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 significantly enhanced ( u,v ) coverage in the 2021 Event Horizon Telescope (EHT) observations. The integration of the 12−m Kitt Peak Telescope (USA) and NOEMA (France) stations into the array introduces two critical intermediate-length baselines to SMT (USA) and IRAM 30−m (Spain), providing sensitivity to emission structures at spatial scales of ∼250 μas and ∼2500 μas (∼ 0.02 pc and ∼ 0.02 pc). Without these new baselines, previous EHT observations of the source in 2017 and 2018 lacked the capability to constrain emission on large scales, where a “missing flux” of order ∼1 Jy is expected to reside. To probe these scales, we analyzed closure phases–robust against station-based gain calibration errors–and model the jet base emission using a simple Gaussian component offset from the compact ring emission at spatial separations > 100 μas. Our analysis revealed a Gaussian feature centered at (ΔRA ≈ 320 μas, ΔDec. ≈ 60 μ as), projected separation of ≈ 5500 AU, with an estimated flux density of only ∼60 mJy, implying that most of the missing flux identified in previous EHT studies had to originate from different, larger scales. Brighter emission at the relevant spatial scales is firmly ruled out, and the data do not favor more complex models. This component aligns with the inferred position of the large-scale jet and is therefore physically consistent with the emission of the jet base. While our findings point to detectable jet base emission at 230 GHz, the limited coverage provided by only two intermediate baselines limits our ability to robustly reconstruct its morphology. Consequently, we treated 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 with higher precision.

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Journal
Astronomy and Astrophysics
Published
2025-11-28
DOI
https://doi.org/10.1051/0004-6361/202557022
Citations
1
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Astrophysics and Cosmic Phenomena
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article
Field-Weighted Citation Impact
1.87
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Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations

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1 citations
Astronomy and Astrophysics
Astrophysics and Cosmic Phenomena
1.87
article

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

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article en
1 citations

Abstract

We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the significantly enhanced ( u,v ) coverage in the 2021 Event Horizon Telescope (EHT) observations. The integration of the 12−m Kitt Peak Telescope (USA) and NOEMA (France) stations into the array introduces two critical intermediate-length baselines to SMT (USA) and IRAM 30−m (Spain), providing sensitivity to emission structures at spatial scales of ∼250 μas and ∼2500 μas (∼ 0.02 pc and ∼ 0.02 pc). Without these new baselines, previous EHT observations of the source in 2017 and 2018 lacked the capability to constrain emission on large scales, where a “missing flux” of order ∼1 Jy is expected to reside. To probe these scales, we analyzed closure phases–robust against station-based gain calibration errors–and model the jet base emission using a simple Gaussian component offset from the compact ring emission at spatial separations > 100 μas. Our analysis revealed a Gaussian feature centered at (ΔRA ≈ 320 μas, ΔDec. ≈ 60 μ as), projected separation of ≈ 5500 AU, with an estimated flux density of only ∼60 mJy, implying that most of the missing flux identified in previous EHT studies had to originate from different, larger scales. Brighter emission at the relevant spatial scales is firmly ruled out, and the data do not favor more complex models. This component aligns with the inferred position of the large-scale jet and is therefore physically consistent with the emission of the jet base. While our findings point to detectable jet base emission at 230 GHz, the limited coverage provided by only two intermediate baselines limits our ability to robustly reconstruct its morphology. Consequently, we treated 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 with higher precision.

