ABSTRACT
The first, primitive X-ray instruments, launched in the 1940s and 1950s on board balloons and rockets, revealed that the Sun emits a modest amount of X-rays. The surface temperature of the Sun is relatively cold (i.e., about 5780 K), and therefore it emits most of the radiation at larger wavelengths. In this framework, the discovery of extrasolar sources, extremely prominent in X-rays, came as a real surprise. Scorpius X-1, later identified as an accreting neutron star [1630], was the first of these sources. It was discovered in 1962 by a team of researchers led by Riccardo Giacconi [640]. Its X-ray power output, 2.3 × 1038 erg s−1, represented about 60,000 times the overall luminosity of the Sun integrated for all wavelengths [230]. Such astonishing figures motivated the launch of different space probes equipped with X-ray detectors, such as Solrad 1, Kosmos 215, and several Vela, OSO, and OGO satellites. But it was not until the launch of NASA’s satellite Uhuru-not Uhura!—in 1970, the first satellite specifically suited for X-ray astronomy, when astronomers opened a new window to observe the violent side of the universe with X-ray eyes.
