A black hole heats up
Explaining the X-ray flare in the Galactic Center Black Hole

A stunning X-ray flare from the Milky Way’s central black hole, recently discovered by the NASA satellite Chandra (Baganoff et al. 2001, Nature, Sep. 6), tells of incredibly hot temperatures in the vicinity of the event horizon, according to a paper by astrophysicists of the Max-Planck-Institute for radio astronomy. Like a boiling kettle the black hole apparently heats and expels plasma in its surrounding, thus sparing it from being eternally captured.

Because it is the closest and best studied case, the supermassive black hole at the Galactic Center, named Sagittarius A*, is considered a key for understanding the physical processes near black holes in general. The source was discovered more than 25 years ago as an unusually compact bright radio source. Observations of star motion in its neighborhood revealed that Sgr A* exerts an incredible gravitational pull on its environment and therefore must be very massive - it weighs in fact 3 Million times the mass of an ordinary star like our sun. Such a combination of unusual radio emission, small size and exceedingly high mass made it a primary black hole candidate.

However, while it is generally thought that black holes swallow matter thereby heating the infalling gas and emitting large doses of emission, Sgr A*, had been detected only with radio telescopes at a rather steady and low luminosity level. The limited amount of information led to a number of speculations about the nature of the source. One popular theory, in fact even suggested that the observed radio emission is not the signature of material falling into the black hole, but rather is indicative of matter that just escaped the laird of the black hole.

Observations with the X-ray satellite Chandra have now not only for the first time detected emission way outside the radio regime, but also discovered an X-ray flash, where the source brightened by a factor of 50 for just three hours. Even more intriguingly, the source was switched on within a period of five minutes indicating that the emission must origin from very close to the event horizon - the point of no return around a black hole.

While the violence and magnitude of the X-ray flare has perplexed astronomers, the presence and the behavior of such flare had in fact been predicted by theoretical models. Earlier Heino Falcke & Sera Markoff of the Max-Planck-Institute of Radio Astronomy in Bonn (Astronomy & Astrophysics, Vol. 362, p. 113) suggested that the X-ray and radio emission are produced in a strong plasma outflow leaving the black hole at almost the speed of light. In this picture the X-ray radiation spectrum is actually a mirrored image of the radio emission, where the radio emitting particles upscatter their own radio photons into the X-ray regime via the so-called inverse-Compton (or synchrotron self-Compton) process.

Because of this mirroring ,the X-ray emission should be much more sensitive to small changes in the physical parameters of the plasma near the black hole. In a recently submitted paper, Markoff and her co-authors (Heino Falcke, Feng Yuan, Peter L. Biermann) now argue that the newly discovered flare allows one to finally tightly constrain these parameters. They conclude that a sudden heating of particles must have been the cause of the X-ray flare. In this process the temperature of the plasma in Sgr A* must have increased from about 200 billion degrees Celsius to at least about 600 billion degrees Celsius within a few minutes. Such an enormously hot plasma can no longer stay gravitationally bound and some of it can escape even the enormous gravitational pull of a black hole.

The black hole almost acts like a tea kettle: when the water starts to boil, the hot outflowing steam produces the most obvious signature of the activity in the kettle. In that respect a black hole may not act much differently: while most of the plasma is heated and swallowed by the black hole a small fraction of it escapes and produces the strong radio and X-ray emission.

Figure for experts: The emission spectrum (energy flux density versus frequency) of the Galactic Center black hole in logarithmic intervals. The box at the right hand side (at 10⁹ GHz) shows the X-ray measurements at two different epochs as it increased by a factor of ~50. The solid line shows the predicted model spectrum for the flare, leading to also detectable mid-infrared emission (at 10⁵ GHz).