NASA | JPL-Caltech | NuSTAR | 2019 Sep 04
Pops of bright 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 NASA's NuSTAR space observatory. Generated by some of the most energetic processes in the universe, these X-ray sources are rare compared to the many visible light sources in the background image. A new study, published in the Astrophysical Journal, offers some possible explanations for the surprise appearance of the green source near the center of the galaxy, which came into view and disappeared in a matter of weeks.
The primary objective of the NuSTAR observations was to study the supernova — the explosion of a star much more massive than our Sun — that appears as a bright blue-green spot at upper right. These violent events can briefly produce enough visible light to outshine entire galaxies consisting of billions of stars. They also generate many of the chemical elements in our universe that are heavier than iron.
The green blob near the bottom of the galaxy 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 — known as an ultraluminous X-ray source, or ULX — had disappeared just as quickly. 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, a fact that most likely rules out the possibility that it is also a supernova. ...
A Broadband Look at the Old and New ULXs of NGC 6946 ~ Hannah P. Earnshaw et al