[show abstract][hide abstract] ABSTRACT: Twenty years ago, the Rapid Burster (MXB 1730-335) was discovered. Its most salient feature was the occurrence of rapidly repetitive type II X-ray bursts, the release of gravitational potential energy due to spasmodic accretion onto a compact object. This is almost certainly due to an accretion disk instability whose origin is still not understood. With the recent appearance of GRO J1744-28, the Rapid Burster is no longer the only system to produce such bursts. Both systems are transient low-mass X-ray binaries in which the accretor is a neutron star. The Rapid Burster, located in a globular cluster, also produces type I bursts which are due to thermonuclear flashes on the neutron star's surface; no X-ray pulsations are observed. Its neutron star magnetic field is therefore relatively weak. In contrast, strong X-ray pulsations have been observed in the persistent flux as well as in the type II bursts from GRO J1744-28, but no type I bursts have been observed. Thus, the magnetic field of the neutron star in this system is probably stronger than is the case of the Rapid Burster. The fact that type II bursts occur in both systems may bring us closer to an understanding of the mechanism(s?) that produces them.
The Astrophysical Journal 01/2009; 462(1):L39. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Dwarf nova outbursts are likely caused by a thermal-viscous instability in the accretion disk around the white dwarf, which can occur if the mass transfer rate, , is below a critical value (which depends on orbital period). Based on data for soft X-ray transients and persistent low-mass X-ray binaries with known distances and orbital periods, Porb, I show that for these systems the distributions in a (Porb, ) diagram can be understood with the dwarf nova instability criterion, if that is adapted to include X-ray heating of the disk. This supports the idea that the disk instability model applies to soft X-ray transients, and shows that X-ray heating must be included in modeling the outbursts.
The Astrophysical Journal 01/2009; 464(2):L139. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The repetitive X-ray bursts from the accretion-powered pulsar GRO J1744-28 show similarities to the type II X-ray bursts from the Rapid Burster. Several authors (notably, Lewin et al.) have suggested that the bursts from GRO J1744-28 are type II bursts (which arise from the sudden release of gravitational potential energy). In this paper, we present another similarity between these sources. Rossi X-ray Timing Explorer observations of GRO J1744-28 show that at least 10 out of 94 bursts are followed by quasi-periodic oscillations (QPOs) with frequencies of ~0.4 Hz. The period of the oscillations decreases over their ~30-80 s lifetime, and they occur during a spectrally hard "shoulder" (or "plateau") that follows the burst. In one case, the QPOs show a modulation envelope that resembles simple beating between two narrow-band oscillations at ~0.325 and ~0.375 Hz. Using EXOSAT observations, Lubin et al. found QPOs with frequencies of 0.039-0.056 Hz following 10 out of 95 type II bursts from the Rapid Burster. As in GRO J1744-28, the period of these oscillations decreased over their ~100 s lifetime, and they occurred only during spectrally hard "humps" in the persistent emission. Even though the QPO frequencies differ by a factor of ~10, we believe that this is further evidence that a similar accretion disk instability is responsible for the type II bursts from these two sources.
The Astrophysical Journal 01/2009; 482(1):L53. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We observed the low-mass X-ray binary and Z source GX 17+2 with the Rossi X-Ray Timing Explorer during 1997 February 6-8, April 1-4, and July 26-27. The X-ray color-color diagram shows a clear Z track. Two simultaneous kHz quasi-periodic oscillations (QPOs) are present in each observation, whose frequencies are well correlated with the position of the source on the Z track. At the left end of the horizontal branch (HB), only the higher frequency peak is observed, at 645 ± 9 Hz, with an rms amplitude of 5.7% ± 0.5% and an FWHM of 183 ± 35 Hz. When the source moves down the Z track to the upper normal branch, the frequency of the kHz QPO increases to 1087 ± 12 Hz, and the rms amplitude and FWHM decrease by a factor of 2. Farther down the Z track, the QPO becomes undetectable, with rms upper limits typically of 2.0%. Halfway down the HB, a second QPO appears in the power spectra with a frequency of 480 ± 23 Hz. The frequency of this QPO also increases when the source moves along the Z track, up to 781 ± 11 Hz halfway down the normal branch, while the rms amplitude and FWHM stay approximately constant at 2.5% and 70 Hz. The QPO frequency difference is constant at 293.5 ± 7.5 Hz. Simultaneously with the kHz QPOs, we detect HB QPOs (HBOs). The simultaneous presence of HBOs and kHz QPOs excludes the magnetospheric beat-frequency model as the explanation for at least one of these two phenomena.
