(CNN) NASA scientists spotted pops of blue and green X-rays in a distant galaxy. What caused them to shine, exactly, is mystifying scientists.

The bright rays belong to ULX-4, short for the fourth ultraluminous X-ray source found in the Fireworks Galaxy , about 22 million light years away from Earth, NASA said

This visible-light image of the Fireworks galaxy (NGC 6946) comes from the Digital Sky Survey, and is overlaid with data from NASA's NuSTAR observatory (in blue and green).

But this ULX had a shorter life than its predecessors. Within 10 days of observations by NASA's NuSTAR and Chandra telescopes, it went from being undetectable to burning bright, then back to invisible just as quickly.

"Ten days is a really short amount of time for such a bright object to appear," Caltech researcher Hannah Earnshaw said.

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