Astronomers Puzzle Over Short-Lived Glowing Green Light Bursts

The ultra luminous X-rays lasted about 10 days in the aptly named Fireworks galaxy

Disappearing Green Blob
The green blob of X-rays in the lower left quadrant of the Fireworks galaxy lasted about 10 days before disappearing. NASA/JPL-Caltech

Earlier this week, NASA released a stunning image of the so-called Fireworks galaxy, which certainly lives up to its nickname. In the frame, two blue beams and one large splash of green paint the already dazzling galaxy.

But don’t dust off your telescope lenses just yet. Those pops of color aren’t within the visible light spectrum. They’re extremely bright bursts of X-ray radiation—and the green glow vanished in the course of ten days.

At first, NASA’s NuSTAR space observatory, which was searching for supernovas, didn’t spot the green X-ray emission, known as an ultraluminous X-ray source (ULX), according to a NASA press release. But ten days later, another look at the Fireworks galaxy revealed that the source—dubbed ULX-4—had sparked to life. Another ten days after that, the Chandra X-ray Observatory failed to relocate ULX-4. The observations are detailed in The Astrophysical Journal.

“Ten days is a really short amount of time for such a bright object to appear,” says the study’s lead author and Caltech astrophysicist Hannah Earnshaw 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.”

Most ULX’s are long lasting. Astronomers think they are created by super-dense objects—like black holes—feeding on nearby stars. The gravity of the black hole rips the star to pieces, creating a disk of debris. The material at the inner edge of the disk accelerates to speeds so fast that it heats up to millions of degrees and begins emitting powerful X-rays.

But that doesn’t explain how the mysterious green blob winked in and out of existence so quickly. The fact that no visible light was associated with the X-ray burst suggests that it was not a supernova, which usually shine brightly. The event was possibly a black hole gobbling up a very small star, which produced the short energetic burst.

Another possibility is that the event was created by a neutron star, the core of a giant star that collapsed in on itself during a supernova. Though they are only about 12.4 miles in diameter, these stars are super dense, with one teaspoon weighing a billion tons. Neutron stars' gravity is also about 2 billion times stronger than gravity on Earth.

These incredibly dense stars create debris fields like black holes. Typically, the strong magnetic field around a neutron star channels debris to the surface, where it produces a steady stream of X-rays as the material is heated up and obliterated. However, if a neutron star spins particularly fast, the study authors hypothesize, the magnetic field would block the material from reaching the surface.

“It would kind of be like trying to jump onto a carousel that’s spinning at thousands of miles per hour,” Earnshaw says.

If that magnetic barrier briefly wavered, allowing some debris through, it would cause the star to light up temporarily. The team hopes to watch the same spot to see if the green blob appears again, which would provide evidence for the neutron star theory.

ULXs were first catalogued in the 1980s, and until NuSTAR began investigating them in 2014, researchers thought they were all caused by black holes. New data, however, has led scientists to consider other possible sources of the bright X-rays. A study last year found that dips in the light spectrum of these ULXs were best explained if they were created by a neutron star, and researchers are beginning to find more and more ultraluminous X-ray sources that appear to be caused by neutron stars, not black holes.

“The discovery that these very bright objects, long thought to be black holes with masses up to 1,000 times that of the sun, are powered by much less massive neutron stars, was a huge scientific surprise,” says Caltech’s Fiona Harrison, principal investigator of the NuSTAR mission. “Now we might actually be getting firm physical clues as to how these small objects can be so mighty.”