It was a spark in the night. A flash of X-rays from a galaxy hovering nearly invisibly on the edge of infinity.
Astronomers say they do not know what caused it.
The orbiting Chandra X-ray Observatory, was in the midst of a 75-day survey of a patch of sky known as the Chandra Deep Field-South, when it recorded the burst from a formerly quiescent spot in the cosmos.
For a few brief hours on Oct 1, 2014, the X-rays were a thousand times brighter than all the light from its home galaxy, a dwarf unremarkable speck almost 11 billion light years from here, in the constellation Fornax. Then whatever had gone bump in the night was over and the X-rays died.
The event as observed does not fit any known phenomena, according to Franz Bauer, an astronomer at Pontifical Catholic University of Chile, and lead author of a report to be published in Science.
The most likely explanation, Dr. Bauer said in the paper and in an interview, is that the X-rays are the afterglow from a gamma ray burst seen sideways. These are caused by the collapse of a massive star into a black hole or the collision of a pair of the dense stellar remnants called neutron stars, and squirt gamma rays but only in one direction. If Earth is out of the beam then all astronomers will see is an “orphan afterglow.”
But if that were the case, a typical afterglow would appear about a hundred times more intense than it is, Dr. Bauer said, unless this was an unusually weak event, or it came from much farther away — from something far behind the little galaxy.
Another possibility, the astronomers said, was that this was the result of a star being torn apart by a black hole, but that would produce a different spectrum of X-rays.
That is to say: None of the usual cosmic catastrophe suspects work.
“Unfortunately there is no ‘smoking gun’ evidence that favors one scenario over the other here,” Dr. Bauer wrote in a recent blog post.
Astronomers’ best chance to understand this “transient,” as he called it, is to find more examples. Chalk up another mystery to nature’s repertoire.