A decade of high-resolution radio observations of GRS 1915+105

ABSTRACT The radio emitting X-ray binary GRS1915+105 shows a wide variety of X-ray andradio states. We present a decade of monitoring observations, with the RXTE-ASMand the Ryle Telescope, in conjunction with high-resolution radio observations usingMERLIN and the VLBA. Linear polarisation at 1.4 and 1.6 GHz has been spatiallyresolved in the radio jets, on a scale of ∼ 150 mas and at flux densities of a few mJy.Depolarisation of the core occurs during radio flaring, associated with the ejection ofrelativistic knots of emission. We have identified the ejection at four epochs of X-rayflaring. Assuming no deceleration, proper motions of 16.5 to 27 mas per day have beenobserved, supporting the hypothesis of a varying angle to the line-of-sight per ejection,perhaps in a precessing jet.Key words: ISM: jets and outflows - X-ray binaries: individual (GRS1915+105). 1 INTRODUCTIONGRS1915+105 has proved to be one of the best labora-tories for the study of relativistic physical environments,due to its high-brightness, periodic outbursts of superlu-minal ejecta and relative proximity. Since the first detec-tion in August 1992 with the WATCH instrument on boardthe X-ray telescope GRANAT (Castro-Tirado et al. 1992),GRS1915+105 has demonstrated some of the most spec-tacular high-energy physics within the Galaxy. Shortly af-ter its discovery, a radio counterpart was discovered byMirabel et al. (1993a) and later an infrared component wasalso identified (Mirabel et al. 1993b). Its radio light-curveshows a highly variable and complex structure, with spa-tially resolved features that can be followed over many days.A clear correlation between the X-ray, in-frared and radio emission was quickly establishedby Mirabel & Rodriguez (1994), with rapid time vari-ability in each band. Radio flaring was found to correspondto a rapid change in the hard X-rays and possible productionof high-energy gamma rays. In March 1994, 20 cm VLA ob-servations of GRS1915+105 detected the first superluminalmotion of a Galactic source (Mirabel & Rodriguez 1994).This major breakthrough provided the direct evidenceof relativistic jets and an extreme physical environmentwithin the Galaxy. The name “microquasar” was coined