arXiv subjects

Christian M. Fromm

Publications and source records attributed to Christian M. Fromm.

At least 19 recordsLinked to original sources

arXiv2026

Dynamics and Spectra-Polarimetric Signatures of GRMHD Simulations with Multiple Magnetic Loops

Relativistic jets are a common outcome of accretion onto black holes, yet their presence and variability depend strongly on the magnetic and dynamical state of the accretion flow. While some systems, such as Blazars and Quasars, launch powerful persistent jets, others, including the Galactic Centre black hole Sgr~A$^\star$, show only weak or transient outflows. The physical conditions leading to the onset or suppression of jet activity remain poorly understood. We investigate the accretion flow conditions that produce transient jets or inhibit jet formation, aiming to improve our understanding of black holes that accrete without strong, steady outflows. We further predict observational signatures in total and polarized emission for comparison with recent observations of Sgr~A$^\star$ in the quiescent state from the radio to the $\gamma$-ray regime. We perform three-dimensional GRMHD simulations of an accreting black hole surrounded by a torus threaded by a poloidal multi-loop magnetic field of alternating polarities. We follow the evolution of the accretion rate, magnetic flux, and jet power, and analyze angular momentum transport. In addition, radiative transfer calculations including Compton scattering are used to derive synthetic total and polarized emission. The simulations show strong variability in jet power while the initial magnetic polarity loops accrete, followed by weaker activity at later times as the system approaches a semi-MAD state. The emission from the disk is relatively stable, weakly polarized and consistent with the quiet state values reported for SgrA$^\star$. The resulting jet emission is strongly suppressed, depolarized by Faraday rotation and conversion in the surrounding cold plasma. Upscattering calculations yield near-infrared (NIR) high energy light curves that respect observational constraints of the quiescent NIR and X-ray fluxes in SgrA$^\star$

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arXiv2026

Dynamics and geometry of the inner sub-parsec-scale jet in 3C 279 observed with the Event Horizon Telescope

The 2021 Event Horizon Telescope observations resolve the innermost jet region of the blazar 3C279 with unprecedented detail. The reconstructed images consistently reveal a compact core elongated nearly orthogonal to the large-scale jet axis. This rarely observed morphology recurs across multiple epochs and from 22-230 GHz and is therefore intrinsic rather than an imaging artifact. Geometric model fitting identifies several components with apparent speeds up to 10c, requiring bulk Lorentz factors greater than 10.3 and constraining viewing angles to extremely small values (smaller than one degree). Rest-frame brightness temperatures are systematically low (between 10^9 and 10^10 K), consistent with optically thin emission at 230 GHz. These results suggest that the jet bends toward the observer on sub-parsec scales, producing strong relativistic beaming. Possible drivers of the observed jet bending and temporal evolution include the jet's interaction with the interstellar medium, kink or Kelvin--Helmholtz instabilities, magnetic reconnection near the horizon, or binary-induced precession. However, the current temporal coverage of VLBI data remains insufficient to distinguish between these mechanisms. Continued multifrequency VLBI monitoring will be essential to constraining the dynamics and geometry of the jet base in 3C279.

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arXiv2026

Probing radiation micro-physics in M 87 I. Total intensity and broad-band spectra

Next generation Very Long Baseline Interferometers (VLBI) will provide dense sampling of the Fourier space together with high signal to noise ratios allowing to reliably observe and image faint jet structure in M 87 at mm-wavelength. The proposed next generation Event Horizon Telescope (ngEHT) and next generation Very Large Array (ngVLA) offers the unique capability to simultaneously resolve and image the accretion flow around the supermassive black hole in M 87 together with the jet launching and acceleration zone. In order to explore these capabilities and to provide theoretical expectations we perform general relativistic magnetohydrodynamic simulations of accretion on to black holes and jet launching. M 87 has been the target for multiple observations across the entire electromagnetic spectrum. Among these VLBI observations provide unique capability to resolve the jet structure down to several gravitational radii. In this work we provide possible observable signatures which will allow us to distinguish between different electron heating models and particle distributions. We use general relativistic magnetohydrodynamics and simulate the accretion of the magnetised plasma onto Kerr-black holes in 3D. The multi-frequency radiative signatures of these simulations are computed taking different electron heating and distribution functions into account. The results of our simulations show that with a dynamical range of $1\times 10^4$ and a frequency range from 86 GHz to 345 GHz observations with future VLBI arrays have the potential to tell turbulent and magnetic reconnection electron heating and the electron distribution function apart.

