Journal Article

Cross-correlations between soft and hard light curves depending on luminosity in the transient neutron star XTE J1701−462

Y. N. Wang, Y. J. Lei, G. Q. Ding, J. L. Qu, M. Y. Ge, C. M. Zhang, L. Chen and X. Ma

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 440, issue 4, pages 3726-3737
ISSN: 0035-8711
Published online April 2014 | e-ISSN: 1365-2966 | DOI:
Cross-correlations between soft and hard light curves depending on luminosity in the transient neutron star XTE J1701−462

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Using all the observations from Rossi X-ray Timing Explorer for the accreting neutron star XTE J1701−462, we carry out a systematic study of the cross-correlation function between its soft and hard light curves. Over the entire outburst, XTE J1701−462 evolves from super-Eddington luminosities to quiescence and shows both Z and atoll behaviours. Following previous work, we divide the outburst into five intervals: one Cyg-like interval, three Sco-like intervals and one atoll interval, according to their different behaviours in the corresponding colour–colour diagrams (CCDs). With cross-correlation analyses, anti-correlation, positive and ambiguous correlations are found in the different intervals in this source. Both anti-correlated soft and hard time lags are detected, where hard lags mean that the hard photons lag behind the soft ones and soft lags mean the reverse. In the Cyg-like interval, anti-correlations are presented in the hard vertex and upper normal branch (NB) and positive correlations dominate the horizontal branch (HB) and lower NB. In the first two Sco-like intervals, anti-correlations are detected first and most of them are found in the HB and/or upper NB, while positive correlations are mostly detected in the lower NB and flaring branch (FB). In the following interval, i.e. the third Sco-like interval, anti-correlations occur on the upper FB and positive correlations are mainly distributed in the lower FB. The different intervals correspond to various luminosities; therefore, the position of anti-correlations in the CCD might depend on the luminosity. It is noted that, in the Cyg-like interval, positive correlations dominate the HB, which is not consistent with the behaviour of the Cyg-like Z sources GX 5-1 and Cyg X-2, the HBs of which host ambiguous correlations and anti-correlations. Hence, for comparison with GX 5-1, we analyse the spectra of the HB and the hard vertex of the Cyg-like interval. The fitting results show that, in contrast to GX 5-1, the ratio of hard emission to soft emission basically remains unvaried from the HB to the hard vertex, which might result in positive correlation. Additionally, we compare the spectra of the third Sco-like interval with atoll source 4U 1735−44 and find that their spectral evolution along the tracks in the CCD is similar, indicating that in this interval XTE J1701−462 could approach an atoll source. The truncated disc model might be responsible for the detected time lags in XTE J1701−462; the possible origins of anti-correlated hard and soft time lags are also discussed.

Keywords: accretion, accretion discs; X-rays: binaries; stars: individual: XTE J1701−462

Journal Article.  5658 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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