We report on an analysis of RXTE data of the transient neutron star low-mass X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst. The X-ray properties of the source changed between those of various types of NS-LMXB subclasses. At high luminosities the source switched between two types of Z source behavior and at low luminosities we observed a transition from Z source to atoll source behavior. These transitions between subclasses primarily manifest themselves as changes in the shapes of the tracks in X-ray color-color and hardness-intensity diagrams, but they are accompanied by changes in the kHz quasi-periodic oscillations, broad-band variability, burst behavior, and/or X-ray spectra. We find that the low-energy X-ray flux is a good parameter to track the gradual evolution of the tracks in color-color and hardness-intensity diagrams, allowing us to resolve the evolution of the source in greater detail than before and relate the observed properties to other NS-LMXBs. We further find that during the transition from Z to atoll, characteristic behavior known as the atoll upper banana can equivalently be described as the final stage of a weakening Z source flaring branch, thereby blurring the line between the two subclasses. Our findings strongly suggest that the wide variety in behavior observed in NS-LXMBs with different luminosities can be linked through changes in a single variable parameter, namely the mass accretion rate, without the need for additional differences in the neutron star parameters or viewing angle. We briefly discuss the implications of our findings for the spectral changes observed in NS LMXBs and suggest that, contrary to what is often assumed, the position along the color-color tracks of Z sources is not determined by the instantaneous mass accretion rate.
Neutron star low-mass X-ray binaries (NS-LMXBs) are systems in which a low-magnetic-field neutron star accretes matter from a low-mass companion star through Roche-lobe overflow. They display a wide range of X-ray spectral and variability properties, both as a class and as individual sources (see van der Klis 2006, for a recent review). Based on their correlated X-ray spectral and rapid variability behavior, NS-LMXBs are commonly divided into two subclasses, the so-called Z sources and the atoll sources (Hasinger & van der Klis 1989), named after the shapes they trace out in X-ray color-color (CD) and hardness-intensity diagrams (HID). The Z sources have luminosities close to the Eddington luminosity (∼0.5-1 L Edd or more), whereas atoll sources have luminosities in the range ∼0.01-0.5 L Edd .
An example of the variety in shapes that is observed in the CD/HID tracks of the Z and atoll subclasses is shown in Figure 1. The Z source tracks typically show three branches, which from top to bottom are called the horizontal branch, the normal branch, and the flaring branch. Based on the presence and orientation of these branches, the Z sources can be further divided into the ‘Cyg-like’ Z sources (Cyg X-2, GX 5-1, and GX 340+0) and the ‘Sco-like’ Z sources (Sco X-1, GX 17+2, and GX 349+2), with the former showing ‘Z’-shaped tracks in the HID (Fig. 1a) and the latter more ‘ν’-shaped tracks (Fig. 1b,c; see also Kuulkers et al. 1994;Homan et al. 2007b (hereafter H07)). The atoll sources can show ‘Z’-shaped tracks as well (Fig. 1g), although the individual branches are thought to have a different physical nature than the Z source branches (Barret & Olive 2002;van Straaten et al. 2003;Reig et al. 2004;van der Klis 2006). The branches of the Z-shaped atoll tracks are called, from top to bottom (Fig. 1g), the extreme island state, the island state, and the lower and upper banana branches; these branches are also frequently referred to as the hard, transitional/intermediate, and soft states, respectively. The low-luminosity atolls are typically found in the (extreme) island state, whereas the high-luminosity ones tend to be found mainly on the (upper) banana branch (Fig. 1d).
In addition to motion along atoll or Z branches, some sources also show changes in the shape and location of their tracks in the CD and HID, which is often referred to as secular motion. Examples of this can be seen in Figure 1, with the horizontal branch of GX 17+2 and the upper-banana branches of GX 9+9 and 4U 1735-44 being traced out at varying intensity levels. Much (left) and HIDs (right) of the Z source NS-LMXBs GX 5-1, GX 17+2, and GX 349+2, and the atoll source NS-LMXBs GX 9+9, 4U 1735-44, 4U 1636-53, and 4U 1705-44. Each data point represents a 256s average. The diagrams are taken from a compilation by Fridriksson et al. (2010, in prep.) and were created following the steps described and referred to in §2 and §3. Z and atoll branches/states are indicated in the CD or HID of each source. They are: horizontal branch (HB), normal branch (NB), flaring branch (FB), upper-banana branch (UB), lower-banana branch (LB), island state (IS), and extreme island state (EIS). For the Z source GX 5-1 we have also labelled the upturn from the horizontal branch (HB-upturn), and the ‘dipping’ flaring branch (dipping FB).
stronger secular motion can be found in the Z source Cyg X-2 (Kuulkers et al. 1996;Wijnands et al. 1997), where it occasionally results in switches between Cyg-like and Sco-like Z source behavior (see Figure 1 in Muno et al. 2002). Finally, we note that transitions between the atoll (extreme) island state and the banana branches, and vice versa, often show strong hysteresis in HIDs, resulting in jumps between those branches (e.g., Maitra & Bailyn 2004;Gladstone et al. 2007;Belloni et al. 2007, see also Fig. 1f,g). It is not clear if and how this behavior is related to the secular motion.
In addition to differences in X-ray luminositiy and CD/HID tracks, the NS-LMXBs subclasses also show differences in X-ray spectral and variability properties (van der Klis 2006), radio behavior (Migliari & Fender 2006), as well as in the rate at which they show type I (thermonuclear) X-ray bursts (Galloway et al. 2008). This disparity in the properties between the subclasses was a long-standing problem in the understanding of the accretion processes in NS-LMXBs. While differences in the mass accretion rate ( Ṁ ) could explain some of the variety in observed properties (e.g., in X-ray luminosity and bursting behavior), it was unclear if one could link the wide variety in CD/HID tracks, rapid variability, and broadband spectral properties through changes in Ṁ alone. Additional parameters have been proposed to explain some of the observed differences, not only between Z and atoll subclasses, but also between the Cyglike and Sco-like Z sources: magnetic field strength and viewing angle (e.g., Psaltis et al. 1995;Hasinger & van der Klis 1989;Kuulkers & van der
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