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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

TRANSPORT AND MIXING OF r -PROCESS ELEMENTS IN NEUTRON STAR BINARY MERGER BLAST WAVES

The r -process nuclei are robustly synthesized in the material ejected during neutron star binary mergers (NSBMs). If NSBMs are indeed solely responsible for the solar system r -process abundances, a galaxy like our own would be required to host a few NSBMs per million years, with each event ejecting, on average, about 5 × 10{sup −2} M {sub ⊙} of r -process material. Because the ejecta velocities in the tidal tail are significantly larger than those in ordinary supernovae, NSBMs deposit a comparable amount of energy into the ISM. In contrast to extensive efforts studying spherical models for supernova remnant evolution, calculations quantifying the impact of NSBM ejecta in the ISM have been lacking. To better understand their evolution, we perform a suite of three-dimensional hydrodynamic simulations of isolated NSBM ejecta expanding in environments with conditions adopted from Milky-Way-like galaxy simulations. Although the remnant morphology is highly complex at early times, the subsequent radiative evolution is remarkably similar to that of a standard supernova. This implies that sub-resolution supernova feedback models can be used in galaxy-scale simulations that are unable to resolve the key evolutionary phases of NSBMs. Among other quantities, we examine the radius, mass, and kinetic energy content of the remnant at shell formation. We find that the shell formation epoch is attained when the swept-up mass is about 10{sup 3}( n {sub H}/1 cm{sup −3}){sup −2/7} M {sub ⊙;} at this point, the mass fraction of r -process material is enhanced up to two orders of magnitude in relation to a solar metallicity ISM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TIMING OBSERVATIONS OF PSR J1023+0038 DURING A LOW-MASS X-RAY BINARY STATE

Transitional millisecond pulsars (tMSPs) switch, on roughly multi-year timescales, between rotation-powered radio millisecond pulsar (RMSP) and accretion-powered low-mass X-ray binary (LMXB) states. The tMSPs have raised several questions related to the nature of accretion flow in their LMXB state and the mechanism that causes the state switch. The discovery of coherent X-ray pulsations from PSR J1023+0038 (while in the LMXB state) provides us with the first opportunity to perform timing observations and to compare the neutron star’s spin variation during this state to the measured spin-down in the RMSP state. Whereas the X-ray pulsations in the LMXB state likely indicate that some material is accreting onto the neutron star’s magnetic polar caps, radio continuum observations indicate the presence of an outflow. The fraction of the inflowing material being ejected is not clear, but it may be much larger than that reaching the neutron star’s surface. Timing observations can measure the total torque on the neutron star. We have phase-connected nine XMM-Newton observations of PSR J1023+0038 over the last 2.5 years of the LMXB state to establish a precise measurement of spin evolution. We find that the average spin-down rate as an LMXB is 26.8 ± 0.4% faster than the rate (−2.39 × 10{sup −15} Hz s{sup −1}) determined during the RMSP state. This shows that negative angular momentum contributions (dipolar magnetic braking, and outflow) exceed positive ones (accreted material), and suggests that the pulsar wind continues to operate at a largely unmodified level. We discuss implications of this tight observational constraint in the context of possible accretion models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

EVOLUTION OF LOW-MASS X-RAY BINARIES: THE EFFECT OF DONOR EVAPORATION

Millisecond pulsars (MSPs) are thought to originate from low-mass X-ray binaries (LMXBs). The discovery of eclipsing radio MSPs, including redbacks and black widows, indicates that evaporation of the donor star by the MSP’s irradiation takes place during the LMXB evolution. In this work, we investigate the effect of donor evaporation on the secular evolution of LMXBs, considering different evaporation efficiencies and related angular momentum loss. We find that for widening LMXBs, the donor star leaves a less massive white dwarf than without evaporation; for contracting systems, evaporation can speed up the evolution, resulting in dynamically unstable mass transfer and possibly the formation of isolated MSPs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ON THE EVOLUTION OF THE INNER DISK RADIUS WITH FLUX IN THE NEUTRON STAR LOW-MASS X-RAY BINARY SERPENS X-1

We analyze the latest Suzaku observation of the bright neutron star (NS) low-mass X-ray binary Serpens X-1 taken in 2013 October and 2014 April. The observation was taken using the burst mode and only suffered mild pile-up effects. A broad iron line is clearly detected in the X-ray spectrum. We test different models and find that the iron line is asymmetric and best interpreted by relativistic reflection. The relativistically broadened iron line is generally believed to originate from the innermost regions of the accretion disk, where strong gravity causes a series of special and general relativistic effects. The iron line profile indicates an inner radius of ∼8 R {sub G}, which gives an upper limit on the size of the NS. The asymmetric iron line has been observed in a number of previous observations, which gives several inner radius measurements at different flux states. We find that the inner radius of Serpens X-1 does not evolve significantly over the range of L / L {sub Edd} ∼ 0.4–0.6, and the lack of flux dependence of the inner radius implies that the accretion disk may be truncated outside of the innermost stable circular orbit by the boundary layer, rather than the stellar magnetic field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Contact binaries as viable distance indicators: new, competitive (v)jhk{sub s} period-luminosity relations

