Bucknell Professor Leads Discovery of Magnetic Loops Around Black Hole

Led by Bucknell University Professor Jack Gallimore, physics & astronomy, an international team of astronomers and students, including Bucknell physics student Virginia Hostetter '25, set out on a new way to study NGC 1068, a well-known, relatively nearby (45 to 50-million light years away), bright galaxy with a supermassive black hole at its center.

Their research had twin goals: astrometric mapping of the galaxy's radio continuum and measurements of polarization for its water masers — naturally occurring sources of microwave radio emissions. In radio astronomy, water masers observed at a frequency of 22 GHz are particularly useful because they can shine through much of the dust and gas that obscures optical wavelengths around the black hole.

Using the High Sensitivity Array (HSA) — a multi-facility network supported by the U.S. National Science Foundation National Radio Astronomy Observatory (NRAO) — the international team observed for the first time evidence of magnetic filaments in the accretion disk surrounding the galaxy's supermassive black hole. Their discovery was announced Wednesday by the NRAO and reported in a recent paper published by The Astrophysical Journal Letters.

By measuring the polarization of water masers and the continuum of radio emissions from NGC 1068, the team generated a map revealing the compact radio source now known as NGC 1068*, as well as mysterious extended structures of more faint emissions.

Mapping the astrometric distribution of NGC 1068 and its water masers revealed that they are spread along filaments of structure.

Professor Jack Gallimore, physics & astronomy. Photo courtesy of Jack Gallimore

"It really came out in these new observations that these filaments of maser spots line up like beads on a string," Gallimore said in the NRAO story. The team was stunned to see that there was a clear offset — a displacement angle — between the radio continuum showing the structures at the galaxy's core and the locations of the masers themselves. "The configuration is unstable, so we are probably observing the source of a magnetically-launched outflow," he said.

HSA measurements of the polarization of these water masers revealed striking evidence of magnetic fields.

"No one has ever seen polarization in water masers outside our galaxy," Gallimore said. Similar to the looping structures seen on our sun's surface as prominences, the polarization pattern of these water masers clearly indicates that magnetic fields are also at the root of these light-year-scale structures as well. "Looking at the filaments and seeing that the polarization vectors are perpendicular to them, that's the key to confirming that they are magnetically driven structures. It's exactly what you'd expect to see."

Previous studies of the region hinted at patterns usually associated with magnetic fields, but such conclusions remained beyond the reach of observing technology until recently. This discovery is the first to confirm the existing theory.

"Only the HSA has the combination of resolution and sensitivity needed to map out magnetic fields using polarized light," said Gallimore.

Their findings reveal evidence of a compact central radio source (the galaxy's supermassive black hole), clear polarization of the water masers indicating structure within NGC 1068's magnetic fields, and spectacular extended features across the continuum of radio frequencies. Together, these findings indicate that magnetic fields are the underlying drivers of these phenomena.

Plenty of mysteries remain, however. Within the radio continuum map, for instance, there is a diffuse, faint protrusion that the team nicknamed "the foxtail," which extends northward from the central region.

"We said to ourselves when we set out to do this, ‘let's see if we can really push the limits and get a good continuum as well as polarization data. And both of those goals succeeded," Gallimore said. "With the NSF NRAO High Sensitivity Array, we detected water megamaser polarization for the first time, and we also made a really amazing continuum map that we're still trying to wrap our minds around."

Special thanks to the NRAO and author Jessi Almstead for the use of their story.