Scientists Observe Eclipse of Quasar in Galaxy

For a team of Harvard astrophysicists who gazed into other galaxies last April, the stars must have been favorably aligned.

That team, led by Harvard researcher Guido Risaliti, witnessed the eclipse of a supermassive black hole in the center of a nearby galaxy, according to a study published in The Astrophysical Journal last week.

The term black hole is misleading in the case of this galaxy, known as NGC 1365, which has a brightly lit core instead of a dark one. The galaxy’s center—known by scientists as a nucleus—emits light and radiation due to the powerful gravitational field of the black hole, and is better known as a quasar.

“Quasars light up because gas is falling down toward them, which heats up through friction like a rocket reentering the atmosphere,” said Martin S. Elvis, a senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics (CFA) and a co-author of the study.

This region of super-heated gas, Elvis said, is shaped like a disk in NGC 1365’s quasar and is a source of X-ray radiation.

But scientists had been unable to determine the precise location of this hot gas disk before Risaliti’s experiment.

Risaliti—who is also a visiting astrophysicist at the CFA—and his colleagues decided to delve deeper into the periodic eclipses of the NGC 1365 quasar’s emissions. Such eclipses are caused by clouds of gas that drift around the black hole, occasionally blocking X-ray radiation from reaching the earth.

A series of six observations of NGC 1365, made every two days over a period of two weeks in April 2006, happened to coincide with one such eclipse.

“In two days it went from being shining bright to being very dark, then back to being bright again,” Elvis said.

The eclipse had occurred for a much shorter time span than expected, compelling Risaliti and his colleagues to reach an unexpected conclusion.

“Originally the idea was that these gas clouds were a long way out, and so would take a long time to cover up the X-ray source,” Elvis said. “Now it’s so close, it’s almost ridiculous.”

Their observations confirmed one structural model for quasars. The cloud that had eclipsed the X-ray source was found to be a mere hundredth of a light year from the black hole’s event horizon, or the point at which material is permanently consumed by the black hole.

Since these clouds surround the emission source, it is now known to be extremely close to the black hole as well.

“People have been trying to understand quasars and active nuclei for many years now,” said Giuseppina Fabbiano, an associate at the Harvard Observatory and another co-author of the study. “This is the first time that we can say with real certainty that there is emission that comes from very near [the black hole].”