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We report on an analysis of Rossi X-Ray Timing Explorer data of the transient neutron star low-mass X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst. The X-ray properties of the source changed between those of various types of NS-LMXB subclasses. At high luminosities, the source switched between two types of Z source behavior and at low luminosities we observed a transition from Z source to atoll source behavior. These transitions between subclasses primarily manifest themselves as changes in the shapes of the tracks in X-ray color-color (CD) and hardness-intensity diagrams (HID), but they are accompanied by changes in the kHz quasi-periodic oscillations, broadband variability, burst behavior, and/or X-ray spectra. We find that for most of the outburst the low-energy X-ray flux is a good parameter to track the gradual evolution of the tracks in CD and HID, allowing us to resolve the evolution of the source in greater detail than before and relate the observed properties to other NS-LMXBs. We further find that during the transition from Z to atoll, characteristic behavior known as the atoll upper banana can equivalently be described as the final stage of a weakening Z source flaring branch, thereby blurring the line between the two subclasses. Our findings strongly suggest that the wide variety in behavior observed in NS-LXMBs with different luminosities can be linked through changes in a single variable parameter, namely the mass accretion rate, without the need for additional differences in the neutron star parameters or viewing angle. We briefly discuss the implications of our findings for the spectral changes observed in NS-LMXBs and suggest that, contrary to what is often assumed, the position along the color-color tracks of Z sources is not determined by the instantaneous mass accretion rate.

On 2008 August 24 the new magnetar SGR 0501+4516 (discovered by Swift) emitted a bright burst with a pronounced double-peaked structure in hard X-rays, reminiscent of the double-peaked temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However, for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24 burst.

We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar (AMXP) XTE J1807-294. We find a 95% confidence interval for the pulse frequency derivative of (+0.7, + 4.7) x 10-14 Hz s-1 and (-0.6, + 3.8) x 10-14 Hz s-1 for the fundamental and second harmonics, respectively. The sinusoidal fractional amplitudes of the pulsations are the highest observed among AMXPs and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental frequency follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion both in longitude and latitude over the surface of the neutron star. An anti-correlation between residuals and X-ray flux suggests an influence of the accretion rate on pulse phase and casts doubts on the interpretation of pulse frequency derivatives in terms of changes of spin rates and torques on the neutron star.

SGR J1550-5418 (previously known as AXP 1E 1547.0-5408 or PSR J1550-5418) went into three active bursting episodes in 2008 October and in 2009 January and March, emitting hundreds of typical soft gamma repeater bursts in soft gamma rays. The second episode was especially intense, and our untriggered burst search on Fermi/Gamma-ray Burst Monitor (GBM) data (8-1000 keV) revealed [?]450 bursts emitted over 24 hr during the peak of this activity. Using the GBM data, we identified a [?]150 s long enhanced persistent emission during 2009 January 22 that exhibited intriguing timing and spectral properties: (1) clear pulsations up to [?]110 keV at the spin period of the neutron star (P [?] 2.07 s, the fastest of all magnetars); (2) an additional (to a power-law) blackbody component required for the enhanced emission spectra with kT [?] 17 keV; and (3) pulsed fraction that is strongly energy dependent and highest in the 50-74 keV energy band. A total isotropic-equivalent energy emitted during this enhanced emission is estimated to be 2.9 x 1040(D/5 kpc)2 erg. The estimated area of the blackbody emitting region of [?]0.046(D/5 kpc)2 km2 (roughly a few x10-5 of the neutron star area) is the smallest "hot spot" ever measured for a magnetar and most likely corresponds to the size of magnetically confined plasma near the neutron star surface.

We present a simultaneous periodic and aperiodic timing study of the accreting millisecond X-ray pulsar SAX J1808.4-3658. We analyze five outbursts of the source and for the first time provide a full and systematic investigation of the enigmatic phenomenon of the 1 Hz flares observed during the final stages of some of the outbursts. We show that links between pulsations and 1 Hz flares might exist, and suggest that they are related with hydrodynamic disk instabilities that are triggered close to the disk-magnetosphere boundary layer when the system is entering the propeller regime.

The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (-5.5 +- 1.2) x 10-18 Hz s-1, which is consistent with the previously determined spin derivative. The spin down occurs mostly during quiescence, and is most likely due to the magnetic dipole torque from a B = 1.5 x 108 G dipolar field at the neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80 +- 0.06) x 10-12 s s-1, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.

We present a phase-coherent timing analysis of the intermittent accreting millisecond pulsar SAX J1748.9-2021. A new timing solution for the pulsar spin period and the Keplerian binary orbital parameters was achieved by phase connecting all episodes of intermittent pulsations visible during the 2001 outburst. We investigate the pulse profile shapes, their energy dependence, and the possible influence of Type I X-ray bursts on the time of arrival and fractional amplitude of the pulsations. We find that the timing solution of SAX J1748.9-2021 shows an erratic behavior when selecting different subsets of data, which is related to substantial timing noise in the timing postfit residuals. The pulse profiles are very sinusoidal and their fractional amplitude linearly increases with energy, and no second harmonic is detected. The reason why this pulsar is intermittent is still unknown but we can rule out a one-to-one correspondence between Type I X-ray bursts and the appearance of the pulsations.

