GRB 180325A: dust grain-size distribution and interstellar iron nanoparticles contribution

We modelled dust grain-size distributions for carbonaceous and silicates dust, as well as for free-flying iron nanoparticles in the environment of a γ-ray burst (GRB) afterglow, GRB 180325A. This GRB, at a redshift z of 2.2486, has an unambiguous detection of the 2175 Å extinction feature with RV = 4.58 and AV = 1.58 mag. In addition to silicates, polycyclic aromatic hydrocarbons (PAH), and graphite, we used iron nanoparticles grain-size distributions for the first time to model the observed extinction curve of GRB 180325A. We fit the observed extinction for four model permutations, using 232 sets of silicates, graphite, carbon abundance in hydrocarbon molecules (bC), and fraction of iron abundance in free-flying nanoparticles (YFe). These four different permutations were chosen to test iron nanoparticles significance and carbon abundance in hydrocarbons. Our results indicate that iron nanoparticles contribution is insignificant and there is a degeneracy of carbon abundances, with the range (0.0 ≤ bC ≤ 0.7) × 10-5 providing the best-fit to the observed extinction curve of GRB 180325A. We therefore favour the simplest model of silicates and PAHs. The silicates are dominant and contribute to the entire wavelength range of the GRB extinction curve while graphite contributes towards both the 2175 Å bump and the ultraviolet (UV) extinction. The afterglow peak luminosity (1.5 × 1051 erg s-1) indicates dust destruction may have taken place. We conclude that further investigations into other potential contributors of extinction are warranted, particularly for steep UV extinction.