A giant cauldron that barely smolders most of the time, but occasionally spits out showers of sparks. That is roughly how the supermassive black hole at the center of our galaxy — Sagittarius A*—behaves. It weighs four million times as much as the Sun and lies 26,000 light-years away. For 25 years, the Chandra X-ray Observatory kept a close eye on this "cauldron" in X-rays, amassing nearly seven million seconds of observing time. As a result, astronomers have compiled a catalog of one hundred bright flares — the most complete collection of such events ever assembled.
Each flare lasts from a few minutes to a couple of hours. Its X-ray luminosity reaches 575 trillion trillion ergs per second — roughly the same amount of energy the Sun emits in an entire day across all wavelengths. The catalog includes 18 previously unnoticed flares, including the second-brightest ever seen. With such a rich dataset, scientists spotted a clear pattern: the brighter the flare, the "harder" (that is, more penetrating and higher-energy) its radiation becomes. Spectral analysis shows that during faint flares the spectral index is about three, while during powerful ones it drops to two, like a campfire flame shifting from reddish to bluish when you toss in dry wood.
These differences likely reflect how the energies are distributed among particles accelerated near the black hole. When more high-energy particles are present, the flare becomes both brighter and harder. The new catalog is not just an archive but a true laboratory for testing theories about how black holes accelerate matter and why they sometimes behave like quiet embers and other times like raging flames.
Over 25 years, Chandra has observed Sagittarius A* for a total of nearly three months of pure observing time, as if the telescope had stared at the galactic center nonstop through an entire summer vacation.
In Dan Simmons's novel Hyperion, black holes are used as portals, but the real flares of Sagittarius A* more closely resemble beacon signals marking the entrance to unexplored regions of spacetime.