SWIFT J1756.9-2508 is one of the few accreting millisecond pulsars (AMPs) discovered to date. We report here the results of our analysis of its aperiodic X-ray variability, as measured with the Rossi X-Ray Timing Explorer during the 2007 outburst of the source. We detect strong (~35%) flat-topped broadband noise throughout the outburst with low characteristic frequencies (~0.1 Hz). This makes SWIFT J1756.9-2508 similar to the rest of AMPs and to other low-luminosity accreting neutron stars when they are in their hard states, and enables us to classify this AMP as an atoll source in the extreme island state. We also find a hard tail in its energy spectrum extending up to 100 keV, fully consistent with such source and state classification.

We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658, an X-ray transient with a recurrence time of [?]2 yr, using data from the Rossi X-Ray Timing Explorer covering four transient outbursts (1998-2005). We verify that the 401 Hz pulsation traces the spin frequency fundamental and not a harmonic. Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous n $˙{ν }$ measurements of this source into question), with typical upper limits of |n| [?] 2.5 x 10[?]14 $&vert; ˙{ν }&vert; &lesssim; 2.5 × 10⁻¹⁴$ Hz s[?]1 (2 s). However, combining data from all the outbursts shows with high (6 s) significance that the pulsar is undergoing long-term spin down at a rate n = ([?]5.6 +- 2.0) x 10[?]16 $˙{ν } = (− 5.6 ± 2.0) × 10⁻¹⁶$ Hz s[?]1, with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1.5 x 108 G for the pulsar's surface dipole magnetic field and Q/I < 5 x 10[?]9 for the fractional mass quadrupole moment. We also measured an orbital period derivative of Porb = (3.5 +- 0.2) x 10[?]12 $˙{P}_orb = (3.5 ± 0.2) × 10⁻¹²$ s s[?]1. This surprisingly large Porb $˙{P}_orb$ is reminiscent of the large and quasi-cyclic orbital period variation observed in the so-called black widow millisecond radio pulsars, which further strengthens previous speculation that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an appendix we derive an improved ( 0.15'') source position from optical data.

Using Suzaku observations of three neutron star low-mass X-ray binaries (Ser X-1, 4U 1820-30, and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ("diskline"), allowing a measurement of the inner radius of the accretion disk and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5-16.5 km for a 1.4 M neutron star. The inner disk radii that we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820-30 and GX 349+2, supporting the inner disk origin for kHz QPOs. In addition, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.

We have analyzed observations of the black hole transient GX 339-4 made with the Rossi X-ray Timing Explorer (RXTE) and the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO). We have found a nearly linear relation between the peak flux during the low/hard (LH) state that occurs at the beginning of an outburst and the time since the flux peak of the latest LH state identified in the previous outburst. Assuming that the rate at which mass accumulates in the accretion disk between these peaks is constant and that any mass that remains in the disk after an outburst has a negligible effect on the next outburst, this nearly linear relation suggests that the peak flux during the LH state that occurs at the beginning of an outburst is related to the mass in the disk, and thus that the entire disk is probably involved in powering these LH states. We have also found a positive correlation between the peak luminosities of the LH state in the three recent outbursts of GX 339-4 and the peak luminosities of the following HS state. This correlation is similar to the correlations reported previously for Aql X-1, 4U 1705-44, and XTE J1550-564, providing further support for the idea that the accretion flow that powers the LH state is related to the accretion flow that powers the following HS state. Although the luminosity of the LH-to-HS transition varies by up to an order of magnitude, the neutron stars and the black holes are distinguishable in the state transition luminosity. We discuss the implications for the mass determination of the compact stars in the ultraluminous X-ray sources (ULXs).

IGR J00291+5934 is one of the seven accreting millisecond pulsars (AMPs) discovered so far. We report on the aperiodic timing and color analysis of its X-ray flux, using all the RXTE observations of the 2004 outburst. Flat-top noise and two harmonically related quasi-periodic oscillations, all of them at very low frequencies (0.01-0.1 Hz), were present in the power spectra during most of the outburst as well as a very high fractional variability (~50%). These properties are atypical not only for AMPs but also for neutron star low-mass X-ray binaries (LMXBs) in general. There are instead some remarkable similarities with the variability observed in black hole systems, reinforcing the connections between these two types of LMXBs, as well as some interesting differences. We note finally that the results of this paper are difficult to reconcile with interpretations where any break frequency of power density spectra scales inversely with the mass of the central object at an accuracy sufficient to distinguish between the masses of neutron stars and black holes in LMXBs.

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 (m) 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 m. 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 m, we further propose that changes in m 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.

We report the discovery of twin kilohertz quasi-periodic oscillations (kHz QPOs) in the X-ray flux of XTE J1807-294, the fourth accreting millisecond pulsar (AMP). This is the second AMP exhibiting twin kHz QPOs. In contrast to the first case, SAX J1808.4-3658, the frequency separation Dn between these kHz QPOs is consistent with the pulse frequency (190.6 Hz), not with half that value, confirming for the first time from pulsation measurements the inference, based on burst oscillations, that "slow rotators" (spin frequency less than 400 Hz) have Dn approximately equal to the spin frequency. While the QPOs move in frequency together over a range of more than 200 Hz, Dn remains constant with an average value of 205 +- 6 Hz. Variability components were found in the 5-130 Hz range, similar to those seen in other LMXBs. The correlations between the QPO and noise frequencies are also similar to those in other sources, but shifted by a factor of 1.59 in kHz QPO frequencies, similar to the factor of 1.45 shift found for SAX J1808.4-3658. Our results argue in favor of a spin-related formation mechanism for twin kHz QPOs and against a spin-related cause of the shift in the frequency correlations.

