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Session 56 - BL Lacs & Blazars.
Oral session, Thursday, January 08

[56.07] VSOP Space VLBI Imaging of the EGRET Blazar 1633+382

J. S. Ulvestad (NRAO), W. T. Vestrand (UNH), J. G. Stacy (LSU and SU), J. A. Biretta (STScI)

The EGRET blazar 1633+382 has been imaged at 5 GHz in one of the first scientific observations made with the VLBI Space Observatory Programme (VSOP). The VSOP satellite (HALCA), launched on February 12, 1997, was used together with the VLBA to observe 1633+382 on July 29/30, with projected baselines longer than an Earth diameter. About five hours of successful space observations with the 8-m spaceborne radio telescope were made in conjunction with nine hours of VLBA observing. A noise-limited image with a dynamic range of better than 1000:1 (peak/rms) was produced, with a synthesized beam of 1.5\times 0.7 mas.

1633+382, at z=1.807, is one of the most luminous and variable \gamma-ray sources detected with the EGRET instrument aboard the Compton Gamma Ray Observatory. Previous ground-based VLBI images have revealed a core-jet structure, with a jet direction varying between the west and northwest on a milliarcsec scale. The VSOP image shows a close double with a separation of 1.3 mas in position angle -85^\circ, and a more extended jet component peaking 4.5 mas to the NW of the nucleus. A VLBA 8.4-GHz image acquired in mid-1995 also indicates the presence of the close double, although the two components are incompletely resolved in the ground-based image. Model-fitting gives a motion of 0.32 mas in the 2.2 years between the ground and space observations, corresponding to an apparent superluminal speed of \sim 8c for H_0 = 75 km s^-1 Mpc^-1, consistent with previous measurements of components more distant from the core. Spectral-index measurements confirm that the easternmost component has an inverted spectrum, and should be identified with the blazar nucleus. Interpretation of the radio jet and its \gamma-ray emission will be discussed briefly.

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Program listing for Thursday