The optical constants and grain sizes of interstellar dust measured directly using the dust-scattered X-ray halo of GRB221009A

¹ Cosmic Dawn Center (DAWN), Denmark
² Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, København 2100, Denmark

^★ Corresponding author: a.sneppen@gmail.com

Received: 25 March 2025
Accepted: 10 July 2025

Abstract

Context. X-ray scattering is a powerful probe of the grain-size distribution of interstellar dust. Bright transient sources are excellent tools for this because they fade rapidly and only leave the expanding scattered X-ray halo.

Aims. We analysed the dust-scattered X-ray halo data of the unprecedentedly bright γ-ray burst GRB 221009A to measure the grain-size distribution of dust in the Galaxy as well as the complex refractive index m, and use these results to infer the likely dust composition.

Methods. GRB 221009A produced 20 distinct rings, as observed with follow-up observations of the GRB afterglow and scattering halo with the EPIC camera on XMM-Newton. We used anomalous diffraction theory to model the ring brightness as a function of angle.

Results. We constrained the complex refractive index m = n + ik at several x-ray energies and found k_{1 keV} = (2.7 ± 0.7) × 10⁻⁴ and 1 − n_{1 keV} = 0.0009 ± 0.0002. This is highly inconsistent with the commonly employed assumptions of the Rayleigh-Gans approximation. These results lie in the expected range for interstellar dust compositions that are dominated by carbon, magnesium silicates, and iron. The absorption results suggest a substantial mass fraction of iron at 35 ± 7%. The Mathis et al. (1977, ApJ, 217, 425) distribution fit returns a maximum grain radius a_max = 0.24 ± 0.01 μm; all fits strongly rule out models with ~0.4 μm grains for this sightline. The soft X-ray spectrum of the prompt GRB can also be inferred from the fitting, and the best fit provides a spectral slope that is consistent with the slope of the low-energy side of the best-fit Band model of the directly measured prompt emission. Forcing a different grain size or composition than the best fit results in an inferred prompt spectrum that is different from the observed prompt emission.

Conclusions. We directly measured the grain-size distribution and refractive index of the interstellar dust. The radius of only a very few grains is larger than about ~0.3 μm. The refractive index is consistent with standard average dust compositions, which shows that X-ray scattering is an effective tool for measuring the optical properties of interstellar dust.