A relativistic helical jet in the γ-ray AGN 1156+295

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
³ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁴ Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11–9DL, UK
⁵ ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁶ Leiden Observatory, PO Box 9513, 2300, RA Leiden, The Netherlands
⁷ Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
⁸ Hartebeesthoek Radio Astronomy Observatory, Krugersdorp 1740, South Africa

Corresponding author: X. Y. Hong, xhong@shao.ac.cn

Received: 31 January 2003
Accepted: 19 November 2003

Abstract

We present the results of a number of high resolution radio observations of the AGN 1156+295. These include multi-epoch and multi-frequency VLBI, VSOP, MERLIN and VLA observations made over a period of 50 months. The 5 GHz MERLIN images trace a straight jet extending to ~ at . Extended low brightness emission was detected in the MERLIN observation at 1.6 GHz and the VLA observation at 8.5 GHz with a bend of ~ at the end of the 2 arcsec jet. A region of similar diffuse emission is also seen about 2 arcsec south of the radio core. The VLBI images of the blazar reveal a core-jet structure with an oscillating jet on a milli-arcsecond (mas) scale which aligns with the arcsecond jet at a distance of several tens of milli-arcseconds from the core. This probably indicates that the orientation of the jet structure is close to the line of sight, with the northern jet being relativistically beamed toward us. In this scenario the diffuse emission to the north and south is not beamed and appears symmetrical. For the northern jet at the mas scale, proper motions of , and are measured in three distinct components of the jet (, km s^-1 Mpc^-1 are used through out this paper). Highly polarised emission is detected on VLBI scales in the region in which the jet bends sharply to the north-west. The spectral index distribution of the source shows that the strongest compact component has a flat spectrum, and the extended jet has a steep spectrum. A helical trajectory along the surface of a cone was proposed based on the conservation laws for kinetic energy and momentum to explain the observed phenomena, which is in a good agreement with the observed results on scales of 1 mas to 1 arcsec.