Astronomers using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) have found a remarkable source of X-rays in NGC 6946, a spiral galaxy located at a distance of about 25 million light-years.

This visible-light image of 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 near the bottom of the galaxy. Image credit: NASA / JPL-Caltech.

This visible-light image of 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 near the bottom of the galaxy. Image credit: NASA / JPL-Caltech.

In 2017, NuSTAR observed the NGC 6946 galaxy twice in order to study a recently-discovered supernova. The two observations were taken 11 days apart.

The extremely bright source of X-rays — the so-called ultraluminous X-ray source (ULX) — wasn’t visible during the first NuSTAR observation but was burning bright at the start of a second observation 10 days later.

NASA’s Chandra X-ray Observatory later observed that the source — named ULX-4, because it is the fourth ULX identified in NGC 6946 — had disappeared just as quickly.

No visible light was detected with ULX-4, a fact that most likely rules out the possibility that it is a supernova.

“Ten days is a really short amount of time for such a bright object to appear,” said Caltech astrophysicist Hannah Earnshaw.

“Usually with NuSTAR, we observe more gradual changes over time, and we don’t often observe a source multiple times in quick

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