NGC 1068: No change in the mid-IR torus structure despite X-ray variability

TL;DR

Despite recent X-ray flux increases in NGC 1068, mid-infrared observations over ten years show no significant structural change in the dusty torus, indicating a patchy obscuring region rather than a uniform response.

Contribution

This study provides direct infrared evidence that the mid-infrared structure of NGC 1068 remains stable despite X-ray variability, supporting the patchy torus model.

Findings

Infrared emission remained unchanged over a decade.

X-ray variability did not affect the mid-infrared structure.

Infrared flux variations are less than 8%.

Abstract

Context. Recent NuSTAR observations revealed a somewhat unexpected increase in the X-ray flux of the nucleus of NGC 1068. We expect the infrared emission of the dusty torus to react on the intrinsic changes of the accretion disk. Aims. We aim to investigate the origin of the X-ray variation by investigating the response of the mid-infrared environment. Methods. We obtained single-aperture and interferometric mid-infrared measurements and directly compared the measurements observed before and immediately after the X-ray variations. The average correlated and single-aperture fluxes as well as the differential phases were directly compared to detect a possible change in the structure of the nuclear emission on scales of $\sim$ 2 pc. Results. The flux densities and differential phases of the observations before and during the X-ray variation show no significant change over a period of ten years. Possible minor variations in the infrared emission are $\lesssim$ 8 %. Conclusions. Our results suggest that the mid-infrared environment of NGC 1068 has remained unchanged for a decade. The recent transient change in the X-rays did not cause a significant variation in the infrared emission. This independent study supports previous conclusions that stated that the X-ray variation detected by NuSTAR observations is due to X-ray emission piercing through a patchy section of the dusty region.