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arXiv2026

Exploring the physics behind the observed magnetic filaments in large scale radio galaxies

Recent low-frequency MeerKAT observations of radio galaxies have revealed an unexpected population of thin, highly collimated synchrotron threads, whose numbers continue to grow with increasing survey depth and sensitivity. These intricate structures display a remarkable diversity of morphologies -- appearing as narrow filaments linking jets and lobes, as well as ring- or ribbon-like features embedded within the jets and radio lobes. Despite their ubiquity, the physical origin and stability of these collimated synchrotron threads remain poorly understood. Proposed mechanisms include shock compression and interactions with the magneto-ionic intracluster medium, magnetic flux tube formation, and reconnection-driven magnetic filaments. In this work, we investigate the formation and evolution of such magnetic filaments using three-dimensional, two-temperature general relativistic magnetohydrodynamic (GRMHD) simulations of realistically launched jets from supermassive black holes. From these simulations, we compute synthetic synchrotron emission maps and polarisation signatures, allowing us to predict the observable characteristics including morphology, brightness profiles, and polarisation patterns. Finally, we assess the detectability and diagnostic potential of these signatures with the Square Kilometre Array Observatory (SKAO), outlining how upcoming SKA observations can distinguish between competing physical models and illuminate the magnetic origin of the collimated synchrotron threads revealed by MeerKAT observations.

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arXiv2026

Broadband multiwavelength properties of the archetypal blazar 3C 279 during the 2017 Event Horizon Telescope campaign

The archetypal blazar 3C 279 hosts a prominent relativistic jet and exhibits strong broadband variability across the electromagnetic spectrum. In April 2017, the Event Horizon Telescope (EHT) observed 3C 279, alongside one of the most extensive quasi-simultaneous multiwavelength (MWL) campaigns ever conducted. With the aim of investigating the physical processes governing 3C 279, we analyzed individual observations and multiband light curves, and constructed a new quasi-simultaneous MWL spectrum. We also performed phenomenological modeling using the turbulent extreme multi-zone (TEMZ) model to constrain the fundamental physical properties of the source. The EHT observations reveal a clear flux increase in the innermost core between April 5 and 11, 2017. Over a broader timescale, radio measurements at longer wavelengths show concurrent enhancements in core flux and polarization around mid-April, coinciding with the ejection of a superluminal knot. Record UV-optical flares with strong polarization variability occurred in late March, followed by gamma-ray activity that declined before the end of the EHT observing period. During this interval, the source remained in a low X-ray state and showed no detectable VHE emission. The TEMZ modeling suggests that the broadband spectrum and variability of 3C 279 can be explained within a jet scenario in which turbulent plasma cells are compressed by a stationary conical shock. However, alternative interpretations, such as magnetic reconnection or a moving shock-in-jet event, remain plausible. This coordinated MWL campaign advances our understanding of the origin of jet and gamma-ray emission in 3C 279, while also providing a comprehensive publicly available dataset that will serve as a valuable reference for future studies.

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arXiv2026

From Morphology to Variability: Radiative Cooling Effects on Horizon-Scale Polarization in Two-Temperature GRMHD Simulations

Polarization signatures provide a new window to investigate the effects of radiative cooling in the horizon-scale accretion flows. Morphology and variability of polarization offer quantifiable diagnostics of how cooling modifies the polarised emission from two-temperature GRMHD simulations. We find that cooling enhances the effective Faraday depth, leading to stronger large-scale Faraday scrambling, particularly at higher accretion rates. In contrast, depolarization associated with higher-order photons is comparable between cooling and non-cooling models. Radiative cooling also increases the intrinsic asymmetry in both the ring structure and the polarization pattern. This effect is quantified by enhanced power in non-axisymmetric azimuthal modes ($β_m$, $m \neq 2$) relative to the dominant quadrupolar component $β_2$. The increased asymmetry is directly linked to stronger temporal variability of the polarization angle $\angleβ_2$, including frequent sign reversals that are absent in non-cooling models. The radial profile of $\angle β_2$ further localizes the physical origin of these effects, distinguishing regions dominated by Faraday rotation from those influenced by photon ring contributions, and providing a clear separation between cooling and non-cooling cases. Additional tests including a non-thermal electron population indicate that the polarization structure at 230 GHz is largely insensitive to the detailed form of the electron distribution functions. Our results demonstrate that horizon-scale polarization asymmetry, variability, and radial structure encode robust signatures of radiative cooling. These findings highlight the diagnostic power of time-resolved polarimetry and high-resolution imaging for constraining radiative processes in black hole accretion flows with EHT-like observations.

