The Variable Quiescent Spectrum Of Cen X-4: A Problem For Neutron Star Radius Measurements?
Abstract
The nearby neutron star low-mass X-ray binary, Cen X-4, has been in a quiescent state since its last outburst in 1979. Typically, quiescent emission from these objects consists of thermal emission from the neutron star surface with an additional hard power-law tail of unknown nature. Variability has been observed during quiescence in Cen X-4 on both timescales as short as hundreds of seconds and as long as years. However, the nature of this variability is still unknown. Early observations seemed to show it was all due to a variable hard X-ray tail. Here, we present a recent Suzaku observation in a historically low-state that contradicts this. During this observation the source was greater than a factor of 5 fainter than a previous XMM observation. Such a large change cannot be explained by just the power-law component, the thermal component has to vary. If the emitting area remains unchanged between observations, then the effective surface temperature must change. Alternatively, if the temperature remains constant, then the emitting area must change. We will discuss possible interpretations of this curious thermal variability.
- Publication:
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AAS/High Energy Astrophysics Division #11
- Pub Date:
- March 2010
- Bibcode:
- 2010HEAD...11.4226C