The Astrophysical Journal 01/2009; 490(2):L157. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have analyzed the Galactic distribution of low-mass X-ray binaries (LMXBs) in which the accreting compact object is a neutron star. The rms value of their distances, z, to the Galactic plane equals ~1 kpc. This wide z-distribution cannot be explained by systemic velocities that increased as a result of sudden symmetric mass loss at the formation of the neutron star alone. Kick velocities imparted on the neutron star, following the radio pulsar velocity distribution recently derived by Lyne & Lorimer (1994) can account for the LMXB z-distribution. This distribution is consistent with formation of the neutron stars in LMXBs from direct collapse of a helium star and also from accretion-induced collapse of a white dwarf. The triple-star evolution proposed by Eggleton & Verbunt is not a dominant production mechanism for LMXBs.
The Astrophysical Journal 01/2009; 447(1):L33. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the discovery, with the Rossi X-Ray Timing Explorer, of a 50-60 Hz quasi-periodic oscillation (QPO) in GX 17+2. The QPO is seen when GX 17+2 is on the normal branch in the X-ray color-color diagram. Its frequency initially increases from 59 to 62 Hz as the source moves down the normal branch, but below the middle of the normal branch it decreases to ~50 Hz. Together with this frequency decrease, the QPO peak becomes much broader, from ~4 Hz in the upper part of the normal branch to ~15 Hz in the lower normal branch. The rms amplitude remains approximately constant between 1% and 2% along the entire normal branch. From a comparison of the properties of this QPO with those of QPOs previously observed along the normal branch in other Z sources, we conclude that it is most likely the horizontal-branch QPO (HBO). However, this QPO displays a number of unusual characteristics. The decrease in the QPO frequency along the lower normal branch is not in agreement with the predictions of the beat-frequency model for the HBO unless the mass flux through the inner disk decreases as the source moves down the lower normal branch. We tentatively suggest that the required decrease in the mass flux through the inner disk is caused by an unusually rapid increase in the mass flux in the radial inflow as GX 17+2 moves down the normal branch. Assuming that this explanation is correct, we can derive an upper bound on the dipole component of the star's magnetic field at the magnetic equator of 5 × 109 G for a 1.4 M☉ neutron star with a radius of 106 cm.
The Astrophysical Journal 01/2009; 469(1):L5. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Pairs of quasi-periodic oscillations (QPOs) at kilohertz frequencies are a common phenomenon in several neutron-star low-mass X-ray binaries. The frequency separation of the QPO peaks in the pair appears to be constant in many sources and directly related to the neutron star spin frequency. However, in Sco X-1 and possibly in 4U 1608-52, the frequency separation between the QPO peaks decreases with increasing inferred mass accretion rate. We show that the currently available Rossi X-Ray Timing Explorer data are consistent with the hypothesis that the peak separations in all sources vary by amounts similar to the variation in Sco X-1. We discuss the implications for models of the kilohertz QPOs.
The Astrophysical Journal 01/2009; 501(1):L95. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: In recent observations with the Rossi X-Ray Timing Explorer, we have detected two simultaneous quasi-periodic oscillation (QPO) peaks in the low-mass X-ray binary and atoll source 4U 1735-44. The lower and higher frequency QPOs have frequencies varying between 632 and 729 Hz and 982 and 1026 Hz, respectively. The fractional rms amplitudes are 3.7%-8.1% and 5.0%-5.8%. The frequency separation between the two QPOs changes from 341±7 to 296±12 Hz. The inferred mass accretion rate during our observations is relatively low compared to that during the previous observations, in which only a single QPO was present. There is weak evidence that the frequency of the QPOs correlates with the mass accretion rate, as observed in other binaries. Five X-ray bursts were recorded with no detectable oscillations with upper limits for the rms fraction of 4%-13%.
