Astron. Astrophys. 362, 780-785 (2000)

1. Introduction

At the end of their life, some stars can lose their thermal-gravitational equilibrium and die in a supernova explosion. A supernova is an event that occurs in extremely short timescales (the explosion itself takes just a few seconds, but the supernova can be visible for years after the explosion). A supernova remnant is composed of a shock wave, generated at the explosion time, the material ejected by the star, and the interstellar medium swept up by the supernova shock wave. In spite of the short lifetime of supernovae, their remnants can hold on for hundreds of thousand years, drastically modifying their environments and emitting energy in the whole electromagnetic spectrum, although the characteristic feature of SNRs is that they are strong sources of radio emission of non-thermal origin (synchrotron radiation).

Astrophysical jets are highly collimated flows which have been observed in many types of astrophysical objects (i.e. the nuclei of active galaxies, regions of stellar formation and compact objects).

Both jets and SNRs have been studied by means of observations (in different wavelengths) and theoretical work. Previously, there has been little interest in studying the interaction between jets and SNRs. However, there is new observational evidence showing that this kind of interactions are possibly occurring in some objects (Dubner et al. 1998for the SNR W 50 case; and Gaensler et al. 1998for SNR G309.2-00.6).

In this work we model the strange morphology observed in the radio emission of the system composed by the source of relativistic jets SS433 and the SNR W 50. To this effect, we use a 2D (axisymmetric) adaptive grid code to simulate the encounter of a jet with a SNR shell.

SS 433 is a compact object (maybe a binary system) which emits two precessing jets in opposite directions, and is located at the center of the supernova remnant W 50. The precession cone has an initial half angle of 20^o and the precession time is of 164 days (Hjellming & Johnston 1988).

The SNR W 50 shows a morphology at radio-frequencies which is far from being a spherical shell (which is the typical morphology of supernova remnants expanding into a homogeneous and isotropic medium) because it has two lateral extensions or lobes, in the East-West direction. Its angular sizes are , which correspond to 10452 pc assuming a distance of 3 kpc to the object (according to the HI study of Dubner et al. 1998). These radio lobes are aligned with the cone axis of the SS 433 jets, as shown by X-ray observations (Safi-Harb & Ögelman 1997). Furthermore, there is optical emission with filamentary structure, located at the beginning of the radio lobes (van den Bergh 1980, Kirshner & Chevalier 1980 and Mazeh et al. 1983).

Several authors have given possible scenarios to describe the SS 433/W 50 system, using analytical and numerical models. These scenarios can be divided into two groups. The models in the first group are based on the hypothesis that this system is a bubble which was formed by the interaction between the SS 433 jets and the surrounding interstellar medium or that the jets are expanding into an ISM swept up by the wind of a stellar partner of SS 433 (e.g. König 1993; Begelman et al. 1980). Kochanek & Hawley (1990) carried out a numerical study inspired by the SS 433 system. Their work was not an attempt to give a complete model of this system but they just wanted to gain some understanding about the physics of hollow conical jets and possible mechanisms to produce a refocusing of the jets. On the other hand, the models in the second group consider that the elongated morphology of W 50 is due to the encounter between the jets from SS 433 and an SNR shell (Zealey et al. 1980; Downes et al. 1986; Murata & Shibazaki 1996).

In view of the alignment of the jets and lobes and observational (radio, optical, and X-ray) evidence which support the framework of a jet propagating inside of a SNR, we consider the models of the second group as the best possibility for describing this problem.

Our aim is to try to understand the process of interaction between jets and SNRs and to simulate the radio emission (synchrotron mechanism) in order to compare the models with observations.

© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000
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