2007ApJ...659.1488B

We report early photospheric-phase observations of the Type IIP supernova (SN) 2005cs obtained by the Swift ultraviolet-optical and X-ray telescopes. Observations started within 2 days of discovery and continued on a regular basis for 3 weeks. During this time the V-band magnitude remained essentially constant, while the UV was initially bright, but steadily faded until below the brightness of an underlying UV-bright H II region. This UV decay is similar to SNe II observed by the International Ultraviolet Explorer. UV grism spectra show the P Cygni absorption of Mg II 2798 Å, indicating a photospheric origin of the UV flux. Based on non-LTE model atmosphere calculations with the CMFGEN code, we associate the rapid evolution of the UV flux with the cooling of the ejecta, the peak of the spectral energy distribution (SED) shifting from ∼700 Å on June 30 to ∼1200 Å on July 5. Furthermore, the corresponding recombination of the ejecta, e.g., the transition from Fe III to Fe II, induces a considerable strengthening of metal line-blanketing at and above the photosphere, blocking more and more effectively this fading UV flux. SN 2005cs was not detected in the X-ray band, and the upper limit to the X-ray luminosity yields a limit to the mass-loss rate of the progenitor of M{dot}≲1x10^–5/yr (v_w/10 km/s). Overall, Swift represents a unique opportunity to capture the early and fast evolution of Type II SNe in the UV, providing additional constraints on the reddening, the SED shortward of 4000 Å, and the ionization state and temperature of the photon-decoupling regions.