Mysterious QPO Signals Reveal New Features of Galactic Black Holes

An international team of scientists reports the first detection of a quasi-periodic oscillation signal in the radio band from a Galactic black hole system.

An international team of scientists observed unique characteristics of a Galactic microquasar, GRS 1915+105, previously unseen in such systems. Using a sophisticated telescope in China, they detected a quasi-periodic oscillation (QPO) signal in the radio band for the first time in microquasar systems. This discovery could offer the first evidence of a “jet” launched by a Galactic stellar-mass black hole. Further observations are needed to fully understand these intriguing signals.

Black holes are among the most mysterious objects in the universe, with features that sound like they come straight from a sci-fi movie.

Stellar-mass black holes with masses of roughly 10 suns, for example, reveal their existence by eating materials from their companion stars. And in some instances, supermassive black holes accumulate at the center of some galaxies to form bright compact regions known as quasars with masses equal to millions or even billions of our sun. A subset of accreting stellar-mass black holes that can launch jets of highly magnetized plasma are called microquasars.

An international team of scientists, including University of Nevada, Las Vegas (UNLV) astrophysicist Bing Zhang, reports in the July 26 issue of Nature a dedicated observational campaign on the Galactic microquasar dubbed GRS 1915+105. The team revealed features of a microquasar system that have never before been seen.

First Detection of Quasi-Periodic Oscillations in Radio Waves

Using the massive Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China, astronomers discovered a quasi-periodic oscillation (QPO) signal in the radio band for the first time from any microquasar systems. QPOs are a phenomenon that astronomers use to understand how stellar systems like black holes function. And while they have been observed in X-rays from microquasars, their presence outside of this manner — as part of the system’s radio emission — is unique.

“The peculiar QPO signal has a rough period of 0.2 seconds, or a frequency of about 5 Hertz,” said Wei Wang, a professor at China’s Wuhan University who led the team that made the discovery. “Such a signal does not always exist and only shows up under special physical conditions. Our team was lucky enough to catch the signal twice — in January 2021 and June 2022, respectively.”

According to UNLV’s Zhang, director of the Nevada Center for Astrophysics and one of the study’s corresponding authors, this unique feature may provide the first evidence of activity from a “jet” launched by a Galactic stellar-mass black hole. Under certain conditions, some black hole binary systems launch a jet — a mix of parallel beams of charged matter and a magnetic field that moves with a swiftness approaching the speed of light.

“In accreting black hole systems, X-rays usually probe the accretion disk around the black hole while radio emission usually probes the jet launched from the disk and the black hole,” said Zhang. “The detailed mechanism to induce temporal modulation in a relativistic jet is not identified, but one plausible mechanism would be that the jet is underlying precession, which means the jet direction is regularly pointing towards different directions and returns to the original direction once every about 0.2 seconds.”

Zhang said that a misalignment between the spin axis of the black hole and its accretion disk (extremely hot, bright spinning gasses surrounding the black hole) could cause this effect, which is a natural consequence of a dragging of spacetime near a rapidly spinning black hole.

“Other possibilities exist, though, and continued observations of this and other Galactic microquasar sources will bring more clues to understand these mysterious QPO signals,” said Zhang.

Reference: “Sub-second periodic radio oscillation in a microquasar” by Pengfu Tian, Ping Zhang, Wei Wang, Pei Wang, Xiaohui Sun, Jifeng Liu, Bing Zhang, Zigao Dai, Feng Yuan, Shuangnan Zhang, Qingzhong Liu, Peng Jiang, Xuefeng Wu, Zheng Zheng, Jiashi Chen, Di Li, Zonghong Zhu, Zhichen Pan, Hengqian Gan, Xiao Chen & Na Sai, 26 July 2023, Nature.
DOI: 10.1038/s41586-023-06336-6

The publication includes 21 co-authors from 13 institutions. Besides UNLV and Wuhan University, other institutions include National Astronomical Observatories of China (NAOC) and several other observatories and universities from China.

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