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arXiv2026

Impacts of radiative cooling on the images of a black hole shadow and extended jets in two-temperature GRMHD simulations

The recent 230 GHz observations from the Event Horizon Telescope collaboration have successfully imaged the supermassive black hole shadow of the M87 galaxy. However, the relatively high radiative efficiency observed in the hot accretion flow suggests that radiative cooling is non-negligible and should be considered when calculating the electron temperature. In this study, we compare accretion models without and with radiative cooling across a range of mass accretion rates, $\dot{M}_{\mathrm{BH}} = (1.0 - 10) \times 10^{-6}\,\dot{M}_{\mathrm{Edd}}$, aiming to assess the impact of cooling on the disk structure, electron temperature distribution (eDF), black hole shadow morphology, broadband spectral energy distributions (SEDs), and flux variability. We performed general relativistic radiative transfer (GRRT) calculations on two-temperature, radiative, general relativistic magnetohydrodynamic (GRMHD) simulations, employing different electron heating prescriptions and nonthermal eDFs, analyzing the radiation transfer due to synchrotron emission at 230 GHz with inclination angle of $163^\circ$. These simulations are targeted toward M87$^{*}$. By comparing density profiles, eDFs, GRRT images, SEDs, and time variability between models, we find that the radiative cooling sharply decreases the electron temperature in the dense inner disk around the equatorial plane ($r\lesssim 10\,r_\mathrm{g}$), while slightly reducing jet sheath temperature. Cooling leads to a dimmer disk, more extended and brighter jets, and reduced total flux. For a given accretion rate, cooling reduces the high-frequency flux. Time variability originates primarily from the midplane in both non-cooling and cooling cases and decreases as accretion rates rise. Although currently below the dynamic range of EHT observations, the features identified in this study could be resolved by next-generation arrays such as the ngEHT.

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arXiv2026

Learning Neural Operator Surrogates for the Black Hole Accretion Code

General-relativistic magnetohydrodynamic (GR-MHD) simulations are essential for studying black hole accretion, relativistic jets, and magnetic reconnection, yet their computational cost severely limits systematic parameter exploration. We investigate neural operator surrogates for two astrophysically relevant simulation scenarios produced by the Black Hole Accretion Code (\texttt{BHAC}). First, a Physics Informed Fourier Neural Operator (PINO) is trained on the special-relativistic resistive MHD (SRRMHD) evolution of the Orszag-Tang vortex over a range of resistivities spanning the Sweet-Parker and fast reconnection regimes. By embedding the governing equations as an additional loss term evaluated at finer temporal resolution than the available data supervision, the model learns dynamics at time steps where no simulation data is provided, enabling recovery of plasmoid formation that a data-only baseline trained on the same sparse snapshots fails to reproduce. To our knowledge, the present work is the first application of a physics informed neural operator to special relativistic resistive MHD, and the first to investigate the capability of such models to resolve plasmoid formation in SRRMHD. In a second line of investigation, an OFormer-style Transformer Neural Operator is trained on the evolution of spine-sheath relativistic jets created with \texttt{BHAC}, in special-relativistic MHD (SRMHD). The model is directly applied on the adaptive mesh, highlighting the need for linear attention due to long sequences. The neural surrogate model is capable of capturing most of the major details, especially in early predictions. To our knowledge, this constitutes the first application of a neural operator directly on a high resolution adaptive mesh refinement grid in the context of MHD simulations.

