A ring-like accretion structure together with jet and black hole shadow in M87 unveiled by GMVA+ALMA+GLT observations

Keiichi Asada^1*, Rusen Lu², Hung-Yi Pu^3,1, Satoki Matsushita¹, Ming-Tang Chen¹, GLT team¹, M87 GMVA collaboration¹

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
²Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China
³Department of Physics, National Taiwan Normal University, Taipei, Taiwan

* Presenter:Keiichi Asada, email:asada@asiaa.sinica.edu.tw

Imaging the shadow of the black hole has been one of the long-standing quests for astrophysics. In 2019, Event Horizon Telescope (EHT) Collaboration reported the first-ever image of the black hole shadow of M87 taken with a global VLBI array at 1.3 mm. Here we report our new findings based on the Global Millimeter Array (GMVA) observations together with ALMA and Greenland Telescope (GLT) at 3.5 mm towards M87. This is the first VLBI observation at 3.5 mm, which includes GLT as a part of a VLBI array. The observations were conducted on 14–15 April 2018 with 16 stations, and correlated in MPIfR. Thanks to the addition of the ALMA and GLT to the GMVA, we improved the angular resolution in the north–south direction by a factor of around 4 compare to the historical GMVA observations.

As a result, we resolved the nuclei into a ring-like structure even at 3.5 mm for the first time. The diameter of the ring is around 8.4 + 0.5 - 1.1 times Schwarzschild radii, which is 50 % larger than that at 1.3 mm. In addition, the outer edge of the ring is larger than that at 1.3 mm. The larger and thicker ring indicates a substantial contribution from the accretion flow in addition to the gravitational lensing effect. By careful comparison with the predictions by theory, we conclude that the ring-like structure we see at 3.5 mm are dominated by emission from the accretion flow, while central depression of the ring is associated with the black hole shadow. In the meantime, the image simultaneously detected the extended jet, which was not seen in previous EHT images as it was limited by the sensitivity. Based on collimation profile analysis of the jet streamline, we revealed that the width of the jet is wider than that is expected from the black-hole driven jet at the vicinity of the black hole, which indicates a presence of the wind from the accretion flow.

Keywords: black hole, accretion flow, relativistic jet, VLBI, Greenland Telescope