Binary Period Research Articles

Chandra revisits WR 48a: testing colliding wind models in massive binaries

ABSTRACT We present results of new Chandra High-Energy Transmission Grating (HETG) observations (2019 November–December) of the massive Wolf–Rayet binary WR 48a. Analysis of these high-quality data showed that the spectral lines in this massive binary are broadened (full width at half-maximum, FWHM = 1400 km s−1) and marginally blueshifted (∼−100 km s−1). A direct modelling of these high-resolution spectra in the framework of the standard colliding stellar wind (CSW) picture provided a very good correspondence between the shape of the theoretical and observed spectra. Also, the theoretical line profiles are in most cases an acceptable representation of the observed ones. We applied the CSW model to the X-ray spectra of WR 48a from previous observations: Chandra-HETG (2012 October) and XMM–Newton (2008 January). From this expanded analysis, we find that the observed X-ray emission from WR 48a is variable on the long time-scale (years) and the same is valid for its intrinsic X-ray emission. This requires variable mass-loss rates over the binary orbital period. The X-ray absorption (in excess of that from the stellar winds in the binary) is variable as well. We note that lower intrinsic X-ray emission is accompanied by higher X-ray absorption. A qualitative explanation could be that the presence of clumpy and non-spherically symmetric stellar winds may play a role.

Photometric Analysis of the TESS Light Curve for the Asynchronous Polar V1500 Cyg

After 40 yr of nova outburst, there are still some difficulties in completely understanding V1500 Cyg. The faintness of V1500 Cyg and its crowded field pose a challenge for high-precision photometry by TESS, while long-term intensive data from TESS provide the first opportunity to accurately model this binary system. The best-fitting model reveals that the high-temperature white dwarf illuminates the surface of its companion star, a low-mass secondary (M 2 ∼ 0.22 M ⊙), and the emission from the heated secondary star still dominates the luminosity of the system. The binary model has determined the white dwarf’s mass to be M 1 ∼ 1.4 M ⊙ and its orbital inclination i ∼ 60°. The residuals after the removal of the fitted model curve present variations in both the white dwarf’s spin period and the binary orbital period, confirming the previous claim that V1500 Cyg is still an asynchronous polar, and the observed spin period demonstrates that V1500 Cyg is approaching synchronicity with the orbital period following the prediction of the polarization ephemeris. The phasing of the spin humps indicates their cyclotron origin, and the residual orbital humps may be produced by the emission from the accretion stream. There is no evidence for the pole-switching effect, while the double-humped profile of the spin modulation suggests that the white dwarf may be accreting matter from a “ring” around it. Further simultaneous polarization and X-ray observations are urged to explore the accretion geometry of V1500 Cyg in detail.

The Observed Mass Distribution of Galactic Black Hole LMXBs Is Biased against Massive Black Holes

Abstract The discovery of gravitational-wave radiation from merging black holes (BHs) also uncovered BHs with masses in the range of ≈20–160 M ⊙. In contrast, the most massive Galactic stellar-mass BH currently known has a mass of ≈21 M ⊙. While low-mass X-ray binaries (LMXBs) will never independently evolve into a binary BH system, and binary evolution effects can play an important role in explaining the different BH masses found through studies of X-ray binaries and gravitational-wave events, (electromagnetic) selection effects may also play a role in this discrepancy. Assuming BH LMXBs originate in the Galactic plane, we show that the spatial distributions of the current samples of confirmed and candidate BH LMXBs are both biased to sources that lie at a large distance from the plane. Specifically, most of the confirmed and candidate BH LMXBs are found at a Galactic height larger than three times the scale height for massive star formation. In addition, the confirmed BH LMXBs are found at larger distances to the Galactic center than the candidate BH LMXBs. Interstellar absorption makes candidate BH LMXBs in the plane and bulge too faint for a dynamical mass measurement using current instrumentation. Given the observed and theoretical evidence for BH natal and/or Blaauw kicks, their relation with BH mass and binary orbital period, and the relation between outburst recurrence time and BH mass, the observational selection effects imply that the current sample of confirmed BH LMXBs is biased against the most massive BHs.

The Double Asteroid Redirection Test (DART): Planetary Defense Investigations and Requirements

Abstract The Double Asteroid Redirection Test (DART) is a Planetary Defense mission, designed to demonstrate the kinetic impactor technique on (65803) Didymos I Dimorphos, the secondary of the (65803) Didymos system. DART has four level 1 requirements to meet in order to declare mission success: (1) impact Dimorphos between 2022 September 25 and October 2, (2) cause at least a 73 s change in its binary orbit period via the impact, (3) measure the change in binary period to an uncertainty of 7.3 s or less, and (4) measure the momentum transfer efficiency (β) of the impact and characterize the resulting effects of the impact. The data necessary to achieve these requirements will be obtained and analyzed by the DART Investigation Team. We discuss the rationales for the data to be gathered, the analyses to be undertaken, and how mission success will be achieved.

