Low-mass X-ray Binary Systems Research Articles

Magnetic effects on the viscous boundary layer damping of ther-modes in neutron stars

This paper explores the effects that magnetic fields have on the viscous boundary layers (VBLs) that can form in neutron stars at the crust-core interface, and it investigates the VBL damping of the gravitational-radiation driven r-mode instability. Approximate solutions to the magnetohydrodynamic equations valid in the VBL are found for ordinary-fluid neutron stars. It is shown that magnetic fields above ${10}^{9}{(10}^{10} \mathrm{K}/T)\mathrm{G}$ significantly change the structure of the VBL, and that magnetic fields decrease the VBL damping time. Furthermore, VBL damping completely suppresses the r-mode instability for $B\ensuremath{\gtrsim}{10}^{12} \mathrm{G}$ (independent of the temperature). These bounds refer to the strength of the radial component of the equilibrium field at the location of the core radius. Magnetic effects on the VBL vanish wherever the radial component of the equilibrium field vanishes. Thus, magnetic fields will profoundly affect the VBL damping of the r-mode instability in hot young pulsars (that are cool enough to have formed a solid crust). One can speculate that magnetic fields can affect the VBL damping of this instability in low-mass x-ray binary systems and other cold old pulsars (if they have sufficiently large internal fields).

SAX J1810.8−2609: A New Hard X‐Ray Bursting Transient

The transient X-ray source SAX J1810.8-2609 was discovered on 1998, March 10 with the Wide Field Cameras on board the BeppoSAX satellite, while observing the Galactic Bulge in the 2-28 keV energy range. On March 11, a strong type-I X-ray burst was detected with evidence of photospheric radius expansion. A follow-up target of opportunity observation with the Narrow-Field Instruments (NFI) was performed on March 11 and 12, for a total elapsed time of 8.51 x 10^{4} s. The wide band spectral data (0.1-200 keV) obtained with the NFI show a remarkable hard X-ray spectrum detected up to ~200 keV, which can be described by a power law with photon spectral index Gamma=1.96\pm0.04, plus a soft component which is compatible with blackbody radiation of temperature kT~0.5 keV. The detection of the type-I X-ray burst is a strong indication that the compact object is a neutron star in a low mass X-ray binary system. Assuming standard burst parameters and attributing the photospheric radius expansion to near Eddington luminosity, we estimate a distance of ~5 kpc. The inferred 2-10 keV X-ray luminosity is ~ 9 x 10^{35} erg/s at the time of the discovery.

The Orbital Period of the Accreting Pulsar GX 1+4.

We report strong evidence for a approximately 304 day periodicity in the spin history of the accretion-powered pulsar GX 1+4 that is most probably associated with the orbital period of the system. We have used data from the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory to show a clear periodic modulation of the pulsar frequency from 1991 to date, in excellent agreement with the ephemeris proposed by Cutler, Dennis, & Dolan in 1986. Our results indicate that the orbital period of GX 1+4 is 303.8+/-1.1 days, making it the widest known low-mass X-ray binary system by more than 1 order of magnitude and putting this long-standing question to rest. A likely scenario for this system is an elliptical orbit in which the neutron star decreases its spin-down rate (or even exhibits a momentary spin-up behavior) at periastron passages due to the higher torque exerted by the accretion disk onto the magnetosphere of the neutron star. These results are not inconsistent with either the X-ray pulsed flux light curve measured by BATSE during the same epoch or the X-ray flux history from the All-Sky Monitor on board the Rossi X-Ray Timing Explorer.

