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 \textit{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 hours.