Ultraluminous X-ray bursts are newly (2016) discovered, and as yet unexplained sources of X-rays which have been observed in nearby galaxies.
The mysterious sources regularly emit X-rays, but have been observed spontaneously flaring up and becoming about 100 times brighter in X-rays in less than a minute before returning to their original X-ray levels after about an hour.“
Source : University of Alberta (archived)
The rise times of all of the flares were less than one minute, and the flares then decayed over about an hour. When not flaring, the sources appear to be normal accreting neutron-star or black-hole X-ray binaries, but they are located in old stellar populations, unlike the magnetars, anomalous X-ray pulsars or soft γ repeaters that have repetitive flares of similar luminosities.”
A number of hypotheses for the source of the flares have been proposed - e.g. the flares could be caused by matter from companion stars falling onto black holes or neutron stars.
Further reading Nature, 538
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