The Optical Counterparts of the Luminous X‐Ray Binary Stars in Globular Clusters

Ten percent of our GalaxyIs luminous ergs (L X 1036 s~1) X-ray point sources are located in globular clusters (GCs), but GCs contribute a far smaller fraction of normal stars to the Galaxy. X-ray bursts have been observed from nearly all GC sources, indicating that they are low-mass X-ray binary (LMXB) systems containing a neutron star and a low-mass companion star from which matter is being transferred. The understanding of LMXB overabundance in GCs may well lead to important insights into the formation and evolution of these exotic binary systems as well as the dynamics of GCs themselves. The goals of this dissertation are to identify the optical counterparts to some GC LMXBs lacking candidates, bring together and compare in the most homogeneous fashion all available Hubble Space T elescope (HST ) optical observations of the current group of GC LMXB counterparts, and discuss the implications for cluster dynamics and LMXB systems in general. In this work, candidates for three additional optical counterparts to luminous X-ray binaries in GCs are presented, thereby doubling the number of optical counterpart candidates. Two, those in NGC 1851 and NGC 6441, are very likely correct although they require additional work to conÐrm the identiÐcations, while the third, in NGC 6652, remains somewhat tentative because of the positional discrepancy with the X-ray coordinates and the fact that the entire error circle is not surveyed. A homogeneous set of HST photometric measurements for all the counterparts identiÐed thus far is presented, and their optical properties are compared with those of Ðeld LMXBs. The mean color of the cluster sources is (U[B)0 identical to that of the Ðeld sources, and the mean is M B0 similar in range, but fainter on average than bursters in the Ðeld. The ratio of optical to X-ray Nux of cluster sources seems to show a signiÐcantly larger dispersion than that of the Ðeld sources. The GC LMXBs seem to conform well to the previously suggested optical luminosity, X-ray luminosity, orbital period relation (J. van Paradijs & J. McClintock, A&A, 290, 133 [1994]). If the source in NGC 1851Efor which no orbital period has yet been determinedEfollows this pattern, then the period will likely be less than 0.8 hr. Similarly, if the object advanced here in NGC 6652 is indeed the correct optical counterpart, this relation suggests that its orbital period will also prove to be less than 1 hr. New and archival HST spectra and imaging data are analyzed to intercompare the UV/optical spectral energy distributions (SEDs) of GC LMXBs. A set of simple model SEDs is introduced and compared with the observations to infer accretion rates, disk diameters, and other properties of these systems. The GC LMXBs are particularly well suited for SED modeling as the distance and reddening of these systems is quite well understood, unlike most Ðeld LMXBs. The sources in NGC 6441 and M15 are found to be optically quite luminous, with comparatively shallow SEDs. The faint sources in NGC 6712 and NGC 1851 have signiÐcantly steeper SEDs and are most likely underluminous because their orbital separations and thus accretion disks are small. The case of NGC 6624 appears slightly di†erent from the others, with an average optical luminosity, yet with the steepest SED slope. This is most likely due to its high accretion rate isD10 times higher than any of the other (L X GC LMXBs) and extremely small size. From its luminosity, it is inferred that the source in NGC 6652 will have similar system parameters and SED to the one in NGC 1851. If these inferences prove correct, it will mean that twothirds of GC LMXBs for which orbital periods are known have periods less than 1 hr and are therefore quite likely double degenerate ultracompact binaries. Such a high fraction of ultracompact systems would be remarkable and may indicate that the processes responsible for creating such short-period systems are more common than previously thought.

High-Resolution Parallax Measurements of Scorpius X-1

The results of eight VLBA observations at 5 GHz, spanning 3 yr, have yielded a measured trigonometric parallax for Sco X-1 of 000036±000004; hence, its distance is 2.8±0.3 kpc. This is the most precise parallax measured to date. Although our measured distance is 40% farther away than previous estimates based on X-ray luminosity, our Rossi X-Ray Timing Explorer observations, with a measured luminosity of 2.3×1038 ergs s-1, and determined distance continue to support the hypothesis that Z-source low-mass X-ray binary systems, like Sco X-1, radiate at the Eddington luminosity at a particular point in their X-ray color-color diagram.

