The jet of Markarian 501 from millions of Schwarzschild radii down to a few hundreds

TL;DR

This study uses high-resolution radio observations to explore the structure and magnetic field of the Markarian 501 jet from hundreds of Schwarzschild radii to hundreds of parsecs, revealing limb brightening and magnetic field alignment.

Contribution

It provides unprecedented imaging of the jet from near the black hole to large scales, extending understanding of jet physics in Markarian 501.

Findings

Detection of the jet up to ~700 mas from the core.

Inner jet regions show brightness temperatures over 6x10^10 K.

Evidence of limb brightening and magnetic field aligned with the jet axis.

Abstract

Aims: The TeV BL Lac object Markarian 501 is a complex, core dominated radio source, with a one sided, twisting jet on parsec scales. In the present work, we attempt to extend our understanding of the source physics to regions of the radio jet which have not been accessed before. Methods: We present new observations of Mrk 501 at 1.4 and 86 GHz. The 1.4 GHz data were obtained using the Very Large Array (VLA) and High Sensitivity Array (HSA) in November 2004, in full polarization, with a final r.m.s. noise of 25 microJy/beam in the HSA total intensity image; the 86 GHz observations were performed in October 2005 with the Global Millimeter VLBI Array (GMVA), providing an angular resolution as good as 110 x 40 microarcseconds. Results: The sensitivity and resolution provided by the HSA make it possible to detect the jet up to ~700 milliarcseconds (corresponding to a projected linear size of ~500 pc) from its base, while the superior resolution of the 86 GHz GMVA observations probes the innermost regions of the jet down to ~200 Schwarzschild radii. The brightness temperature at the jet base is in excess of 6e10 K. We find evidence of limb brightening on physical scales from <1 pc to ~40 pc. Polarization images and fits to the trend of jet width and brightness vs. distance from the core reveal a magnetic field parallel to the jet axis.