The relative wavelength independence of IR lags in active galactic nuclei: implications for the distribution of the hot dust

TL;DR

Most active galactic nuclei exhibit nearly simultaneous IR variability across different wavelengths, implying a specific hot dust distribution, which simplifies using IR lags for cosmological measurements.

Contribution

This study reveals the near wavelength independence of IR lags in AGNs and proposes a hot dust outflow model to explain it, challenging previous simple predictions.

Findings

Most AGNs show minimal increase in IR lags with wavelength.

NGC 4151 exhibited a sharp increase in IR lag at one epoch.

The results support a hollow bi-conical hot dust outflow model.

Abstract

We show that, contrary to simple predictions, most AGNs show at best only a small increase of lags in the J, H, K, and L bands with increasing wavelength. We suggest that a possible cause of this near simultaneity of the variability from the near-IR to the mid-IR is that the hot dust is in a hollow bi-conical outflow of which we only see the near side. Although most AGNs show near simultaneity of IR variability, there was at least one epoch when NGC 4151 showed the sharply increasing IR lag with the increase of the wavelength. This behaviour might also be present in GQ Comae. We discuss these results briefly. The relative wavelength independence of IR lags simplifies the use of IR lags for estimating cosmological parameters.