The Absolute Magnitudes of Type Ia Supernovae in the Ultraviolet
  We examine the absolute magnitudes and light-curve shapes of 14
nearby(redshift z = 0.004--0.027) Type Ia supernovae (SNe~Ia) observed in the
ultraviolet (UV) with the Swift Ultraviolet/Optical Telescope. Colors and
absolute magnitudes are calculated using both a standard Milky Way (MW)
extinction law and one for the Large Magellanic Cloud that has been modified by
circumstellar scattering. We find very different behavior in the near-UV
filters (uvw1_rc covering ~2600-3300 A after removing optical light, and u
~3000--4000 A) compared to a mid-UV filter (uvm2 ~2000-2400 A). The uvw1_rc-b
colors show a scatter of ~0.3 mag while uvm2-b scatters by nearly 0.9 mag.
Similarly, while the scatter in colors between neighboring filters is small in
the optical and somewhat larger in the near-UV, the large scatter in the
uvm2-uvw1 colors implies significantly larger spectral variability below 2600
A. We find that in the near-UV the absolute magnitudes at peak brightness of
normal SNe Ia in our sample are correlated with the optical decay rate with a
scatter of 0.4 mag, comparable to that found for the optical in our sample.
However, in the mid-UV the scatter is larger, ~1 mag, possibly indicating
differences in metallicity. We find no strong correlation between either the UV
light-curve shapes or the UV colors and the UV absolute magnitudes. With larger
samples, the UV luminosity might be useful as an additional constraint to help
determine distance, extinction, and metallicity in order to improve the utility
of SNe Ia as standardized candles.
