Mystery Green Flash at Center of Distant Galaxy That Vanished After Just Two Weeks Discovered by Scientists

A surprise green flash from a mystery source at the center of a distant galaxy has been spotted by scientists using NASA's NuSTAR satellite. Follow-up observations revealed the flash had vanished within two weeks, leaving scientists to ponder what could have caused it.

Researchers led by Caltech's Hannah Earnshaw noticed a green blob in observations returned by NuSTAR—a mission where one of the objectives is to study supernova. These stellar explosions produce so much light they can, at points, outshine galaxies. The satellite image, which shows the 'Fireworks galaxy' NGC 6946, normally shows supernova as green and blue spots, with one currently appearing in the top right of the image provided.

This image, however, also showed something else. At the bottom of the image was another green dot that had not been present in the first observation 10 days earlier. Follow-up observations with the Chandra X-ray Observatory showed the source had disappeared. Since then, the team has named the flash ULX-4 (ultraluminous X-ray), as it is the fourth ULX from this galaxy so far identified.

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

nustar flash
The image from NuSTAR. The green flash can be seen towards the bottom left of the picture. NASA/JPL-Caltech

In a study published in the Astrophysical Journal, Earnshaw and colleagues look at what could have caused the bright flash. Observations showed there was no visible light from the burst, indicating it could not have been a supernova.

Instead, the team say it could be a black hole consuming a nearby star—as gravity from the black hole drags it inwards, material at the edge moves extremely fast and heats up, producing X-rays. Normally, ULXs are long lived, having been created by an object like a black hole slowly feeding on a nearby star for an extended period. A black hole that swiftly consumes a star in a single event would be a far rarer event.

Another event that could have produced the flash is a neutron star—an incredibly small, dense object that forms from the collapsed core of a giant star. Neutron stars generate extremely strong magnetic fields that pull material down to the surface, which produces X-rays. This idea has its problems, however. Because it is spinning so fast, the X-rays should not be able to escape the surface of the star: "It would kind of be like trying to jump onto a carousel that's spinning at thousands of miles per hour," Earnshaw explained. Potentially, the rays may break through if there was a blip in the barrier. If this is the case, it should, at some point, flash again.

The source could also be some currently unknown phenomenon. "Ultimately, further detections of ULX-4 would be required to establish whether it is an [neutron star] ULX or other relatively persistently accreting object rather than a transient event," they conclude.

Rosanne Di Stefano, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, who was not involved in the study, said the last time a supernova was observed in the Milky Way was 1604—the Fireworks Galaxy, in contrast, is something of a supernova factory. She told Newsweek that most supernovae are high mass stars with relatively short lifespans. Normally, these come in binary pairs and when one member dies, it explodes leaving behind a black hole or neutron star. If it is close enough to its companion star, it will consume it, sometimes producing ULXs in the process. Several features of the emission of ULX-4 are highly unusual for ULXs, suggesting that what Earnshaw and colleagues observed does not fit with current models of ULXs.

"This source had never been seen before, not even during a NuSTAR observation 11 days earlier," she said, adding that astronomers are now working to find out the physical nature of ULX-4—and that intriguingly, they may be looking at a previously unknown type of source.

"The most likely possibility suggested so far is that the X-ray emission could have been emitted by mass stripped from a low-mass object like a brown dwarf that was tidally disrupted by a black hole in NGC 6946. If this explanation is correct, we will almost certainly not see emission from ULX-4 again. Instead, the flash we saw would be our first view of radiation from a tidally disrupted substellar object. We would expect in the future to see other tidal disruptions of brown dwarfs or even of planets. These could occur in NGC 6946, or possibly in other galaxies.

"Hannah Earnshaw and her collaborators have conducted a thorough analysis of the data available from the region that hosted the flash from ULX-4. Theoretical work can determine which physical models are most consistent with the data. Observers and theorists together will work to plan future observing campaigns to discover and study examples of this new type of X-ray flasher."

This article has been updated with additional comments and information on NuSTAR.