NASA's NuSTAR X-ray observatory recently picked up a green flash of light in the aptly named fireworks galaxy (NGC 6946). Within a matter of days, the green light had already disappeared.
While the exact origin of this mystery space light is not fully understood, a new study, published in the Astrophysical Journal, offers some possible explanations for the event.
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A mystery flash
The main objective for the NuSTAR observations was to study a supernova in the NGC 6946 galaxy, NASA said in a post about the green flash. The supernova appears in the image above as a bright turquoise spot on the upper right.
During the first observations, the green blob wasn't visible near the bottom. However, ten days later, during the second observation, it had appeared and was burning bright.
"Ten days is a really short amount of time for such a bright object to appear," Hannah Earnshaw, a postdoctoral researcher at Caltech in Pasadena, and lead author on the new study said in the NASA post.
"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."
NASA's Chandra X-ray Observatory observed that the bright green light had disappeared just as quickly. The object has been named ULX-4.
What is the green space blob?
Importantly, no visible light was detected emanating from the green blob. It was only picked up by X-ray observatories. This NASA, says rules out the possibility that it is a supernova.
The study says that the light might have been caused by a black hole swallowing another space object, such as a star. If a star gets sucked in too close to a black hole, gravity can tear it apart, causing a huge flash of energy.
— NASA JPL (@NASAJPL) September 4, 2019
The source of ULX-4 could also be a neutron star.
Neutron stars are extremely dense space objects that are formed when a star explodes. They are similar to black holes but originate in smaller stars. These, NASA says, can generate slow-feeding ultraluminous X-ray sources, such as the ones picked up by the NuSTAR satellite.
We might never know 100 percent what caused the flash, though the readings, Earnshaw says, are "a step towards understanding some of the rarer and more extreme cases in which matter accretes onto black holes or neutron stars."