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 first radius-expansion X-ray burst from GX 3+1

During several observations in 1999 August with RXTE of the low-mass X-ray binary GX 3+1, we found a single short and strong X-ray burst. This is the first burst from GX 3+1 which clearly shows evidence for radius expansion of the neutron-star photosphere during the thermo-nuclear runaway. We show that the cooling phase of the neutron star photosphere starts already just before the end of the contraction phase. Considering the fact that the radius expansion is due to the burst luminosity being at the Eddington luminosity, assuming standard burst parameters and accounting for gravitational redshift effects we derive a distance to the source of ~4.5 kpc, although relaxing these assumptions may lead to uncertainties up to ~30%. By comparing the persistent flux with that observed at the peak of the burst we infer that near the time of the X-ray burst the persistent luminosity of GX 3+1 is ~0.17*L_edd, confirming predictions from theoretical modeling of X-ray spectra of bright sources like GX 3+1.Comment: 5 pages, 4 figures, accepted for publication in A&A Letter

Phase-resolved spectroscopy of low frequency quasi-periodic oscillations in GRS 1915+105

X-ray radiation from black hole binary (BHB) systems regularly displays quasi-periodic oscillations (QPOs). In principle, a number of suggested physical mechanisms can reproduce their power spectral properties, thus more powerful diagnostics which preserve phase are required to discern between different models. In this paper, we first find for two Rossi X-ray Timing Explorer (RXTE) observations of the BHB GRS 1915+105 that the QPO has a well defined average waveform. That is, the phase difference and amplitude ratios between the first two harmonics vary tightly around a well defined mean. This enables us to reconstruct QPO waveforms in each energy channel, in order to constrain QPO phase-resolved spectra. We fit these phase resolved spectra across 16 phases with a model including Comptonisation and reflection (Gaussian and smeared edge components) to find strong spectral pivoting and a modulation in the iron line equivalent width. The latter indicates the observed reflection fraction is changing throughout the QPO cycle. This points to a geometric QPO origin, although we note that the data presented here do not entirely rule out an alternative interpretation of variable disc ionisation state. We also see tentative hints of modulations in the iron line centroid and width which, although not statistically significant, could result from a non-azimuthally symmetric QPO mechanism.Comment: Accepted for publication in MNRA

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

Pulse amplitude depends on kHz QPO frequency in the accreting millisecond pulsar SAX J1808.4-3658

We study the relation between the 300-700 Hz upper kHz quasi-periodic oscillation (QPO) and the 401 Hz coherent pulsations across all outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the Rossi X-ray Timing Explorer. We find that the pulse amplitude systematically changes by a factor of ~2 when the upper kHz QPO frequency passes through 401 Hz: it halves when the QPO moves to above the spin frequency and doubles again on the way back. This establishes for the first time the existence of a direct effect of kHz QPOs on the millisecond pulsations and provides a new clue to the origin of the upper kHz QPO. We discuss several scenarios and conclude that while more complex explanations can not formally be excluded, our result strongly suggests that the QPO is produced by azimuthal motion at the inner edge of the accretion disk, most likely orbital motion. Depending on whether this azimuthal motion is faster or slower than the spin, the plasma then interacts differently with the neutron-star magnetic field. The most straightforward interpretation involves magnetospheric centrifugal inhibition of the accretion flow that sets in when the upper kHz QPO becomes slower than the spin.Comment: 5 pages, 4 figures, Accepted for publication in ApJ

The aperiodic X-ray variability of the accreting millisecond pulsar SAX J1808.4-3658

We have studied the aperiodic variability of the 401 Hz accreting millisecond X-ray pulsar SAX J1808.4-3658 using the complete data set collected with the Rossi X-ray Timing Explorer over 14 years of observation. The source shows a number of exceptional aperiodic timing phenomena that are observed against a backdrop of timing properties that show consistent trends in all five observed outbursts and closely resemble those of other atoll sources. We performed a detailed study of the enigmatic ~410 Hz QPO, which has only been observed in SAX J1808.4-3658. We find that it appears only when the upper kHz QPO frequency is less than the 401 Hz spin frequency. The difference between the ~410 Hz QPO frequency and the spin frequency follows a similar frequency correlation as the low frequency power spectral components, suggesting that the ~410 Hz QPO is a retrograde beat against the spin frequency of a rotational phenomenon in the 9 Hz range. Comparing this 9 Hz beat feature with the Low-Frequency QPO in SAX J1808.4-3658 and other neutron star sources, we conclude that these two might be part of the same basic phenomenon. We suggest that they might be caused by retrograde precession due to a combination of relativistic, classical and magnetic torques. Additionally we present two new measurements of the lower kHz QPO, allowing us, for the first time, to measure the frequency evolution of the twin kHz QPOs in this source. The twin kHz QPOs are seen to move together over 150 Hz, maintaining a centroid frequency separation of $(0.446 \pm 0.009) \nu_{spin}$.Comment: 18 pages, 9 figures, 7 tables. Accepted for publication in Ap

The Timing Properties of Sco X-1 along its Z track as seen with EXOSAT

We present a systematic, homogeneous analysis of all the EXOSAT ME, high time resolution data on Sco X-1. We investigate all power spectral properties of the $<$100 Hz quasi-periodic oscillations (QPO) and noise of Sco X-1 as a function of position on the Z-shaped track traced out in the X-ray colour-colour diagram. Generally, the properties of Sco X-1 vary smoothly as a function of position along the Z track. However, some variability parameters change abruptly at either of the vertices of the Z track indicating that the branches of the Z track represent distinct source states not only in spectral state but also in rapid variability characteristics. We show that the very low frequency noise (VLFN) is consistent with being solely due to motion along the Z track. The QPO show a remarkably rapid change in frequency at or just before the normal-branch/flaring-branch (NB/FB) vertex. This transition happens within 1.5% of the entire extent of the Z track. The QPO themselves are visible for 17% of the Z. We find a new sort of behaviour near the NB/FB vertex, i.e., rapid excursions from the NB into the FB and back again taking only a few minutes.Comment: 30 pages, 15 figures, MNRAS in pres

The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity

We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out a set of nearly parallel lines in a frequency vs. X-ray count rate diagram. We find that between two of these tracks, for similar QPO frequency, the source count rate can differ by up to a factor of ~ 4, whereas at the same time the rms amplitude of the kHz QPOs is only a factor of ~ 1.1 different. We also find that, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does not depend upon which track the source occupies in the frequency vs. X-ray count rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are inconsistent with scenarios in which the properties of the kHz QPOs are only determined by the mass accretion rate through the disk, whereas X-ray count rate also depends upon other sources of energy that do not affect the QPOs.Comment: Submitted to ApJ.(7 pages; 9 figures

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