Spectral Study of Aql~X-1 during \textit{Pulse-on} and \textit{Pulse-off} Stages

T. Kocabıyık, C. Güngör, M. T. Sağlam, T. Güver, Z. F. Bostancı

Published 2025-01-08Version 1

We present a detailed spectral study of an intermittent-AMXP Aql X-1 during the \textit{pulse-on} and \textit{pulse-off} stages by using the archival RXTE data. We first perform temporal analysis by using Z$_n^2$ technique in three different energy bands, 3.0 -- 13.0~keV, 13.0 -- 23.0~keV and 23.0 -- 33.0~keV, for the last 128~s time segment of the RXTE data including \textit{pulse-on} region. We show that the pulse is the most significant in the softest band. We, then, show that the spectrum is represented the best via combination of absorbed blackbody, disk blackbody and a gaussian line. We modeled the last four segments of the data 30188-03-05-00 to better compare \textit{pulse-on} and \textit{pulse-off} stages. We found a vague residual in the spectral fit of the \textit{pulse-on} segment between $\sim$3.0 -- 13.0~keV which agrees with the result of temporal analysis. We show that the residual may be represented with an extra blackbody component with the temperature of 1.75~keV and the radius of 0.75$\pm$0.49 km. For deeper analysis, we performed phase-resolved spectroscopy to the last 128~s, \textit{pulse-on}, segment. We obtain two separate spectra for the spin phase range of 0.75 -- 0.25, \textit{pulse-high} and 0.25 -- 0.75, \textit{pulse-low} and followed the same procedure. We display that the residual becomes more clear for \textit{pulse-high} compared to the \textit{pulse-low}. We report that the additional blackbody component, which models the residual, indicates a hotspot from the surface of the neutron star with the radius of 1.65$\pm$0.74 km whose temperature is 1.65 keV.