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A peculiar infrared ring-like structure was discovered by {\em Spitzer}around the strongly magnetised neutron star SGR 1900$+$14. This infraredstructure was suggested to be due to a dust-free cavity, produced by the SGRGiant Flare occurred in 1998, and kept illuminated by surrounding stars. Usinga 3D dust radiative transfer code, we aimed at reproducing the emissionmorphology and the integrated emission flux of this structure assumingdifferent spatial distributions and densities for the dust, and differentpositions for the illuminating stars. We found that a dust-free ellipsoidalcavity can reproduce the shape, flux, and spectrum of the ring-like infraredemission, provided that the illuminating stars are inside the cavity and thatthe interstellar medium has high gas density ($n_H\sim$1000 cm$^{-3}$). Wefurther constrain the emitting region to have a sharp inner boundary and to besignificantly extended in the radial direction, possibly even just a cavity ina smooth molecular cloud. We discuss possible scenarios for the formation ofthe dustless cavity and the particular geometry that allows it to be IR-bright.

Dust radiative transfer modelling of the infrared ring around the magnetar SGR 1900$+$14