Effects of Rapid Stellar Rotation on Equation‐of‐State Constraints Derived from Quasi‐periodic Brightness Oscillations

Quasi-periodic X-ray brightness oscillations (QPOs) with frequencies around a kilohertz have now been discovered in more than a dozen neutron stars in low-mass X-ray binary systems using the Rossi X-ray Timing Explorer. There is strong evidence that the frequencies of the kilohertz oscillations are the orbital frequencies of accreting gas in nearly circular orbits around these stars. Some stars that produce kilohertz QPOs may have spin frequencies greater than 400 Hertz. For spin rates this high, first-order analytic treatments of the effects of the star's rotation on its structure and the spacetime are inaccurate. Here we use the results of a large number of fully relativistic, self-consistent numerical calculations of the stellar structure of rapidly rotating neutron stars and the interior and exterior spacetime to investigate the constraints on the properties of such stars that can be derived if stable circular orbits of various frequencies are observed. We have computed the equatorial radius of the star, the radius of the innermost stable circular orbit, and the frequency of the highest-frequency stable circular orbit as functions of the stellar spin rate, for spin rates up to the maximum possible and for several illustrative equations of state. Our calculations show that the upper bounds on the stiffness of neutron star matter implied by a given orbital frequency are typically significantly stricter for stars with spin frequencies more than 400 Hertz than for slowly rotating stars.

Constraints on the equation of state of neutron star matter from observations of kilohertz QPOs

The frequencies of the highest-frequency kilohertz QPOs recently discovered in some sixteen neutron stars in low-mass X-ray binary systems are most likely the orbital frequencies of gas in Keplerian orbit around these neutron stars. If so, these QPOs provide tight upper bounds on the masses and radii of these neutron stars and important new constraints on the equation of state of neutron star matter. If the frequency of a kilohertz QPO can be established as the orbital frequency of gas at the innermost stable circular orbit, this would confirm one of the key predictions of general relativity in the strong-gravity regime. If the spin frequency of the neutron star can also be determined, the frequency of the QPO would fix the mass of the neutron star for each assumed equation of state. Here we show how to derive mass and radius bounds, using the kilohertz QPOs, for nonrotating and slowly rotating stars, and discuss how these bounds are affected by rapid stellar rotation and radial radiation forces. We also describe observational results that would be strong evidence for the presence of an innermost stable circular orbit. No such strong evidence is present in current data, but future prospects are excellent.

The two-hour orbit of a binary millisecond X-ray pulsar

Typical radio pulsars are magnetized neutron stars that are born rapidly rotating and slow down as they age on time scales of 10 to 100 million years. In contrast, millisecond radio pulsars spin very rapidly even though many are billions of years old1. The most compelling explanation is that they have been ‘spun up’ by the transfer of angular momentum during the accretion of material from a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs consist of a neutron star or black hole accreting matter from a companion with mass less than one solar mass2.) The recent detection of coherent X-ray pulsations with a millisecond period from a suspected low-mass X-ray binary system appears to confirm this link3. Here we report observations showing that the orbital period of this binary system is two hours, which establishes it as an LMXB. We also find an apparent modulation of the X-ray flux at the orbital period (at the two per cent level), with a broad minimum when the pulsar is behind the low-mass companion star. This system seems closely related to the ‘black-widow’ millisecond radio pulsars, which are evaporating their companions through irradiation4,5,6,7,8. It may appear as an eclipsing radio pulsar during periods of X-ray quiescence.

The Mass Distribution of Stellar Black Holes

We examine the distribution of masses of black holes in transient low mass X-ray binary systems. A Bayesian analysis suggests that it is probable that six of the seven systems with measured mass functions have black hole masses clustered near seven solar masses. There appears to be a significant gap between the masses of these systems and those of the observed neutron stars. The remaining source, V404 Cyg, has a mass significantly larger than the others, and our analysis suggests that it is probably drawn from a different distribution. Selection effects do not appear to play a role in producing the observed mass distribution, which may be explained by currently unknown details of the supernova explosions and of binary evolution prior to the supernova.

