Rumblings at the Galactic Center
Left to themselves, black holes are almost completely inert. They turn ferocious only when they encounter any outside material, which gets swept into a rapidly orbiting swirl, or accretion disk, just outside the event horizon. As that disk spirals inward, it grows tremendously hot and radiates intensely. The fact that Sagittarius A* is currently so dim indicates that it must be on a starvation diet, with only a trickle of material leaking in.
But what happens, Clavel wondered, when it is feeding time at the galactic center? She and her colleagues pored over observations from NASA’s space-based Chandra X-ray Observatory to get some answers. From 1999 to 2011, Chandra has monitored the region around Sagittarius A*, recording an intriguing series of X-ray flickers.
In a new paper published in Astronomy & Astrophysics, Clavel identified those flickers as “light echoes” of some long-ago event. Brilliant outbursts of radiation from near the black hole had spread outward, struck iron atoms in surrounding gas clouds, and then reflected toward Earth, becoming visible here long after the original eruption. (Visit chandra.harvard.edu to watch a cool video of the light echoes.)
By meticulously analyzing the changing pattern of illumination, Clavel and company deduced that Sagittarius A* had experienced not one but two separate outbursts in the past few centuries. Many of the details got lost in the echo, unfortunately. “The precise dating of these two flares, their chronological order and the level of emission in between are difficult to assess,” Clavel laments. But the brightness of the echoes gives a good sense of the peak luminosity of the original events: When Sagittarius A* was acting up, it was at least a million times as energetic as it is today. Put another way, it briefly shone as brilliantly as a million suns.
As for why Sagittarius A* went haywire in the first place, Clavel and her colleagues offer several explanations, all of them pointing to the black hole’s intermittently savage nature. It might have stripped off the outer layers of a nearby star, caused a pair of stars to interact and consumed the released gas, or swallowed some planets whole.
In fact, other researchers may have already caught Sagittarius A* in the act of such a feeding frenzy, albeit on a much smaller scale. Every day or so, Chandra captures modest flare-ups during which the black hole brightens up to a factor of 160 for a few hours. Astronomer Kastytis Zubovas interprets those events as the burps caused when an asteroid or comet, at least 10 kilometers across, passes within about 100 million kilometers of the black hole, quickly getting shredded and consumed.
The centuries-old twin eruptions of Sagittarius A* deduced by Clavel may also explain why our monster black hole is such a shrinking violet today. One of the many paradoxes of black holes is that in the act of feeding, they tend to starve themselves. The outbursts probably cleared gas from the immediate region: With no material falling in, Sagittarius A* fell into a near-dormant state.