Deep in the constellation Ophiuchus lies the galaxy NGC 6447. Until recently, it was quiet and unremarkable, like a house with its windows dark. But over the past 14 years, something has ignited at its center, and now it's visible in infrared light.
The infrared telescope WISE has been monitoring the galaxy and saw its brightness in the mid-infrared (wavelengths 3.5 and 4.6 microns) increase by 1.2 magnitudes. This means the light flux has roughly tripled. Meanwhile, in visible light, the galaxy remained unchanged, as if a bright flame is hidden behind a thick curtain of dust.
By 2018, the color of the infrared emission changed in a way that made it clear: a true "cosmic engine" had switched on at the galaxy's center — a supermassive black hole pulling in surrounding gas. As the gas falls into the black hole, it heats up to millions of degrees and begins to glow. This process is called an active galactic nucleus.
The X-ray telescope NuSTAR confirmed that X-rays are coming from the galaxy's center with a luminosity of 8.4×$10^{41}$ erg/s. This is on the low end for active nuclei, but still billions of times brighter than our Sun across all wavelengths. Over the course of a year, this X-ray flux increased by more than a factor of 7—meaning the black hole is flaring up more and more.
Most likely, the black hole was there all along, but hidden behind a dense ring of dust. Now that ring is dispersing, and we see the active nucleus "switching on." Such events are estimated to last from 10,000 to a million years — by cosmic standards, that's a flash, like a lighthouse blinking.
NGC 6447 is the first such case where the awakening of a galaxy has been tracked in such detail. This will help us find other galaxies that are just beginning their active lives.
If our Sun suddenly became as bright in X-rays as the nucleus of NGC 6447, life on Earth would evaporate in seconds. Fortunately, the Sun is a calm star, and the nearest such "lighthouse" is millions of light-years away.
In Isaac Asimov's novel "The Gods Themselves," stars in a parallel universe exchange energy — vaguely reminiscent of how black holes "feed" on gas and flare up.