An elaborate search for coherent pulsations from Intermittent–AMXPs

Abstract

We present a detailed systematic pulse search for three Intermittent-Accreting Millisecond X-ray Pulsars (Intermittent-AMXPs), HETE J1900.1-2455, SAX J1748.9-2021& Aql X-1, via Z${}_1^2$ and maximum likelihood (ML) techniques by using 16 years data of Rossi X-ray Timing Explorer/Proportional Counter Array (RXTE/PCA) in the energy range of 3.0–13.0 keV. We first performed a pulse scan using the Z${}_1^2$ technique in millisecond sensitivities for every 25 s time interval with 1 s shifts to cover all data set around the detected frequencies given in the literature. We tracked the Z${}_1^2$ power over time and flagged the time intervals exceeding defined threshold levels for each source as pulse candidates. The detected pulse list throughout our scan has new discoveries while covering the pulsed regions presented in the literature. For a deeper search, using the pulses obtained from the Z${}_1^2$ method as a probability density function as an input parameter, we re-scanned the time intervals centered on the detected pulse via ML. The detected pulse-on duration via ML is slightly longer than the one via Z${}_1^2$ method. This phenomenon allows us to argue for the existence of the smooth transition between pulse-on and pulse-off stages. For SAX J1748.9-2021, we also obtained orbital period by using the systematic pulse arrival phase patterns throughput of ML to be 8.76 h.