Early Ultraviolet, Optical, and X‐Ray Observations of the Type IIP SN 2005cs in M51 withSwift

We report early photospheric-phase observations of the Type IIP Supernova(SN) 2005cs obtained by Swift's Ultraviolet-Optical and X-Ray Telescopes.Observations started within two days of discovery and continued regularly forthree weeks. During this time the V-band magnitude remained essentiallyconstant, while the UV was initially bright but steadily faded until below thebrightness of an underlying UV-bright HII region. This UV decay is similar toSNe II observed by the International Ultraviolet Explorer. UV grism spectrashow the P-Cygni absorption of MgII 2798A, indicating a photospheric origin ofthe UV flux. Based on non-LTE model atmosphere calculations with the CMFGENcode, we associate the rapid evolution of the UV flux with the cooling of theejecta, the peak of the spectral energy distribution (SED) shifting from ~700Aon June 30th to ~1200A on July 5th. Furthermore, the correspondingrecombination of the ejecta, e.g., the transition from FeIII to FeII, inducesconsiderable strengthening of metal line-blanketing at and above thephotosphere, blocking more effectively this fading UV flux. SN2005cs was notdetected in X-rays, and the upper limit to the X-ray luminosity yields a limitto the mass loss rate of the progenitor of about 10^-5 solar masses per year.Overall, Swift presents a unique opportunity to capture the early and fastevolution of Type II SNe in the UV, providing additional constraints on thereddening, the SED shortward of 4000A, and the ionization state and temperatureof the photon-decoupling regions.Comment: 15 pages, 6 figures. Accepted for publication by Astrophysical Journa