The role of Violent Magnetic Reconnection on the Production of the Large Scale Superluminal Ejections of the Microquasar GRS 1915+105

Abstract

We here propose that the large scale superluminal ejections observed in the galactic microquasar GRS 1915+105 during radio flare events are produced by violent magnetic reconnection episodes in the corona just above the inner edge of the magnetized accretion disk that surrounds the central $\sim 10M_{\odot }$ black hole. The process occurs when a large scale magnetic field is established by turbulent dynamo in the inner disk region with a ratio between the gas$+$radiation and the magnetic pressures $\beta \simeq 1$, implying a magnetic field intensity of $\sim 7\times 10^8$ G. During this process, substantial angular momentum is removed from the disk by the wind generated by the vertical magnetic flux therefore increasing the disk mass accretion to a value near (but below) the critical one ($\dot{M}\sim 10^{19}$ g s${}^{-1}$). Part of the magnetic energy released by reconnection heats the coronal gas ($T_c\lesssim 5\times 10^8$ K) that produces a steep, soft X-ray spectrum with luminosity $L_X\simeq 10^{39}$ erg s${}^{-1}$, in consistency with observations. The remaining magnetic energy released goes to accelerate the particles to relativistic velocities ($v\sim v_A\sim c$, where $v_A$ is the Alfvén speed) in the reconnection site through first-order Fermi processes. In this context, two possible mechanisms have been examined which produce power-law electron distributions $N(E)\propto E^{-{\alpha }_E}$, with ${\alpha }_E=5/2,2$, and corresponding synchrotron radio power-law spectra with spectral indices which are compatible with that observed during the flares ($S_{\nu }\propto {\nu }^{-0.75,-0.5}$).

A&A, in press; 3 figures; revised version has suffered few important modifications

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