Ultraviolet Spectroscopy of Supernovae: The First Two Years of Swift
  Observations
  We present the entire sample of ultraviolet (UV) spectra of supernovae (SNe)
obtained with the Ultraviolet/Optical Telescope (UVOT) on board the Swift
satellite during the first 2 years of observations (2005/2006). A total of 29
UV-grism and 22 V-grism spectra of 9 supernovae (SNe) have been collected, of
which 6 are thermonuclear (type Ia) and 3 core collapse (type Ibc/II) SNe. All
the spectra have been obtained during the photospheric phase. After a
comparison of the spectra of our sample with those in the literature (SNe
1992A, 1990N and 1999em), we confirm some degree of diversity in the UV
emission of Type Ia SNe and a greater homogeneity in the Type II Plateau SN
sample. Signatures of interaction between the ejecta and the circumstellar
environment have been found in the UV spectrum of SN 2006jc, the only SN Type
Ib/c for which UVOT grism data are available. Currently, Swift UVOT is the best
suited instrument for early SN studies in the UV due to its fast response and
flexible scheduling capabilities. However, in order to increase the quality of
the data and significantly improve our understanding of the UV properties of
SNe and to fully maximize the scientific potential of UVOT grism observations,
a larger investment in observing time and longer exposures are needed.
