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Astron. Astrophys. 356, 975-988 (2000)

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1. Introduction

The number of stellar mass black hole candidates known to produce jets has increased considerably over recent years. While SS433 was for a long time thought to be a rather exotic object, advances in X-ray astronomy have dramatically increased the number of known galactic X-ray binaries (van Paradijs 1995). At least nine of these sources subsequently showed evidence for the production of relativistically moving jets. Mirabel & Rodrguez (1999) give an excellent review on the observational and theoretical status of these objects, the microquasars.

The presence of relativistically moving material in these sources was discovered during radio observations. During times of strongly enhanced radio emission, which in the following we will refer to as radio outbursts, the emission region can often be resolved into at least two components. These components are observed to separate in opposite directions over a fews tens of days (i.e. Mirabel & Rodríguez 1994). The projected velocity of the component traveling on a trajectory towards the observer can exceed the speed of light, if its intrinsic velocity is large (e.g. Rybicki & Lightman 1979). In addition to the apparently superluminal nature of the jet components, their propagation has been observed to slow down only in one case (XTE J1748-288, Hjellming et al. 1999). This constant expansion speed led to the interpretation of practically ballistic trajectories of discrete plasmon ejections as explanation for the observed radio components (e.g. Mirabel & Rodríguez 1999).

Although suggested for the jets of SS433 (Hjellming & Johnston 1988), models with quasi-permanent jet production have received little attention in the case of microquasars. This is somewhat surprising given the large number of similarities they show with jet producing extragalactic sources like quasars (hence the name microquasars) and radio galaxies (e.g. Mirabel & Rodríguez 1998). In these cases there is little doubt that apart from some possible minor intermittency of the jet production mechanism for some sources (i.e. Reynolds & Begelman 1997) the jet flow is practically continuous.

In this paper we endeavour to close this gap by the development of a continuous jet model for microquasars to explain the radio outbursts observed in these sources. The model is based on the idea of internal shocks in jets (Rees 1978) which was successfully applied to Gamma Ray Bursts (GRB) (Rees & Meszaros 1994). In Sect. 2 we briefly review the plasmon model and discuss possible improvements on this within the jet picture. We develop the treatment of the relativistic jet flow in Sect. 3 and discuss the evolution of the synchrotron emission resulting from the internal shock in Sect. 4. The model is then applied to the probably best studied radio outburst of any microquasar, the March 1994 event in GRS 1915+105 (Mirabel & Rodríguez 1994) in Sect. 5. The properties of the jet of GRS 1915+105 like its energy content are derived from the model in Sect. 6. In Sect. 7 we consider the implications of a continuous jet for the interaction of microquasars with their environment. Finally, in Sect. 8 we summarise some observational consequences of our model which may be used to test the model with future observations.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000