Testing a Model for Interstellar Dust

We use published ROSAT observations of the X-ray Nova V1974 Cygni 1992 to test a model for interstellar dust grains consisting of a mixture of carbonaceous grains and silicate grains. The time-dependent X-ray emission from the nova is modelled as the sum of emission from a ONe white dwarf plus a thermal plasma, and X-ray scattering is calculated for a dust mixture with a realistic size distribution, and with various spatial distributions. Model results are compared with the scattering halos measured by ROSAT at 9 different epochs, including the early period of rising X-ray emission, the " plateau " phase of steady emission, and the X-ray decline at late times. We find that the observed X-ray halos appear to be consistent with the halos calculated for the size distribution of Weingartner & Draine which reproduces the Milky Way extinction with R V = 3.1, provided that the reddening to the nova is E(B − V) ≈ 0.19, consistent with E(B − V) = 0.19 inferred from the late-time Balmer decrement. The time delay of the scattered halo relative to the direct flux from the nova is clearly detected. Models with smoothly-distributed dust give good overall agreement with the observed scattering halo, but tend to produce somewhat more scattering than observed at 200–300 ′′ , and insufficient scattering at 50–100 ′′. While an additional population of large grains can increase the scattered intensity at 50–100 ′′ , this could also be achieved by having ∼20% of the dust in a cloud at a distance from us equal to ∼90% of the distance to the nova. Such a model also improves agreement with the data at larger angles, and illustrates the sensitivity of X-ray scattering halos to the presence of line-of-sight density fluctuations. The observations therefore do not require a population of micron-sized dust grains. Future observations by Chandra of X-ray scattering halos around point sources can provide more stringent tests of interstellar dust models.