Mechanisms for High‐Frequency Quasi‐periodic Oscillations in Neutron Star and Black Hole Binaries

We explain the millisecond variability detected by Rossi X-Ray Timing Explorer (RXTE) in the X-ray emission from a number of low-mass X-ray binary systems (Sco X-1, 4U 1728-34, 4U 1608-522, 4U 1636-536, 4U 0614+091, 4U 1735-44, 4U 1820-30, GX 5-1) in terms of dynamics of the centrifugal barrier, a hot boundary region surrounding a neutron star (NS). We demonstrate that this region may experience the relaxation oscillations and that the displacements of a gas element both in radial and vertical directions occur at the same main frequency, of order of the local Keplerian frequency. We show the importance of the effect of a splitting of the main frequency produced by the Coriolis force in a rotating disk for the interpretation of a spacing between the quasi-periodic oscillation (QPO) peaks. We estimate a magnitude of the splitting effect and present a simple formula for the whole spectrum of the split frequencies. It is interesting that the first three lowest order overtones (corresponding to the azimuthal numbers m = 0, -1, and -2) fall in the range of 200-1200 Hz and match the kHz QPO frequencies observed by RXTE. Similar phenomena should also occur in black hole (BH) systems, but, since the QPO frequency is inversely proportional to the mass of a compact object, the frequency of the centrifugal-barrier oscillations in the BH systems should be a factor of 5-10 lower than that for the NS systems. The X-ray spectrum formed in this region is a result of upscattering of a soft radiation (from a disk and an NS surface) off relatively hot electrons in the boundary layer. The typical size of the emission region should be 1-3 km, which is consistent with the time-lag measurements. We also briefly discuss some alternative QPO models, including the possibility of acoustic oscillations in the boundary layer, the proper stellar rotation, and g-mode disk oscillations.

New Results for Two Optically Faint Low‐Mass X‐Ray Binary Systems

We present optical photometry of the low-mass X-ray binary systems GX 349+2 (X1702-363) and Ser X-1 (X1837+049). Extensive VRI photometry of the faint optical counterpart (V = 18.4) to GX 349+2 reveals a period of 22.5 ± 0.1 hr and a half-amplitude of 0.2 mag. This result confirms and extends our previously reported 22 hr period (Wachter & Margon). No color change is detected over the orbit, although the limits are modest. We also report the discovery of two new variable stars in the field of GX 349+2, including a probable W Ursae Majoris system. Ser X-1 is one of the most intense persistent X-ray burst sources known. It is also one of only three burst systems for which simultaneous optical and X-ray bursts have been observed. The faint blue optical counterpart MM Ser (B ~ 19.2) has long been known to have a companion 21 distant. Our images indicate that MM Ser is itself a further superposition of two stars separated by only 1''. At the very least, the ratio of inferred burst to quiescent optical flux is affected by the discovery of this additional component. In the worst case, the wrong object may have previously been assumed to be the optical counterpart.

ITAL]Rossi X-Ray Timing Explorer[/ITAL] Discovery of Coherent Millisecond Pulsations during an X-Ray Burst from KS 1731−260

A highly coherent 523.92 ± 0.05 Hz periodic X-ray signal has been observed during a type I X-ray burst from the low-mass X-ray binary system KS 1731-260 with the Proportional Counter Array on Rossi X-Ray Timing Explorer. The spectral evolution of the burst indicates photospheric-radius expansion and contraction. The 524 Hz signal occurred at the end of the contraction phase, lasted for ~2 s, was highly coherent (Q 900), and had a pulse fraction (ratio of sinusoidal amplitude to mean count rate) of 6.2% ± 0.6%. KS 1731-260 is one of only three systems that have exhibited high-coherence millisecond oscillations during X-ray bursts and the first reported in which the pulsations are associated with photospheric contraction. These coherent signals may be interpreted as a direct indication of the neutron star spin.

