NEAR-ULTRAVIOLET PROPERTIES OF A LARGE SAMPLE OF TYPE Ia SUPERNOVAE AS OBSERVED WITH THE<i>Swift</i>UVOT

We present ultraviolet (UV) and optical photometry of 26 Type Ia supernovae\n(SNe~Ia) observed from March 2005 to March 2008 with the NASA {\\it Swift}\nUltraviolet and Optical Telescope (UVOT). The dataset consists of 2133\nindividual observations, making it by far the most complete study of the UV\nemission from SNe~Ia to date. Grouping the SNe into three subclasses as derived\nfrom optical observations, we investigate the evolution of the colors of these\nSNe, finding a high degree of homogeneity within the normal subclass, but\ndramatic differences between that group and the subluminous and SN 2002cx-like\ngroups. For the normal events, the redder UV filters on UVOT ($u$, $uvw1$) show\nmore homogeneity than do the bluer UV filters ($uvm2$, $uvw2$). Searching for\npurely UV characteristics to determine existing optically based groupings, we\nfind the peak width to be a poor discriminant, but we do see a variation in the\ntime delay between peak emission and the late, flat phase of the light curves.\nThe UV light curves peak a few days before the $B$ band for most subclasses (as\nwas previously reported by Jha et al. 2006a), although the SN 2002cx-like\nobjects peak at a very early epoch in the UV. That group also features the\nbluest emission observed among SNe~Ia. As the observational campaign is\nongoing, we discuss the critical times to observe, as determined by this study,\nin order to maximize the scientific output of future observations.\n