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arXiv2026

Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign

Sagittarius A* (Srg A*), the supermassive black hole at the center of the Milky Way, provides a unique laboratory to study accretion dynamics and plasma processes near the event horizon. We investigated the variability and polarization properties of Srg A* using ALMA observations during the 2018 Event Horizon Telescope campaign. We analyzed high-cadence full-polarization light curves from ALMA at millimeter wavelengths, performed time-series analysis, and investigated the temporal behavior during an X-ray flare observed by Chandra on 2018 April 24. The variability characteristics are compared with expectations from standard accretion flow models. We find low variability in total intensity ($σ/μ< 10\%$), but significantly higher variability in linear and circular polarization (~ 30% and ~ 50%, respectively). A time-series analysis reveals red-noise variability, with power spectral densities between -2 and -3 across all Stokes parameters. Polarized intensity shows stable intra-day timescales, while total intensity exhibits more variable timescales, suggesting distinct emission regions, with polarization likely arising from a coherent structure. On April 24, a statistically significant inter-band delay in polarized intensity coincides with a near-simultaneous X-ray and millimeter peak that deviates from the typical delayed flare scenario. This event also features enhanced millimeter variability and coherent polarization loop evolution. The observed simultaneity challenges standard models of transient synchrotron emission with cooling delays, favoring instead a scenario of continuous energy injection in an optically thin region. Our results offer new constraints on the physical mechanisms driving variability in Srg A*, and provide key observational input for refining theoretical models of accretion and plasma behavior in the vicinity of supermassive black holes.

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arXiv2026

Ring Asymmetry and Spin in M87*

Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $a_{*}$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $|a_{*}| \lesssim 0.2$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

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arXiv2026

Probing anisotropic particle acceleration and limb-brightening in Centaurus A's jet

Relativistic jets are among the most fascinating objects in the Universe, and recent high-resolution Very Long Baseline Interferometric (VLBI) observations, including the Global mm-VLBI Array and the Event Horizon Telescope (EHT), are able to resolve their structure close to their launching site. These observations reveal strongly limb-brightened jet structures for Centaurus A (Cen A), M 87 and 3C 84. Thus, the question arises which physical mechanism can generate the limb-brightened structure, and if this structure is common for jets from low-luminosity active galactic nuclei (LLAGN) seen under large viewing angles. Therefore, as a pilot study, we aim to model the EHT observations of Cen A. We performed a 3D two-temperature general-relativistic magnetohydrodynamic (GRMHD) simulation of an accreting supermassive black hole (SMBH) and jet launching to study the plasma dynamics and computed the connected emission via general relativistic radiative transfer (GRRT) calculations considering possible anisotropies in the distribution of the radiating particles. In order to adjust our simulations to the EHT observations of Cen A, we carried out a Bayesian fitting in the Fourier plane. We find that GRMHD simulations of magnetically arrested disks (MADs) combined with anisotropically emitting particle distributions along the direction of the magnetic field, parametrized by a value $η=0.07$, are able to mimic the recent EHT observations of Cen A. In addition, we extracted a black hole mass of $M_\mathrm{BH} = 6\times10^7 M_\odot$ and a viewing angle of $\vartheta=72°$. Our obtained model can reproduce key features of the EHT and Atacama Large Millimeter/submillimeter Array (ALMA) observations in total and polarized emission. Finally, we predict that the black hole shadow in Cen A will be observable at a frequency of $\sim$ 3 THz.

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arXiv2026

The magnetic filling in magnetically arrested accretion disk simulations and its impact on the jet in M87

Magnetically arrested accretion disks (MADs) in black hole jet launching simulations are very successful in modelling low-luminosity active galactic nuclei (AGN) like M87*. The Fishbone-Moncrief torus is well established for this purpose in numerical astrophysics. The extent of the magnetic vector potential inside the torus that we coin the filling factor has not been studied before in the case of MAD simulations. We employ five 3D general relativistic magneto-hydrodynamics (GRMHD) simulations initialized with large-scale tori, that are immersed in weak, poloidal magnetic fields. To study the impact of the spatial extent of the initial magnetic field, hence the magnetic energy content in the torus, we scale it with the filling factor w.r.t. the poloidal geometric area of the mass density distribution. A common choice of the filling factor is complimented and investigated in terms of altered energetics and angular momentum transport. Further, we investigate the polarized, radiative imprints of synchrotron emission on M87 at 86 GHz, comparing them with VLBI observations. Our simulations show that elevated filling factors significantly increase the electromagnetic energy contributions and outward angular momentum transport in the jet, due to the initially increased magnetic energy-content in the torus. High magnetic fillings exhibit increased linear polarization fractions, agreeing with the observed 15$\%$ in M87*. We find the jet morphology more prone to disk-vertical flux tubes generated by MAD events. We show, that GRMHD simulations bracket the jet width measurements at the jet base in M87*. Increased magnetic filling of the torus produces jets that are noticeably brighter downstream compared to our reference models, hence, we find high fillings well suited for extended GRMHD jet models of other low-luminosity AGN, as well.