Evidence from Quasi-periodic Oscillations for a Millisecond Pulsar in the Low-Mass X-Ray Binary 4U 0614+091

We have detected quasi-periodic oscillations (QPOs) near 1 kHz from the low mass X-ray binary 4U 0614+091 in observations with RXTE. The observations span several months and sample the source over a large range of X-ray luminosity. In every interval QPOs are present above 400 Hz with fractional RMS amplitudes from 3 to 12% over the full PCA band. At high count rates, two high frequency QPOs are detected simultaneously. The difference of their frequency centroids is consistent with a constant value of 323 Hz in all observations. During one interval a third signal is detected at 328 +/- 2 Hz. This suggests the system has a stable 'clock' which is most likely the neutron star with spin period 3.1 msec. Thus, our observations of 4U 0614+091 and those of 4U 1728-34 provide the first evidence for millisecond pulsars within low-mass X-ray binary systems and reveal the 'missing-link' between millisecond radiopulsars and the late stages of binary evolution in low mass X-ray binaries. The constant difference of the high frequency QPOs sug,,ests a beat-frequency interpretation. In this model, the high frequency QPO is associated with the Keplerian frequency of the inner accretion disk and the lower frequency QPO is a 'beat' between the differential rotation frequency of the inner disk and the spinning neutron star. Assuming the high frequency QPO is a Keplerian orbital frequency for the accretion disk, we find a maximum mass of 1.9 solar mass and a maximum radius of 17 km for the neutron star.

The Galactic Distribution of Black Hole Candidates in Low-Mass X-Ray Binary Systems

We find that for black hole candidates (BHC) in low-mass X-ray binaries (LMXRB) the dispersion of their distances,z, to the Galactic plane is smaller than for LMXRB with neutron stars by more than a factor 2. This result indicates that any kick velocities that the black hole systems may have received at formation are substantially smaller than those of neutron star systems. We infer a value of 40 km s 21 for the radial-velocity dispersion of the BHC systems from their z dispersion, consistent with the value obtained directly from radial-velocity measurements. From the surface density of black hole X-ray transients (BHXT) in the solar vicinity, and an estimate for the typical recurrence time of1100 yr, we infer that the total number of BHXT in the Galaxy is1500. Subject headings: binaries: close—galaxy: kinematics and dynamics—X-rays: general

Quasi-periodic X-Ray Brightness Oscillations of GRO J1744−28

The newly discovered source GRO J1744-28 is remarkable in many respects. It is a low-mass X-ray binary system that shows 2.1 Hz coherent pulsations. It also produces type II X-ray bursts similar to those seen in the Rapid Burster. In this Letter we report results from a series of observations with the Rossi X-Ray Timing Explorer (RXTE) during which GRO J1744-28 showed quasi-periodic X-ray brightness oscillations (QPOs) at 20, 40, and 60 Hz. Their fractional rms amplitudes are 0.19%, 5.9%, and 0.42%, respectively. The QPO centroid frequency and its rms amplitude as a function of source brightness in the band of 2-60 keV indicate that these QPOs are of a different kind from the beat-frequency QPOs that have been observed among Z sources as well as X-ray pulsars. We discuss possible interpretations of these QPOs in the context of neutron star g-mode oscillations and photon bubble oscillations.

A population synthesis study of low-mass X-ray binary systems

The origin of low-mass X-ray binary systems from the helium star-main-sequence binary remnants of a common-envelope phase is studied. Using a statistical Monte Carlo approach, the evolution of an initial distribution of zero-age binaries is followed from the main sequence through a phase of common-envelope evolution and subsequent supernova explosion of the helium star. The resultant population of low-mass main-sequence stars (≾3 M⊙) with a neutron star companion is studied as a function of the kick velocity associated with the possible asymmetry of the supernova explosion, the efficiency of mass ejection during the common-envelope phase, the minimum mass for evolution to a neutron star and the mass distribution of the secondary components in the system. A comparison of the results with the observed low-mass X-ray binary system population suggests that kick velocities are required. For an average kick velocity of ∼450 km s−1, as inferred from the radio pulsar distribution, birth rates in the range ∼3 × 10−6-10−5 yr−1 are indicated for efficiencies of the mass ejection process during the common-envelope phase of 0.25–1. The implications of the results for the general population of low-mass X-ray binaries, and for the formation of Her X-l type systems and long-period detached systems similar to PSR 1820 —11, are briefly discussed.

