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 on a regular basis for three 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 2798Å , 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 \sim 700Å on June 30th to \sim 1200Å on July 5th .
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-rays , and the upper limit to the X-ray luminosity yields a limit to the mass loss rate of the progenitor of { { { { \dot { M } \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle < % $ \cr$ \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \textstyle < $ \cr% $ \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptstyle < $ \cr$% \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptscriptstyle < $% \cr$ \scriptscriptstyle \sim$ } } } } 1 \times 10 ^ { -5 } ~ { } M _ { \odot } ~ { } { yr } ^ { -1 } ~ { } ( % v _ { w } / 10 ~ { } { km~ { } s } ^ { -1 } ) .
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 4000Å , and the ionization state and temperature of the photon-decoupling regions .