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arXiv2026

Electromagnetic Signatures of Supermassive Binary Black Holes: Synchrotron, Self-Lensing Flares, and Jet Precession

The recent evidence for a nanohertz gravitational wave background from Pulsar Timing Arrays highlights the urgent need to identify electromagnetic counterparts to supermassive binary black holes. Here, we perform global 3D general relativistic magnetohydrodynamic (GRMHD) simulations of a secondary black hole (mass ratio $q=0.1$) interacting with a Magnetically Arrested Disk around a primary black hole using a time-dependent superposed Kerr-Schild metric and post-processed general relativistic radiation transfer calculations based on thermal electron distribution function (eDF). We explore three orbital configurations: a vertical impact orbit, a coplanar embedded orbit, and a high-spin, eccentric, inclined scenario. Despite clear orbital periodicity and recurrent shock formation, the thermal synchrotron light curves frequently lack expected shock-induced flares. In vertical impacts, shock brightenings are typically sub-dominant to the stochastic MAD variability of the primary black hole, unless viewed at specific alignment phases. Conversely, coplanar orbits produce distinctive, rapid flares driven by gravitational self-lensing. We identify a frequency-dependent emission hierarchy: the primary black hole dominates sub-millimeter flux, while the secondary dominates near-infrared emission due to higher electron temperatures in thermal eDF. Finally, spin-orbit coupling drives Lense-Thirring precession, yielding twisted, wobbling jets that following the tilt and precession of the primary BH. Crucially, we show that intrinsic MAD turbulence can easily mask shock-induced radio flares, making self-lensing flares a more reliable electromagnetic counterpart candidate for supermassive binary black holes.

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arXiv2026

Locating the missing large-scale emission in the jet of M87* with short EHT baselines

In Very-Long Baseline Interferometric arrays, nearly co-located stations probe the largest scales and typically cannot resolve the observed source. In the absence of large-scale structure, closure phases constructed with these stations are zero and, since they are independent of station-based errors, they can be used to probe data issues. Here, we show with an expansion about co-located stations, how these trivial closure phases become non-zero with brightness distribution on smaller scales than their short baseline would suggest. When applied to sources that are made up of a bright compact and large-scale diffuse component, the trivial closure phases directly measure the centroid relative to the compact source and higher-order image moments. We present a technique to measure these image moments with minimal model assumptions and validate it on synthetic Event Horizon Telescope (EHT) data. We then apply this technique to 2017 and 2018 EHT observations of M87* and find a weak preference for extended emission in the direction of the large-scale jet. We also apply it to 2021 EHT data and measure the source centroid about 1 mas northwest of the compact ring, consistent with the jet observed at lower frequencies.

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arXiv2025

Supermassive black-hole imaging with a self-consistent electron-temperature prescription

The recent 230 GHz observations by the Event Horizon Telescope have resolved the innermost structure of the M87 galaxy, revealing a ring-like feature consistent with thermal synchrotron emission from a magnetized torus surrounding a rotating supermassive black hole. Moreover, Global Millimeter VLBI Array observations at 86 GHz have revealed a larger-scale, edge-brightened jet with clear signatures of non-thermal emission. The theoretical modelling of these observations involves advanced general-relativistic magnetohydrodynamic simulations of magnetized accretion disks around rotating black holes, together with the associated synchrotron emission, which is normally treated with simplified expressions for the electron temperature and assuming a purely thermal distribution. However, an important non-thermal component is expected to be present, making the thermal-emission model not only an approximation, but also a source of degeneracy in the modelling. In view of this, we here present the first application of an ab-initio approach to the electron temperature derived from microscopic simulations of turbulent collisionless plasmas. The novel method, which has no tuneable coefficients and is fully specified by the thermodynamical and magnetic properties of the plasma, provides a better description of the jet morphology and width at 86 GHz, as well as of the broadband spectral emission. These findings highlight the importance of incorporating microscopic plasma physics in black-hole imaging and emphasise the crucial role of magnetic reconnection in electron heating and acceleration processes.

