New X-ray observations suggest how the Milky Way's supermassive black hole stays so trim when faced with a feast.
Our galaxy’s black hole has an eating disorder. This is not a surprise. Young, massive stars surrounding the black hole throw off thick winds of particles, and these particles should feed the black hole at a rate of about an Earth’s worth of mass each year. But that influx would make X-ray emission from the beast’s tutu-like accretion disk 100 million times brighter than it actually is.
Astronomers have come up with a few ideas to explain the mystery but haven’t been able to settle on one solution. “It is as though you filled up your car’s gas tank, drove two feet, and ran out of gas,” Jeremy Schnittman (NASA Goddard) writes in a perspective piece in this week’s Science. “Is there a massive leak in the fuel line, or is the mileage really that horrible?” It’s hard to know without hoisting the car and taking a look underneath.
New high-resolution X-ray observations by the Chandra X-ray Observatory now offer insight into why the black hole, nicknamed Sgr A*, is so dim: like a viciously self-denying dieter, it’s throwing over its shoulder almost all of the feast set before it.
Chandra peered deep into the obscured galactic center for an impressive five weeks to find that insight. The X-ray observations reveal emission from hot ionized gas, laid out in a shape similar to the disk of stars around Sgr A*. But one particular spectral line from ionized iron was very weak. This line comes from the hottest material spiraling toward the black hole’s event horizon. A weak line implies that there’s not much gas in that innermost region.
Q. Daniel Wang (University of Cambridge, UK, and University of Massachusetts, Amherst) and colleagues calculate that a measly 1% or less of the initially captured material makes it into the innermost region. The other 99% escapes as an outflow.
“The outflow [is] powered by a tiny fraction of the gas that (altruistically) sacrifices itself so that its fellow protons can escape to freedom,” explains black hole researcher Roger Blandford (Stanford University). This theory has been one of several proposed to explain black holes’ inefficient feeding habits.
Wang explains that the 1% transfers energy to the outflow and, although no one is quite sure of the details, the transfer probably involves magnetic fields. The inner disk is hotter and rotates faster than the outer disk, and this rotation would take the turbulent magnetic fields woven through the gas and wind them up. Wrapped tightly, the fields increase the gas pressure, which in turn pushes an outflow from the disk’s outer regions that would carry with it some of the disk’s energy.
Q. D. Wang et al. "Dissecting X-ray-Emitting Gas Around the Center of Our Galaxy." Science, 30 August 2013.
J. D. Schnittman. "The Curious Behavior of the Milky Way's Central Black Hole." Science, 30 August 2013.