Journal Article

Timing and spectral study of XB 1254−690 using new <i>RXTE</i> PCA data

Arunava Mukherjee and Sudip Bhattacharyya

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 411, issue 4, pages 2717-2725
Published in print March 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI:
Timing and spectral study of XB 1254−690 using new RXTE PCA data

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We have analysed 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 × 10−8 and ≈7.39 × 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 × 10−4, and hence a low-frequency quasi-periodic oscillation can be considered to be detected with a significance of ∼ 4.5 × 10−4, or ∼ 3.5σ 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-power-law 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-power-law component to describe the 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-fitting spectral model, and concluded that a reliable estimation of these budgets requires correlations among time variations of spectral properties in different wavelengths.

Keywords: accretion, accretion discs; methods: data analysis; techniques: miscellaneous; stars: neutron; X-rays: binaries; X-rays: individual: XB 1254−690

Journal Article.  5709 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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