The Physics of Accretion Disk Atmospheres: Observing a 35d Turn On of Her X-1

We propose to study the evolution of the 0.510keV spectrum and pulse profile ofHer X1 in order to resolve the properties of the photo ionized accretion diskatmosphere of Her X1 as the outer rim of the warped accretion disk frees ourline of sight onto the neutron star. The proposed campaign consists of five 10ksobservations that cover the 2.5d of one turn on of the main on state of the 35dcycle. The observations will be spaced 0.5d apart and cover the whole turn onfrom quiescence to almost the peak of the main on. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]