We present a variability study of the black hole candidate and X-ray transient H1743-322 during its 2003-2004 outburst. We analyzed five Rossi X-Ray Timing Explorer observations that were performed as part of a multiwavelength campaign, as well as six observations from the early rise of the outburst. The source was observed in several black hole states and showed various types of X-ray variability, including high-frequency quasi-periodic oscillations (QPOs) at 240 and 160 Hz (i.e., with a 3 : 2 frequency ratio), several types of low-frequency QPOs, and strong variations on a timescale of a few hundred seconds. The discovery of high-frequency QPOs in H1743-322 supports predictions that these QPOs should be more easily observed in high inclination systems. In one of our observations a transition in count rate and color occurred, during which we were able to follow the smooth evolution of the low-frequency QPOs from type B to type A. We classify the X-ray observations and QPOs and briefly discuss the QPOs in terms of recently proposed models.

We have studied the aperiodic X-ray timing and color behavior of the accreting millisecond pulsars SAX J1808.4-3658, XTE J1751-305, XTE J0929-314, and XTE J1814-338 using large data sets obtained with the Rossi X-Ray Timing Explorer. We find that the accreting millisecond pulsars have timing properties very similar to atoll sources and low-luminosity bursters. On the basis of the correlation of timing and color behavior, SAX J1808.4-3658 can be classified as an atoll source, and XTE J0929-314 and XTE J1814-338 are consistent with atoll sources, but the color behavior of XTE J1751-305 is different. Unlike the color in other atoll sources, the hard color in the extreme island state of XTE J1751-305 is strongly correlated with soft color and intensity and is not anticorrelated with any of the characteristic frequencies. We found previously that the frequencies of the variability components of atoll sources follow a universal scheme of correlations. The frequency correlations of the accreting millisecond pulsars are similar, but in the cases of SAX J1808.4-3658 and XTE J0929-314 they are shifted compared with those of the other atoll sources in a way that is most easily described as a shift in upper and lower kilohertz quasi-periodic oscillation (QPO) frequency by a factor close to 1.5. However, we note that the shift in the lower kilohertz QPO frequency is based on only one observation for SAX J1808.4-3658. XTE J1751-305 and XTE J1814-338, as well as the low-luminosity bursters, show no or possibly only small shifts.

We present the long-term X-ray light curves and detailed spectral and timing analyses of XTE J1908+094 using the Rossi X-Ray Timing Explorer Proportional Counter Array observations covering two outbursts in 2002 and early 2003. At the onset of the first outburst, the source was found in a spectrally low/hard state lasting for ~40 days, followed by a 3 day long transition to the high/soft state. The source flux (in 2-10 keV) reached ~100 mcrab on 2002 April 6, then decayed rapidly. In power spectra, we detect strong band-limited noise and varying low-frequency quasi-periodic oscillations that evolved from ~0.5 to ~5 Hz during the initial low/hard state of the source. We find that the second outburst closely resembled the spectral evolution of the first. The X-ray transient's overall outburst characteristics led us to classify XTE J1908+094 as a black hole candidate. Here we also derive precise X-ray position of the source using Chandra observations that were performed during the decay phase of the first outburst and following the second outburst.

We report on the Chandra X-Ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is LX [?] 1029 ergs s-1 for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify six X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar, and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.

We have studied the atoll source 4U 1608-52 using a large data set obtained with the Rossi X-Ray Timing Explorer. We find that the timing properties of 4U 1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09 and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a transient covering 2 orders of magnitude in luminosity. The frequencies of the variability components of these three sources follow a universal scheme when plotted versus the frequency of the upper kilohertz quasi-periodic oscillation, suggesting a very similar accretion flow configuration. If we plot the Z sources on this scheme, only the lower kilohertz quasi-periodic oscillation and horizontal branch oscillation follow identical relations. Using the mutual relations between the frequencies of the variability components, we tested several models: the transition layer model, the sonic point beat frequency model, and the relativistic precession model. None of these models described the data satisfactory. Recently, it has been suggested that the atoll sources (among them 4U 1608-52) trace out similar three-branch patterns as the Z sources in the color-color diagram. We have studied the relation between the power spectral properties and the position of 4U 1608-52 in the color-color diagram and conclude that the timing behavior is not consistent with the idea that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram along which the timing properties vary gradually, as Z sources do.

A group of persistently faint Galactic X-ray sources exist that, based on their location in the Galaxy, high LX/Lopt, association with X-ray bursts, and absence of low-frequency X-ray pulsations, are thought to be low-mass X-ray binaries (LMXBs). We present results from Chandra observations for eight of these systems: 4U 1708-408, 2S 1711-339, KS 1739-304, SLX 1735-269, GRS 1736-297, SLX 1746-331, 1E 1746.7-3224, and 4U 1812-12. Locations for all these sources, excluding GRS 1736-297, SLX 1746-331, and KS 1739-304 (which were not detected), were improved to 0.''6 error circles (90% confidence). Our observations support earlier findings of transient behavior of GRS 1736-297, KS 1739-304, SLX 1746-331, and 2S 1711-339 (which we detect in one of two observations). Energy spectra for 4U 1708-408, 2S 1711-339, SLX 1735-269, 1E 1746.7-3224, and 4U 1812-12 are hard, with power-law indices typically 1.4-2.1, which is consistent with typical faint LMXB spectra.

