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 $\sim 10 M_{\odot}$ 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 Alfv\'en 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}$).