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

Abstract

Using all the observations from Rossi X-ray Timing Explorer for the accreting neutron star XTE J1701-462, we carry out a systematic study on the cross-correlation function between its soft and hard light curves. During the entire outburst, XTE J1701-462 evolves from super-Eddington luminosities to quiescence, and shows both Z and atoll behaviours. Following the 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 behaviors in the corresponding color-color diagrams (CCDs). With cross-correlation analyses, the anti-correlation, positive and ambiguous correlations are found in the different intervals in this source. Both the anti-correlated soft and hard time lags are detected, where the hard lags mean that the hard photons lag behind the soft ones and the soft lags are reverse. In the Cyg-like interval, the anti-correlations are presented in the hard vertex and upper normal branch (NB), and the positive correlations dominate the horizontal branch (HB) and lower NB. In the first two Sco-like intervals, the anti-correlations are firstly detected and most of them show up the HB and/or upper NB, and the positive correlations are mostly detected in the lower NB and flaring branch (FB). While in the following interval, i.e., the third Sco-like interval, the anticorrelations occur on the upper FB, and the positive correlations mainly distribute in the lower FB. The different intervals are corresponding to the 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, the positive correlations dominate the HB, which is not consistent with that of the Cyg-like Z sources GX 5-1 and Cyg X-2 whose HBs host ambiguous correlations and anti-correlations. Hence, for comparing with GX 5-1, we analyze the spectra of the HB and the hard vertex of the Cyg-like interval. The fitting results show that, different from GX 5-1, the ratio of the hard emission to the soft emission basically keeps unvaried from the HB to the hard vertex, which might result in the positive correlation. Additionally, we compare the spectra of the third Sco-like interval with atoll source 4U 1735-44, and find 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 disk model might be responsible for the detected time lags in XTE J1701-462, and the possible origins of the anti-correlated hard time lags and soft time lags are also discussed.

A study of the cross-correlation and time lag in black hole X-ray binary XTE J1859+226

“…Based on the systematic cross-correlation analysis of 4U 1608-52 (atoll source), Lei et al (2014) proposed that the distribution of the cross-correlations on the CCDs might be in evolution with the luminosity of the source. Similar results were also found in a Z-source XTE J1701-462 by Wang et al (2014). However, the evolution of cross-correlations along with the luminosity of the source and the spectral states has never been investigated in BHXBs, nor has the crosscorrelation analysis been performed on a whole outburst of BHXBs.…”

“…Tens of seconds to over 1500 seconds anti-correlated time lags were detected in the IMS of other BHXBs that are GRS 1915+105 (Choudhury et al 2005), XTE J1550-564, 4U 1630-47 (Sriram et al 2007), H1743-322 Raghurama Rao 2009) andGX 339-4 (Sriram, Rao, &Choi 2010). Similar kinds of anti-correlated time lags were observed in six other NS LMXBs, including four Z-sources (Cyg X-2 (Lei et al 2008), GX 5-1 (Sriram, Choi, & Rao 2012), XTE J1701-462 (Wang et al 2014) and GX 349+2 (Ding et al 2016)) and two atoll sources (4U 1735-44 (Lei et al 2013) and 4U 1608-52 ), and positive correlated time lags were also observed in them. Furthermore, the evolution of the crosscorrelation along the branches in the CCD or hardness intensity diagram (HID) was studied for the six NS LMXBs.…”

“…This phenomenon is also detected in atoll source 4U 1608-52 ) and Z-source GX 349+2 (Ding et al 2016). However, for atoll source 4U 1735-44 (Lei et al 2013) and Z-source XTE J1701-462 (Wang et al 2014), the length of time lags in positive correlation observations does not show obvious difference from that in anti-correlation observations.…”

“…The atoll sources show island state (IS) and banana state (BS) in CCDs, and BS can be subdivided into three states, viz., lower left banana (LLB), lower banana (LB), and upper banana (UB) states. Anti-correlations are found in HB, NB and FB of three Z-sources, viz., Cyg X-2 (Lei et al 2008), GX 5-1 (Sriram, Choi, & Rao 2012) and XTE J1701-462 (Wang et al 2014). For the atoll source, the anti-correlated observations are detected in the LB and UB of 4U 1735-44 (Lei et al 2013), and in the IS, LLB and UB of 4U 1608-52 .…”

“…Based on the 114 observations, we study the characteristics of XTE J1859+226 covering the entire duration of the single outburst. Given that the light curves from some observations are discontinued, we follow Sriram, Choi, & Rao (2012); Lei et al (2013Lei et al ( , 2014; Wang et al (2014) by dividing these observations into several individual segments with uninterrupted duration (> 1000 s). We finally obtain 140 observations/segments.…”