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   72033
   Recent re-analysis of archival ASCA observations of three transiently
   accreting neutron stars (Cen X-4, Aql X-1, and 4U 1608--52) suggests
   that some of the quiescent emission is powered by the heated core.
   Accretion may still contribute to the quiescent luminosity. Variability
   provides a discriminant between these two sources of thermal surface
   emission. We propose to re-observe these objects while they are in
   quiescence. This, combined with the archival ASCA data, will allow us
   to search for variability, on timescales of weeks to years, in the
   quiescent luminosities and spectra. The proposed observations will also
   provide benchmarks for future observations with, e.g., AXAF and XMM.
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