Discovery of Polarization Reverberation in NGC 4151

TL;DR

This study detects polarization reverberation in NGC 4151, revealing that polarized light lags total flux by about 8 days, indicating electron scattering in a flattened broad-line-region component.

Contribution

It provides the first evidence of polarization reverberation in NGC 4151, supporting a scattering origin and constraining the scattering region's geometry and location.

Findings

Polarized flux varies with an 8-day lag behind total flux.

Polarization is consistent with electron scattering in a flattened region.

Long-term polarization changes suggest evolving scattering conditions.

Abstract

Observations of the optical polarization of NGC 4151 in 1997-2003 show variations of an order of magnitude in the polarized flux while the polarization position angle remains constant. The amplitude of variability of the polarized flux is comparable to the amplitude of variability of the total U-band flux, except that the polarized flux follows the total flux with a lag of 8 +/- 3 days. The time lag and the constancy of the position angle strongly favor a scattering origin for the variable polarization rather than a non-thermal synchrotron origin. The orientation of the position angle of the polarized flux (parallel to the radio axis) and the size of the lag imply that the polarization arises from electron scattering in a flattened region within the low-ionization component of the broad-line-region. Polarization from dust scattering in the equatorial torus is ruled out as the source of the lag in polarized flux because it would produce a larger lag and, unless the half-opening angle of the torus is more than 53 degrees, the polarization would be perpendicular to the radio axis. We note a long-term change in the percentage of polarization at similar total flux levels and this could be due either to changing non-axisymmetry in the optical continuum emission, or a change in the number of scatterers on a timescale of years.