Internal shock model for microquasars

Abstract

We present a model for the radio outbursts of microquasars based on the assumption of quasi-continuous jet ejection. The jets are 'lit up' by shock fronts traveling along the jets during outbursts. The shocks accelerate relativistic particles which emit the observed synchrotron radiation. The observed comparatively flat decay light curves combined with gradually steepening spectral slopes are explained by a superposition of the radiation of the aging relativistic particle population left behind by the shocks. This scenario is the low energy, time-resolved equivalent to the internal shock model for GRBs. We show that this model predicts energy contents of the radiating plasma similar to the plasmon model. At the same time, the jet model relaxes the severe requirements on the central source in terms of the rate at which this energy must be supplied to the jet. Observations of 'mini-bursts' with flat spectral slopes and of infrared emission far from the source centre suggest two different states of jet ejections: (i) A 'mini-burst' mode with relatively stable jet production and weak radio emission with flat spectra and (ii) an outburst mode with strong variations in the jet bulk velocities coupled with strong radio emission with steeper spectra. We also show that the continuous jets in microquasars should terminate in strong shocks and possibly inflate radio lobes similar to extragalactic jet sources. We investigate the possibility of testing the predictions of this model with resolved radio observations. Finally, we suggest that Doppler-shifted X-ray iron lines, and possibly H-alpha lines, may be emitted by the jet flow of microquasars if thermal instabilities analogous to those in SS433 exist in their jets.

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