A Detailed Spectral Study of Intermittent-Accreting Millisecond X-ray Pulsar Aql X-1 during Pulse-on and Pulse-off Stages

TL;DR

This study provides a detailed spectral analysis of the intermittent millisecond X-ray pulsar Aql X-1 during different pulse stages, revealing a potential hotspot on the neutron star surface through phase-resolved spectroscopy.

Contribution

It introduces a combined spectral and temporal analysis of Aql X-1, identifying an additional blackbody component linked to a neutron star hotspot during pulse-on stages.

Findings

Pulse is most significant in the softest energy band.

An extra blackbody component suggests a neutron star hotspot.

Residual spectral features are more prominent during pulse-high phases.

Abstract

We present a detailed spectral study of an intermittent-AMXP Aql X-1 during the pulse-on and 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 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 pulse-on and pulse-off stages. We found a vague residual in the spectral fit of the 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$0.49 km. For deeper analysis, we performed phase-resolved spectroscopy to the last 128 s, pulse-on, segment. We obtain two separate spectra for the spin phase range of 0.75 -- 0.25, pulse-high and 0.25 -- 0.75, pulse-low and followed the same procedure. We display that the residual becomes more clear for pulse-high compared to the pulse-low. We report that the additional blackbody component, which models the residual, indicates a hotspot from the surface of the neutron star with the radius of 1.65$\pm$0.74 km whose temperature is 1.65 keV.