Astronomers move closer to solving Earth-pointing 'blazars'

A new study provides compelling evidence on the dynamics of active galactic nuclei (AGN), a class of objects known as 'blazars.'
Sade Agard
Blazar Artist Concept
Blazar Artist Concept


In the celestial tapestry of active galactic nuclei (AGN), a class of objects known as "blazars" has long captivated astronomers. Blazars make their cosmic debut when one of their emitted jets points directly towards the Earth.

For years, scientists believed that the frequent and significant surges in brightness in blazars, often referred to as flare activity, were intimately tied to the ejection of jet components from the core, resulting in enhanced emissions.

However, a recent study published in The Astrophysical Journal challenges this conventional wisdom.

The crucial role of precession

Traditionally, variations in brightness and their enigmatic, meandering jet structures were thought to result from the ejection of jet components from the core.

Yet, as researchers accumulated detailed observations over the years, a more complex and intricate picture emerged, casting doubt on this simplified explanation.

The recent paper introduces an alternative hypothesis. Silke Britzen, the lead author from the Max Planck Institute for Radio Astronomy in Bonn, Germany, proposes that the precession of the jet source might hold the key.

When these jets swirl due to precession, they introduce periodic changes in intensity, a phenomenon known as the Doppler effect.

Notably, this precession-driven variability has been observed in multiple AGN jets. In a prior study focused on OJ 287, a prime candidate for hosting a supermassive black hole binary, Britzen's team established that the variations in brightness and jet bending originate from precession.

The researchers extended their investigation to 12 prominent AGNs to validate their theory. Their findings provided compelling evidence that the observed brightness variations and jet curvature could indeed be attributed to precession.

While the researchers do not dismiss the role of internal interactions within the jet, they suggest that jet precession plays a pivotal role in modulating and altering the appearance of these cosmic jets.

It's this precession that endows them with their characteristic curvy and luminous demeanor.

One of the study's most profound implications is that jet curvature could serve as a crucial signature for the presence of binary black holes at the centers of galaxies.

The gravitational influence of a second black hole on the jet-emitting black hole compels the jet to take on its meandering path.

Furthermore, the research team detected traces of a smaller amplitude nutation motion in the radio light curves and jet component kinematics, providing additional support for the role of precession.

Like 'simple gyroscopes'

"Physics of accretion disks and jets is rather complex, but their bulk kinematics can be compared to simple gyroscopes," said co-author Michal Zajaček from the Masaryk University (Brno, Czech Republic) in a press release.

"If you exert an external torque on an accretion disk, for instance by an orbiting secondary black hole, it will precess and nutate, and along with it the jet as well, similar to the Earth's rotation axis that is affected by the Moon and the Sun."

To achieve this discovery, the researchers harnessed the power of radio observations, employing Very Long Baseline Radio Interferometry (VLBI) techniques.

This approach links radio telescopes across vast distances, offering unparalleled resolution in astronomical observations.

The same technique enabled the Event Horizon Telescope collaboration to capture the first-ever image of a black hole's shadow in 2019.

Despite the immense progress made, the quest to directly probe the existence of supermassive binary black holes continues. Currently, the resolution required for this monumental task remains just out of reach.

The complete study was published in The Astrophysical Journal and can be found here.

Study abstract:

The combined study of the flaring of active galactic nuclei (AGNs) at radio wavelengths and parsec-scale jet kinematics with Very Long Baseline Interferometry has led to the view that (i) the observed flares are associated with ejections of synchrotron blobs from the core, and (ii) most of the flaring follows a one-to-one correlation with the ejection of the component. Recent results have added to the mounting evidence showing that the quasi-regular component injections into the relativistic jet may not be the only cause of the flux variability. We propose that AGN flux variability and changes in jet morphology can both be of deterministic nature, i.e., having a geometric/kinetic origin linked to the time-variable Doppler beaming of the jet emission as its direction changes due to precession (and nutation). The physics of the underlying jet leads to shocks, instabilities, or ejections of plasmoids. The appearance (morphology, flux, etc.) of the jet can, however, be strongly affected and modulated by precession. We demonstrate this modulating power of precession for OJ 287. For the first time, we show that the spectral state of the spectral energy distribution (SED) can be directly related to the jet's precession phase. We model the SED evolution and reproduce the precession parameters. Further, we apply our precession model to 11 prominent AGNs. We show that for OJ 287 precession seems to dominate the long-term variability (≳1 yr) of the AGN flux, SED spectral state, and jet morphology, while stochastic processes affect the variability on short timescales (≲0.2 yr).