Based on the largest catalogs currently available, comprising 6090 contact binaries (CBs) and 2167 open clusters (OCs), we determine the near-infrared (NIR) JHK {sub s} CB period–luminosity (PL) relations, for the first time achieving the low levels of intrinsic scatter that make these relations viable as competitive distance calibrators. To firmly establish our distance calibration on the basis of OC CBs, we require that (i) the CB of interest must be located inside the core radius of its host cluster, (ii) the CB’s proper motion must be located within the 2σ distribution of that of its host OC, and (iii) the CB’s age, t, must be comparable to that of its host cluster, i.e., Δlog(tyr{sup −1})<0.3. We thus select a calibration sample of 66 CBs with either OC distances or accurate space-based parallaxes. The resulting NIR PL relations, for both late-type (i.e., W Ursae Majoris-type) and—for the first time—early-type CBs, are as accurate as the well-established JHK {sub s} Cepheid PL relations, (characterized by single-band statistical uncertainties of σ < 0.10 mag). We show that CBs can be used as viable distance tracers, yielding distances with uncertainties of better than 5% for 90% of the 6090 CBs in our full sample. By combining the full JHK {sub s} photometric data set, CBs can trace distances with an accuracy, σ = 0.05 (statistical) ± 0.03 (systematic) mag. The 102 CBs in the Large Magellanic Cloud are used to determine a distance modulus to the galaxy of (m−M{sub V}){sub 0}{sup LMC}=18.41±0.20 mag.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A VERY BRIGHT, VERY HOT, AND VERY LONG FLARING EVENT FROM THE M DWARF BINARY SYSTEM DG CVn

On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DG CVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3–100 keV bandpass by either a single very high-temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at T{sub X} of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be >10{sup 20} cm{sup 2}, and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T ∼ 10{sup 4} K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3–10 keV bandpass of 4 × 10{sup 35} and 9 × 10{sup 35} erg, and optical flare energies at E{sub V} of 2.8 × 10{sup 34} and 5.2 × 10{sup 34} erg, respectively. The results presented here should be integrated into updated modeling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

KOI-1003: a new spotted, eclipsing RS CVn binary in the Kepler field

Using the high-precision photometry from the Kepler space telescope, thousands of stars with stellar and planetary companions have been observed. The characterization of stars with companions is not always straightforward and can be contaminated by systematic and stellar influences on the light curves. Here, through a detailed analysis of starspots and eclipses, we identify KOI-1003 as a new, active RS CVn star—the first identified with data from Kepler. The Kepler light curve of this close binary system exhibits the system’s primary transit, secondary eclipse, and starspot evolution of two persistent active longitudes. The near equality of the system’s orbital and rotation periods indicates the orbit and primary star’s rotation are nearly synchronized (P{sub orb}=8.360613±0.000003 days; P{sub rot}∼8.23 days). By assuming the secondary star is on the main sequence, we suggest the system consists of a 1.45{sub −0.19}{sup +0.11} M{sub ⊙} subgiant primary and a 0.59{sub −0.04}{sup +0.03} M{sub ⊙} main-sequence companion. Our work gives a distance of 4400 ± 600 pc and an age of t=3.0{sub +2.0}{sup −0.5} Gyr, parameters which are discrepant with previous studies that included the star as a member of the open cluster NGC 6791.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Observations of Binary Stars with the Differential Speckle Survey Instrument. VIII. Measures of Metal-poor and Triple Stars from 2015 to 2018

We present 248 speckle observations of 43 binary and 19 trinary star systems chosen to make progress in two main areas of investigation: the fundamental properties of metal-poor stars and star formation mechanisms. The observations were taken at the Gemini North and South telescopes during the period 2015 July to 2018 April, mainly with the Differential Speckle Survey Instrument but also with a few early results from the new ’Alopeke speckle camera at Gemini North. We find that the astrometry and photometry of these observations as a whole are consistent with previous work at Gemini. We present five new visual orbits for systems important in understanding metal-poor stars, three of which have orbital periods of less than 4 yr, and we indicate the degree to which these and future observations can impact our knowledge of stellar properties and star formation. In particular, we find a decrease in mass at fixed spectral type for metal-poor stars versus their solar-metallicity analogs that is consistent with predictions that are made from current stellar models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Visual Orbits of Spectroscopic Binaries with the CHARA Array. III. HD 8374 and HD 24546

