Variability in the quiescent state of neutron star transients

Variability studies represent a powerful tool to assess the emission mechanism(s) at work in the quiescent state of transient low mass X-ray binaries. Here we report on Chandra and XMM-Newton observations of two well known transient sources: Aql X-1 and Cen X-4. Long and short term variability is observed. This variability can be easily interpreted in 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.