The discovery of a 7-14 Hz Quasi-Periodic Oscillation in the X-ray Transient XTE J1806-246

We have studied the correlated X-ray spectral and X-ray timing behavior of the X-ray transient XTE J1806-246 using data obtained with the proportional counter array onboard the Rossi X-ray Timing Explorer. In the X-ray color-color diagram two distinct patterns are traced out. The first pattern is a curved branch, which is observed during the rise and the decay of the outburst. This pattern resembles the so-called banana branch of those low-luminosity neutron star low-mass X-ray binaries (LMXBs) which are referred to as atoll sources. The power spectrum of XTE J1806-246 on this curved branch consisted of a power law and a cutoff power law component. The presence of these components and their dependence on position of the source on the branch is also identical to the behavior of atoll sources on the banana branch. Near the end of it outburst XTE J1806-246 formed patches in the color-color diagram, the spectrum was harder, and the power spectrum showed strong band limited noise, characteristic of the atoll sources in the island state. A second pattern was traced out during the only observation at the peak of the outburst. It consists of a structure which we interpret as formed by two distinct branches. This pattern resembles the normal-flaring branches of the high-luminosity neutron star LMXBs (the Z sources). The discovery of a 7-14 Hz QPO during this observation strengthens this similarity. We conclude that if XTE J1806-246 is a neutron star, it is most likely an atoll source that only at the peak of its outburst reached a luminosity level sufficiently high to show the type of QPO that Z sources who on their normal and flaring branches.Comment: Accepted for publication in ApJ Main Journal (14 April 1999). Only minor changes to the text and to some of the figure

The X-ray Timing Behavior of the X-ray Burst Source SLX 1735-269

We report for the first time on the rapid X-ray variability of the galactic bulge source and X-ray burster SLX 1735-269. The power spectrum as observed with the Rossi X-ray Timing Explorer is characterized by a strong band-limited noise component which is approximately flat below a 0.1-2.3 Hz break frequency; above this frequency the power spectrum declines as a power law of index 0.9. At the highest observed count rate a broad bump is superimposed on this band-limited noise. The power spectrum is very similar to that of other low-luminosity neutron-star low-mass X-ray binaries (LMXBs) and to black-hole candidates when these types of source accrete at their lowest observed mass accretion rates. However, we identify one unusual aspect of the X-ray variability of SLX 1735-269: the break frequency increases when the inferred mass accretion rate decreases. This is the opposite to what is normally observed in other sources. The only source for which the same behavior has been observed is the accretion-powered millisecond X-ray pulsar SAX J1808.4-3658. No coherent millisecond pulsations were observed from SLX 1735-269 with an upper limit on the amplitude of 2.2% rms. Observing this behavior in SLX 1735-269 increases the similarities between SAX J1808.4-3658 and the other neutron star LMXBs for which so far no coherent pulsations have been observed. We expect that other sources will show the same behavior when these sources are studied in detail at their lowest mass accretion rates.Comment: Accepted for publication in A&A Letter

Accreting millisecond pulsars: one on each hand

We report on the X-ray aperiodic timing analysis of two accreting millisecond pulsars: XTE J1807-294 and IGR J00291+5934. On the one hand, we discovered in XTE J1807-294 seven pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) separated in frequency by nearly the spin frequency of the neutron star. This confirms the suspected dichotomy in the frequency separation of kHz QPOs: sometimes once and sometimes half the spin frequency. On the other hand, we found an extreme behavior in the power spectra of IGR J00291+5934: very strong variability at very low frequencies. Namely, the fractional amplitude of the variability was ~50%, the highest value found so far in a neutron star system.Comment: To appear in the proceedings of the conference `The Multicoloured Landscape of Compact Objects and their Explosive Origins' (Cefalu, Sicily, June 2006), AI

Non-detection of kHz QPOs in GX 9+1 and GX 9+9

In numerous low-mass X-ray binaries kHz quasi-periodic oscillations (kHz QPOs) are detected. We observed the atoll sources GX 9+1 and GX 9+9 with the RXTE satellite. Both sources were on the (upper) banana branch during our observations. No kHz QPOs were detected, with upper limits of 1.3-1.8 %. It is known from other atoll sources (e.g. 4U 1636-53 and 4U 1820-30) that when they are in the upper banana branch the kHz QPOs are not detected. Thus, it remains possible that when GX 9+1 and GX 9+9 are observed longer on the lower banana, or even in the island state, kHz QPOs are detected in these sources.Comment: 2 pages, 1 figure. To appear in the proceedings of IAU Symposium 188 "The Hot Universe

The Complex Phase Lag Behavior of the 3-12 Hz Quasi-Periodic Oscillations during the Very High State of XTE J1550-564