We present X-ray imaging, timing, and phase-resolved spectroscopy of the anomalous X-ray pulsar 4U 0142+61 using the Chandra X-Ray Observatory. The spectrum is well described by a power-law plus blackbody model with G = 3.35(2), kTBB = 0.458(3) keV, and NH = 0.91(2) x 1022 cm-2; we find no significant evidence for spectral features (0.5-7.0 keV). Time-resolved X-ray spectroscopy shows evidence for evolution in phase in either G, kTBB, or some combination thereof as a function of pulse phase. We derive a precise X-ray position for the source and determine its spin period, P = 8.68866(30) s. We have detected emission beyond 4'' from the central source and extending beyond 100'', likely due to dust scattering in the interstellar medium.

We report the detection of high-frequency variability in the black hole X-ray transient XTE J1650-500. A quasi-periodic oscillation (QPO) was found at 250 Hz during a transition from the hard to the soft state. We also detected less coherent variability around 50 Hz that disappeared when the 250 Hz QPO showed up. There are indications that when the energy spectrum hardened the QPO frequency increased from ~110 to ~270 Hz, although the observed frequencies are also consistent with being 1 : 2 : 3 harmonics of each other. Interpreting the 250 Hz as the orbital frequency at the innermost stable orbit around a Schwarzschild black hole leads to a mass estimate of 8.2 M. The spectral results by Miller et al., which suggest considerable black hole spin, would imply a higher mass.

Soft gamma repeaters undergo pulse profile changes in connection with their burst activity. Here we present a comprehensive pulse profile history of SGR 1806-20 and SGR 1900+14 in three energy bands using Rossi X-Ray Timing Explorer/Proportional Counter Array observations performed between 1996 and 2001. Using the Fourier harmonic powers of pulse profiles, we quantify the pulse shape evolution. Moreover, we determined the rms pulsed count rates (PCRs) of each profile. We show that the pulse profiles of SGR 1806-20 remain single pulsed, showing only modest changes for most of our observing span, while those of SGR 1900+14 change remarkably in all energy bands. Highly significant pulsations from SGR 1900+14 following the 1998 August 27 and 2001 April 18 bursts enabled us to study not only the decay of PCRs in different energy bands but also their correlations with each other.

We have monitored the pulse frequencies of the two soft gamma repeaters SGR 1806-20 and SGR 1900+14 through the beginning of year 2001 using primarily Rossi X-Ray Timing Explorer Proportional Counter Array observations. In both sources, we observe large changes in the spin-down torque up to a factor of ~4, which persist for several months. Using long-baseline phase-connected timing solutions as well as the overall frequency histories, we construct torque noise power spectra for each SGR. The power spectrum of each source is very red (power-law slope ~-3.5). The torque noise power levels are consistent with some accreting systems on timescales of ~1 yr, yet the full power spectrum is much steeper in frequency than any known accreting source. To the best of our knowledge, torque noise power spectra with a comparably steep frequency dependence have been seen only in young, glitching radio pulsars (e.g., Vela). The observed changes in spin-down rate do not correlate with burst activity; therefore, the physical mechanisms behind each phenomenon are also likely unrelated. Within the context of the magnetar model, seismic activity cannot account for both the bursts and the long-term torque changes unless the seismically active regions are decoupled from one another.

As part of our campaign to determine the nature of the various source populations of the low-luminosity globular cluster X-ray sources, we have obtained a Chandra X-Ray Observatory ACIS-S3 image of the globular cluster NGC 6440. We detect 24 sources to a limiting luminosity of ~2 x 1031 ergs s-1 (0.5-2.5 keV) inside the cluster's half-mass radius, all of which lie within ~2 core radii of the cluster center. We also find excess emission in and around the core that could be due to unresolved point sources. Based on X-ray luminosities and colors, we conclude that there are 4-5 likely quiescent low-mass X-ray binaries and that most of the other sources are cataclysmic variables. We compare these results to Chandra results from other globular clusters and find the X-ray luminosity functions differ among the clusters.

We study an empirical model for a unified description of the power spectra of accreting neutron stars and black holes. This description is based on a superposition of multiple Lorentzians and offers the advantage that all quasi-periodic oscillation and noise components are dealt with in the same way, without the need of deciding in advance the nature of each component. This approach also allows us to compare frequencies of features with high and low coherences in a consistent manner and greatly facilitates comparison of power spectra across a wide range of source types and states. We apply the model to six sources: the low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269, and GS 1826-24; the high-latitude transient XTE J1118+480; the bright system Cir X-1; and the Z source GX 17+2. We find that it provides a good description of the observed spectra without the need for a scale-free (1/f) component. We update previously reported correlations between characteristic frequencies of timing features in the light of this new approach and discuss similarities between different types of systems that may point toward similar underlying physics.

We have observed the millisecond X-ray pulsar SAX J1808.4-3658 on three occasions during its 2000 outburst with the BeppoSAX satellite. The source was highly variable and erratic during this outburst, and by coincidence we obtained data only during times when the source had very low luminosities. During our observations, we detected four faint sources. The source closest to the position of SAX J1808.4-3658 is still ~1.'6 away. This source can be identified with SAX J1808.4-3658 only if we assume that the BeppoSAX positional reconstruction is not completely understood. We also reanalyzed a BeppoSAX observation taken in 1999 March by Stella et al. when the source was in quiescence and during which the source was thought to have been detected. Based on the similarities (position and luminosity) of this source with the above-mentioned source ~1.'6 away from SAX J1808.4-3658, it is possible that they are the same source. If this source is not the millisecond pulsar, then during all BeppoSAX observations of SAX J1808.4-3658 (the 2000 outburst ones and the 1999 quiescent one), the millisecond pulsar was not detected. A reanalysis of the ASCA quiescent data of SAX J1808.4-3658 recently performed by Dotani, Asai, & Wijnands confirms that during this observation the source was securely detected in quiescence. We discuss our results for SAX J1808.4-3658 in the context of the quiescent properties of low-mass X-ray binary transients.

