A Relativistically Expanding Radio Source associated with GRO J1655-40
Session 120 -- Galactic Gamma Rays
Oral presentation, Thursday, 12, 1995, 2:00pm - 3:30pm

## [120.05] A Relativistically Expanding Radio Source associated with GRO J1655-40

D. W. Murphy (JPL), S. J. Tingay (MSSO), D. L. Jauncey (ATNF), R. A. Preston (JPL), J. E. Reynolds (ATNF), D. L. Meier (JPL), A. K. Tzioumis (ATNF), D. L. Jones (JPL)

GRO J1655-40 was discovered as a new, bright X-ray source with the BATSE detector of the Gamma Ray Observatory (GRO) on July 27, 1994. During the subsequent radio outburst we completed VLBI synthesis imaging observations at a frequency of 2.29 GHz. An unprecedented angular motion of $65\pm5$ mas day$$^{-1}$$ was observed between two components of complex and disparate morphology, or equivalently, an apparent transverse motion of $1.3c\pm^{0.7c}_{0.6c}$ using the likely distance range of $3.5 \pm 1.5 \: kpc$ as inferred from HI absorption measurements. This separation rate indicates that the two components were at zero separation near the onset of the radio flare. Similar to the recently reported GRS 1915+105 it seems that the motion of material ejected from a stellar mass compact object with at least a mildly relativistic velocity may explain the observed radio structural changes in GRO J1655-40. The minimum intrinsic expansion speed of the radio source must be (with minimal assumptions) $0.65c\pm^{0.18c}_{0.30c}$ if the expansion is two-sided, and $0.79c\pm^{0.11c}_{0.22c}$ if one of the two components is a core and the expansion is one-sided. The two-week delay between the X-ray outburst and the production of radio components may indicate that the X-rays were produced by a super-critical accretion process onto the compact member of this stellar binary system. This process inhibited or smothered the ejection of radio components until the accretion disk stabilized.