Rossi X-Ray Timing Explorer Observations of the First Transient Z Source XTE J1701–462: Shedding New Light on Mass Accretion in Luminous Neutron Star X-Ray Binaries

We report on the first 10 weeks of RXTE observations of the X-ray transient XTE J1701-462 and conclude that it had all the characteristics of the neutron star Z sources, i.e., the brightest persistent neutron star low-mass X-ray binaries. These include the typical Z-shaped tracks traced out in X-ray color diagrams and the variability components detected in the power spectra, such as kHz QPOs and normal and horizontal branch oscillations. XTE J1701-462 is the first transient Z source and provides unique insights into mass accretion rate (ṁ) and luminosity dependencies in neutron star X-ray binaries. As its overall luminosity decreased, we observed a switch between two types of Z source behavior, with the branches of the Z track changing their shape and/or orientation. We interpret this as an extreme case of the more moderate long-term changes seen in the persistent Z sources and suggest that they result from changes in ṁ. We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1, and GX 340+0) are substantially more luminous (>50%) than the Sco-like Z sources (Sco X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior of kHz QPOs, which involves a prompt as well as a filtered response to changes in ṁ, we further propose that changes in ṁ can explain both movement along the Z track and changes in the shape of the Z track. We discuss some consequences of this and consider the possibility that the branches of the Z will smoothly evolve into the branches observed in X-ray color diagrams of the less luminous atoll sources, although not in a way that was previously suggested.

[1]  W. Lewin,et al.  Compact stellar X-ray sources , 2006 .

[2]  P. Casella,et al.  A continuous flaring- to normal-branch transition in Scorpius X-1 , 2005, astro-ph/0506483.

[3]  P. Reig,et al.  Timing Properties and Spectral States in Aquila X-1 , 2003, astro-ph/0311281.

[4]  M. Rupen,et al.  Simultaneous radio and X-ray observations of the low-mass X-ray binary GX 13+1 , 2003, astro-ph/0309042.

[5]  E. Fomalont,et al.  The X-Ray Spectral Changes of Scorpius X-1 , 2003 .

[6]  M. Klis,et al.  The Atoll Source States of 4U 1608-52 , 2003, astro-ph/0307041.

[7]  M. Klis,et al.  Peculiar spectral and power spectral behaviour of the LMXB GX 13+1 , 2003, astro-ph/0305161.

[8]  E. Kuulkers,et al.  The Complete Z Diagram of LMC X-2 , 2003, astro-ph/0303133.

[9]  J. Olive,et al.  A Peculiar Spectral State Transition of 4U 1705–44: When an Atoll Looks Like a Z , 2002, astro-ph/0205184.

[10]  P. G. Jonker,et al.  Low- and high-frequency variability as a function of spectral properties in the bright X-ray binary GX 5-1 , 2002, astro-ph/0202420.

[11]  D. Psaltis,et al.  A Unified Description of the Timing Features of Accreting X-Ray Binaries , 2002, astro-ph/0202213.

[12]  M. Gierliński,et al.  A comment on the colour–colour diagrams of low-mass X-ray binaries , 2001, astro-ph/0111378.

[13]  Ronald A. Remillard,et al.  How Do Z and Atoll X-Ray Binaries Differ? , 2001, astro-ph/0111370.

[14]  R. Wijnands,et al.  RXTE Observations of the Neutron Star Low-Mass X-Ray Binary GX 17+2: Correlated X-Ray Spectral and Timing Behavior , 2001, astro-ph/0104323.

[15]  M. Klis,et al.  The X-ray Fast-time Variability of Sco X-2 (GX 349+2) with RXTE , 2002, astro-ph/0204362.

[16]  M. Klis.  A Possible Explanation for the "Parallel Tracks" Phenomenon in Low-Mass X-Ray Binaries , 2001, astro-ph/0106291.

[17]  L. Bildsten,et al.  Magnetic Screening in Accreting Neutron Stars , 2001, astro-ph/0102178.

[18]  Rudy WijnandsMichiel van der Klis.  Normal-branch quasi-periodic oscillations during the high-intensity state of Cygnus X-2 , 2000, astro-ph/0010166.

[19]  R. Wijnands,et al.  Simultaneous Measurements of X-Ray Luminosity and Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries , 2000, astro-ph/0002074.

[20]  R. Wijnands,et al.  The Power Spectral Properties of the Z Source GX 340+0 , 2000, astro-ph/0002022.

[21]  M. Klis,et al.  The timing properties of Sco X-1 along its Z track with EXOSAT , 1999, astro-ph/9909472.

[22]  R. Wijnands,et al.  The Complex Phase-Lag Behavior of the 3-12 Hz Quasi-Periodic Oscillations during the Very High State of XTE J1550–564 , 1999, The Astrophysical journal.

[23]  Rudy WijnandsMichiel van der Klis.  The discovery of a 7-14 Hz quasi-periodic oscillation in the x-ray transient XTE J1806-246 , 1999, astro-ph/9902383.

[24]  D. Psaltis,et al.  Correlations in Quasi-periodic Oscillation and Noise Frequencies among Neutron Star and Black Hole X-Ray Binaries , 1999, astro-ph/9902130.

[25]  E. Fomalont,et al.  High-Resolution Parallax Measurements of Scorpius X-1 , 1999 .

[26]  Mit,et al.  The Complete “Z” Track of Circinus X-1 , 1999, astro-ph/9901003.

[27]  D. Psaltis,et al.  Discovery of Kilohertz Quasi-periodic Oscillations in the Z Source GX 5–1 , 1998, astro-ph/9804070.

[28]  N. White,et al.  Millisecond X-Ray Pulsars in Low-mass X-Ray Binaries , 1997 .

[29]  C. B. Markwardt,et al.  Calibration of the Rossi X-Ray Timing Explorer Proportional Counter Array , 2005, astro-ph/0511531.

[30]  K. Jahoda,et al.  Discovery of Submillisecond Quasi-periodic Oscillations in the X-Ray Flux of Scorpius X-1 , 1996, astro-ph/9607047.

[31]  D. Psaltis,et al.  X-Ray Spectra of Z Sources , 1995, astro-ph/9506096.

[32]  Kazuhiro Kimura,et al.  X-ray variability of GX 339 - 4 in its very high state , 1991 .

[33]  R. Kelley,et al.  Long-term temporal variability of Cygnus X-2 , 1988 .

[34]  M. van der Klis,et al.  Fourier Techniques in X-Ray Timing , 1988 .

[35]  G. Hasinger,et al.  Bimodal quasi-oscillatory and spectral behavior in Scorpius X-1 , 1986 .

[36]  M. A. Alpar,et al.  Quasi-periodic oscillations in bright galactic-bulge X-ray sources , 1985, Nature.

[37]  J. Shaham,et al.  Is GX5 – 1 a millisecond pulsar? , 1985, Nature.

[38]  John B. Hutchings,et al.  The halo population X-ray source Cygnus X-2. , 1979 .

[39]  E. Wright,et al.  Optical studies of UHURU sources. XI. A probable period for Scorpius X- 1=V818 Scorpii , 1975 .