A surprise green flash from a mystery source at the center of a distant galaxy has been spotted by scientists using NASA's NuSTAR satellite.
Since then, the team has named the flash ULX-4 (ultraluminous X-ray), as it is the fourth unexplained burst from this galaxy so far identified.
Instead, the team say it could be a black hole consuming a nearby star—as gravity from the black hole drags it inwards, material at the edge moves extremely fast and heats up, producing X-rays.
Normally, ULXs are long lived, having been created by an object like a black hole slowly feeding on a nearby star for an extended period.
Another event that could have produced the flash is a neutron star—an incredibly small, dense object that forms from the collapsed core of a giant star.
"Ultimately, further detections of ULX-4 would be required to establish whether it is an [neutron star] ULX or other relatively persistently accreting object rather than a transient event," they conclude.
"The most likely possibility suggested so far is that the X-ray emission could have been emitted by mass stripped from a low-mass object like a brown dwarf that was tidally disrupted by a black hole in NGC 6946.