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

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2. Plasmons or jets?

Atoyan & Aharonian (1999) developed a model which is intended to reconcile the idea of discrete magnetised plasmon ejections during radio outbursts with the observed lightcurve and spectral behaviour in the radio waveband. They find that a single population of relativistic particles accelerated at the time of the ejection of the plasmons cannot explain the observations. The energy losses of the relativistic particles due to synchrotron radiation would lead to a sharp cut-off in the radio spectrum moving to lower frequencies as the plasmons expand and travel outwards. This is quite different from the observed rather gentle steepening of the spectrum. Atoyan & Aharonian (1999) also show that a continuous replenishment of relativistic particles to the plasmons alone cannot solve this problem because in this case the spectral cut-off moves to higher frequencies with time. They therefore postulate that the relativistic particles in the plasmons during the March 1994 event in GRS 1915+105 were continuously replenished, presumably by a shock at the side of the plasmons pointing towards the source centre, but also suffered energy dependent escape losses.

To fit the observations the scenario proposed by Atoyan & Aharonian (1999) requires that the mean free path of the most energetic relativistic particles in the magnetised plasmons is comparable to or exceeds the physical dimensions of the plasmons. This implies that these particles travel through the plasmons producing synchrotron emission but then leave them without scattering once off irregularities in the magnetic field or other particles. This is difficult to reconcile with the requirement that in order to be accelerated to relativistic velocities in the shock regions the mean free path in these regions must be short to ensure many shock crossings. If the accelerating shocks are close to the plasmons, this then means that the properties of the plasma change dramatically over short distances. Moreover, it is not clear in this scenario why the synchrotron emission is not completely dominated by the contribution of the shocks themselves.

In this paper we propose a different scenario to explain the observed properties of the radio emission of microquasars during outbursts. This is based on the assumption that microquasars may produce continuous jets for a long time before the actual outburst occurs and may well do so permanently (see also Levinson & Blandford 1996a, b). The outbursts in our model are then caused by two shocks traveling along these continuous jets which accelerate the required relativistic particles in situ. After the shock has passed a particular region in one of the jets, this region continues to contribute to the total emission until the cut-off in the specific spectrum of this region moves below the observing frequency. The jet components observed in microquasars are in general not resolved and so the measured flux is the integrated emission from all the jet regions passed by the shock which are still emitting at the relevant frequency. This implies that the observed spectrum is steeper than that of the jet region immediately behind the shock where radiation losses are still negligible. The variation of the strength of the magnetic field and of the number of relativistic particles accelerated by the shock along the jet then give rise to a slowly steepening radio spectrum. This effect was discussed in the case of the radio hot spots of powerful extragalactic radio sources by Heavens & Meisenheimer (1987).

Our investigation is based on the jet model of Blandford & Rees (1974) and Marscher & Gear (1985). A similar approach to explain the synchrotron self-Compton emission of extragalactic jets was taken by Ghisellini et al. (1985). They develop a numerical scheme to follow the evolution of the energy spectrum of the relativistic particles downstream of the jet shock taking into account radiative as well as adiabatic energy losses. They also include synchrotron self-absorption in this calculation. Since we are mainly interested in the radio emission of the jets of microquasars on rather large scales ([FORMULA]m), we can neglect any Compton scattering and absorption effects on the energy spectrum. In this case only adiabatic and synchrotron losses are important and we can use the analytic solution for the evolution of the energy spectrum of the relativistic electrons derived by Kaiser et al. (1997).

The underlying physical processes of the model presented here are very similar to the internal shock model proposed as explanation for GRB (Rees & Meszaros 1994). The same scenario has also been invoked to explain the X-ray and [FORMULA]-ray emission of extragalactic jets (Ghisellini 1999). In the internal shock model the energy of the shock traveling along the jet is thought to be supplied by the collision of fast shells of jet material with slower ones (Rees 1978). In the case of GRB this energy is released practically instantaneously leading to the extremely short duration of the observed bursts of emission. In extragalactic jets the shell collision may take longer but the large distance to these objects makes it difficult to separate the contributions of multiple collision to the total emission. As outlined above, we argue in this paper that the jets of microquasars provide us with the possibility to observe the development of internal shocks in jets resolved both in space and time.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000