Millisecond phenomena in mass accreting neutron stars

The past twelve years have seen the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), of several long-predicted phenomena associated with the accretion of matter onto a neutron star in a binary (double) star system. These phenomena are observed in the strong X-ray emission produced by these neutron stars, at luminosities up to several 100,000 Solar luminosities, and take place at the dynamical time scale of the strong field gravity region surrounding a neutron star (~0.001 sec). Physical models for these millisecond phenomena include the neutron star spin, thermonuclear burning and release of gravitational energy at the neutron star surface, and general relativistic orbital and epicyclic motions in the accreting plasma. The detection and characterization of these phenomena (some of which are themselves best modeled as noise phenomena, others of which have a more deterministic nature but always involve random-process aspects) is done by the estimation of the Fourier power spectrum of the intrinsic millisecond time variations in the X-ray luminosity in the presence of dominant photon counting statistics noise which can be characterized as Poisson noise modified by instrumental deadtime effects. To a large extent these methods work well: they give astronomers the quantitative answers they need. I discuss some of the methodological challenges as well as some of the main results in this field.