Unusual X-Rays

The blobs of blue and green in this image of the Fireworks Galaxy (NGC 6946) show the locations of extremely bright sources of X-ray light captured by the NuSTAR space observatory. They were generated by some of the most energetic processes in the universe, and the surprise appearance and rapid disappearance of the green source near the center of the galaxy is puzzling.

The primary objective of the NuSTAR observations was to study the supernova that appears as a bright blue-green spot at upper right. The green blob near the bottom of the galaxy wasn’t visible during the first NuSTAR observation, but it was burning bright at the start of a second observation 10 days later. When the Chandra X-ray Observatory later observed that the ultraluminous X-ray source, it had disappeared. The object has since been named ULX-4 because it is the fourth ULX identified in this galaxy. No visible light was detected with the X-ray source, so it’s unlikely that it is also a supernova.

A new study explores the possibility that the light came from a black hole consuming another object, such as a star. If an object gets too close to a black hole, gravity will pull the object apart, and the debris will enter a close orbit around the black hole. Material at the inner edge of the newly formed disk will move so fast that it heats up to millions of degrees and radiates X-rays. Most ULXs are typically long-lived because they’re created by a black hole, that will feed on the victim star for an extended period of time. Short-lived, or “transient,” X-ray sources like ULX-4 are rare, but a single dramatic event such as a black hole swallowing a star in one gulp might explain ULX-4.

Image Credit: NASA

The Black Holes of the Circinus Galaxy

CircinusThe magenta spots in this image show a couple of black holes in the Circinus galaxy—the supermassive black hole at its heart and a smaller one closer to the edge. The smaller one belongs to a class of objects called ultraluminous X-ray sources, or ULXs. ULXs are black holes actively feeding off material drawn in from a partner star.

The ULX was spotted by NuSTAR which sees high-energy X-ray light. The magenta X-ray data in the image above come from the NuSTAR and are overlaid on a visible/infrared image from the Digitized Sky Survey.

The Circinus galaxy is located 13 million light-years away in the southern sky constellation Circinus.

Image Credit: NASA