Magnetic reconnection and plasmoid formation in three-dimensional accretion flows around black holes

Magnetic reconnection is thought to be one of the main energy-dissipation mechanisms fueling energy to the plasma in the vicinity of a black hole. Indeed, plasmoids formed through magnetic reconnection may play a key role in γ-ray, X-ray and near-infrared flares from the black hole at the centre of our galaxy, SgrA*. We report the results of 3D general-relativistic ideal and resistive magnetohydrodynamics simulations modelling magnetic reconnection in accretion flows around astrophysical black holes. As an important difference with similar works, our accretion discs have an initial dipolar magnetic-field configuration with loops of alternating polarity. We show that current sheets are formed and destroyed rapidly in the turbulent environment of black hole accretion. Plasmoids are formed from current sheets close to the event horizon, in a region of ~2-15 gravitational radii. We further quantify the magnetic dissipation and the process of energy transfer to the plasmoids, reporting the reconnection rate, the relative current density with respect to the local magnetic field, and the size of the plasmoids. We find that plasmoids gain energy through reconnection and heat up to relativistic temperatures, with the largest ones being sufficiently energetic to leave the black hole near the polar regions. During their evolution, plasmoids are stretched and elongated, becoming disrupted when the shear is sufficiently large, although some plasmoids survive as well-distinguished structures at distances of ~30-40 gravitational radii from the black hole. Finally, we find that in some cases the plasmoids acquire a super-Keplerian azimuthal velocity, as suggested by recent observations of flares from Sgr A*

A multi-band study and exploration of the radio wave-γ -ray connection in 3C 84

Funding Information: We thank the anonymous referee for the detailed comments, which improved this manuscript. G. F. Paraschos is supported for this research by the International Max-Planck Research School (IMPRS) for Astronomy and Astrophysics at the University of Bonn and Cologne. J.-Y. Kim acknowledges support from the National Research Foundation (NRF) of Korea (grant no. 2022R1C1C1005255). This work makes use of 37 GHz, and 230 and 345 GHz light curves kindly provided by the Aalto University Metsähovi Radio Observatory and the Submillimeter Array (SMA), respectively. The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. Maunakea, the location of the SMA, is a culturally important site for the indigenous Hawaiian people; we are privileged to study the cosmos from its summit. This research has made use of data from the University of Michigan Radio Astronomy Observatory which has been supported by the University of Michigan and by a series of grants from the National Science Foundation, most recently AST-0607523. This work makes use of the Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under licence. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has also made use of NASA’s Astrophysics Data System Bibliographic Services. This research has also made use of data from the OVRO 40-m monitoring program (Richards et al. 2011), supported by private funding from the California Institute of Technology and the Max Planck Institute for Radio Astronomy, and by NASA grants NNX08AW31G, NNX11A043G, and NNX14AQ89G and NSF grants AST-0808050 and AST-1109911. S.K. acknowledges support from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme under grant agreement No. 771282. Finally, this research made use of the following python packages: numpy (Harris et al. 2020), scipy (Virtanen et al. 2020), matplotlib (Hunter 2007), astropy (Astropy Collaboration 2013, 2018) and Uncertainties: a Python package for calculations with uncertainties. Publisher Copyright: © The Authors 2023.Total intensity variability light curves offer a unique insight into the ongoing debate about the launching mechanism of jets. For this work, we utilised the availability of radio and γ-ray light curves over a few decades of the radio source 3C 84 (NGC 1275). We calculated the multi-band time-lags between the flares identified in the light curves via discrete cross-correlation and Gaussian process regression. We find that the jet particle and magnetic field energy densities are in equipartition (kr-=-1.08-±-0.18). The jet apex is located z91.5-GHz-=-22-645-Rs (2- -20 ×10-3-pc) upstream of the 3 mm radio core; at that position, the magnetic field amplitude is Bcore91.5-GHz-=-3-10 G. Our results are in good agreement with earlier studies that utilised very-long-baseline interferometry. Furthermore, we investigated the temporal relation between the ejection of radio and γ-ray flares. Our results are in favour of the γ-ray emission being associated with the radio emission. We are able to tentatively connect the ejection of features identified at 43 and 86 GHz to prominent γ-ray flares. Finally, we computed the multiplicity parameter λ and the Michel magnetisation Ï M, and find that they are consistent with a jet launched by the Blandford & Znajek (1977, MNRAS, 179, 433) mechanism.Peer reviewe