We present the visual orbits of two long-period spectroscopic binary stars, HD 8374 and HD 24546, using interferometric observations acquired with the CHARA Array and the Palomar Testbed Interferometer. We also obtained new radial velocities from echelle spectra using the APO 3.5 m and Fairborn 2.0 m telescopes. By combining the visual and spectroscopic observations, we solve for the full, three-dimensional orbits and determine the stellar masses and distances to within 3% uncertainty. We then estimate the effective temperature and radius of each component star through Doppler tomography and spectral energy distribution analyses, in order to compare the observed stellar parameters to the predictions of stellar evolution models. For HD 8374, we find masses of M {sub 1} = 1.636 ± 0.050M {sub ⊙} and M {sub 2} = 1.587 ± 0.049M {sub ⊙}, radii of R {sub 1} = 1.84 ± 0.05R {sub ⊙} and R {sub 2} = 1.66 ± 0.12R {sub ⊙}, temperatures of T{sub eff1}=7280±110 K and T{sub eff2}=7280±120 K, and an estimated age of 1.0 Gyr. For HD 24546, we find masses of M {sub 1} = 1.434 ± 0.014M {sub ⊙} and M {sub 2} = 1.409 ± 0.014M {sub ⊙}, radii of R {sub 1} = 1.67 ± 0.06R {sub ⊙} and R {sub 2} = 1.60 ± 0.10R {sub ⊙}, temperatures of T{sub eff1}=6790±120 K and T{sub eff2}=6770±90 K, and an estimated age of 1.4 Gyr. HD 24546 is therefore too old to be a member of the Hyades cluster, despite its physical proximity to the group.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Kepler Binary Stars in the NGC 6819 Open Cluster: KIC 5113146 and KIC 5111815

In this study, an investigation of two double-lined binary stars KIC 5113146 and KIC 5111815 in the NGC 6819 open cluster is presented based on both photometric and spectroscopic data. Simultaneous analysis of light and radial velocity curves was made and the absolute parameters of the systems’ components were determined for the first time. We find that both systems have F-type main-sequence components. The masses and radii were found to be M 1 = 1.29 ± 0.02M ⊙ , R 1 = 1.47 ± 0.03 R ⊙ and M 2 = 1.19 ± 0.02 M ⊙ , R 2 = 1.13 ± 0.02 R ⊙ for the primary and secondary components of KIC 5113146; and M 1 = 1.51 ± 0.08 M ⊙ , R 1 = 2.02 ± 0.05 R ⊙ and M 2 = 1.19 ± 0.07 M ⊙ , R 2 = 1.32 ± 0.04 R ⊙ for the components of KIC 5111815, respectively. The evolutionary status of the components was evaluated based on the MESA evolutionary tracks and isochrones. The ages of KIC 5111815 and KIC 5113146 were derived to be about 2.50 ± 0.35 Gyr and 1.95 ± 0.40 Gyr, respectively. Photometric distances were calculated to be 2850 ± 185 pc for KIC 5113146 and 3120 ± 260 pc for KIC 5111815. The results reveal that both KIC 5113146 and KIC 5111815 systems are the most likely members of NGC 6819.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

EXPLICIT SYMPLECTIC-LIKE INTEGRATORS WITH MIDPOINT PERMUTATIONS FOR SPINNING COMPACT BINARIES

We refine the recently developed fourth-order extended phase space explicit symplectic-like methods for inseparable Hamiltonians using Yoshida’s triple product combined with a midpoint permuted map. The midpoint between the original variables and their corresponding extended variables at every integration step is readjusted as the initial values of the original variables and their corresponding extended ones at the next step integration. The triple-product construction is apparently superior to the composition of two triple products in computational efficiency. Above all, the new midpoint permutations are more effective in restraining the equality of the original variables and their corresponding extended ones at each integration step than the existing sequent permutations of momenta and coordinates. As a result, our new construction shares the benefit of implicit symplectic integrators in the conservation of the second post-Newtonian Hamiltonian of spinning compact binaries. Especially for the chaotic case, it can work well, but the existing sequent permuted algorithm cannot. When dissipative effects from the gravitational radiation reaction are included, the new symplectic-like method has a secular drift in the energy error of the dissipative system for the orbits that are regular in the absence of radiation, as an implicit symplectic integrator does. In spite of this, it is superior to the same-order implicit symplectic integrator in accuracy and efficiency. The new method is particularly useful in discussing the long-term evolution of inseparable Hamiltonian problems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The 2015 decay of the black hole X-ray binary V404 Cygni: robust disk-jet coupling and a sharp transition into quiescence

