D. J. McKay

The radio flaring behaviour of GRO J1655—40: an analogy with extragalactic radio sources?

At radio frequencies, the current evidence for the microquasar-quasar connection is based on imaging observations showing that relativistic outflows/jets are found in both classes of objects. Some microquasars also display superluminal motion, further strengthening the view that microquasars are in fact Galactic miniatures of quasars. Here we demonstrate that this connection can be extended to incorporate timing and spectral observations. Our argument is based on the striking similarity found in the radio flaring behaviour of the Galactic superluminal source GRO J1655-40 and of extragalactic sources, such as the blazar 3C 273. We find that the variability of GRO J1655-40 can be explained within the framework of the successful generalized shock model for compact extragalactic radio sources in which the radio emission arises from shocked plasma in relativistic jets. Specifically, the multifrequency flare amplitudes, time-delays and radio polarization position angle measurements are consistent with the predictions of the growth stage of this model.

VLBI imaging of GRO J1655-40 with the sheve array

Abstract On 27 July 1994 the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory detected an outburst of high energy X-rays from a previously unknown source in the constellation Scorpius. This source was designated GRO J1655-40. Approximately 12 days after the start of the X-ray outburst, a strong outburst of radio emission occurred. Here we present very long baseline interferometry (VLBI) observations of GRO J1655-40 made with an array of telescopes in Australia, South Africa, and the western United States. These observations show that the radio source which appeared two weeks after the initial X-ray outburst consisted of two prominent components which separated with an apparent speed of 1.5 ± 0.4c. When the various possibilities for the geometry of the radio source are taken into account the apparent speed implies an intrinsic speed between 0.5c and 0.9c. Our results and those of other investigators imply a strong link between the accretion of material onto a highly compact object and the ejection of relativistic components of radio emission.

Southern Hemisphere VLBI Observations of the Expansion of GRO J1655–40

Abstract. We present high-resolution radio observations of the second Galactic superluminal radio source GRO1655-40, which was detected as an X-ray transient on 1994 July 27. Our radio radio images reveal two components moving away from each other at an angular speed of 65±5 mas/day, corresponding to superluminal motion (v/c= 1.4 ± 0.4) at the estimated distance of 3-5 kpc. The 12-day delay between the X-ray and radio outbursts suggests that the ejection of material at relativistic speeds occurs during a stable phase of accretion onto a black hole, which follows an unstable phase with a high accretion rate. A complete description and discussion of these observations can be found in Tingay et al 1995 (Nature, 374, pp 141-143).