On the Nature of the Bursting X-Ray Pulsar GRO J1744−28

The unusual properties of the bursting X-ray pulsar GRO J1744-28 are explained in terms of a low-mass X-ray binary system consisting of an evolved stellar companion transferring mass through Roche-lobe overflow onto a neutron star, implying that the inclination of the system is ≲18°. Interpretation of the QPO at frequency νQPO = 40 Hz using the beat-frequency model of Alpar & Shaham and the measured period derivative with the Ghosh & Lamb accretion-torque model implies that the persistent X-ray luminosity of the source is approximately equal to the Eddington luminosity and that the neutron star has a surface equatorial magnetic field Beq ≅ 2 × 1010 (νQPO/40 Hz)-1 G for standard neutron star parameters. This implies a distance to GRO J1744-28 of ≅5 (νQPO/40 Hz)1/6b1/2 kpc, where b < 1 is a correction factor that depends on the orientation of the neutron star.

The Spectrum of the 8.7 s X-ray Pulsar 4U 0142+61

We investigate the properties of the 8.7 s X-ray pulsar 4U 0142+61 using new data obtained with the ASCA observatory and archival data from the Einstein and ROSAT observatories. New measurements of the pulse period from 1979 and 1994 confirm that 4U 0142+61 is spinning down on a timescale of 127,000 yr. The ASCA spectrum is featureless and requires two components consisting of a ~0.4 keV blackbody plus a power law with a photon index of ~3.7. The blackbody flux is ~40% the total and for a distance greater than 0.5 kpc covers more than 12% of the neutron star surface. This covering fraction is 2 orders of magnitude larger than expected for thin disk accretion onto a magnetized neutron star. These results suggest 4U 0142+61 is probably not a low-mass X-ray binary system, but rather is an isolated pulsar undergoing a combination of spherical and disk accretion. The observed properties seem consistent with the suggestion by van Paradijs, Taam, & van den Heuvel that this pulsar is powered by accretion from the remnant of a Thorne-Żytkow object (TŻO). The ROSAT PSPC image shows a dust-scattering halo that is a factor of 2 less than predicted by the measured equivalent hydrogen column density of 8 × 1021 cm-2, suggesting half of the absorbing material is located in the vicinity of the pulsar and possibly the remains of the TŻO envelope.

PSR B1745−20 and Young Pulsars in Globular Clusters

Recent searches for radio pulsars in globular clusters have targeted the millisecond pulsars (MSPs) that are expected to result from the spin-up of old neutron stars in low-mass X-ray binary systems. These surveys have been very successful, discovering 34 pulsars, most of which are old and probably recycled by accretion-powered spin-up in a binary system. However, three objects have properties that are more like those of the normal population of Galactic pulsars. Timing measurements of one such pulsar, PSR B1745-20, show that it is clearly associated with the cluster NGC 6440 and that it is solitary and young, with a large magnetic field. This pulsar, together with PSR B1820-30B in NGC 6624, establishes long-period pulsars associated with globular clusters as a distinct group that seems to have a completely different genesis to the MSP population in globular clusters. Produced at a comparable rate to the MSPs, the origin of such apparently young objects in very old stellar systems is not understood.

Observations of Low Mass X-Ray Binaries

AbstractIn this paper I review the properties and behavior of low-mass X-ray binary systems (LMXBs) that contain neutron stars (NS), concentrating on the Galactic bulge sources and bursters. I describe the observed characteristics of LMXBs including their light curves, spectra, eclipses, dips, bursts, flares, pulsations, QPO, long-term periodicities and orbital period changes, and explain how fast timing results and the distinction between ‘Z’ and ‘atoll’-type sources provide the key to a unified model of LMXB behavior.