🔭 astrophysics

Multiband ALMA polarimetry of the jet base and kiloparsec-scale jet of 3C 273: A multicomponent Faraday structure in the nucleus

Using multiband ALMA polarimetry, this study reveals that the nucleus of quasar 3C 273 contains a time-variable, multicomponent Faraday screen with distinct high and low rotation measure components, while its kiloparsec-scale jet exhibits a complex magnetic field structure consistent with centimeter-wavelength observations.

Original authors: T. Hovatta, S. P. O'Sullivan, I. Marti-Vidal, T. Savolainen

Published 2026-07-09

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CC BY 4.0

Original authors: T. Hovatta, S. P. O'Sullivan, I. Marti-Vidal, T. Savolainen

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Cosmic Lighthouse

Imagine 3C 273 as a giant cosmic lighthouse powered by a supermassive black hole. This black hole is shooting out a massive, high-speed jet of superheated gas and magnetic fields that stretches for thousands of light-years.

Astronomers have studied this jet for decades, but they wanted to look at it with a new pair of "glasses." Instead of using radio waves (which are like long, lazy ocean waves), they used the Atacama Large Millimeter Array (ALMA) to look at the jet using millimeter waves. These are much shorter, like the ripples on a pond, allowing them to see finer details and understand the invisible magnetic fields that guide the jet.

Part 1: The Core (The Engine Room)

The most exciting discovery happened right at the heart of the lighthouse—the "nucleus" where the jet begins.

The Problem:
In previous studies, astronomers saw the light from the core getting twisted as it traveled through space. This twisting is called Faraday rotation. Imagine looking at a straight arrow through a thick, swirling fog; the arrow appears to rotate. The fog here is magnetized plasma (charged gas).

The New Discovery:
The team realized that the "fog" wasn't just one uniform layer. It was actually two different layers mixed together, acting like a complex filter.

Layer 1 (The Heavy Fog): This layer is incredibly dense and magnetic. It twists the light a huge amount (a "High Rotation Measure"). Think of this as a thick, turbulent storm cloud right next to the engine.
Layer 2 (The Light Mist): This layer is less dense and twists the light much less. Think of this as a lighter mist further out.

The "Chameleon" Effect:
The paper also found that this core is unstable. If you look at it one week, the "fog" looks one way; a week later, it looks different. It's like a chameleon changing colors rapidly. This tells us the environment right next to the black hole is chaotic and constantly shifting, which explains why previous single-frequency observations couldn't figure out what was going on. They needed to look at the object across a wide range of "colors" (frequencies) to see the full picture.

Part 2: The Jet (The Long Road)

Moving away from the core, the team looked at the jet itself, which stretches out for thousands of light-years (the "kiloparsec-scale" jet).

The Magnetic Highway:
They mapped the magnetic fields along this cosmic highway.

The Middle: In the center of the jet, the magnetic fields run straight along the direction of the flow, like lanes on a highway.
The Edges: As you get to the edges of the jet, the magnetic fields wrap around the sides, like the ribbing on a sock or the spiral of a seashell.
The Hotspot: At the very front of the jet, where it slams into the surrounding gas (creating a bright "hotspot"), the magnetic fields turn sideways, perpendicular to the flow. This is like a shockwave hitting a wall and bouncing back.

Why it matters:
The structure they saw at these high frequencies (millimeter waves) looked almost identical to what was seen years ago at lower frequencies (radio waves). This suggests that the same group of high-speed electrons is responsible for the light we see across the entire spectrum. It's like seeing the same car in a rearview mirror and a side mirror; the shape is the same, just the angle is different.

The Takeaway

This paper is like taking a high-definition, multi-spectral photo of a cosmic engine.

At the source: They found a complex, two-layered magnetic environment that is constantly changing, proving that the area right next to the black hole is a turbulent, magnetized mess.
Along the jet: They confirmed that the magnetic fields act like a structured highway, guiding the energy from the core out into the universe, with specific patterns at the edges and the front.

To truly separate these two layers at the core and see exactly where the "heavy fog" starts, the authors suggest we need even sharper eyes in the future, like the Event Horizon Telescope, which can zoom in closer than ever before.

Technical Summary: Multiband ALMA Polarimetry of 3C 273

Problem and Motivation
The archetypal quasar 3C 273 exhibits extreme Faraday rotation in its nucleus, previously detected by Hovatta et al. (2019, hereafter H19) within the ALMA Band 6 (1.3 mm) range. However, the limited wavelength coverage of that single-band study prevented a definitive distinction between two competing physical scenarios: (1) the presence of multiple polarized components, each undergoing distinct Faraday rotation, or (2) internal Faraday rotation where the jet rotates its own polarized emission. Furthermore, previous ALMA polarization calibration limitations restricted analysis to the inner third of the primary beam, leaving the kiloparsec-scale jet's polarization structure uncharacterized at millimeter wavelengths. This study addresses these gaps by utilizing multiband ALMA data to model the nuclear Faraday screen and to map the magnetic field evolution along the kiloparsec-scale jet.

