  Recent observations with dust detectors on board the interplanetary
spacecraft Ulysses and Galileo have recorded a substantial flux of large
interstellar grains with radii between 0.25 and 2.0 mu entering the solar
system from the local interstellar cloud. The most commonly used interstellar
grain size distribution is characterized by a a^-3.5 power law in grain radii
a, and extends to a maximum grain radius of 0.25 mu. The extension of the
interstellar grain size distribution to such large radii will have a major
effect on the median grain size, and on the amount of mass needed to be tied up
in dust for a given visual optical depth. It is therefore important to
investigate whether this population of larger dust particles prevails in the
general interstellar medium, or if it is merely a local phenomenon. The
presence of large interstellar grains can be mainly inferred from their effect
on the intensity and radial profiles of scattering halos around X-ray sources.
In this paper we examine the grain size distribution that gives rise to the
X-ray halo around Nova Cygni 1992. The results of our study confirm the need to
extend the interstellar grain size distribution in the direction of this source
to and possibly beyond 2.0 mu. The model that gives the best fit to the halo
data is characterized by: (1) a grain size distribution that follows an a^-3.5
power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0
mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044
and 0.0022, respectively, consistent with solar abundances constraints.
Additional observations of X-ray halos probing other spatial directions are
badly needed to test the general validity of this result.
