Production of the large scale superluminal ejections of the microquasar GRS 1915+105 by violent magnetic reconnection

Abstract

We propose here 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 ∼10M ⊙ black hole. The process occurs when a large-scale magnetic field is established by a turbulent dynamo in the inner disk region with a ratio between the gas+radiation and the magnetic pressures β ≃ 1, implying a magnetic field intensity of ∼7 × 108 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 (Ṁ ∼ 10 19 g s-1). Part of the magnetic energy released by reconnection heats the coronal gas (Tc ≲ 5 × 10 8 K) that produces a steep X-ray spectrum with luminosity L X ≃ 1039 erg s-1, consistent with observations. The remaining magnetic energy released goes to accelerate the particles to relativistic velocities (ν - νA - c, where νA is the Alfvén speed) in the reconnection site through first-order Fermi processes. In this context, two possible mechanisms have been examined that produce power-law electron distributions N(E) ∝ E -αE, with αE = 5/2, 2, and corresponding synchrotron radio power-law spectra with spectral indices which are compatible with that observed during the flares (Sν ∝ ν-0.75,-0.5). © ESO 2005.