Mid-infrared VISIR and Spitzer observations of the surroundings of the magnetar SGR 1806-20

Context. SGR 1806-20 is the soft gamma-ray repeater that has exhibited the highest activity since its discovery, including a giant flare in 2004 December 27. Previous studies of this source - probably a magnetar - have shown that it was associated with a massive star cluster embedded in a gas and dust cloud. Moreover, several stars in the cluster are peculiar hypergiants - luminous blue variable (LBV) and Wolf-Rayet stars - exhibiting strong and likely dusty stellar winds.Aims. We aimed at studying the mid-infrared emission of the stars associated with the same cluster as SGR 1806-20, to detect variations that could be due to the high-energy activity of the magnetar through interaction with the dust. We also studied the morphology of the cloud close to the centre of the cluster.Methods. We carried out mid-infrared observations of SGR 1806-20 and its environment, with the highest spatial resolution in this domain to date, using ESO/VISIR in 2005 and 2006, and we retrieved Spitzer/IRAC-MIPS archival data of the same field. We performed broadband photometry of three stars - LBV 1806-20, a WC9 and an O/B supergiant - on our VISIR images, as well as on the IRAC data. We then built and fitted their broadband spectral energy distributions with a combination of two absorbed black bodies, representing their stellar components, as well as a possible mid-infrared excess, to derive their physical parameters.Results. We show that LBV 1806-20 and the WC9 star exhibit a mid-infrared excess, likely because of the presence of circumstellar dust related to their winds. We also show that only LBV 1806-20 has had a variable flux over a period of two years, variability that is due to its LBV nature rather than to a heating of the gas and dust cloud by the high-energy emission of SGR 1806-20. Finally, differences in the intrinsic absorptions of the three stars show an inhomogeneous structure of the density of the gas and dust cloud in the massive star cluster..