X-ray long-term variations in the low-luminosity AGN NGC835 and its
  circumnuclear emission
  Obscured active galactic nuclei (AGNs) are thought to be very common in the
Universe. Observations and surveys have shown that the number of sources
increases for near galaxies and at the low-luminosity regime (the so-called
LLAGNs). Furthermore, many AGNs show changes in their obscuration properties at
X-rays that may suggest a configuration of clouds very close to the accretion
disk. However, these variations could also be due to changes in the intrinsic
continuum of the source. It is therefore important to study nearby AGN to
better understand the locus and distribution of clouds in the neighbourhood of
the nucleus. We aim to study the nuclear obscuration of LLAGN NGC835 and its
extended emission using mid-infrared observations. We present mid-infrared 11.5
microns imaging of the LLAGN galaxy NGC835 obtained with the instrument
CanariCam in the Gran Telescopio CANARIAS (GTC), archival Spitzer/IRS
spectroscopy, and archival Chandra data observed in 2000, 2008, and 2013. The
GTC/CanariCam 11.5 microns image reveals faint extended emission out to ~6
arcsec. We obtained a nuclear flux of F(11.5 microns) ~18 mJy, whereas the
extended emission accounts for 90% of the total flux within the 6 arcsec. This
means that the low angular resolution (~4 arcsec) IRS spectrum is dominated by
this extended emission and not by the AGN, clearly seen in the Spitzer/IRS
spectrum. Although the extended soft X-ray emission shows some resemblance with
that of the mid-infrared, the knots seen at X-rays are mostly located in the
inner side of this mid-infrared emission. The nuclear X-ray spectrum of the
source has undergone a spectral change between 2000/2008 and 2013. We argue
that this is most probably due to changes in the hydrogen column density from ~
8x10E+23 cm-2 to ~ 3x10E+23 cm-2. NGC835 therefore is one of the few LLAGN,
together with NGC1052, in which changes in the absorber can be claimed.
