First evidence of jet precession in a galactic black hole

Black holes, those enigmatic entities of the universe, possess characteristics that seem straight out of science fiction. For instance, stellar-mass black holes, weighing about 10 times our Sun, betray their presence by devouring materials from neighboring stars. Meanwhile, supermassive black holes congregate at the centers of galaxies, forming luminous compact regions known as quasars, with masses millions to billions of times that of our Sun. A specific subset of these stellar-mass black holes, known as microquasars, exhibit a fascinating trait: they can emit jets of magnetized plasma.

A team of international scientists, including astrophysicist Bing Zhang from UNLV, embarked on a dedicated observational campaign focused on a galactic microquasar named GRS 1915+105. In their study published in Nature, they uncovered previously unseen features of this microquasar system.

The breakthrough came using China’s colossal Five-hundred-meter Aperture Spherical radio Telescope (FAST), where astronomers detected a quasi-periodic oscillation (QPO) signal in the radio band, a first for any microquasar system. QPOs are essential for understanding how stellar systems, like black holes, operate. While astronomers have previously observed QPOs in X-rays from microquasars, this new revelation of QPOs in the radio emission of the system is entirely unique.

The QPO signal displayed a rough period of 0.2 seconds, equivalent to a frequency of about 5 Hertz. Interestingly, this signal is not a constant presence and only manifests under specific physical conditions. The team was fortunate enough to capture this signal twice, once in January 2021 and the other in June 2022.

UNLV’s Bing Zhang suggested that this distinctive feature might offer the initial evidence of activity from a “jet” launched by a galactic stellar-mass black hole. In some cases, black hole binary systems generate a jet composed of charged matter and a magnetic field, racing close to the speed of light.

Zhang explained that, typically, X-rays probe the accretion disk around the black hole, while radio emission investigates the jet launched from the disk and the black hole. The mechanism responsible for inducing temporal modulation in a relativistic jet remains unidentified, though one plausible explanation could be the precession of the jet’s direction, meaning it regularly points towards different directions and returns to its original orientation every approximately 0.2 seconds.

Such an effect could be caused by a misalignment between the spin axis of the black hole and its accretion disk, resulting from the dragging of spacetime near a rapidly spinning black hole.

Although the study provides significant insights, Zhang emphasizes that various other possibilities exist. Continued observations of this and other galactic microquasars are essential to unravel the mysteries behind these enigmatic QPO signals.

Source: University of Nevada, Las Vegas

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