Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar.

The relativistic plasma flows onto neutron stars that are accreting material from stellar companions can be used to probe strong-field gravity as well as the physical conditions in the supra-nuclear-density interiors of neutron stars. Plasma inhomogeneities orbiting a few kilometres above the stars are observable as X-ray brightness fluctuations on the millisecond dynamical timescale of the flows. Two frequencies in the kilohertz range dominate these fluctuations: the twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the origins of these oscillations (based on orbital motions) all predict that they should be related to the stellar spin frequency, but tests have been difficult because the spins were not unambiguously known. Here we report the detection of kHz QPOs from a pulsar whose spin frequency is known. Our measurements establish a clear link between kHz QPOs and stellar spin, but one not predicted by any current model. A new approach to understanding kHz QPOs is now required. We suggest that a resonance between the spin and general relativistic orbital and epicyclic frequencies could provide the observed relation between QPOs and spin.

An x-ray nebula associated with the millisecond pulsar B1957+20.

We have detected an x-ray nebula around the binary millisecond pulsar B1957+20. A narrow tail, corresponding to the shocked pulsar wind, is seen interior to the known Halpha bow shock and proves the long-held assumption that the rotational energy of millisecond pulsars is dissipated through relativistic winds. Unresolved x-ray emission likely represents the shock where the winds of the pulsar and its companion collide. This emission indicates that the efficiency with which relativistic particles are accelerated in the postshock flow is similar to that for young pulsars, despite the shock proximity and much weaker surface magnetic field of this millisecond pulsar.

Linear and nonlinear time series analysis of the black hole candidate cygnus X-1

We analyze the variability in the x-ray lightcurves of the black hole candidate Cygnus X-1 by linear and nonlinear time series analysis methods. While a linear model describes the overall second order properties of the observed data well, surrogate data analysis reveals a significant deviation from linearity. We discuss the relation between shot noise models usually applied to analyze these data and linear stochastic autoregressive models. We debate statistical and interpretational issues of surrogate data testing for the present context. Finally, we suggest a combination of tools from linear and nonlinear time series analysis methods as a procedure to test the predictions of astrophysical models on observed data.

The Rapid X-Ray Variability of V4641 Sagittarii (SAX J1819.3-2525 = XTE J1819-254).

We report on the rapid X-ray variability of the variable star and X-ray transient V4641 Sagittarii (SAX J1819.3-2525 = XTE J1819-254) as observed on 1999 September 15 by the proportional counter array (PCA) on board the Rossi X-Ray Timing Explorer. During the first approximately 900 s of the first PCA observation, V4641 Sgr showed very strong X-ray fluctuations by a factor of 4 on timescales of seconds to about 500 on timescales of minutes. The spectrum of the source during this flaring episode became harder when the count rate decreased. After this flaring episode, V4641 Sgr entered a quiescent state in which it remained for the rest of this and subsequent PCA observations. The X-ray spectrum was considerably softer in this quiescent state than during the flaring episode. The intrinsic X-ray luminosity (during both the flaring episode and the quiescent state) and the rapid X-ray variability do not strongly constrain the nature of the compact object (neutron star or black hole) in the system, although a black hole seems to be more likely. The very short duration of the bright X-ray phase of V4641 Sgr and its likely close proximity suggest that many similar objects could be present in our Galaxy, most of which are not noticed when they are in outburst because of the short duration of these outbursts. A considerable number of the black holes present in our Galaxy might be contained in systems similar to V4641 Sgr.

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 less than 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 greater than 6 and with harmonics at 3, 12, 18, and possibly at 9 Hz. Not only are the Q-values and the harmonic content of the two types different, but their phase-lag behavior also differs. 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 rad. The phase lags of the second type of QPO are more complex. The soft photons (<5 keV) of the 3 and 12 Hz QPOs lag the hard photons (>5 keV) by as much as 1.0 rad. However, the soft photons of the 6 Hz QPO precede the hard ones by as much as 0.6 rad. 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 in which the QPOs are formed.