Timing and spectral study of XB 1254-690 using new RXTE PCA data

TL;DR

This study analyzes RXTE PCA data of XB 1254-690, detecting a low-frequency QPO and performing the first systematic spectral analysis, revealing that a combined broken-powerlaw and Comptonization model best describes its X-ray spectrum.

Contribution

First detection of a low-frequency QPO in XB 1254-690 with significance, and first application of a broken-powerlaw component in spectral modeling of this source.

Findings

Detected a low-frequency QPO with ~3.5σ significance.

Identified that a combined broken-powerlaw and Comptonization model best fits the spectrum.

Plasma temperature (~3 keV) and optical depth (~7) are consistent with other sources.

Abstract

We have analyzed the new Rossi X-ray Timing Explorer Proportional Counter Array data of the atoll neutron star (NS) low-mass X-ray binary (LMXB) system XB 1254-690. The colour-colour diagram shows that the source was in the high-intensity banana state. We have found two low-frequency candidate peaks with single trial significances of ~ 2.65 X 10^{-8} and ~ 7.39 X 10^{-8} in the power spectra. After taking into account the number of trials, the joint probability of appearance of these two peaks in the data set only by chance is ~ 4.5 X 10^{-4}, and hence a low-frequency QPO can be considered to be detected with a significance of ~ 4.5 X 10^{-4}, or, ~ 3.5\sigma for the first time from this source. We have also done the first systematic X-ray spectral study of XB 1254-690, and found that, while one-component models are inadequate, three-component models are not required by the data. We have concluded that a combined broken-powerlaw and Comptonization model best describes the source continuum spectrum among 19 two-component models. The plasma temperature (~ 3 keV) and the optical depth (~ 7) of the Comptonization component are consistent with the previously reported values for other sources. However, the use of a broken-powerlaw component to describe NS LMXB spectra has recently been started, and we have used this component for XB 1254-690 for the first time. We have attempted to determine the relative energy budgets of the accretion disc and the boundary layer using the best-fit spectral model, and concluded that a reliable estimation of these budgets requires correlations among time variations of spectral properties in different wavelengths.