The Astrophysical Journal 01/2009; 508(2):L155. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: In observations with the Australia Telescope Compact Array, we have resolved the radio counterpart of the unusual X-ray binary Circinus X-1 into an asymmetric, extended structure on arcsecond scales. In order to quantify the asymmetry, we have redetermined as accurately as possible both the optical and radio coordinates of the source. The extended emission can be understood as a compact, absorbed core at the location of the X-ray binary and as extended emission up to 2'' to the southeast of the core. The arcsecond-scale-extended emission aligns with the larger, more symmetric arcminute-scale-collimated structures in the surrounding synchrotron nebula. This suggests that the transport of mass and/or energy from the X-ray binary to the synchrotron nebula is occurring via the arcsecond-scale structures. The ratio of extended flux from the southeast to that from the northwest of the core is at least 2:1. Interpreted as a relativistic aberration of an intrinsically symmetric jet from the source, this implies a minimum outflow velocity of 0.1c. Alternatively, the emission may be intrinsically asymmetric, perhaps as a result of the high space velocity of the system.
The Astrophysical Journal 01/2009; 506(2):L121. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on the results of R-band observations of the error box of the γ-ray burst of 1997 August 28 made between 4 hr and 8 days after this burst occurred. No counterpart was found varying by more than 0.2 mag down to R=23.8. We discuss the consequences of this nondetection for relativistic blast wave models of γ-ray bursts and the possible effect of redshift on the relation between optical absorption and the low-energy cutoff in the X-ray afterglow spectrum.
The Astrophysical Journal 01/2009; 493(1):L27. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present new Rossi X-Ray Timing Explorer observations of the low-mass X-ray binary 4U 1608-52 during the decay of its 1998 outburst. We detect, by a direct fast Fourier transform method, the existence of a second kilohertz quasi-periodic oscillation (kHz QPO) in its power density spectrum, which was previously only seen by means of the sensitivity-enhancing "shift and add" technique. This result confirms that 4U 1608-52 is a twin kHz QPO source. The frequency separation between these two QPOs decreased significantly, from 325.5±3.4 to 225.3±12.0 Hz, as the frequency of the lower kHz QPO increased from 470 to 865 Hz, in contradiction with a simple beat-frequency interpretation. This change in the peak separation of the kHz QPOs is closely similar to that previously seen in Scorpius X-1 but takes place at a 10 times lower average luminosity. We discuss this result within the framework of models that have been proposed for kHz QPO. Beat-frequency models where the peak separation is identified with the neutron star spin rate, as well as the explanations previously proposed to account for the similar behavior of the QPOs in Sco X-1, are strongly challenged by this result.
The Astrophysical Journal 01/2009; 505(1):L23. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We discovered a single kilohertz (kHz) quasi-periodic oscillation (QPO) near 1150 Hz in the Rossi X-Ray Timing Explorer X-ray light curve of the low-mass X-ray binary and atoll source 4U 1735-44. The rms amplitude of this peak was 2%-3%, and the FWHM was 6-40 Hz. There are indications that the kHz QPO frequency decreased from 1160 to 1145 Hz when the count rate increased, which would be quite different from what is observed in other atoll sources for which kHz QPOs have been discovered. In the X-ray color-color diagram and hardness-intensity diagram, the source traced out the curved branch (the so-called banana branch) that has been found by previous instruments. The kHz QPO was detected only when the source was at the lowest count rates during our observations, i.e., on the lower part of the banana branch. When 4U 1735-44 was at higher count rates, i.e., on the upper part of the banana branch and at a higher inferred mass accretion rate with respect to that on the lower part of the banana branch, the QPO was not detected. Besides the kHz QPO, we discovered a low-frequency QPO with a frequency near 67 Hz, together with a complex, broad-peaked noise component below 30 Hz. This 67 Hz QPO may be related to the magnetospheric beat-frequency QPO, which is observed on the horizontal branch of Z sources. This idea is supported by the (peaked) noise found in both 4U 1735-44 and Z sources at frequencies just below the QPO frequency. We discovered a single kilohertz (kHz) quasi-periodic oscillation (QPO) near 1150 Hz in the Rossi X-Ray Timing Explorer X-ray light curve of the low-mass X-ray binary and atoll source 4U 1735-44. The rms amplitude of this peak was 2%-3%, and the FWHM was 6-40 Hz. There are indications that the kHz QPO frequency decreased from 1160 to 1145 Hz when the count rate increased, which would be quite different from what is observed in other atoll sources for which kHz QPOs have been discovered. In the X-ray color-color diagram and hardness-intensity diagram, the source traced out the curved branch (the so-called banana branch) that has been found by previous instruments. The kHz QPO was detected only when the source was at the lowest count rates during our observations, i.e., on the lower part of the banana branch. When 4U 1735-44 was at higher count rates, i.e., on the upper part of the banana branch and at a higher inferred mass accretion rate with respect to that on the lower part of the banana branch, the QPO was not detected. Besides the kHz QPO, we discovered a low-frequency QPO with a frequency near 67 Hz, together with a complex, broad-peaked noise component below 30 Hz. This 67 Hz QPO may be related to the magnetospheric beat-frequency QPO, which is observed on the horizontal branch of Z sources. This idea is supported by the (peaked) noise found in both 4U 1735-44 and Z sources at frequencies just below the QPO frequency.
