The Variable Quiescence of Cen X - 4

Cen X-4 is one of the best studied low-mass neutron star transients in quiescence. Thanks to XMM-Newton large throughput, Cen X-4 was observed at the highest signal to noise ever. This allowed us to disclose rapid (> 100 s), large (45 ± 7 rms in the 10 −4 − 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 occuring 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. 1. introduction Many Low Mass X–ray Binaries (LMXRB) accrete matter at very high rates, and therefore shine as bright X–ray sources, only sporadically. Among these systems are Soft X–ray Transients (SXRTs) hosting an old neutron star (for a review see Campana et al. 1998a). These systems alternate periods (weeks to months) of high X–ray luminosity, during which they share the same properties of persistent LMXRBs, to long (1–few 10 years) intervals of quiescence in which the X–ray luminosity drops by up to 5–6 orders of magnitude.