Constraints on the dust extinction law of the Galaxy with Swift/UVOT,
  Gaia and 2MASS
  We explore variations of the dust extinction law of the Milky Way by
selecting stars from the Swift/UVOT Serendipitous Source Catalogue,
cross-matched with Gaia DR2 and 2MASS to produce a sample of 10,452 stars out
to ~4kpc with photometry covering a wide spectral window. The near ultraviolet
passbands optimally encompass the 2175A bump, so that we can simultaneously fit
the net extinction, quoted in the V band (A$_V$), the steepness of the
wavelength dependence ($\delta$) and the bump strength (E$_b$). The methodology
compares the observed magnitudes with theoretical stellar atmospheres from the
models of Coelho. Significant correlations are found between these parameters,
related to variations in dust composition, that are complementary to similar
scaling relations found in the more complex dust attenuation law of galaxies -
that also depend on the distribution of dust among the stellar populations
within the galaxy. We recover the strong anticorrelation between A$_V$ and
Galactic latitude, as well as a weaker bump strength at higher extinction.
$\delta$ is also found to correlate with latitude, with steeper laws towards
the Galactic plane. Our results suggest that variations in the attenuation law
of galaxies cannot be fully explained by dust geometry.
