The variability of the Crab Nebula in radio: No radio counterpart to gamma-ray flares

TL;DR

This study used VLA radio imaging to investigate the Crab Nebula's variability and searched for a radio counterpart to gamma-ray flares, finding none but detecting decade-scale brightness changes.

Contribution

First detailed radio imaging of the Crab Nebula post gamma-ray flare with no detected radio counterpart, revealing complex, decade-scale brightness variability.

Findings

No significant radio emission change associated with gamma-ray flare.

Detected 10% brightness variability over decade timescales.

Complex filamentary and knotty structures observed in brightness changes.

Abstract

We present new Jansky Very Large Array (VLA) radio images of the Crab Nebula at 5.5 GHz, taken at two epochs separated by 6 days about two months after a gamma-ray flare in 2012 July. We find no significant change in the Crab's radio emission localized to a region of <2 light-months in radius, either over the 6-day interval between our present observations or between the present observations and ones from 2001. Any radio counterpart to the flare has a radio luminosity of <~ $2 \times 10^{-4}$ times that of the nebula. Comparing our images to one from 2001, we do however find changes in radio brightness, up to 10% in amplitude, which occur on decade timescales throughout the nebula. The morphology of the changes is complex suggesting both filamentary and knotty structures. The variability is stronger, and the timescales likely somewhat shorter, nearer the centre of the nebula. We further find that even with the excellent uv~coverage and signal-to-noise of the VLA, deconvolution errors are much larger than the noise, being up to 1.2% of peak brightness of the nebula in this particular case.