Blob Formation From The Accretion Flow Around The Black Hole Of SGR A*

In 2018,Very Large Telescope Interferometer(VLTI-Gravity)observed several near-infrared superflares which might be the hot spots at about 6 to 10 Gravitational radii from SGR A*.It is believed that these hot spots are likely due to the magnetic reconnection event.We study the small-scale magnetic reconnection above the radiative inefficient accretion flow around a massive black hole via 2D magnetohydrodynamics(MHD)numerical simulations,in order to model the blob formation and ejection from the accretion flow around SGR A*.We calculate the electron spectrum at specific time and in specific region,in order to find the peak of the synchrotron radiation energy during the magnetic reconnection process.Firstly,in our simulations,we use the 2D MHD numerical simulations to study the reconnection between the newly emerging magnetic field and the pre-existing magnetic field on the accretion disk.At the beginning of the reconnections,the magnetic fields line with opposite polarities come closer and form a current sheet,and the fluid appears along the current sheet.Both the velocity and temperature of the plasma can be comparable to the inferred velocity and the temperature based on the observations.The fluid is pulled towards the central black hole due to the strong gravitational attraction and the current sheet produced by the reconnection is also pulled towards the same direction.The whole structure of the magnetic reconnection region moves to the central black hole,and the huge blob is formed in the current sheet.Then a flux structure is formed through the magnetic reconnection under the current sheet.We analysis the numerical simulation result during the magnetic reconnection events,and find that the blobs are formed through the Kelvin-Helmholtz instability and the tear-mode instability.Thus,we use the numerical simulation to confirm that the magnetic reconnection is one of the important physical mechanisms of blob formation.Secondly,using the simulation results,we also calculate the electron spectrum and the synchrotron radiation energy at specific time and in specific region around SGR A*.We assume that the electrons in the magnetic reconnection region are non-thermal and emit synchrotron radiation,while the electrons in other regions are thermal electrons and emit thermal synchrotron radiation.Based on the assumption,we calculate the light curve during the whole process of the magnetic evolution and reconnection on the two Radio frequencies(230 GHz and 43 GHz)and two Near Infrared(NIR)wavelengths(3.8 μm and 2.2 μm).We find two peaks in the temporal light curve during the magnetic reconnection,and both of the peaks are caused by the non-thermal electron synchrotron radiation.The first peak appears before the blob is formed,and the second peak appears after the blob is ejected.The luminosities of both peaks exceed 1026 erg/s.For the NIR band,the highest luminosity can reach more than 1028 erg/s.Thus,we calculate the emission energy on the radio and near-infrared band of the reconnection region.Last,we study the tearing-mode instability and plasmid merger process during the magnetic reconnection.We find that the different physics environment can cause different phenomena of the magnetic reconnection.When the temperature becomes larger,the number of the plasmoids becomes more.When the density becomes higher or the guide field is added,the blob formed by the tear-mode instability will appear earlier.When the the Alfvén Mach number around the blob is higher than 1,more blobs are formed during the magnetic reconnection,which even cause the plasmoid merger.The process of the plasmoid merger appears everywhere on the current sheet.It even appears in the centre of the huge blob.Thus,we study the tearing mode instability and the phenomenon of the plasmoid merger.In summary,in this thesis,the magnetic reconnection process between the newly emerging magnetic flux and the pre-existing magnetic field is researched by numerical simulation,and it is confirmed that the magnetic reconnection is one of the mechanisms of the blob formation.The synchrotron radiation energy changed via the time around the black hole of the SGR A*are also calculated.