NGC 1068: No change in the mid-infrared torus structure despite X-ray variability ^⋆,⋆⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: nlopez@strw.leidenuniv.nl
² European Southern Observatory, Alonso de Córdova 3107 Vitacura, Santiago, Chile
³ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 306, Santiago 22, Chile
⁴ Millennium Institute of Astrophysics, MAS, Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
⁵ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁶ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Gießenbachstr., 85741 Garching, Germany
⁷ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁸ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA

Received: 29 August 2016
Accepted: 5 October 2016

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 ~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 ≲6%.

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.