Caught in action: An Ultraluminous X-ray source escaping its star cluster


Credit: Egorov et al. 2017


Although the nature of Ultraluminous X-ray sources (ULXs) is debatable (they either are intermediate mass black holes or stellar mass black holes) all studies agree that they are associated with young stellar clusters. Nevertheless, they do not reside in them but are located outside these clusters due to close encounters that cause their ejection at high speeds (~80 Kilometer per second). Despite this fact, scientists have never observed an ULX as it moves outwards from its star cluster.

In a recent study, astronomers observed the ULX HoII in the Holmerg galaxy. The galaxy is located in the eastern part of a nebula known as ‘foot nebula’, due to its geometry. Analysis of their data revealed an ‘arc’ surrounding the ‘foot nebula’. Further investigation of the origin of this arc showed that it actually is a bow shock caused by the ULX’s motion as it escapes the nebula. Up to now all the conclusions about ULXs’ escape were based on their location outside the cluster. If this finding is confirmed by other studies, it is the first direct evidence of an ULX moving outwards from its parental star cluster.


Publication: Egorov et al. 2017