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

Free access article

Issue		A&A Volume 441, Number 3, October III 2005


Page(s)		845 - 853
Section		Astrophysical processes
DOI		http://dx.doi.org/10.1051/0004-6361:20042590

A&A 441, 845-853 (2005)
DOI: 10.1051/0004-6361:20042590

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

E. M. de Gouveia Dal Pino¹ and A. Lazarian²

¹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil
e-mail: dalpino@astro.iag.usp.br
² Department of Astronomy, University of Wisconsin, Madison, USA
e-mail: lazarian@astro.wisc.edu

(Received 21 December 2004 / Accepted 13 June 2005)

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 ~ $10 \, M_{\odot}$ 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 $\beta \simeq 1$ , implying a magnetic field intensity of ~ $7 \times 10^8$ 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 ( $T_{\rm c} \lesssim 5 \times 10^8$ K) that produces a steep X-ray spectrum with luminosity $L_{\rm X} \simeq 10^{39}$ erg s^{- 1}, consistent with observations. The remaining magnetic energy released goes to accelerate the particles to relativistic velocities ( $v \, \sim \, v_{\rm A} \, \sim \, c$ , where $v_{\rm A}$ 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 $N(E) \propto E^{-\alpha_E}$ , 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 ( $S_{\nu} \propto \,\nu^{-0.75, - 0.5}$ ).

Key words: acceleration of particles -- accretion, accretion disks -- black hole physics -- magnetic fields

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