VLBI detection of the HST-1 feature in the M87 jet at 2 cm

TL;DR

This study used VLBI at 2 cm to detect and analyze the HST-1 feature in the M87 jet, revealing its motion and structure, and concluding it is not blazar-like.

Contribution

First VLBI detection of HST-1 at 2 cm, providing detailed analysis of its evolution and ruling out blazar characteristics.

Findings

Projected apparent speed of 0.61 +/- 0.31 c

HST-1 is mostly resolved on milliarcsecond scales

HST-1 is optically thin between 2 cm and 20 cm

Abstract

A bright feature 80 pc away from the core in the powerful jet of M87 shows highly unusual properties. Earlier radio, optical and X-ray observations have shown that this feature, labeled HST-1, is superluminal, and is possibly connected with the TeV flare detected by HESS in 2005. It has been claimed that this feature might have a blazar nature, due to these properties. To examine the possible blazar-like nature of HST-1, we analyzed lambda 2 cm VLBA archival data from dedicated full-track observations and the 2 cm survey/MOJAVE VLBI monitoring programs obtained between 2000 and 2009. Applying VLBI wide-field imaging techniques, the HST-1 region was imaged at milliarcsecond resolution. Here we present the first 2 cm VLBI detection of this feature in observations from early 2003 to early 2007, and analyze its evolution over this time. Using the detections of HST-1, we find that the projected apparent speed is 0.61 +/- 0.31 c. A comparison of the VLA and VLBA flux densities of this feature indicate that is mostly resolved on molliarcsecond scales. This feature is optically thin between lambda 2 cm and lambda 20 cm. We do not find evidence of a blazar nature for HST-1.