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

TL;DR

This study uses high-resolution mid-infrared observations from ESO/VISIR and archival Spitzer data to analyze the environment of magnetar SGR 1806-20, revealing dust interactions, stellar variability, and cloud inhomogeneity.

Contribution

It provides the first high-resolution mid-infrared imaging of SGR 1806-20's environment, identifying dust-related excesses and variability in cluster stars, and characterizing the dust cloud structure.

Findings

LBV 1806-20 and WC9 stars show mid-infrared excess due to circumstellar dust.

Only LBV 1806-20 exhibited flux variability over two years.

The dust cloud has an inhomogeneous density structure.

Abstract

We report 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 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 aimed at studying the morphology of the cloud close to the centre of the cluster. 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, in order to derive their physical parameters. 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 had a variable flux over a period of two years, variability which 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.