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arXiv2025

Relativistic reflection within an extended hot plasma geometry

The reflection of X-rays at the inner accretion disk around black holes imprints relativistically broadened features in the observed spectrum. Aside from the black hole properties and the ionization and density of the accretion disk, these features also depend on the location and geometry of the primary source of X-rays, often referred to as the corona. We present a fast general relativistic model for spectral fitting of a radially extended, ring-like corona above the accretion disk. A common approach used to explain observed X-ray reflection spectra is the lamp post geometry, which assumes a point-like source on the rotational axis of the black hole. While it is typically able to explain the observations, this geometric model does not allow for any constraint to be placed on the radial size of the corona. We therefore extended the publicly available relativistic reflection model relxill by implementing a radially extended, ring-like primary source. With the new RELXILL model allowing us to vary the position of the primary source in two dimensions, we present simulated line profiles and spectra and discuss the implications of carrying out a data fitting, in comparison to the lamp post model. We applied this extended RELXILL model to XMM-Newton and NuSTAR data of the radio-quiet Seyfert-2 active galactic nucleus (AGN) ESO 033-G002. The new model describes the data well and we are able to constrain the distance of the source to the black hole to be less than three gravitational radii, while the angular position of the source is poorly constrained. We show that a compact, radially extended corona close to the innermost stable circular orbit is able to explain the observed relativistic reflection as well as the lamp post corona does. This model has been made freely available to the community.

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arXiv2025

Probing the disk-jet coupling in M87

Context. Recent GMVA observations of M 87 at event horizon scales revealed a ring-like structure which is 50% larger at 86 GHz than the ring observed by the Event Horizon Telescope at 230 GHz. Aims. In this paper, we study a possible origin of the increased ring size at 86 GHz. We specifically aim to study the role the nonthermal electron population plays in the observed event horizon scales. Methods. We carry out 3D general relativistic magnetohydrodynamic simulations followed by radiative transfer calculations. We incorporate into the latter synchrotron emission from both thermal and nonthermal electrons. To better compare our results to observations, we generate synthetic interferometric data adjusted to the properties of the observing arrays. We fit geometrical models to this data in Fourier space through Bayesian analysis to monitor the variable ring size and width over the simulated time span of years. Results. We find that the 86 GHz ring is always larger than the 230 GHz ring, which can be explained by the increased synchrotron self-absorption at 86 GHz and the mixed emission from both the accretion disk and the jet footpoints, as well as flux arcs ejected from a magnetized disk. We find agreement with the observations, particularly within the error range of the observational value of M/D for M 87. Conclusions. We show that state-of-the art 3D GRMHD simulations combined with thermal and nonthermal emitting particles can explain the observed frequency-dependent ring size in M 87. Importantly we found that MAD events triggered in the accretion disk can significantly increase the lower frequency ring sizes.

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arXiv2025

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

We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the enhanced uv coverage in the 2021 Event Horizon Telescope (EHT) observations. The addition of the 12-m Kitt Peak Telescope and NOEMA provides two key intermediate-length baselines to SMT and the IRAM 30-m, giving sensitivity to emission structures at scales of $\sim250~μ$as and $\sim2500~μ$as (0.02 pc and 0.2 pc). Without these baselines, earlier EHT observations lacked the capability to constrain emission on large scales, where a "missing flux" of order $\sim1$ Jy is expected. To probe these scales, we analyzed closure phases, robust against station-based gain errors, and modeled the jet base emission using a simple Gaussian offset from the compact ring emission at separations $>100~μ$as. Our analysis reveals a Gaussian feature centered at ($Δ$RA $\approx320~μ$as, $Δ$Dec $\approx60~μ$as), a projected separation of $\approx5500$ AU, with a flux density of only $\sim60$ mJy, implying that most of the missing flux in previous studies must arise from larger scales. Brighter emission at these scales is ruled out, and the data do not favor more complex models. This component aligns with the inferred direction of the large-scale jet and is consistent with emission from the jet base. While our findings indicate detectable jet base emission at 230 GHz, coverage from only two intermediate baselines limits reconstruction of its morphology. We therefore treat the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate-baseline coverage will be essential to constrain the structure and nature of this component.

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