Annu. Rev. Astron. Astrophys. 1999. 37: 409-443
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10. INTERACTION OF RELATIVISTIC JETS WITH THE ENVIRONMENT

If a compact source (black hole or neutron star) injects collimated relativistic jets into its cold environment, it is expected that some fraction of the injected power will be dissipated by shocks in the circumstellar gas and dust. The collision of relativistic ejecta with environmental material has been observed in real time in XTE J1748-288 (Hjellming et al 1998), where the leading edge of the jet decelerates while strongly brightening. The interaction of the mildly relativistic jets from CI Cam (Hjellming & Mioduszewski 1998) with an HII and dust shell nebula has been reported by García et al (1998). Other signatures of the interaction of relativistic jets with the environment are the radio lobes of 1E 1740.7-2942 (Mirabel et al 1992) and GRS 1758-258 (Rodríguez et al 1992), the twisted arcmin jets of Circinus X-1 (Steward et al 1993, Fender et al 1998), and the two lateral extensions of tens of pc in the radio shell W50 that hosts at its center SS 433. The interaction of SS 433 with the nebula W50 has been studied in the X-rays (Brinkmann et al 1996), infrared (Mirabel et al 1996b), and radio wavelengths (Dubner et al 1998 and references therein).

SS 433 is a high mass X-ray binary at a distance of ~ 3 kpc near the centre of the radio shell W50 (Margon 1984). The latter may be either the supernova remnant from the formation of the compact object (Velusamy & Kundu 1974), or a bubble evacuated by the energy outflow of SS 433 (Begelman et al 1980). Besides the well-known relativistic jets seen at sub-arcsec scales in the radio, large-scale jets become visible in the X-rays at distances ~ 30 arcmin (~ 25 pc) from the compact source (Brinkmann et al 1996). In the radio, the lobes reach distances of up to 1° (~ 50 pc). These large-scale X-ray jets and radio lobes are the result of the interaction of the mass outflow with the interstellar medium. From optical and X-ray emission lines it is found that the sub-arcsec relativistic jets have a kinetic energy of ~ 1039 erg s-1 (Margon 1984, Spencer 1984), which is several orders of magnitude larger than the energy radiated in the X-rays and in the radio.

In Figure 8 is shown the lambda20 cm map with 55 arcsec resolution by Dubner et al (1998). It shows the connection between the subarcsec relativistic jets and the extended nebula over ~ 105 orders of magnitude in distance scales. Dubner et al (1998) estimate that the kinetic energy transferred into the ambient medium is ~ 2 × 1051 ergs, thus confirming that the relativistic jets from SS 433 represent an important contribution to the overall energy budget of the surrounding nebula W50. Begelman et al (1980) characterized W50 as a "beambag," interpreting the elongated shape and filled-in radio structure of W50 as evidence for continuing injection of magnetic field and high-energy particles from SS 433.

Figure 8

Figure 8. Very Large Array continuum mosaic of W50 at 1.5 GHz (Dubner et al 1998). The radio counterpart of SS 433 is the bright unresolved source at the center of the image. The lateral E-W extension of the nebula over ~ 1° (~ 50 pc) is caused by the injection of the relativistic jets from SS 433. The greyscale varies from 1 to 25 mJy beam-1. The angular resolution is 56 × 54 arcsec.

Evidences for the interaction of jets with the environmental medium have also been searched in the two well-established superluminal sources. In GRS 1915+105 Chaty, Mirabel & Rodríguez (1999) searched at millimeter, infrared, and X-rays for evidences of the physical association between the relativistic jets and two IRAS sources projected symmetrically on each side at ~ 15 arcmin of angular distance from the compact source that at first glance could be lobes caused by the impact of the jets in interstellar molecular clouds (Rodríguez & Mirabel 1999b). Besides the good alignment of the IRAS sources with the subarcsec jets and the presence of an intriguing non-thermal jet-like source in the SE IRAS source (Rodríguez & Mirabel 1999b), no conclusive physical evidence for association with GRS 1915+105 has been found, with the IRAS sources most probably being normal HII regions. On the other hand, Hunstead et al (1999) find regions of extended low-surface-brightness emission aligned with the radio jets of GRO J1655-40, but their real association with the high energy source has not been confirmed. The jets in GRS 1915+105 and GRO J1655-40 are faster than those in SS 433, but much more sporadic, and this probably accounts for the lack of obvious lobes associated with them.

It has been proposed that the interaction of relativistic jets with the environment may induce high energy radiation. Positrons released impulsively from the compact source could annihilate locally in the hot plasma producing a broad 511 keV spectral feature (Sunyaev et al 1991, Ramaty et al 1992). Alternatively, a fraction of the positrons could stream up to the interstellar gaseous environment, slowing down and annihilating in such cold medium, thus emitting 511 keV narrow line emission, and inducing radio lobe synchrotron emission and bremsstrahlung gamma-ray continuum emission (Laurent & Paul 1994).

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