Very early ultraviolet and optical observations of the type Ia supernova 2009ig

Supernova (SN) 2009ig was discovered 17hr after explosion by the Lick Observatory Supernova Search, promptly classified as a normal Type Ia SN (SNIa), peaked at V = 13.5mag, and was equatorial, making it one of the foremost SNe for intensive study in the last decade. Here, we present ultraviolet (UV) and optical observations of SN2009ig, starting about 1day after explosion until around maximum brightness. Our data include excellent UV and optical light curves, 25 premaximum optical spectra, and 8UV spectra, including the earliest UV spectrum ever obtained of an SNIa. SN2009ig is a relatively normal SNIa, but does display high-velocity ejecta - the ejecta velocity measured in our earliest spectra (v ≈ -23, 000 kms^-1 for Si II λ6355) is the highest yet measured in an SNIa. The spectral evolution is very dramatic at times earlier than 12days before maximum brightness, but slows after that time. The early-time data provide a precise measurement of 17.13 ± 0.07days for the SN rise time. The optical color curves and early-time spectra are significantly different from template light curves and spectra used for light-curve fitting and K-corrections, indicating that the template light curves and spectra do not properly represent all SNeIa at very early times. In the age of wide-angle sky surveys, SNe like SN2009ig that are nearby, bright, well positioned, and promptly discovered will still be rare. As shown with SN2009ig, detailed studies of single events can provide significantly more information for testing systematic uncertainties related to SNIa distance estimates and constraining progenitor and explosion models than large samples of more distant SNe.