NASA Snaps Bright Flash Of X-Rays But Its Source Remains A Mystery

This visible-light image of the Fireworks galaxy (NGC 6946) comes from the Digital Sky Survey, and is overlaid with data from NASA's NuSTAR observatory (in blue and green). ULX-4 is the green blob towards the middle of the galaxy. NASA/JPL-Caltech

NASA’s telescope NuSTAR is designed to spot energetic X-ray lights coming from the cosmos. These signals often come from supernovae and supermassive black holes but a team of researchers has recently found one without a straightforward explanation.

When looking at the Fireworks galaxy (also known as NGC 6946), they discovered a transient event that appeared and disappeared over less than three weeks. The green blob in the main image wasn’t there in the first set of observations; it suddenly appeared in the second set, which was conducted 10 days later. When the researchers checked again after another 10 days, the signal had gone for good.

The observations are reported in The Astrophysical Journal and were part of a wider investigation of NGC 6946’s ultra-luminous X-ray sources (ULX). Three have been known to exist in the galaxy and the flash-in-the-pan event is the fourth one, named ULX-4. The team also used ESA's XMM-Newton telescope and NASA's Chandra.

"Ten days is a really short amount of time for such a bright object to appear," lead author Hannah Earnshaw, a postdoctoral researcher at Caltech in Pasadena, California, 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."

The researchers are pretty confident that ULX-4 was not a supernova given that they couldn’t find the specific visible light signature for such an event coming from the same region. In their study, the team proposes two explanations, and both have to do with matter accumulating on extremely compact objects.

One possibility is that the team caught a black hole disrupting a star. The gravity near a black hole is high enough to break objects apart and as they spiral inwards towards the black hole they are heated up by extreme forces. The disrupted material can easily reach millions of degrees, emitting bright X-rays.

The second scenario sees a neutron star being the compact object in question. These objects are what remain after certain stars go supernova. They have a mass larger than the Sun packed into a sphere not much larger than a big city. They also have incredible magnetic fields that often trap matter into columns funneling material down to the surface.

But in certain cases, the material cannot reach the surface. Some neutron stars spin hundreds of times per second and this acts as a barrier to the material falling towards the star. The authors describe it as jumping on a carousel that is spinning at thousands of miles per hour.

But it is possible that the magnetic field changed for a brief time and that was enough for matter to come barging in, hitting the surface, and emit a burst of X-rays.

The team will keep an eye on the galaxy to see if whatever caused ULX-4 repeats. Future observations could provide us with new clues and help us better understand these occasional phenomena.

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