawn from a companion falls into a deep gravitational well and releases energy as it is heated and slowed. This inflow is called accretion. It can make a small region immensely bright in X-rays. Scorpius X-1 was eventually understood as such a system with a neutron star. Its brightness was pointing toward the release of gravitational energy, a different source of power from the nuclear reactions inside an ordinary star.
Making images required a different instrument
Counting X-rays and focusing them are different tasks. X-rays are usually absorbed when they strike a mirror face-on, but they can reflect when they skim a surface at a shallow angle. Carefully shaped, nested mirrors use this grazing reflection to bring photons to a focus. Rocket experiments imaged the Sun with such optics in 1963; the Einstein Observatory, launched in 1978, brought large focusing optics to cosmic X-ray observations.
Giacconi's 2002 Nobel Prize recognized pioneering work that led to the discovery of cosmic X-ray sources, across the long development from counters to observatories. The first sky peak and the later images belong to different stages of that work. One revealed a source nobody expected to be so bright; the next generations learned where such sources were, how they changed and what powered them.