We present a study of the complex phase lag behavior of the low-frequency (<20 Hz) quasi-periodic oscillations (QPOs) in the X-ray transient and black-hole candidate XTE J1550-564 during its very high state. We distinguish two different types of low-frequency QPOs, based on their coherence and harmonic content. The first type is characterized by a 6 Hz QPO with a Q (the QPO frequency divided by the QPO width) of <3 and with a harmonic at 12 Hz. The second type of QPO is characterized by a 6 Hz QPO with a Q value of >6 and with harmonics at 3, 12, 18, and possibly at 9 Hz. Not only the Q values and the harmonic content of the two types are different, but also their phase lag behavior. For the first type of QPO, the low energy photons (<5 keV) of both the 6 Hz QPO and its harmonic at 12 Hz lag the hard energy photons (>5 keV) by as much as 1.3 radian. The phase lags of the second type of QPO are more complex. The soft photons (<5 keV) of the 3 and the 12 Hz QPOs lag the hard photons (>5 keV) by as much as 1.0 radian. However, the soft photons of the 6 Hz QPO precede the hard ones by as much as 0.6 radian. This means that different harmonics of this type of QPO have different signs for their phase lags. This unusual behavior is hard to explain when the lags are due to light-travel-time differences between the photons at different energies, e.g., in a Comptonizing region surrounding the area where the QPOs are formed.Comment: Accepted for publication in ApJ Letters, 29 September 199

KiloHertz Quasi-Periodic Oscillations in the Z sources GX 340+0, Cygnus X-2, GX 17+2, GX 5-1, and Scorpius X-1

We have discovered kiloHertz quasi-periodic oscillations (kHz QPOs) in five Z sources: GX 340+0, Cygnus X-2, GX 17+2, GX 5-1, and Scorpius X-1. In all sources the properties of these kHz QPOs are very similar and closely related to the position of the sources on the Z track traced out in the X-ray color-color diagram and the hardness-intensity diagram. The frequencies of the kHz QPOs increase when the sources move from the left end of the horizontal branch to horizontal/normal branch vertex, thus with inferred mass accretion rate. Only for Scorpius X-1 the kHz QPOs have been observed down the normal branch unto the flaring branch. The strength and the FWHM of the higher-frequency kHz QPOs decrease with mass accretion rate, but when the lower-frequency kHz QPOs are detected the strength and the FWHM of this QPO stay approximately constant with mass accretion rate. In Scorpius X-1 the frequency separation between the kHz QPOs decreases with mass accretion rate, but in the other Z sources the separation remains approximately constant, although a similar decrease in peak separation as found in Scorpius X-1 can not be excluded.Comment: Aipproc LaTeX (4 pages, 2 ps-figures), to appear in "Accretion Processes in Astrophysical Systems", Proc. of the 8th Annual Astrophysics Conference in Maryland, S. S. Holt & T. Kallman (eds.

Discovery of Two Simultaneous Kilohertz Quasi-Periodic Oscillations in KS 1731-260

We have discovered two simultaneous quasi-periodic oscillations (QPOs) at 898.3+/-3.3 Hz and 1158.6+/-9.0 Hz in the 1996 August 1 observation of the low-mass X-ray binary KS 1731-260 with the Rossi X-ray Timing Explorer. The rms amplitude and FWHM of the lower frequency QPO were 5.3+/-0.7 % and 22+/-8 Hz, whereas those of the higher frequency QPO were 5.2+/-1.0 % and 37+/-21 Hz. At low inferred mass accretion rate both QPOs are visible, at slightly higher mass accretion rate the lower frequency QPO disappears and the frequency of the higher frequency QPO increases to ~1178 Hz. At the highest inferred mass accretion rate this QPO is only marginally detectable (2.1 sigma) near 1207 Hz, which is the highest frequency so far observed in an X-ray binary. The frequency difference (260.3+/-9.6 Hz) between the QPOs is equal to half the frequency of the oscillations observed in a type I burst in this source (at 523.92+/-0.05 Hz, Smith, Morgan and Bradt 1997). This suggests that the neutron star spin frequency is 261.96 Hz (3.8 ms), and that the lower frequency QPO is the beat between the higher frequency QPO, which could be a preferred orbital frequency around the neutron star, and the neutron star spin. During the 1996 August 31 observation we detected an additional QPO at 26.9+/-2.3 Hz, with a FWHM and rms amplitude of 11+/-5 Hz and 3.4+/-0.6 %.Comment: 6 pages including 3 figures, Astrophysical Journal Letters, in press (issue 482

4U 1957+11: a persistent low-mass X-ray binary and black-hole candidate in the high state?

We report on several pointed Rossi X-ray Timing Explorer observations of the enigmatic low-mass X-ray binary (LMXB) 4U 1957+11 at different X-ray luminosities. The luminosity of the source varied by more than a factor of four on time scales of months to years. The spectrum of the source tends to get harder when its luminosity increases. Only very weak (1%-2% rms amplitude; 0.001-10 Hz; 2-60 keV) rapid X-ray variability was observed during the observations. A comparison of the spectral and temporal behaviour of 4U1957+11 with other X-ray binary systems, in particular LMC X-3, indicates that 4U 1957+11 is likely to be a persistent LMXB harboring a black hole and which is persistently in the black-hole high state. If confirmed, it would be the only such system known.Comment: Accepted for publication in MNRAS, 8 Nov. 200

An alternative interpretation of the timing noise in accreting millisecond pulsars

The measurement of the spin frequency in accreting millisecond X-ray pulsars (AMXPs) is strongly affected by the presence of an unmodeled component in the pulse arrival times called 'timing noise'. We show that it is possible to attribute much of this timing noise to a pulse phase offset that varies in correlation with X-ray flux, such that noise in flux translates into timing noise. This could explain many of the pulse frequency variations previously interpreted in terms of true spin up or spin down, and would bias measured spin frequencies. Spin frequencies improved under this hypothesis are reported for six AMXPs. The effect would most easily be accounted for by an accretion rate dependent hot spot location.Comment: Submitted to ApJ Letter