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

The neutron star LMXB, Cen X-4, has been in quiescence since 1979. Its quiescent X-ray emission consists of both a thermal and power-law component, with variability observed on timescales as short as hundreds of seconds and as long as years. Yet, this variability is not understood. We recently found Cen X- 4 in a historically low-state, over 5 times fainter than before. Such a difference requires the thermal component to vary. This poses a big problem for using thermal emission to measure neutron star radii and constrain the dense matter equation of state. To investigate the nature of this variability and the emission mechanisms from X-rays through to the optical, we propose four, 12 ks observations of Cen X-4 to obtain good X-ray spectra and accurate photometry in 5 optical/UV filters.