New Insights from the Global VLBI Alliance: Edge-Brightening in AGN Jets is More common Than Previously Thought!

Jongho Park

A fraction of Active Galactic Nuclei (AGN) hosts relativistic jets that can extend to kiloparsecs and megaparsecs from the central supermassive black holes. Interestingly, jets observed in certain nearby radio galaxies, such as M87 (Walker et al. 2018), 3C 84 (Giovanninni et al. 2018), and Centaurus A (Janssen et al. 2021), exhibit an edge-brightened morphology. This is characterized by brighter jet edges compared to the jet axis region in their transverse intensity profiles. Conversely, jets observed in most other AGN, usually in distant blazars, exhibit a center-brightened morphology.

The reason why only certain AGN jets exhibit an edge-brightened morphology, while most others do not, remains unclear. One popular explanation for this morphological dichotomy is the "spine-sheath" or "structured" jet model (Sol et al. 1989; Ghisellini et al. 2005). This model predicts that the jet's outer edges move substantially slower than the inner on-axis region due to interactions with the surrounding slower medium. As a result, a differential Doppler boosting effect is expected: the central axis emission will be significantly beamed into a narrow angle relative to the jet axis due to its ultra-relativistic speed. This interpretation aligns with the observation that jets in nearby radio galaxies, viewed at large angles, are edge-brightened, while those in blazars, viewed at small angles, are center-brightened.

We conducted a new observation of the giant radio galaxy NGC 315 using a global VLBI array organized by the Global VLBI Alliance (GVA) at 22 GHz. A total of 22 stations across five continents participated in the observation, including the EVN, VLBA, phased VLA, Korean VLBI Network (KVN), and the Long Baseline Array (LBA). The resulting (u,v)-coverage (Figure 1, right, blue) is significantly improved compared to previous centimeter-VLBI observations at the same frequency (Figure 1, right, magenta).

Figure 1. Left: Image of the parsec-scale jet in NGC 315 observed with the VLBA at 22 GHz from our previous study (Park et al. 2021). The jet exhibits a center-brightened morphology. Center: Image obtained with the global VLBI array at 22 GHz from our recent study (Park et al. 2024). The jet now shows a distinct edge-brightened morphology, which could not be resolved in the previous VLBA-only observation due to limited angular resolution. Right: Comparison of the (u,v)-coverage between the VLBA (magenta) and the global VLBI observation (blue). The (u,v)-coverage has been significantly enhanced due to the large number of stations that participated in the GVA observation.

In Figure 1, we compare the pc-scale jet image of NGC 315 observed with the VLBA at 22 GHz (left) and with the global VLBI array at the same frequency (center). Previously known for its center-brightened morphology like many distant blazars, the new GVA image reveals a clear edge-brightened morphology. This finding indicates that the jet is intrinsically edge-brightened, a feature unresolved in earlier centimeter-VLBI observations due to limited angular resolution. Our study suggests that blazar jets, long known for their center-brightened morphology, may actually be intrinsically edge-brightened. We plan to conduct dedicated studies with new GVA and Event Horizon Telescope (EHT) observations in the near future to test this hypothesis.

We investigated whether the observed edge-brightened morphology can be explained by the spine-sheath jet model. We constrained the speed of the central spine emission, if present, with upper limits on the bulk Lorentz factors ranging from 6.0 to 12.9 at this scale. Achieving such fast speeds at a short distance from the black hole is challenging according to General Relativistic Magnetohydrodynamic (GRMHD) simulations (Nakamura et al. 2018), though we cannot entirely rule out this model based on our data alone.

Another explanation involves models with intrinsically higher synchrotron emissivity at the edges than at the axis. Recent GRMHD simulations suggest that magnetic reconnection frequently occurs in the magnetically arrested disk near the black hole. This creates numerous MeV energy photons, quickly converted into e+e- pairs through interactions. These pairs can efficiently load onto the jet edges along magnetic field lines anchored to the equatorial plane of the event horizon (Ripperda et al. 2022, Kimura et al. 2022). This suggests that synchrotron-emitting particles are preferentially located at the jet edges from the jet's inception.

Furthermore, recent GRMHD simulations have shown that distributed particle acceleration may occur at the jet edges over long distances, due to magnetic reconnection and/or velocity shear at the jet boundary (Fromm et al. 2022). Thus, the jet edges can naturally exhibit higher synchrotron emissivity than the on-axis region.

Our study demonstrates that global VLBI observations by the Global VLBI Alliance are powerful tools for uncovering the true nature of supermassive black holes and their relativistic jets. It also implies that edge-brightening in AGN jets may be more common than previously thought, necessitating a reevaluation and deeper investigation of the simple spine-sheath jet model in the future.

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More information: The Astrophysical Journal Letters, Volume 973, Issue 2, id.L45, 18 pp.

Contact: Jongho Park, jparkastro@khu.ac.kr