We present simultaneous X-ray and radio observations of the black hole X-ray binary V404 Cygni at the end of its 2015 outburst. From 2015 July 11–August 5, we monitored V404 Cygni with Chandra, Swift, and NuSTAR in the X-ray, and with the Karl G. Jansky Very Large Array and the Very Long Baseline Array in the radio, spanning a range of luminosities that were poorly covered during its previous outburst in 1989 (our 2015 campaign covers 2×10{sup 33}≲L{sub X}≲10{sup 34} erg s{sup −1}). During our 2015 campaign, the X-ray spectrum evolved rapidly from a hard photon index of Γ≈1.6 (at L{sub X}≈10{sup 34} erg s{sup −1}) to a softer Γ≈2 (at L{sub X}≈3×10{sup 33} erg s{sup −1}). We argue that V404 Cygni reaching Γ≈2 marks the beginning of the quiescent spectral state, which occurs at a factor of ≈3–4 higher X-ray luminosity than the average pre-outburst luminosity of ≈8×10{sup 32} erg s{sup −1}. V404 Cygni falls along the same radio/X-ray luminosity correlation that it followed during its previous outburst in 1989, implying a robust disk-jet coupling. We exclude the possibility that a synchrotron-cooled jet dominates the X-ray emission in quiescence, leaving synchrotron self-Compton from either a hot accretion flow or from a radiatively cooled jet as the most likely sources of X-ray radiation, and/or particle acceleration along the jet becoming less efficient in quiescence. Finally, we present the first indications of correlated radio and X-ray variability on minute timescales in quiescence, tentatively measuring the radio emission to lag the X-ray by 15±4 minute, suggestive of X-ray variations propagating down a jet with a length of <3.0 au.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

SPECTRAL-TIMING ANALYSIS OF THE LOWER kHz QPO IN THE LOW-MASS X-RAY BINARY AQUILA X-1

Spectral-timing products of kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass X-ray binary (LMXB) systems, including energy- and frequency-dependent lags, have been analyzed previously in 4U 1608-52, 4U 1636-53, and 4U 1728-34. Here, we study the spectral-timing properties of the lower kHz QPO of the neutron star LMXB Aquila X-1 for the first time. We compute broadband energy lags as well as energy-dependent lags and the covariance spectrum using data from the Rossi X-ray Timing Explorer . We find characteristics similar to those of previously studied systems, including soft lags of ∼30 μ s between the 3.0–8.0 keV and 8.0–20.0 keV energy bands at the average QPO frequency. We also find lags that show a nearly monotonic trend with energy, with the highest-energy photons arriving first. The covariance spectrum of the lower kHz QPO is well fit by a thermal Comptonization model, though we find a seed photon temperature higher than that of the mean spectrum, which was also seen in Peille et al. and indicates the possibility of a composite boundary layer emitting region. Lastly, we see in one set of observations an Fe K component in the covariance spectrum at 2.4- σ confidence, which may raise questions about the role of reverberation in the production of lags.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Minidisks in Binary Black Hole Accretion

Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole (“minidisks”), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using 2D hydrodynamical simulations performed with a new general relativistic version of the moving-mesh code Disco. We introduce a comoving energy variable that enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the innermost stable circular orbit, providing a Reynolds stress that causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov–Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly wound linear waves. We measure the shock-induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing-angle-dependent images for comparison with observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Gamma–Gamma Absorption in the γ-ray Binary System PSR B1259-63/LS 2883

The observed TeV light curve from the γ -ray binary PSR B1259-63/LS 2883 shows a decrease in the flux at periastron that has not been fully explained by emission mechanisms alone. This observed decrease can, however, be explained by γγ absorption due to the stellar and disk photons. We calculate the γγ absorption in PSR B1259-63/LS 2883 taking into account photons from both the circumstellar disk and star, assuming that the γ -rays originate at the position of the pulsar. The γγ absorption due to the circumstellar disk photons produces a ≈14% decrease in the flux, and there is a total decrease of ≈52% (>1 TeV) within a few days before periastron, accompanied by a hardening of the γ -ray photon index. While the γγ absorption alone is not sufficient to explain the full complexity of the H.E.S.S. γ -ray light curve, it results in a significant decrease in the predicted flux, which is coincident with the observed decrease. In addition, we have calculated an upper limit on the γγ absorption, assuming that the emission is produced at the apex of the bow shock. Future observations with CTA during the 2021 periastron passage may be able to confine the location of the emission based on the degree of γγ absorption, as well as measure the hardening of the spectrum around periastron.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Giant Metrewave Radio Telescope Monitoring of the Black Hole X-Ray Binary, V404 Cygni during Its 2015 June Outburst