Methodology
The authors utilized full-polarization ALMA observations from Cycle 6 (Project 2018.1.00357.S) conducted in November 2018 across three bands: Band 4 (~~145 GHz), Band 6 (~~230 GHz), and Band 7 (~345 GHz).

Observational Setup: Two separate pointings were executed in each band: one centered on the nucleus and another on the bright kiloparsec-scale hotspot H2 (~21.3" away).
Calibration: Standard calibration was performed using 3C 279 as the bandpass/polarization calibrator. Specific modifications were applied to Bands 6 and 7 to correct for unrealistic instrumental polarization (D-term) slopes and electric vector position angle (EVPA) jumps between spectral windows. This involved adopting an Event Horizon Telescope (EHT) approach of setting polarization models independently per spectral window and applying a self-bandpass calibration using the target source itself.
Nuclear Analysis: Stokes parameters for the unresolved core were extracted using UVMULTIFIT with Monte Carlo simulations to quantify uncertainties. The wavelength-dependent polarization was modeled using QU-fitting in the complex plane, testing four parametric models: a single Faraday-thin component (S), a single inverse-depolarizing component (Tinv), two Faraday-thin components (S2), and two external dispersive components (T2). Model selection was based on Bayesian evidence ( $\ln Z$ ) and reduced $\chi^2$ .
Jet Analysis: Total intensity and polarization maps were generated for the kiloparsec-scale jet. Spectral index maps were created by fitting total intensity against frequency across all 12 spectral windows. EVPA vectors were analyzed to determine magnetic field topology.

Key Results

Nuclear Polarization Structure:
- The nuclear data are best fit by the two-component external-dispersion model (T2), which is decisively favored over single-component or two-thin-component models based on Bayesian evidence ( $\Delta \ln Z \approx 140$ ).
- The best-fit solution reveals two distinct polarized components:
  - Component 1: A high Rotation Measure (RM) component with $RM_1 = +(2.58 \pm 0.08) \times 10^5$ rad m $^{-2}$ and a comparable Faraday dispersion ( $\sigma_{RM,1} \approx 2.36 \times 10^5$ rad m $^{-2}$ ).
  - Component 2: A moderate RM component with $RM_2 = +(1.48 \pm 0.18) \times 10^4$ rad m $^{-2}$ and significantly lower dispersion.
- Variability: Comparison with previous epochs (H19 and Goddi et al. 2021) reveals significant variability in the polarization angle and RM on timescales of weeks to months. The 2018 data show a nearly flat polarization angle across Band 6, contrasting with the steep rotation seen in 2016/2017.
Kiloparsec-Scale Jet Polarization:
- The millimeter-wavelength polarization structure closely resembles that observed at centimeter wavelengths.
- Magnetic Field Topology: The magnetic field is predominantly parallel to the jet direction in the outer regions. Near the brightest hotspot (H2), the field in the inner jet becomes perpendicular to the jet axis, while at the jet edges, the field aligns parallel to the edges. This circumferential structure suggests compression and shearing of a tangled magnetic field at the shock front.
- Spectral Index: The spectral index is generally steep ( $\alpha \approx -0.9$ ), steepening further at the hotspot, indicating severe synchrotron losses. No significant Faraday rotation was detected in the jet ( $RM \approx 0$ ), consistent with the high frequencies where Faraday effects are negligible.

Significance and Claims
The paper claims that the multiband coverage was essential to distinguish between Faraday rotation models, ruling out the single-component internal rotation scenario that could not be excluded with single-band data. The detection of a multicomponent, time-variable Faraday screen in the nucleus suggests a complex, dense, and magnetized environment close to the jet base.

The authors emphasize that while the high-RM component is likely associated with the jet base (within $\sim 10 r_g$ ), the current data cannot spatially resolve these components. Consequently, the paper concludes that future spatially resolved millimeter polarimetry, specifically with the Event Horizon Telescope (EHT), is crucial to disentangle these components and directly localize the high-RM emission region. The study also reinforces the connection between the nuclear polarization variability and the beamed emission from the parsec-scale jet, highlighting the dynamic nature of the jet launching region in 3C 273.

The Big Picture: A Cosmic Lighthouse

Part 1: The Core (The Engine Room)

Part 2: The Jet (The Long Road)

The Takeaway

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