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

## TL;DR

This study models the infrared ring around magnetar SGR 1900+14 using 3D dust radiative transfer, showing a dust-free cavity illuminated by stars can explain the observed emission, with implications for understanding the environment around magnetars.

## Contribution

The paper presents a detailed 3D radiative transfer model demonstrating that a dust-free ellipsoidal cavity can reproduce the infrared ring around SGR 1900+14, considering different dust distributions and star positions.

## Key findings

- A dust-free ellipsoidal cavity explains the IR ring's shape and flux.
- High gas density ($n_H extasciitilde 1000$ cm$^{-3}$) is required for the model.
- The emitting region likely has a sharp inner boundary and is extended radially.

## Abstract

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