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A search of archival X-ray data for 70 nearby galaxies has yielded two flaring sources in globular clusters or ultracompact dwarf companions of parent elliptical galaxies. One source flared once, and the other five times. When not flaring, these sources appear to be normal accreting neutron-star or black-hole X-ray binaries. Unlike magnetars and other bodies that produce repetitive flares of similar luminosity, they are located in old stellar populations.


This is the Accepted Manuscript of an article that was published in Nature, volume 538, 2016. The Version of Record is available online at this link: SJSU users: use the following link to login and access the article via SJSU databases.