The Astrophysical Journal 01/2009; 495(1):L39. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on the results of optical follow-up observations of the counterpart of the gamma-ray burst GRB 970508, starting 7 hr after the event. Multicolor U-, B-, V-, Rc-, and Ic-band observations were obtained during the first three consecutive nights. The counterpart was monitored regularly in Rc until ~4 months after the burst. The light curve after the maximum follows a decline that can be fitted with a power law with exponent α = -1.141 ± 0.014. Deviations from a smooth power-law decay are moderate (rms = 0.15 mag). We find no flattening of the light curve at late times. The optical afterglow fluence is a significant fraction, ~5%, of the GRB fluence. The optical energy distribution can be well represented by a power law, the slope of which changed at the time of the maximum (the spectrum became redder).
The Astrophysical Journal 01/2009; 497(1):L13. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The bursting pulsar GRO J1744-28 exhibits a unique combination of persistent X-ray pulsations (with a pulse period Ppulse ≈ 0.467 s) and X-ray bursts. The pulsations are also present (at an enhanced amplitude) during the bursts, but the arrival times of the pulses during the burst are delayed with respect to those of the persistent emission. We present the results of a detailed study of the pulse delays using data obtained with the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory. We find that the average delay, as measured during a 1.5 s interval at the peak of the burst, is independent of energy in the energy range ~25-75 keV and has a magnitude Δt = 74±13 ms. We also find that the phase delay measured near the peak of the bursts remained approximately constant throughout the first outburst of the source, although the peak flux of the bursts varied by a factor of ~3.3.
The Astrophysical Journal 01/2009; 496(2):L101. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using a new technique to improve the sensitivity to weak quasi-periodic oscillations (QPOs), we discovered a new QPO peak at about 1100 Hz in the March 1996 outburst observations of 4U 1608-52, simultaneous with the ~600-900 Hz peak previously reported from these data. The frequency separation between the upper and the lower QPO peaks varied significantly from 232.7±11.5 Hz on March 3 to 293.1±6.6 Hz on March 6. This is the first case of a variable kilohertz peak separation in an atoll source. We discuss to what extent this result could be accommodated in beat-frequency models such as proposed for the kilohertz QPOs. We measured the rms fractional amplitude of both QPOs as a function of energy, and we found that the relation is steeper for the lower than for the upper frequency peak. This is the first source where such a difference between the energy spectrum of the two kilohertz QPOs could be measured.