We report on the Chandra X-Ray Observatory ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect six X-ray sources within the 10.''5 core radius and 13 more within the 115'' half-mass radius down to a limiting luminosity of LX [?] 1030 ergs s-1 for cluster sources. We reanalyze archival data from the Hubble Space Telescope and the Australia Telescope Compact Array and make 12 optical identifications and one radio identification. Based on X-ray and optical properties of the identifications, we find 10 likely cataclysmic variables (CVs), one to three likely RS CVn or BY Dra systems, and one or two possible background objects. Of the seven sources for which no optical identifications were made, we expect that approximately two to four are background objects and that the rest are either CVs or some or all of the five millisecond pulsars whose radio positions are not yet accurately known. These and other Chandra results on globular clusters indicate that the dozens of CVs per cluster expected by theoretical arguments are being found. The findings to date also suggest that the ratio of CVs to other types of X-ray sources is remarkably similar in clusters of very different structural parameters.

Optical spectroscopic observations of the companion star (type G8 IV to K4 III) in the microquasar system XTE J1550-564 reveal a radial velocity curve with a best-fitting spectroscopic period of Psp = 1.552 +- 0.010 days and a semiamplitude of K2 = 349 +- 12 km s-1. The optical mass function is f(M) = 6.86 +- 0.71 M (1 s). We tentatively measure the rotational velocity of the companion star to be Vrot sin i = 90 +- 10 km s-1, which when taken at face value implies a mass ratio of Q [?] M1/M2 = 6.6img1.gif (1 s), using the above value of K2. We derive constraints on the binary parameters from simultaneous modeling of the ellipsoidal light and radial velocity curves. We find 1 s ranges for the photometric period (1.5430 days [?] Pph [?] 1.5440 days), K-velocity (350.2 [?] K2 [?] 368.6 km s-1), inclination (67.deg0 [?] i [?] 77.deg4), mass ratio (Q [?] 12.0), and orbital separation (11.55 R [?] a [?] 12.50 R). Given these geometrical constraints, we find that the most likely value of the mass of the compact object is 9.41 M with a 1 s range of 8.36 M [?] M1 [?] 10.76 M. If we apply our tentative value of Vrot sin i = 90 +- 10 km s-1 as an additional constraint in the ellipsoidal modeling, we find 1 s ranges of 1.5432 days [?] Pph [?] 1.5441 days for the photometric period, 352.2 [?] K2 [?] 370.1 km s-1 for the K-velocity, 70.deg8 [?] i [?] 75.deg4 for the inclination, 6.7 [?] Q [?] 11.0 for the mass ratio, and 12.35 R [?] a [?] 13.22 R for the orbital separation. These geometrical constraints imply the most likely value of the mass of the compact object of 10.56 M with a 1 s range of 9.68 M [?] M1 [?] 11.58 M. In either case the mass of the compact object is well above the maximum mass of a stable neutron star, and we therefore conclude that XTE J1550-564 contains a black hole.

We present the results of a multi-Lorentzian fit to the power spectra of two kilohertz quasi-periodic oscillation (QPO) sources: 4U 0614+09 and 4U 1728-34. This work was triggered by recent results of a similar fit to the black hole candidates (BHCs) GX 339-4 and Cyg X-1 by Nowak in 2000. We find that one to six Lorentzians are needed to fit the power spectra of our two sources. The use of exactly the same fit function reveals that the timing behavior of 4U 0614+09 and 4U 1728-34 is almost identical at luminosities that are about a factor of 5 different. As the characteristic frequency of the Lorentzians we use the frequency nmax, at which each component contributes most of its variance per log frequency, as proposed by Belloni, Psaltis, & van der Klis in 2001. When using nmax instead of the centroid frequency of the Lorentzian, the recently discovered hectohertz Lorentzian is practically constant in frequency. We use our results to test the suggestions by Psaltis, Belloni, & van der Klis in 1999 and Nowak in 2000 that the two Lorentzians describing the high-frequency end of the broadband noise in BHCs in the low state can be identified with the kilohertz QPOs in the neutron star low-mass X-ray binaries. The prediction for the neutron star sources is that if the two kilohertz QPOs are present, then these two high-frequency Lorentzians should be absent from the broadband noise. We find that when the two kilohertz QPOs are clearly present, the low-frequency part of the power spectrum is too complicated to draw immediate conclusions from the nature of the components detected in any one power spectrum. However, the relations we observe between the characteristic frequencies of the kilohertz QPOs and the band-limited noise, when compared to the corresponding relations in BHCs, hint toward the identification of the second highest frequency Lorentzian in the BHCs with the lower kilohertz QPO. They do not confirm the identification of the highest frequency Lorentzian with the upper kilohertz QPO.

