Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations

Technical Summary: Probing Jet Base Emission of M87 with the 2021 Event Horizon Telescope Observations*

Problem Statement
Previous Event Horizon Telescope (EHT) observations of M87* in 2017 and 2018 successfully resolved the compact ring structure (the black hole shadow) at 230 GHz. However, these datasets suffered from a "missing flux" problem, where the total flux density measured on trivial (co-located) baselines (e.g., ALMA/APEX, JCMT/SMA) was approximately 1.0–1.4 Jy, significantly higher than the ~0.5–1.0 Jy recovered by compact ring-only models. This discrepancy suggested the presence of extended emission on large spatial scales, likely associated with the jet base. The 2017 and 2018 arrays lacked the necessary short and intermediate-length baselines to constrain or image this extended structure, leading to unresolved degeneracies in the reconstruction of the jet base.

Methodology
This study leverages the significantly enhanced $(u, v)$ coverage of the 2021 EHT observing campaign, specifically the integration of the 12-m Kitt Peak (KP) telescope (USA) and the NOEMA array (France). These new stations introduced two critical intermediate-length baselines:

1. PV-NOEMA: 1100 km, sensitive to spatial scales of ~250 $\mu$as (0.02 pc).
2. SMT-KP: 100 km, sensitive to spatial scales of ~2500 $\mu$as (0.2 pc).

Due to the sparsity of these intermediate baselines, direct imaging of the faint, extended jet base is not robust. Instead, the authors employ a geometric model-fitting approach using closure phases. Closure phases are robust against station-based gain calibration errors and are sensitive to asymmetries in the brightness distribution. The analysis focuses on closure triangles formed with the highly sensitive ALMA station (ALMA-PV-NOEMA and ALMA-SMT-KP).

The authors compare observed closure phases against models consisting of:

• A compact ring component (reconstructed via DoG-HIT and other algorithms).
• An additional, faint, offset Gaussian component representing the jet base emission.

The model parameters (position, flux density, and full width at half maximum) are optimized using Markov chain Monte Carlo (MCMC) sampling to minimize the reduced chi-squared ($\chi^2_\nu$) residuals between the model and the observed closure phases. Alternative models, including asymmetric Gaussians, double Gaussians, and GMVA-derived jet structures, are also tested.

Key Results

• Detection of Residuals: The compact ring-only models fail to fully reproduce the observed closure phases on the ALMA-PV-NOEMA and ALMA-SMT-KP triangles, exhibiting significant residuals (a "bump" and an "offset" of a few degrees).
• Gaussian Component Fit: Introducing a single, offset Gaussian component significantly improves the fit. The best-fit model places a faint Gaussian component at a projected separation of $\approx 5500$ AU (centered at $\Delta$R.A. $\approx 320$ $\mu$as, $\Delta$Dec. $\approx 60$ $\mu$as, relative to the ring center) with an estimated flux density of ~60 mJy and a size (FWHM) of ~180 $\mu$as.
• Statistical Improvement: The inclusion of this component reduces the $\chi^2_\nu$ for the ALMA-PV-NOEMA triangle from 2.73 to 1.29 and for the ALMA-SMT-KP triangle from 2.52 to 1.12. The global fit across all triangles improves from 1.50 to 1.39.
• Model Constraints: More complex models (asymmetric Gaussians or a second Gaussian component) do not provide a statistically significant improvement over the single Gaussian model. The data do not favor brighter emission at these scales; a significantly brighter component would produce residuals larger than observed.
• Physical Consistency: The location of the recovered Gaussian aligns with the southern/southwestern arm of the edge-brightened jet observed in 86 GHz GMVA images, consistent with the expected direction of the large-scale jet.
• Impact on Previous Data: The study confirms that this faint component would have been undetectable in 2017 and 2018 data due to the lack of intermediate baselines, validating the robustness of previous ring-only reconstructions for those epochs.

Significance and Claims
The paper claims to provide the first constraints at 230 GHz on faint, asymmetric emission at intermediate scales near M87*. The primary contribution is the identification of a plausible jet base component that accounts for a portion of the previously unexplained "missing flux."

However, the authors maintain a modest and conservative stance regarding the definitive nature of this detection:

1. Morphological Limitations: Due to the limited number of intermediate baselines (only two), the authors cannot robustly reconstruct the detailed morphology of the jet base. The recovered Gaussian is treated as an upper limit on the resolved jet base flux density at these specific spatial scales.
2. Systematic Uncertainties: While polarization leakage and other known systematics are ruled out as the source of the signal, the authors acknowledge that unknown systematic errors cannot be entirely excluded given the reliance on single-baseline/small-triangle measurements.
3. Future Requirements: The paper concludes that definitive confirmation and higher-precision constraints on the structure and nature of the jet base will require future EHT observations with expanded intermediate-baseline coverage (more stations) and potentially expanded frequency ranges.

In summary, the work demonstrates that the 2021 EHT data reveals evidence for a faint, southwest-offset emission component consistent with the jet base, but the current data density limits the ability to fully resolve its structure, necessitating future observations for a complete physical characterization.