Abstract
BOTH nebular and stellar objects have been identified as discrete sources of cosmic X-rays. Representative of the first is the Crab Nebula, the size of which has been measured to be 1 to 2 arc min in the energy range 1–6 keV according to observations during a lunar occupation1 and with a modulation collimator2. This size is comparable with the optical size of the nebula. Representative of stellar sources is Sco X–1, the X-ray size of which has been shown to be less than 20 arc s by a measurement with a modulation collimator, also in the energy range 1–6 keV (ref. 3). The optical counterpart of Sco X–1 is an irregularly variable blue star4. The tentative identifications of Cas XR–1 with the supernova remnant, Cas A (ref. 5, and of several other X-ray sources with blue stars (Cyg X–2 (ref. 6), Cen XR–2 (ref. 7)) obviously suggest that these particular sources can be assigned to the nebular and the stellar classes, respectively.
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W. FLOYD, F. Limit on the Angular Size of Cyg XR–1. Nature 222, 967 (1969). https://doi.org/10.1038/222967a0
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DOI: https://doi.org/10.1038/222967a0
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