The object, called SCP 06F6, was first spotted in the constellation Bootes in February 2006 in a search for supernovae by the Hubble Space Telescope.
Nothing had been seen at its location before it started to brighten, and nothing was spotted after it dimmed. That suggests it is normally too faint to observe and that it brightened by at least 120 times during its firefly-like episode.
Stars are known to brighten dramatically when they explode as supernovae. But supernovae reach their maximum brightness after about 20 days, and this object took a leisurely 100 days to hit its peak.
The object’s spectrum is also bizarre. It does not match that of anything seen in the mammoth Sloan Digital Sky Survey, which has mapped more than a quarter of the sky.
Near or far
The spectrum shows a handful of spectral lines, but when astronomers try to trace any one of them to an element – such as magnesium, the other lines fail to match up with known elements.
“Because we can’t see anything we recognise in the spectrum, we can’t tell if it’s even in the galaxy or in another galaxy,” says Kyle Barbary of the University of California, Berkeley, lead author of the new study.
If it’s inside the galaxy, it might be a dim stellar ember called a white dwarf. White dwarfs can brighten suddenly when they steal matter from a nearby stellar companion or suck in matter from a disc of debris around them.
But that process of sucking in matter would have to happen in a “strange way” to explain the odd spectrum, Barbary says: “It would still leave something unanswered.”
If the object lies outside the galaxy, explaining its provenance is no easier. …