We report results from a Giant Metrewave Radio Telescope (GMRT) monitoring campaign of the black hole X-ray binary V404 Cygni during its 2015 June outburst. The GMRT observations were carried out at observing frequencies of 1280, 610, 325, and 235 MHz, and extended from June 26.89 UT (a day after the strongest radio/X-ray outburst) to July 12.93 UT. We find the low-frequency radio emission of V404 Cygni to be extremely bright and fast-decaying in the outburst phase, with an inverted spectrum below 1.5 GHz and an intermediate X-ray state. The radio emission settles to a weak, quiescent state ≈11 days after the outburst, with a flat radio spectrum and a soft X-ray state. Combining the GMRT measurements with flux density estimates from the literature, we identify a spectral turnover in the radio spectrum at ≈1.5 GHz on ≈ June 26.9 UT, indicating the presence of a synchrotron self-absorbed emitting region. We use the measured flux density at the turnover frequency with the assumption of equipartition of energy between the particles and the magnetic field to infer the jet radius (≈4.0 × 10{sup 13} cm), magnetic field (≈0.5 G), minimum total energy (≈7 × 10{sup 39} erg), and transient jet power (≈8 × 10{sup 34} erg s{sup −1}). The relatively low value of the jet power, despite V404 Cygni’s high black hole spin parameter, suggests that the radio jet power does not correlate with the spin parameter.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Diverse Long-term Variability of Five Candidate High-mass X-Ray Binaries from Swift Burst Alert Telescope Observations

We present an investigation of long-term modulation in the X-ray light curves of five little-studied candidate high-mass X-ray binaries using the Swift Burst Alert Telescope. IGR J14488-5942 and AX J1700.2-4220 show strong modulation at periods of 49.6 and 44 days, respectively, which are interpreted as orbital periods of Be star systems. For IGR J14488-5942, observations with the Swift X-ray Telescope show a hint of pulsations at 33.4 s. For AX J1700.2-4220, 54 s pulsations were previously found with XMM-Newton . Swift J1816.7-1613 exhibits complicated behavior. The strongest peak in the power spectrum is at a period near 150 days, but this conflicts with a determination of a period of 118.5 days by La Parola et al. AX J1820.5-1434 has been proposed to exhibit modulation near 54 days, but the extended BAT observations suggest modulation at slightly longer than double this at approximately 111 days. There appears to be a long-term change in the shape of the modulation near 111 days, which may explain the apparent discrepancy. The X-ray pulsar XTE J1906+090, which was previously proposed to be a Be star system with an orbital period of ∼30 days from pulse timing, shows peaks in the power spectrum at 81 and 173 days. The origins of these periods are unclear, although they might be the orbital period and a superorbital period respectively. For all five sources, the long-term variability, together with the combination of orbital and proposed pulse periods, suggests that the sources contain Be star mass donors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Parallel Tracks as Quasi-steady States for the Magnetic Boundary Layers in Neutron-star Low-mass X-Ray Binaries

The neutron stars in low-mass X-ray binaries (LMXBs) are usually thought to be weakly magnetized objects accreting matter from their low-mass companions in the form of a disk. Albeit weak compared to those in young neutron-star systems, the neutron-star magnetospheres in LMXBs can play an important role in determining the correlations between spectral and temporal properties. Parallel tracks appearing in the kilohertz (kHz) quasi-periodic oscillation (QPO) frequency versus X-ray flux plane can be used as a tool to study the magnetosphere–disk interaction in neutron-star LMXBs. For dynamically important weak fields, the formation of a non-Keplerian magnetic boundary layer at the innermost disk truncated near the surface of the neutron star is highly likely. Such a boundary region may harbor oscillatory modes of frequencies in the kHz range. We generate parallel tracks using the boundary region model of kHz QPOs. We also present the direct application of our model to the reproduction of the observed parallel tracks of individual sources such as 4U 1608–52, 4U 1636–53, and Aql X-1. We reveal how the radial width of the boundary layer must vary in the long-term flux evolution of each source to regenerate the parallel tracks. The run of the radial width looks similar for different sources and can be fitted by a generic model function describing the average steady behavior of the boundary region over the long term. The parallel tracks then correspond to the possible quasi-steady states the source can occupy around the average trend.

79 ASTRONOMY AND ASTROPHYSICS↗