December 14, 2012
All Black Holes Launch Fundamentally Similar High-Speed Jets
April Flowers for redOrbit.com - Your Universe Online
It would be a mistake to think of black holes as having a uniformity of size or mass. They range from modest objects formed from the end of an individual stars' life to behemoths billions of times more massive that rule the centers of galaxies.
"What we're seeing is that once any black hole produces a jet, the same fixed fraction of energy generates the gamma-ray light we observe with Fermi and Swift," said Rodrigo Nemmen, a NASA Postdoctoral Program (NPP) fellow at NASA's Goddard Space Flight Center in Greenbelt, Md.
An accretion disk forms when gas falling toward a black hole spirals inward and piles up. The disk becomes compressed and heated. A pair of jets flowing in opposite directions along the black hole's spin axis are formed when some of the material near the inner edge of the disk — on the threshold of the black hole's event horizon, the point of no return — accelerates and races outward. Particles moving at nearly the speed of light are contained in the jets. These particles produce the most extreme form of light — gamma rays — when they interact.
"We don't fully understand how this acceleration process occurs, but in active galaxies we see jets that have operated so long that they've produced trails of gas extending millions of light-years," said Sylvain Guiriec, an NPP fellow at Goddard .
The most powerful explosions in the universe — gamma-ray bursts (GRBs) — are at the other end of the scale. The most common type of GRB foretells the death of a massive star and the birth of a black hole of stellar-mass, astronomers believe. The black hole is formed as the star's energy-producing core runs through its store of fuel and collapses. The accretion disk is formed as the star's overlying layers cascade inward, and the black hole launches a jet.
Some GRB jet particles have been clocked at speeds exceeding 99.9 percent the speed of light. The gamma-ray pulses produced as the jet breaches the star's surface typically last only a few seconds. If the jet is approximately directed towards us, Swift and Fermi are able to detect the emission.
Scientists looked at the galactic-scale equivalent of GRB jets, which come from the brightest classes of active galaxies called blazars and quasars, to search for a trend across a wide range of masses. Blazars and quasars sport jets that also happen to point our way.
To put the energy of these bursts into perspective, in order to match the energy output of a typical blazar in one second, our Sun must shine for 317,000 years. In contrast, to equal the energy output of a run-of-the-mill GRB in one second, the Sun would need to shine for an additional 3 billion years.
The team observed 54 GRBs and 234 blazars and quasars to complete this study. The scientists learned how much light the jets radiate by studying the gamma-ray brightness recorded by Swift, Fermi and other observatories, while radio and X-ray observations allowed the team to determine the power of the particle acceleration in each jet. The researchers discovered that the GRB and blazar samples both exhibited the same relationship by analyzing how these two properties related to each other.
"Here we have a situation where the mechanism that launches material from a black hole either has to be very similar on both ends of the mass scale -- from a few to a billion solar masses -- or we need different mechanisms that manage to produce very similar efficiencies," explained Eileen Meyer, a post-doctoral researcher at the Space Telescope Science Institute.
Astronomers' understanding of black holes was simplified by the findings of this study. It showed that their activity is governed by the same set of physical rules — whatever they happen to be — independent of mass, age, or brightness and power of the jet. The energy wrapped up in the motion of the accelerated particles to power the emission of gamma rays and other forms of light have similar fractions — between 3 and 15 percent.
"It's a bit like a poor man and a billionaire spending the same percentage of their incomes on their heating bills," said team member Markos Georganopoulos, an associate professor of physics at the University of Maryland, Baltimore County.
The authors expect to continue their research, extending it to other black-hole-powered events that launch jets. These events include the tidal disruption of stars by supermassive black holes.
"One especially useful outcome of this research will be to foster greater communication between astronomers studying GRBs and those working on active galaxies, which in the past we've tended to regard as separate areas of study," said Neil Gehrels, the principal investigator on NASA's Swift.