The Astrophysical Journal 01/2009; 494(1):L65. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: During observations with the Rossi X-Ray Timing Explorer from 1997 June 31 to July 3 we discovered two simultaneous kHz quasi-periodic oscillations (QPOs) near 500 and 860 Hz in the low-mass X-ray binary and Z source Cygnus X-2. In the X-ray color-color diagram and hardness-intensity diagram (HID), a clear Z track was traced out, which shifted in the HID within 1 day to higher count rates at the end of the observation. Z track shifts are well known to occur in Cyg X-2; our observation for the first time catches the source in the act. A single kHz QPO peak was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731±20 Hz and an amplitude of 4.7+ 0.8−0.6% rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532±43 Hz was detected with an rms amplitude of 3.0+ 1.0−0.7%. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher frequency QPO increased to 839±13 Hz, and its amplitude decreased to 3.5+ 0.4−0.3% rms. The higher frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8+ 0.6−0.4% and a frequency of 1007±15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower frequency QPO was most of the time undetectable, with typical upper limits of 2% rms; no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in some beat-frequency models), then the neutron star spin period is 2.9±0.2 ms (346±29 Hz). This discovery makes Cyg X-2 the fourth Z source that displays kHz QPOs. The properties of the kHz QPOs in Cyg X-2 are similar to those of other Z sources. Simultaneous with the kHz QPOs, the well-known horizontal-branch QPOs (HBOs) were visible in the power spectra. At the left end of the HB, the second harmonic of the HBO was also detected. We also detected six small X-ray bursts. No periodic oscillations or QPOs were detected in any of them, with typical upper limits of 6%-8% rms.
The Astrophysical Journal 01/2009; 493(2):L87. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: One year after its discovery, the Bursting Pulsar (GRO J1744-28) went into outburst again, displaying the hard X-ray bursts and pulsations that make this source unique. We report on BATSE observations of both the persistent and burst emission for this second outburst and draw comparisons with the first. The second outburst was smaller than the first in both duration and peak luminosity. The persistent flux, burst peak flux, and burst fluence were all reduced in amplitude by a factor of ~1.7. Despite these differences, the two outbursts were very similar with respect to the burst occurrence rate, the durations and spectra of bursts, the absence of spectral evolution during bursts, and the evolution of the ratio α of average persistent to burst luminosity. Although no spectral evolution was found within individual bursts, we find evidence for a small (20%) variation of the spectral temperature during the course of the second outburst.
The Astrophysical Journal 01/2009; 517(1):431. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have used the Fourier cross spectra of GRO J1719-24, obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20-50 and 50-100 keV energy bands as a function of Fourier frequency in the interval 0.002-0.488 Hz. Our analysis covers the entire ~80 day X-ray outburst of this black hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.072 ± 0.010 rad in the frequency interval 0.02-0.20 Hz. The peak phase lags in the interval 0.02-0.20 Hz are about twice those of Cyg X-1 and GRO J0422+32. These results are consistent with models for Comptonization regions composed of extended nonuniform clouds around the central source.
The Astrophysical Journal 01/2009; 519(1):332. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope (HST) and the Keck I telescope. The gamma-ray burst (GRB) was observed approximately 6 months after outburst, on 1997 September 4, using the HST/STIS CCD, and approximately 1 year after outburst, on 1998 February 24, using HST/NICMOS, and on 1998 April 4 using the NIRC on Keck. The unresolved counterpart is detected by STIS at V=28.0 ± 0.25, consistent with a continued power-law decline with exponent -1.10 ± 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~08 and integrated magnitude V=25.8 ± 0.25. A reanalysis of HST and New Technology Telescope observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Although the optical transient is not detected in the NICMOS images (H≥25.3), the extended source is visible and has a total magnitude H=23.3 ± 0.1. The Keck observations find K=22.8 ± 0.3. Comparison with observations obtained shortly after outburst suggests that the nebular luminosity has also been stable in the infrared. We find that several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with AV=0.75 ± 0.2. After adjusting for this Galactic extinction, we find that the size of the observed extended emission is consistent with that of galaxies of comparable magnitude found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the sky is covered by galaxies of similar or greater surface brightness. We therefore conclude that the extended source observed about GRB 970228 is almost certainly its host galaxy. Additionally, we find that independent of assumed redshift, the host is significantly bluer than typical nearby blue dwarf irregulars. With the caveat that the presently available infrared observations of the HDF are only fully complete to a limit about one-half magnitude brighter than the host, we find that the extinction-corrected V-H and V-K colors of the host are as blue as any galaxy of comparable or brighter magnitude in the HDF. Taken in concert with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work suggests that all four GRBs with spectroscopic identification or deep multicolor broadband imaging of the host lie in rapidly star-forming galaxies.
The Astrophysical Journal 01/2009; 516(2):683. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
The Astrophysical Journal 01/2009; 520(2):763. · 6.73 Impact Factor