The Relative Wavelength Independence of IR Time Lags in NGC4151 During the Years 2010-2015

TL;DR

This study shows that in NGC 4151, infrared time lags relative to optical light are consistent across HKL bands over 2010-2015, supporting models where dust clouds are located beyond the sublimation radius and contributing to IR emission.

Contribution

It provides observational evidence that IR lags are wavelength-independent in NGC 4151, challenging previous reports of lag decrease and supporting the hollow bi-conical outflow model.

Findings

Infrared lags in HKL bands are approximately 37 days and consistent.

No significant decrease in IR lag during 2013-2014 as previously reported.

IR lag independence supports dust clouds being beyond the sublimation radius.

Abstract

We present results of a study of the correlation between the infrared (JHKL) and optical(B) fluxes of the nucleus of the Seyfert galaxy NGC 4151 for the years 2010 - 2015 using our own data (partially published) in combination with published data of Roberts and Rumstey (2012), Guo et al. (2014) and Schnulle et al. (2013, 2015).We find similar lags for each of the HKL passbands relative to the optical of 37 +-3 days. The lags are the same to within the accuracy of measurement. We do not confirm a significant decrease in the lag for HKL in 2013-2014 previously reported by Schnulle et al. (2015), but we find that the lag of the short-lag component of J increased. We discuss our results within the framework of the standard model, where the variable infrared radiation is mainly due to the thermal re-emission of short-wave radiation by dust clouds close to a variable central source. There is also some contribution to the IR emission from the accretion disk, and this contribution increases with decreasing wavelength. The variability in J and K is not entirely simultaneous, which may be due to the differing contributions of the radiation from the accretion disk in these bands. The absence of strong wavelength-dependent changes in infrared lag across the HKL passbands can be explained by having the dust clouds during 2010-2015 be located beyond the sublimation radius. The relative wavelength independence of the infrared lags is also consistent with the hollow bi-conical outflow model of Oknyansky et al. (2015).