X-ray emission from the Magellanic Clouds was first observed in a five-minute rocket flight from Johnston Atoll in the South Pacific on October 29, 1968. The LMC was detected as an ∼ 4 σ excess in two adjacent 5° bins, the flux was ∼ 1.5 × 10−9 erg cm−2 s−1 and the spectrum was slightly softer than that of the diffuse background (Mark et al. 1969). Two years later two source regions in the LMC were identified by Price et al. (1971) and emission from the SMC was recorded. The same year Leong et al. (1971) showed that the LMC emission could be resolved by the collimated detector system of the Uhuru satellite into three steady and one possible highly variable source; they were designated LMC X-1, X-2, X-3, and X-4. There was also a possible diffuse emission extending over much of the Cloud. The SMC emission was located to a single, highly variable source, called SMC X-1, in the Wing region. It was the first stellar X-ray source to be confirmed in an external galaxy.

Confirmation of the existence of LMC X-4 was presented in the second Uhuru catalogue (Giaconi et al. 1972).

The LMC sources X-1, X-2, and X-3 were confirmed by Copernicus satellite observations (Rapley and Tuohy 1974) and given more accurate positions. Also Markert and Clark (1975), using the OSO 7 satellite confirmed these three sources, defined an upper limit for LMC X-4, and introduced a fifth source, LMCX-5.