Astronomy and AstrophysicsVol. 706
University of Lethbridge (CA), Canadian Institute for Advanced Research (CA), Boston University (US), King University (US), Hiroshima University (JP), Goethe University Frankfurt (DE), Tokyo Electron (Japan) (JP), Tokyo Institute of Technology (JP), Kogakuin University (JP), University of Hawaiʻi at Mānoa (US), University of Science and Technology (YE), Leiden University (NL), California Institute of Technology (US), Las Cumbres Observatory Global Telescope Network (US), Google (United States) (US), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), Space Science Institute (US), Planetary Science Institute (US), Pontificia Universidad Católica de Valparaíso (CL), Georgia Institute of Technology (US), Consejo Nacional de Ciencia y Tecnología (GT), Institute of Astronomy and Astrophysics, Academia Sinica (TW), Fermi National Accelerator Laboratory (US), National Taiwan Normal University (TW), Harvard University (US), Joint Institute for Laboratory Astrophysics (US), University of Arizona (US), Seoul National University (KR), University of Trieste (IT), National Sun Yat-sen University (TW), Radboud University Nijmegen (NL), University of Waterloo (CA), University of Turku (FI), Korea Advanced Institute of Science and Technology (KR), University of Illinois Urbana-Champaign (US), Universitat de València (ES), Kavli Institute for the Physics and Mathematics of the Universe (JP), National Taiwan University (TW), University of New Mexico (US), University of Cagliari (IT), University of Concepción (CL), University of Southern Denmark (DK), Universidade de São Paulo (BR), Shanghai Jiao Tong University (CN), Turku Centre for Computer Science (FI), Perimeter Institute (CA), University of Toronto (CA), Joint Institute for Nuclear Astrophysics (US), University of the Witwatersrand (ZA), Yonsei University (KR), Chinese Academy of Sciences (CN), The Graduate University for Advanced Studies, SOKENDAI (JP), Princeton University (US), Tohoku University (JP), Peking University (CN), Trinity College (CA), Université Paris Cité (FR), Cornell University (US), Academy of Athens (GR), Heidelberg University (DE), University of Würzburg (DE), National Radio Astronomy Observatory (US), Université Paris Sciences et Lettres (FR), Atacama Large Millimeter Submillimeter Array (CL), Shanghai Astronomical Observatory (CN), Observatoire de Paris (FR), Czech Academy of Sciences, Astronomical Institute (CZ), Korea Astronomy and Space Science Institute (KR), Dutch Research Council (NL), Yunnan Observatories (CN), Frankfurt Institute for Advanced Studies (DE), Radboud University Medical Center (NL), Kyungpook National University (KR), Yale University (US), University of Malaya (MY), Nagoya City University (JP), Kyung Hee University (KR), Sorbonne Université (FR), University of Chicago (US), Central China Normal University (CN), Kavli Institute for Particle Astrophysics and Cosmology (US), National Institute of Technology, Ichinoseki College (JP), Institut de Radioastronomie Millimétrique (FR), Gemini North Observatory (US), Galaxies, Etoiles, Physique et Instrumentation (FR), Max Planck Institute for Astrophysics (DE), Istituto Nazionale di Fisica Nucleare, Sezione di Napoli (IT), Radboud Institute for Molecular Life Sciences (NL), Canadian Institute for Theoretical Astrophysics (CA), Delta Institute for Theoretical Physics (NL), Istituto di Radioastronomia di Bologna (IT), Inception Institute of Artificial Intelligence (AE), East Asian Observatory (US), Centre of Plasma Physics - Institute for Plasma Research (IN), Hachinohe National College of Technology (JP), Laboratoire d’études spatiales et d’instrumentation en astrophysique (FR), Gemini South Observatory (CL), Center for Astrophysics Harvard & Smithsonian (US), Northeast Radio Observatory Corporation (US), Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari (IT), Applied Mathematics (United States) (US), Astro-Med (United States) (US), The Institute of Statistical Mathematics (JP), National Center for Supercomputing Applications (US), Astronomical Institute of the Slovak Academy of Sciences (SK), Astronomy and Space (AU), Osservatorio Astronomico di Cagliari (IT), Istituto Nazionale di Fisica Nucleare, Sezione di Trieste (IT), Foundation for Common Land (GB), Instituto de Geofísica y Astronomía (CU), Laboratório Nacional de Astrofísica (BR), Max Planck Institute for Radio Astronomy (DE), Instituto de Astrofísica de Andalucía (ES), National Astronomical Observatory of Japan (JP), National Astronomical Observatories (CN), Applied Materials (Germany) (DE), Imaging Center (US), University of Wisconsin–Milwaukee (US), University College London (GB), The University of Texas at San Antonio (US), Huazhong University of Science and Technology (CN), Ulsan National Institute of Science and Technology (KR), McGill University (CA), Universitat de les Illes Balears (ES), Inter-University Centre for Astronomy and Astrophysics (IN), Massachusetts Institute of Technology (US), Chalmers University of Technology (SE), University of Maryland, College Park (US), Brandeis University (US), University of Pretoria (ZA), University of Naples Federico II (IT), Niigata University (JP), Rice University (US), The University of Tokyo (JP), Villanova University (US), Joint Institute for VLBI ERIC (NL), Heidelberg University (US), Michigan State University (US), Nanjing University (CN), University of Amsterdam (NL), Universidad Nacional Autónoma de México (MX), Netherlands Institute for Radio Astronomy (NL), Aalto University (FI)
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