We have analyzed ~600 ks of Rossi X-Ray Timing Explorer data of the neutron star low-mass X-ray binary and Z source GX 17+2. A study was performed of the properties of the noise components and quasi-periodic oscillations (QPOs) as a function of the broadband spectral properties, with the main goal to study the relation between the frequencies of the horizontal branch (HBO) and upper kHz QPOs. It was found that when the upper kHz QPO frequency is below 1030 Hz these frequencies correlate, whereas above 1030 Hz they anticorrelate. GX 17+2 is the first source in which this is observed. We also found that the frequency difference of the high-frequency QPOs was not constant and that the quality factors (Q-values) of the HBO, its second harmonic, and the kHz QPOs are similar and vary almost hand in hand by a factor of more than 3. Observations of the normal branch oscillations during two type I X-ray bursts showed that their absolute amplitude decreased as the flux from the neutron star became stronger. We discuss these and other findings in terms of models that have been proposed for these phenomena. We also compare the behavior of GX 17+2 and other Z sources with that of black hole sources and consider the possibility that the mass accretion rate might not be the driving force behind all spectral and variability changes.

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 tracks in a frequency versus X-ray count rate diagram. We find that between two of these tracks, for similar QPO frequency, the 2-60 keV 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 versus X-ray count rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are inconsistent with simple "extra source of X-rays" scenarios for the parallel tracks, such as those in which the properties of the kHz QPOs are determined only 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.

We report on the highly variable and erratic long-term X-ray luminosity behavior of the only known accretion-driven millisecond X-ray pulsar SAX J1808.4-3658 during its 2000 outburst, as observed with the Rossi X-ray Timing Explorer (RXTE) satellite. The maximum observed luminosity is ~2.5 x 1035 ergs s-1 (3-25 keV; for a distance of 2.5 kpc), which is approximately a factor of 10 lower than that observed during the 1996 and 1998 outbursts. Due to solar constraints, the source could not be observed for several months with RXTE before 2000 January 21. Therefore, the exact moment of the outburst onset is unknown, and the peak luminosity could have been significantly higher. On some occasions SAX J1808.4-3658 was observed with luminosities of ~1035 ergs s-1, but on other occasions it could not be detected with RXTE, resulting in typical upper limits of a few times 1033 ergs s-1 (3-25 keV). The nondetections of the source during its 2000 outburst obtained with the BeppoSAX satellite demonstrate that its luminosity was at times less than 1032 ergs s-1 (0.5-10 keV). However, only a few days after these BeppoSAX observations, we detected the source again with RXTE at high luminosities, giving a factor of greater than 1000 of luminosity swings in this systems on timescales of days. The last detection of SAX J1808.4-3658 with RXTE was on 2000 May 13, almost 4 months after the first detection during this outburst. Due to the lack of sensitivity and observations during the 1996 and 1998 outbursts, it cannot be excluded that after those outbursts the source remained active for months and that the source behavior during the 2000 outburst is not unique. Long duration activity at low luminosities has been observed in other transients (both neutron stars and black holes), although not with such extreme variability, which might point to a different origin for this behavior for the millisecond X-ray pulsar.

Spectroscopic observations of the fast X-ray transient and superluminal jet source SAX J1819.3-2525 (V4641 Sgr) reveal a best-fitting period of Pspect = 2.81678 +- 0.00056 days and a semiamplitude of K2 = 211.0 +- 3.1 km s-1. The optical mass function is f(M) = 2.74 +- 0.12 M. We find a photometric period of Pphoto = 2.81730 +- 0.00001 days using a light curve measured from photographic plates. The folded light curve resembles an ellipsoidal light curve with two maxima of roughly equal height and two minima of unequal depth per orbital cycle. The secondary star is a late B-type star that has evolved off the main sequence. Using a moderate resolution spectrum (R = 7000) we measure Teff = 10500 +- 200 K, log g = 3.5 +- 0.1, and Vrot sin i = 123 +- 4 km s-1 (1 s errors). Assuming synchronous rotation, our measured value of the projected rotational velocity implies a mass ratio of Q [?] M1/M2 = 1.50 +- 0.08 (1 s). The lack of X-ray eclipses implies an upper limit to the inclination of i [?] 70.deg7. On the other hand, the large amplitude of the folded light curve ([?]0.5 mag) implies a large inclination (i [?] 60deg). Using the above mass function, mass ratio, and inclination range, the mass of the compact object is in the range 8.73 [?] M1 [?] 11.70 M and the mass of the secondary star is in the range 5.49 [?] M2 [?] 8.14 M (90% confidence). The mass of the compact object is well above the maximum mass of a stable neutron star, and we conclude that V4641 Sgr contains a black hole. The B-star secondary is by far the most massive, the hottest, and the most luminous secondary of the dynamically confirmed black hole X-ray transients. We find that the a-process elements nitrogen, oxygen, calcium, magnesium, and titanium may be overabundant in the secondary star by factors of 2-10 times with respect to the Sun. Finally, assuming E(B-V) = 0.32 +- 0.10, we find a distance 7.40 [?] d [?] 12.31 kpc (90% confidence). This large distance and the high proper motions observed for the radio counterpart make V4641 Sgr possibly the most superluminal galactic source known, with an apparent expansion velocity of [?]9.5c and a bulk Lorentz factor of G [?] 9.5, assuming that the jets were ejected during one of the bright X-ray flares observed with the Rossi X-ray Timing Explorer.

