|Annu. Rev. Astron. Astrophys. 1999. 37:
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3.2. Superluminal Motions in GRO J1655-40
The relativistic ejections observed in the radio in GRO J1655-40 have striking similarities as well as differences with those in GRS 1915+105. Bright components moving apart with proper motions in the range of 40 to 65 mas d-1 were independently observed with the Southern Hemisphere VLBI Experiment array (Tingay et al 1995), and the VLA and VLBA (Hjellming & Rupen 1995). In Figure 5 is shown a sequence of seven VLBA radio images of GRS J1655-40 from Hjellming & Rupen (1995). At a distance of 3.2 kpc the motions of the ejecta have been fit - using a kinematic model - with a velocity of 0.92c, and a jet axis inclined 85° to the line of sight at a position angle of 47°, about which the jets rotate every three days at an angle of 2°.
Figure 5. A sequence of seven VLBA images of GRO J1655-40 at 1.6 GHz, each rotated anticlockwise by 43°, and each having an angular resolution of 3.0 × 0.4 arcsec (Hjellming & Rupen 1995). Each image is labeled with the date of the observations. The solid lines between images identify motions of 54 mas day-1 (left) and 45.5 mas day-1 (right). The vertical line marks the position of the central source, assumed to be the brightest point on each image.
In contrast to what has been observed in the repeated ejections of GRS 1915+105, the flux ratios of the blobs on either side of GRO J1655-40 cannot be ascribed to relativistic Doppler boosting. In GRO J1655-40 the asymmetry in brightness appears to flip from side to side (Hjellming & Rupen 1995). Not only do the jets appear to be intrinsically asymmetric, but also the sense of that asymmetry changes from event to event. Therefore, although similar intrinsic velocities greater than 0.9c are found in both superluminal sources, due to the asymmetries in GRO J1655-40, the ultimate physical interpretation of the superluminal expansions in this source remains uncertain.
We point out that in SS 433 flux asymmetries between knots ejected simultaneously on both sides have also been observed (Fejes 1986). This asymmetry could be due to intrinsic variations, so perhaps GRO J1655-40 is not unusual in this respect. However, VLBA multiwavelength monitoring of SS 433 (Paragi et al 1998) shows that it is always the receding part of the core-complex which is fainter compared to the approaching one, and that this effect cannot be explained simply by Doppler beaming. It is possible that free-free absorption and the different pathlengths through an absorbing medium could explain some of these asymmetries in SS 433 and other jet sources. Furthermore, in SS 433 more than 90% of the radio emission is in knots rather than in continuous jets, and the core complex disappears after large outbursts, as observed in GRS 1915+105 during the years 1992 and 1993 (Mirabel & Rodríguez 1994).