NASA’s NuSTAR X-ray observatory has spotted something a little weird. While imaging the Fireworks galaxy, NuSTAR spotted several mysterious bright sources of X-ray light, appearing as green and blue spots. Within days, the blobs had disappeared.
The main goal of the NuSTAR observations was to examine a supernova — a huge star explosion. The green blob, known as an ultraluminous X-ray source (ULX), shown at the bottom of the galaxy in the image above didn’t appear during the first observation, but showed up during a second one 10 days later. Another space telescope, NASA’s Chandra X-ray Observatory, then looked again and found the object, ULX-4, had quickly disappeared.
“Ten days is a really short amount of time for such a bright object to appear,” Hannah Earnshaw, a postdoctoral researcher at Caltech and lead author on the study, said in a statement. “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.”
It’s possible the light was from a black hole consuming another object like a star, the study suggests. When objects get too close to a black hole, they can get torn apart by gravity, and their debris is pulled into a close orbit around the black hole. Material at the disk’s inner edge moves so quickly that it “heats up to millions of degrees and radiates X-rays,” NASA says. For reference, the surface of the sun is around 10,000 degrees Fahrenheit.
Most ULXs last for a long time because they’re formed by dense objects like black holes that “feed” on a star for a long time. Short-lived X-ray sources like this ULX aren’t as common, so its appearance could be explained by a scenario such as a black hole quickly destroying a small star.
There could be other possible explanations for the green blob’s appearance. The study’s authors suggest its source could be a neutron star, which is an extremely dense object created from the explosion of a star that wasn’t big enough to create a black hole. The mass of a neutron star is similar to that of the sun, but it’s only around the size of a large city. Therefore, neutron stars can pull in material and cause debris to move really quickly in a disk.
Neutron stars can create strong magnetic fields that form “columns” and take material to the surface. In the process, they generate strong X-rays. However, if the star is spinning super-fast, material can’t reach the surface and create these X-ray bursts. It’s kind of like an invisibility cloak — astronomers can’t see the neutron star’s X-ray signature. But if material does somehow sneak through, the invisibility cloak fails. That could explain why ULX-4 quickly appeared and then disappeared.
“This result is a step towards understanding some of the rarer and more extreme cases in which matter accretes onto black holes or neutron stars,” Earnshaw said in the statement.