The variable quiescence of Centaurus X-4

Centaurus X-4 is one of the best-studied low-mass neutron star transients in quiescence. Thanks to XMM-Newton's large throughput, Centaurus X-4 was observed at the highest signal-to-noise ratio ever. This allowed us to discern rapid (> 100 s), large (45% +/- 7% rms in the 10(-4) to 1 Hz range) intensity variability, especially at low energies. In order to highlight the cause of this variability, we divided the data into intensity intervals and fit the resulting spectra with the canonical model for neutron star transients in quiescence, i.e., an absorbed power law plus a neutron star atmosphere. The fit is consistent with a variable column density plus variability in ( at least) one of the spectral models. Variations in the neutron star atmosphere might suggest that accretion onto the neutron star surface is occurring in quiescence; variations in the power-law tail should support the view of an active millisecond radio pulsar emitting X-rays at the shock between a radio pulsar wind and inflowing matter from the companion star.