Energy‐dependent ∼100 μs Time Lags as Observational Evidence of Comptonization Effects in the Neutron Star Plasma Environment

We present a Comptonization model for the observed properties of theenergy-dependent soft/hard time lags and pulsed fraction (amplitude) associatedwith the pulsed emission of a neutron star (NS). We account for the soft lagsby downscattering of hard X-ray photons in the relatively cold plasma of thedisk or NS surface. A fraction of the soft X-ray photons coming from the diskor NS surface upscatter off hot electrons in the accretion column. This effectleads to hard lags as a result of thermal Comptonization of the soft photons.This model reproduces the observed soft and hard lags due to the down- andupscattered radiation as a function of the electron number densities of thereflector, n_{e}^{ref}, and the accretion column, n_{e}^{hot}. In the case ofthe accretion-powered millisecond pulsars IGR J00291+5934, XTE J1751-305, andSAX J1808.4-3658, the observed time lags agree well with the model. Soft lagsare observed only if n_e^{ref} << n_e^{hot}. Scattering of the pulsed emissionin the NS environment may account for the observed time lags as a non-monotonicfunction of energy. The time lag measurements can be used as a probe of theinnermost parts of the NS and accretion disk. We determine the upper and lowerlimits of the density variation in this region using the observed time lags.The observed energy-dependent pulsed amplitude allows us to infer a variationof the Thomson optical depth of the Compton cloud in which the accretion columnis embedded.Comment: 4 pages, 3 figures, ApJ accepte