The curious activity in the nucleus of NGC 4151: jet interaction causing
  variability?
  A key characteristic of many active galactic nuclei (AGN) is their
variability, but its origin is poorly understood, especially in the radio
domain. Williams et al. (2017) reported a ~50 per cent increase in peak flux
density of the AGN in the Seyfert galaxy NGC 4151 at 1.5 GHz with the e-MERLIN
array. We present new high resolution e-MERLIN observations at 5 GHz and
compare these to archival MERLIN observations to investigate the reported
variability. Our new observations allow us to probe the nuclear region at a
factor three times higher-resolution than the previous e-MERLIN study. We
separate the core component, C4, into three separate components: C4W, C4E and
X. The AGN is thought to reside in component C4W, but this component has
remained constant between epochs within uncertainties. However, we find that
the Eastern-most component, C4E, has increased in peak flux density from
19.35$\pm$1.10 to 37.09$\pm$1.86 mJy/beam, representing a 8.2 sigma increase on
the MERLIN observations. We attribute this peak flux density increase to
continued interaction between the jet and the emission line region (ELR),
observed for the first time in a low-luminosity AGN such as NGC 4151. We
identify discrete resolved components at 5 GHz along the jet axis, which we
interpret as areas of jet-ELR interaction.
