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First Transient Z Source XTE J1701–462

25/07/2011

It was in 1962 when the first low-mass X-ray binary (LMXB) was discovered: Scorpius X-1. Such objects can be divided into two classes: Z sources and Atoll sources (Hasinger & van der Klis 1989). So far, only six galactic LMXBs have be classified as Z sources (Sco X-1, GX 17+2, GX 349+2, GX 5-1, GX 340+0 and Cyg X-2). In addition to these six Z sources, two other LMXBs also show the characteristics of Z sources in most cases. One is Circinus X-1 (Cir X-1), but was neither conclusively an Atoll source or a Z source (see Shirey, Bradt, & Levine 1999 ~ Z source classification; Oosterbroek et al. 1995 ~ Atoll classification).

Homan et al. (2006a, 2006b, 2007a) suggested that XTE J1701-462 was a Z source, because it exhibited typical CDs of Z sources, and its timing properties were also consistent with those of Z sources. However, it was also reported that XTE J1701-462 evolved into an atoll source at the end of its outburst (Homan et al. 2007b). As shown in Figure 4 in the paper of Homan et al. (2007a), XTE J1701-462 showed two types of tracks on the CDs: the “Z” tracks and the “ν” tracks during the Z-source stages.

N.B. For a comprehesive review of Z and ν see Muno, Remillard & Chakrabarty, 2002 (How Do Z & Atoll X-Ray Binaries Differ?). The source names are derived from their shape on soft colour-hard colour plots; e.g. 5 years of pointed RXTE PCA observations.

Lin, Remillard, & Homan (2009b) went on to analyse the Rossi X-Ray Timing Explorer (RXTE) observations of XTE J1701-462 during the 2006-2007 quasi-periodic outburst and concluded that during the Atoll-source stage the disk or neutron star (NS) evolved roughly along the * track, both the inner disk radius and blackbody radius of the NS remained constant, and the inner disk radius, comparable with the NS radius, was set by the innermost stable circular orbit (ISCO).

N.B. The  track is essentially an expression of Stefan-Boltzmann’s Law. For a more intensive overview of this track see van der Klis et al. (1996).

However, contrary to Lin, Remillard, & Homan’s (2009b) findings, during the Z-source stages the disc or NS was concluded to depart from the  track and the inner disc radius increased with the source intensity (Ding et al. 2011), which was believed to be due to the local Eddington limit in the disc having some sort of effect. It has also been noted that during the Atoll-source stage, the gas pressure from the disc pushes the magnetosphere to near the ISCO, so the inner disc radius is set by the ISCO. However, as the disc thickness increases due to an added radiative photon pressure the gas pressures decreases and consequently the inner disc radius has to increase (as both a result of the magnetosphere expanding and to conserve angular momentum).

Hasinger & van der Klis (1989) considered two scenarios for the relation between Z and Atoll sources: (i) the two types of sources are similar and their differences are due to the difference of their ; (ii) their differences are resulted from some additional parameters such as neutron star surface magnetic field strength, disk inclination, etc., rather than . They argued that under scenario (i), one could expect that in the CD the atoll and Z patterns would evolve into each other in the sources with a large range in and these sources then switch between Z and Atoll behaviours. Because this kind of evolution had never been observed, they excluded this scenario.

Therefore, they proposed scenario (ii) is responsible for the difference between Atoll sources and Z sources and suggested that both the mass accretion rate and neutron star surface magnetic field strength might be larger in Z sources than in atoll sources, which might be due to different evolutionary history. Conversely, it was observed that during the 2006-2007 outburst of XTE J1701-462, the track in CD evolved from the Z track, via the ν track, and finally to the Atoll pattern (Lin, Remillard, & Homan 2009b; Homan et al. 2010). This finding thus strongly supports the first scenario considered by Hasinger & van der Klis (1989).

Thus, it is now possible for astronomers to consider XTE J1701-462 as a major find, and the first transient source, useful in linking between Z and Atoll sources (Lin, Remillard, & Homan 2009b).

Journal References:

  • Hasinger, G.; van der Klis, M. (1989) Two Patterns Of Correlated X-ray Timing & Spectral Behaviour In Low-Mass X-ray Binaries. Astronomy & Astrophysics25 (1) pp. 79-96.
  • Homan, J. et al. (2007) RXTE Observations Of The First Transient Z Source XTE J1701-462: Shedding New Light On Mass Accretion In Luminous Neutron Star X-Ray Binaries. The Astrophysical Journal, 656 (1) pp. 420-430.
  • Lin, D.; Remillard, R.A.; Homan, J. (2009) Spectral States Of XTE J1701 – 462: Link Between Z & Atoll Sources. The Astrophysical Journal, 696 (2) pp. 1257-1277.
  • Ding. G.Q. et al. (2011) On The Nature Of The First Transient Z Source XTE J1701–462: Its Accretion Disk Structure, Neutron Star Magnetic Field Strength & Hard Tail. The Astrophysical Journal142 (2) Article I.D.: 34.
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