Early ultraviolet observations of a Type IIn supernova (2007pk)

We present some of the earliest UV observations of a TypeIIn supernova (SN) - SN 2007pk, where UV and optical observations using Swift's Ultra-Violet/Optical Telescope began 3 days after discovery or ∼5 days after shock breakout. The SN observations commence at approximately maximum light in the UV and u-band filters, suggesting that the UV light curve peaks begin very rapidly after the initial explosion, and subsequently exhibit a linear decay of 0.20, 0.21, 0.16 mag day^-1 in the UVOT uvw2, uvm2, uvw1 (λ_c = 1928, 2246, 2600 Å) filters. Meanwhile the b- and v-band light curves begin approximately seven days before v-band peak and exhibit a shallow rise followed by a subsequent decay. A series of optical/near-IR spectra taken with the Hobby-Eberly Telescope at days 3-26 after discovery show spectra similar to that of the peculiar TypeIIn 1998S. The emission from 2007pk falls below detection ∼20 days after discovery in the UV and 50 days in the optical, showing no sign of the long duration emission seen in other TypeIIn SNe. We examine the physical and spectral characteristics of 2007pk and compare its UV light curve and decay rate with other TypeII SNe.