Scientists at NASA Goddard Space Flight Center and the University of Maryland have developed a theory of how radiation pressure from the region closest to a black hole may produce two types of winds, with wind closer to the black hole shielding the wind from the outer region in such a way to allow matter to escape.

The theory, which helps confirm previously unexplained observational data, is presented today at the 33rd Committee on Space Research (COSPAR) Scientific Assembly in Warsaw, Poland, and will appear in the November 10 issue of the Astrophysical Journal. The authors are Dr. Daniel Proga of Goddard, Dr. James Stone of University of Maryland, and Dr. Timothy Kallman, also at Goddard.

"This looks like a solid result; we have enough information now to avoid hand-waving explanations," said Proga, a National Research Council fellow within Goddard's Theory Group. "For years we have seen observational evidence of fast outflows of matter from the region surrounding very massive black holes. We couldn't explain exactly what was going on, and this led to a certain level of uncertainty in what the data were saying. Now we can start to explain what we've been seeing."

Black holes are objects so dense that nothing, not even light, can escape their gravitational attraction. But this only applies once matter crosses the theoretical border of a black hole, called the event horizon. Outside the event horizon, the tug of gravity is strong, but matter and light can escape.

Matter falling into a black hole can escape in two ways: through particle jets shooting from the poles of the black hole region, and as supersonic winds generated in the swirling flow of matter, called an accretion disk, spiraling into the black hole at its equatorial region. Today's announcement concerns the latter.

For particles to be "blown" by ultraviolet light, they must be atoms with electrons bound to them. Atoms fully stripped of their electrons will not be efficiently propelled by ultraviolet light. But if an atom still has a few electrons orbiting its nucleus, then that is enough of a "sail" to catch a UV "wind".

Intense X-ray radiation in the vicinity of the black hole serves to separate atoms from their electrons, a process called ionization. A fully-ionized atom has no electrons. How then could the UV-induced wind blow matter away from the accretion disk, as seen by observational astronomers, if all the matter is ionized by the X-rays?

Proga and his colleagues confirmed the concept of a double wind generated by UV light that pervades the entire accretion disk. The first wind, from the inner regions of the accretion disk, pushes matter up and away from the plane of the accretion disk. But with the black hole's gravity pulling it back and X-ray radiation stripping away electrons, the wind just barely makes it to the middle regions of the accretion disk.

As the particles fall back to this middle region, however, they shield the outer region from the X-ray radiation produced very close to the black hole. This shielding in the outer disk allows the gas that is present there to keep their electrons. This non-ionized or partially- ionized gas still has its electron sails needed to catch a UV-induced wind. Without this shield, the wind from the outer disk would suffer the same fate as the inner-disk wind at the hands of gravity and ionizing X-ray radiation.

"We think that matter can blow off the accretion disk in significant quantities," said Proga. Knowing this mass-loss rate is crucial in understanding the dynamics of the black hole. Also, this theory fits the observational evidence for a supersonic outflow, and it can be applied to many types of black hole systems, such as quasars."

"We often think of black holes pulling in matter, but actually what we observe with our current instruments is the matter flying out," said Kallman. "Observations say outflows; previous theory says inflow. This new theory reconciles this conflict."

COSPAR is an international, interdisciplinary scientific organization concerned with scientific research carried out with space vehicles, rockets, and balloons. The annual meeting convenes this year from July 16-23 at the Warsaw University of Technology.