It is a dark companion of a type o star . its lower mass limit is 7 solar masses and it emits x - rays 这物体的伴星是一颗o型星，质量下限是七个太阳质量，并会放射出x射线。
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Astronomers are almost sure that the dark companions of many x - ray binaries are black holes and not neutron stars . v404 cygni is one of the most well - known examples 天文学家已经几乎可以肯定很多x射线双星所包含的黑暗天体是黑洞而非中子星，其中最著名的例子包括天鹅座v404等。