We report the discovery of a kilohertz quasi-periodic oscillation (kHz QPO) in the low-mass X-ray binary (LMXB) 2S 0918-549. The kHz QPO has a frequency of 1156 +- 9 Hz, an FWHM of 70 +- 20 Hz, and a fractional rms amplitude of 18% +- 2%. We also detected for the first time a type I X-ray burst in this source. The compact object must therefore be a neutron star. Finally, we were able to classify the source as an atoll source exhibiting the full complement of spectral/timing states of this class (island and banana). From the peak burst flux, an upper limit on the persistent luminosity can be derived of 0.5% of the Eddington luminosity, making 2S 0918-549 one of the least luminous LMXBs showing a kHz QPO. We compare the fractional rms amplitudes of the upper kHz QPO across the ensemble of LMXBs. We find a strong anticorrelation with luminosity. In LMXBs with luminosities ~100 times lower than those of Z sources, the fractional rms amplitude is a factor ~10 larger.

We study correlations between the burst properties and the position in the color-color diagram of the low mass X-ray binary 4U 1728-34 using 21 bursts found in two data sets obtained with the Rossi X-Ray Timing Explorer. Using spectral fits we analyze the spectral evolution during the bursts and determine the burst peak flux, temperature, rise time, and fluence. Using dynamical power spectra, we study the properties of the ~363 Hz burst oscillations. We find correlations between fluence, peak flux, the occurrence of radius expansion, and the presence of burst oscillations and the position of the source in the color-color diagram. The radius expansion bursts with and without burst oscillations differ both with respect to where they occur in the color-color diagram and in how the radius expansion takes place. We compare our results with those of a similar study by Muno and coworkers for KS 1731-26. Both KS 1731-260 and 4U 1728-34 show more or less the same behavior with respect to the dependence of the presence/absence of the burst oscillations on the position of the source in the color-color diagram, but the dependence of most of the spectral burst parameters on the position in the color-color diagram is clearly different. We find that the systematic absence in 4U 1728-34 of burst oscillations in the "island" state of the color-color diagram is not explained by the bursts involving mixed H-He burning, as Cumming & Bildsten found may be the case in KS 1731-260.

We study the correlations between timing and X-ray spectral properties in the low-mass X-ray binary 4U 0614+09 using a large (265 ks) data set obtained with the Rossi X-Ray Timing Explorer. We find strong quasi-periodic oscillations (QPOs) of the X-ray flux, like the kilohertz QPOs in many other X-ray binaries with accreting neutron stars, with frequencies ranging from 1329 Hz down to 418 Hz and perhaps as low as 153 Hz. We report the highest frequency QPO yet from any low-mass X-ray binary at 1329 +- 4 Hz, which has implications for neutron star structure. This QPO has a 3.5 s single-trial significance; for an estimated 40 trials the significance is 2.4 s. Besides the kilohertz QPOs, the Fourier power spectra show four additional components: high-frequency noise (HFN), described by a broken power law with a break frequency between 0.7 and 45 Hz, very low frequency noise (VLFN), which is fitted as a power law below 1 Hz, and two broad Lorentzians with centroid frequencies varying from 6 to 38 Hz and from 97 to 158 Hz, respectively. We find strong correlations between the frequencies of the kilohertz QPOs, the frequency of the 6-38 Hz broad Lorentzian, the break frequency of the HFN, the strength of both the HFN and the VLFN, and the position of the source in the hard X-ray color versus intensity diagram. The frequency of the 97-158 Hz Lorentzian does not correlate with these parameters. We also find that the relation between power density and break frequency of the HFN is similar to that established for black hole candidates in the low state. We suggest that the changing mass accretion rate is responsible for the correlated changes in all these parameters.

We report the discovery of a 695 Hz quasi-periodic oscillation (QPO) in data taken with the Rossi X-Ray Timing Explorer of the low-mass X-ray binary (LMXB) EXO 0748-676. This makes EXO 0748-676 the second dipping LMXB, after 4U 1915-05, that shows kilohertz QPOs. Comparison with other sources suggests that the QPO corresponds to the lower frequency peak of the kilohertz QPO pair often observed in other LMXBs. The QPO was found in the only observation done during an outburst of the source in early 1996. This observation is also the only one in which the ~1 Hz QPO recently found in EXO 0748-676 is not present.

We have discovered a ~1 Hz quasi-periodic oscillation (QPO) present in the persistent X-ray emission of the globular cluster source, dipper, and low-mass X-ray binary (LMXB) 4U 1746-37. The QPO was also observed during type I X-ray bursts. The QPO properties resemble those of QPOs found recently in the LMXB dippers 4U 1323-62 and EXO 0748-676, which makes 4U 1746-37 the third source known to exhibit this type of QPO. We present evidence for X-ray spectral changes in this source similar to those observed in LMXBs referred to as atoll sources. We detect two states, a lower intensity and spectrally hard state and a higher intensity and spectrally soft state. This may explain the different spectral characteristics reported for 4U 1746-37 earlier. The high-intensity state resembles the banana branch state of atoll sources. The QPOs are seen only in the low-intensity state and are absent when the source is in the banana branch. This strongly suggests that either the accretion disk or an extended central source changes shape between the low-intensity state and the banana branch. Twelve bursts were detected, of which five took place while the source was on the banana branch and seven while the source was in the low-intensity state. The bursts occurring on the banana branch had an e-folding time ~3 times longer than those that occurred in the low-intensity state. Whereas previously detected dips showed a decrease in count rate of only ~15%, we found a dip in one observation for which the count rate dropped from ~200 counts s-1 to ~20 counts s-1. This dip lasted only ~250 s, during which clear spectral hardening occurred. This is the first time strong evidence for spectral changes during a dip have been reported for this source.

