NASA was watching a ferocious supernova, but what it spotted was even stranger

While observing a large supernova in the 'Fireworks' galaxy, researchers accidentally captured imagery of a brief, bright spot in the same galaxy that didn't originate from the supernova. This spot wasn't visible in the initial NASA NuSTAR observation but was spied a little over a week later in a followup observation by the space agency's Chandra X-ray Observatory.

The image shared by NASA (above) shows three distinctly bright blue/green and green spots located in NGC 6946, also known as the Fireworks galaxy. According to a new study from the space agency, researchers were using NuSTAR space observatory to study the blue-green supernova located in the upper right corner of the image. The supernova was the result of a large star explosion.

The green spot located near the center of the galaxy wasn't visible when NuSTAR first observed the supernova. However, the spot appeared 10 days later when the Chandra X-ray Observatory focused on the galaxy. The spot wasn't producing any detectable visible light; it is an ultraluminous X-ray (ULX) source that has been named ULX-4 due to being the fourth of its kind discovered in the Fireworks galaxy.

Though other ULX spots have been observed, NASA explains that ULX-4 is quite rare due to how briefly visible it was. The majority of ULXs are described as lasting for extended periods of time. This latest spot, however, is thought to have possibly been the result of a black hole decimating a small star, resulting in what was essentially a brief blip in the cosmos.

The study's lead author Hannah Earnshaw explains:

Ten days is a really short amount of time for such a bright object to appear. Usually with NuSTAR, we observe more gradual changes over time, and we don't often observe a source multiple times in quick succession. In this instance, we were fortunate to catch a source changing extremely quickly, which is very exciting.