Starship Asterisk*

HEAPOW: Bewildering Outburst (2016 Oct 24)

The X-ray sky is characterized by enormous variability. Some sources wink on and off, while others, more tempermentally choleric, erupt in extreme outbursts that garner enormous attention before flaming out. But the X-ray outbursts detected by three undergraduate students have astronomers scratching their heads. These students surveyed the archive of X-ray observations obtained over the past 16 years by the Chandra X-ray Observatory, looking for sources that showed evidence of unusually variable X-ray emission. They found two especially variable sources apparently associated with external elliptical galaxies, NGC 4636 and the radio galaxy NGC 5128 (also known as Centaurus A). The image above is an X-ray image of Cen A, with a box around the variable source. The images along the bottom show the variability of this source. These two sources increased their X-ray output by more than a factor of 100 in about a minute, so that, at their peaks, they joined the rare class of "ultraluminous X-ray sources" (ULXs). But these sources are extreme even for ULXs, since few objects have been identified that increase their X-ray output by this large an amount in this short a time without destroying themselves. Young highly-magnetized neutron stars, the so-called magnetars, can show such rapid, bright X-ray flares, but the two mysterious sources seem to be associated with much older types of stars. Another alternative might be an especially large amount of matter being gobbled by a black hole. Astronomers are searching for answers, hoping that these sources spout off again, or that other such sources can be identified in similar environments.