After years of watching the supermassive black hole at the center of the Milky Way, researchers were rewarded with a megaflare of X-rays, the largest one ever recorded.
The black hole in the middle of our galaxy is called Sagittarius A*, and astronomers believe that it contains about 4.5 million times the mass of the sun. A team led by Daryl Haggard of Amherst College has been observing Sgr A* with NASA’s Chandra X-ray Observatory since 1999, waiting to see if the black hole would eat up material from a nearby gas cloud called G2 during the spring of last year when it made its closest approach. “Unfortunately, the G2 gas cloud didn’t produce the fireworks we were hoping for when it got close to Sgr A*,” Haggard says in a university statement.
She adds: “However, nature often surprises us and we saw something else that was really exciting.” Last September, the team detected an X-ray outburst that was 400 times brighter than the typical outputs from Sgr A* and nearly three times brighter than the previous record holder (which was observed in 2012). Then in October, they observed a second giant X-ray flare that was 200 times brighter than Sgr A* in its quiet state.
The team has two main hypotheses on how the supermassive black hole erupted in this extremely bright way: Perhaps its gravity tore apart a couple of asteroids that wandered too close, or the black hole experienced a massive sun-like magnetic flare.
“If an asteroid were torn apart, it would go around the black hole for a couple of hours—like water circling an open drain—before falling in,” MIT’s Fred Baganoff explains. “That’s just how long we saw the brightest X-ray flare last, so that is an intriguing clue for us to consider.” Debris from this sort of event, called a tidal disruption, would become very hot and produce X-rays before disappearing forever. And if that’s the case, then that asteroid was probably the largest-ever asteroid torn apart by Sgr A*.
On the other hand, it could be the tightly packed and tangled magnetic field lines within the gas flowing towards the black hole. When field lines interconnect and reconfigure themselves, their magnetic energy gets converted into kinetic energy, heat, and particle acceleration. Together, these could produce an X-ray flare like the ones seen on the sun.
But right now the researchers aren’t able to distinguish between the two ideas. Regardless, “such rare and extreme events give us a unique chance to use a mere trickle of infalling matter to understand the physics of one of the most bizarre objects in our galaxy,” Gabriele Ponti of the Max Planck Institute for Astrophysics adds in a NASA release.
The findings were announced at a press conference during the American Astronomical Society’s meeting in Seattle this week.
Images: NASA/CXC/Northwestern Univ/D.Haggard et al. (top), Daryl Haggard via Amherst (middle)