The nature of quiescent emission in the neutron star Cen X-4

The neutron star LMXB, Cen X4, has been in quiescence since 1979. Its quiescentXray emission consists of both a thermal and powerlaw component, withvariability observed on timescales as short as hundreds of seconds and as longas years. Yet, this variability is not understood. We recently found Cen X 4 ina historically lowstate, over 5 times fainter than before. Such a differencerequires the thermal component to vary. This poses a big problem for usingthermal emission to measure neutron star radii and constrain the dense matterequation of state. To investigate the nature of this variability and theemission mechanisms from Xrays through to the optical, we propose four, 12 ksobservations of Cen X4 to obtain good Xray spectra and accurate photometry in 5 optical/UV filters. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]