We analyze the rapid, aperiodic X-ray variability of different types of X-ray binaries (black hole candidates, atoll sources, the recently discovered millisecond X-ray pulsar, and Z sources) at their lowest inferred mass accretion rates. At these accretion rates, the power spectra of all sources are dominated by a strong band-limited noise component, which follows a power law with an index of roughly 1 at high frequencies and breaks at a frequency between 0.02 and 32 Hz, below which the spectrum is relatively flat. Superimposed on this, a broad bump (sometimes a quasi-periodic oscillation) is present, with a 0.2-67 Hz centroid frequency that varies in good correlation with the frequency of the break. The black hole candidates and the low-luminosity neutron star systems (including the millisecond X-ray pulsar) have the same relation between the frequency of the bump and the frequency of the break. These similar characteristics strongly suggest that in all those different types of sources, the band-limited noise and the bump are produced by the same physical mechanism. This mechanism cannot then depend on the presence or absence of either a small magnetosphere or a solid surface, so that it is most likely related to an instability in the flow in the accretion disk that modulates the accretion rate. The Z sources, which are more luminous than the other sources discussed here, follow a similar, but slightly shifted correlation between the break frequency and the frequency of the bump. The data suggest that the band-limited noise in Z sources is more complex than that in the other sources.

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 on board the Rossi X-Ray Timing Explorer (RXTE). 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 decay of the outburst. This pattern resembles the so-called banana branch of those low-luminosity neutron star low-mass X-ray binaries (LMXBs) that 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 the position of the source on the branch is also identical to the behavior of atoll sources on the banana branch. Near the end of its 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 that 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 quasi-periodic oscillation (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 show on their normal and flaring branches.

Three types of quasi-periodic oscillations (QPOs) have been discovered so far in the persistent emission of the most luminous neutron star low-mass X-ray binaries, the Z sources: ~10-60 Hz horizontal-branch and ~6-20 Hz normal/flaring-branch oscillations and ~200-1200 Hz kilohertz QPOs, which usually occur in pairs. Here we study the horizontal-branch oscillations and the two simultaneous kilohertz QPOs, which were discovered using the Rossi X-Ray Timing Explorer, comparing their properties in five Z sources with the predictions of the magnetospheric beat-frequency and Lense-Thirring precession models. We find that the variation of the horizontal-branch oscillation frequency with accretion rate predicted by the magnetospheric beat-frequency model for a purely dipolar stellar magnetic field and a radiation-pressure-dominated inner accretion disk is consistent with the observed variation. The model predicts a universal relation between the horizontal-branch oscillation, stellar spin, and upper kilohertz QPO frequencies that agrees with the data on five Z sources. The model implies that the neutron stars in the Z sources are near magnetic spin equilibrium, that their magnetic field strengths are ~109-1010 G, and that the critical fastness parameter for these sources is [?]0.8. If the frequency of the upper kilohertz QPO is an orbital frequency in the accretion disk, the magnetospheric beat-frequency model requires that a small fraction of the gas in the disk does not couple strongly to the stellar magnetic field at 3-4 stellar radii but instead drifts slowly inward in nearly circular orbits until it is within a few kilometers of the neutron star surface. The Lense-Thirring precession model is consistent with the observed magnitudes of the horizontal-branch oscillation frequencies only if the moments of inertia of the neutron stars in the Z sources are ~4-5 times larger than the largest values predicted by realistic neutron star equations of state. If instead the moments of inertia of neutron stars have the size expected and their spin frequencies in the Z sources are approximately equal to the frequency separation of the kilohertz QPOs, Lense-Thirring precession can account for the magnitudes of the horizontal-branch oscillation frequencies only if the fundamental frequency of the horizontal-branch oscillation is at least 4 times the precession frequency. We argue that the change in the slope of the correlation between the frequency of the horizontal-branch oscillation and the frequency of the upper kilohertz QPO, when the latter is greater than 850 Hz, is directly related to the varying frequency separation of the kilohertz QPOs.

We study systematically the [?]0.1-1200 Hz quasi-periodic oscillations (QPOs) and broad noise components observed in the power spectra of nonpulsing neutron star and black hole low-mass X-ray binaries. We show that among these components we can identify two, occurring over a wide range of source types and luminosities, whose frequencies follow a tight correlation. The variability components involved in this correlation include neutron star kilohertz QPOs and horizontal-branch oscillations, as well as black hole QPOs and noise components. Our results suggest that the same types of variability may occur in both neutron star and black hole systems over 3 orders of magnitude in frequency and with coherences that vary widely but systematically. Confirmation of this hypothesis will strongly constrain theoretical models of these phenomena and provide additional clues to understanding their nature.

We present a detailed analysis of simultaneous Ginga and ROSAT observations of the X-ray burst source 1735-444, during which the source was in an unusually high luminosity state. The source shows the characteristics of an atoll source in the high-intensity (banana) state, with a general correlation between intensity and spectral hardness. A blackbody component contributes ~15%-20% of the total flux when the conventional two-component model is used to fit the spectra. We also observed a bright burst with radius expansion at this very high luminosity. The burst spectrum could not be described by a simple blackbody model. We discuss possible explanations.

As a result of an error at the publisher, the name of the author Michiel van der Klis and his affiliation were misspelled. The corrected version appears above. IOP Publishing sincerely regrets this error.