The science of the galactic burp

Certain galaxies among the multitudes of the universe spit flares of radio emission so energetic that astronomers did not understand what could power this phenomenon. Recent work suggests that a burping black hole may be responsible.

Professor Alan Marscher, of Boston University’s Institute for Astrophysical Research, led an international team of astronomers through a three-year study to monitor what he calls the “galactic belching” of one such mysterious galaxy, the one called 3C 120.

3C 120 is 450 million light years from Earth, but it is not merely another sparkling dot in the night sky. It is an “active galaxy,” a type of cosmic body that has long puzzled scientists. Early radio astronomers in the 1950’s discovered these objects emit very powerful radio signals. Development in astronomy allowed scientists to detect equally strong production throughout the electromagnetic spectrum, a total energy equal to the combined output of thousands of conventional galaxies.

Obviously, enormous forces are at work, but now Marscher may have an explanation.

“The jets in active galaxies,” Marscher reports, “are powered by disks of hot gas orbiting around supermassive black holes.”

How massive is supermassive? Try a weight equal to millions or billions of stars. Marscher’s study provides the first direct observational link between a supermassive black hole and accompanying jets of rapidly moving particles.

A supermassive black hole is paired with an enormously strong gravitational field. Interstellar gas and dust is drawn toward the black hole. The gas forms a doughnut-shaped ‘accretion disk’ around the black hole, moving in a flat circle like water around a drain. The swirling accretion disk generates a correspondingly swirling magnetic field. This field gets stronger as it approaches the event horizon where the gas is hottest and most energetic and fuels a jet of particles (moving at 98 percent the speed of light) spewed from both ends of the hole perpendicular to the rotation of the accretion disk.

Giant black holes had long been suspected of being the engines powering the high radiation and jets of active galaxies, but the intervening distances and time scales involved made confirmation of the hypothesis difficult. The study of much closer, and much less massive, microquasars helped to flesh out this hypothesis with readily obtained data.

Microquasars are star systems that contain a large star plus a black hole of five to 15 times the mass of the sun. They exhibit effects similar to those of active galaxies but can be studied with much greater resolution since they are only thousands of lightyears away, some within our own Milky Way galaxy.

Guided by microquasar observations, Marcher’s team developed a theoretical model for the behavior of accretion disks and their own electromagnetic forces that included the formation of high-velocity jets of plasma, hot gas whose atoms have been ionized.

In 3C 120, the accretion disk constantly emits radiation powered by energy the hot gas gains from the black hole’s attractive force. Every 300 days, on average, the inner portion of the disk breaks off and is pulled to the hole.

To the astronomer, this is marked by a sudden dip in X-ray output as that portion of the disk passes the event horizon and is unavailable to emit radiation.

A small fraction of this inner portion escapes being swallowed. Instead, electromagnetic forces twist and propel this matter and energy along the jets. A bright spot appears over a light year from the black hole, moving at what appears to be five times the speed of light.

“We see it,” explains Marscher, “as a knot of radio emission shooting away from the black hole.”

Although the spots appear to be moving at faster-than-light speeds, this is only an intergalactic illusion. Because the jets are pointed almost directly at us, the spots appear to be moving at relativistic velocities.

“Because the jets are pointed nearly directly at us, the slightly-less-than-light-speed spots move toward us almost as fast as the radio waves do. This causes the appearance of faster-than-light velocities when observed from Earth.

Meanwhile, the accretion disk accretes even more gas, and the system prepares to reload and fire off another volley.

“About 10 months later, something disturbs the accretion disk orbit and the whole thing flushes and blows again,” said Marscher.

The researchers observed these ultra fast “smoke signals” moving along the jets of 3C 120 four times during the three-year period of the study: confirmation of the theory that a supermassive black hole was gassing up and powering one of the most impressive displays seen in the sky.

The study team focused state-of-the-art astronomical tools to 3C 120 to see if its activity matched their model’s prediction. The Rossi X-ray Timing Explorer, launched by NASA in 1995, carries the X-ray detector that was used to observe the accretion disk as it approached the even horizon.

The Very Long Baseline Array (VBLA), a radio telescope system that spans nearly an entire hemisphere, was used to record the movement of the bright radio sources as they shot along the jets of 3C 120. The VBLA’s 10 component radio telescopes — one each in Hawaii and St. Croix and eight others spread across the continental United States, including an antenna at BU’s Sargent Camp in New Hampshire — are linked together to act as a single receiver with the highest radio resolution currently possible with any telescope system.

However, the theoretical link between active galaxies and microquasars is not rock solid.

“Our main worry is that the X-ray dips in 3C 120 aren’t as deep as in the microquasars and that we haven’t yet seen the rapid oscillations in brightness for which we don’t yet have a really good explanation yet,” Marscher said.

Marscher’s team is continuing to monitor 3C 120 as they accumulate data to confirm their study.

“We’re now observing 3C 120 four times per week with RXTE and monthly with the VLBA in order to observe more events and get better time coverage of them,” Marscher said. “We’ll also be looking for rapid oscillations in brightness as seen in microquasars.”

Alan Marscher, professor and associate dean of the College of Arts and Sciences, specializes in exotic astronomy. He currently teaches solely for the CAS Core Curriculum. His research interests include quasars; high-energy astrophysics; interstellar clouds and radio, infrared, X-ray, and gamma-ray astronomy. His research group at BU continues to focus on the gas dynamics and radiative processes in the jets of active galaxies.

This research is detailed in the report, “Observational Evidence For The Accretion-Disk Origin For A Radio Jet In An Active Galaxy,” in the June 6, 2002, issue of Nature. Alan Marscher is lead author with co-authors Margo Aller, of the University of Michigan; BU co-researcher Svetlana Jorstad; Matthew Lister, of the National Radio Astronomy Observatory; Alastair Stirling, of the University of Central Lancashire, England; Harri Terasranta of the Helsinki University of Technology, Finland; and Jose-Luis Gomez, of the Astrophysical Institute of Andalucia in Granada, Spain. Funding was provided by NASA, the National Science Foundation and the Fulbright Commission.

The team members and equipment used to observe 3C 120 are scattered across the globe. According to Marscher, this sort of international collaboration is vital to research of this magnitude.

“The experts that you want to recruit, and the most suitable telescopes you want to use are scattered around the globe,” Marscher said.

Though Marcher admits that the benefits of his research to society are not immediately obvious, he defended his work as contributing to general knowledge about the universe.

“This is strictly curiosity-driven research to find out more about how the universe causes bizarre high-energy phenomena with such apparent ease,” Marscher said.