The excited spin state of Dimorphos resulting from the DART impact

The NASA Double Asteroid Redirection Test (DART) mission is a planetary defense-driven test of a kinetic impactor on Dimorphos, the satellite of the binary asteroid 65803 Didymos. DART will intercept Dimorphos at a relative speed of ∼6.5km s−1, perturbing Dimorphos’s orbital velocity and changing the binary orbital period. We present three independent methods (one analytic and two numerical) to investigate the post-impact attitude stability of Dimorphos as a function of its axial ratios, a/b and b/c (a≥b≥c), and the momentum transfer efficiency β. The first method uses a novel analytic approach in which we assume a circular orbit and a point-mass primary that identifies four fundamental frequencies of motion corresponding to the secondary’s mean motion, libration, precession, and nutation frequencies. At resonance locations among these four frequencies, we find that attitude instabilities are possible. Using two independent numerical codes, we recover many of the resonances predicted by the analytic model and indeed show attitude instability. With one code, we use fast Lyapunov indicators to show that the secondary’s attitude can evolve chaotically near the resonance locations. Then, using a high-fidelity numerical model, we find that Dimorphos enters a chaotic tumbling state near the resonance locations and is especially prone to unstable rotation about its long axis, which can be confirmed by ESA’s Hera mission arriving at Didymos in late 2026. We also show that a fully coupled treatment of the spin and orbital evolution of both bodies is crucial to accurately model the long-term evolution of the secondary’s spin state and libration amplitude. Finally, we discuss the implications of a post-impact tumbling or rolling state, including the possibility of terminating BYORP evolution if Dimorphos is no longer in synchronous rotation.

X-ray view of colliding winds in WR 25

AstractThe long-term behavior of a colliding wind binary WR 25 is presented using archival X-ray data obtained over a time span of : 16 years. The present analysis reveals phase-locked variations repeating consistently over many consecutive orbits of the source (with binary orbital period : 208 days). A significant deviation of the X-ray flux with respect to the theoretical 1/D trend (D is the binary separation) close to periastron passage has been observed. This may occur due to the shifting of the adiabatic wind collision to the radiative regime in that part of the orbit. Further, no signature of X-ray emission in 10.0-79.0 keV energy range attributable to inverse Compton scattering is detected by NuSTAR.

Discovery of orbital eccentricity and evidence for orbital period increase of SS433

ABSTRACT The examination of long-term (1979–2020) photometric observations of SS433 enabled us to discover a non-zero orbital eccentricity of e = 0.05 ± 0.01. We have also found evidence for a secular increase in the orbital period at a rate of $\dot{P}_\mathrm{b}=(1.0\pm 0.3)\times 10^{-7}$ s s−1. The binary orbital period increase rate makes it possible to improve the estimate of the binary mass ratio q = MX/MV > 0.8, where MX and MV are the masses of the relativistic object and the optical star, respectively. For an optical star mass of 10 M⊙, the mass of the relativistic object (a black hole) is MX > 8 M⊙. A neutron star in SS433 is reliably excluded because in that case the orbital period should decrease, in contradiction to observations. The derived value of $\dot{P}_\mathrm{b}$ sets a lower limit on the mass-loss rate in the Jeans mode from the binary system ≳7 × 10−6 M⊙ yr−1. The discovered orbital ellipticity of SS433 is consistent with the model of the slaved accretion disc tracing the precession of the misaligned optical star’s rotational axis.

NuSTAR Observation of LS 5039

Abstract LS 5039 is a high-mass γ-ray binary hosting a compact object of unknown type. NuSTAR observed LS 5039 during the entire 3.9-day binary period. We performed a periodic signal search up to 1000 Hz, which did not produce credible period candidates. We do see the 9.05 s period candidate, originally reported by Yoneda et al. using the same data, in the Fourier power spectrum, but we find that the statistical significance of this feature is too low to claim it as a real detection. We did not find significant bursts or quasiperiodic variability either. The modulation with orbital period is clearly seen and remains unchanged over a timescale of 10 years when compared to the earlier Suzaku light curve. The joint analysis of the NuSTAR and Suzaku XIS data shows that the 0.7–70 keV spectrum can be satisfactory described by a single absorbed power-law model with no evidence of a cutoff at higher energies. The slope of the spectrum is anticorrelated with the flux during the binary orbit. If LS 5039 hosts a young neutron star, its X-ray pulsations therefore appear to be outshone by the intrabinary shock emission. The lack of spectral lines and/or an exponential cutoff at higher energies suggests that the putative neutron star is not actively accreting. Although a black hole scenario still remains a possibility, the lack of variability or Fe Kα lines, which typically accompany accretion, makes it less likely.

RX J0123.4-7321 – the story continues: major circumstellar disc loss and recovery

ABSTRACT RX J0123.4-7321 is a well-established Be star X-ray binary system in the Small Magellanic Cloud. Like many such systems, the variable X-ray emission is driven by the underlying behaviour of the mass donor Be star. Previous work has shown that the optical and X-ray were characterized by regular outbursts at the proposed binary period of 119 d. However, around 2008 February the optical behaviour changed substantially, with the previously regular optical outbursts ending. Reported here are new optical (OGLE) and X-ray (Swift) observations covering the period after 2008 that suggest an almost total circumstellar disc loss followed by a gradual recovery. This indicates the probable transition of a Be star to a B star, and back again. However, at the time of the most recent OGLE data (2020 March) the characteristic periodic outbursts had yet to return to their early state, indicating that the disc still had some re-building yet to complete.

Approximations of the Spin of Close Black Hole–Wolf–Rayet Binaries

Abstract Population synthesis studies of binary black hole mergers often lack robust black hole spin estimates as they cannot accurately follow tidal spin-up during the late black hole–Wolf–Rayet evolutionary phase. We provide an analytical approximation of the dimensionless second-born black hole spin given the binary orbital period and Wolf–Rayet stellar mass at helium depletion or carbon depletion. These approximations are obtained from fitting a sample of around 105 detailed MESA simulations that follow the evolution and spin up of close black hole–Wolf–Rayet systems with metallicities in the range [ 10 − 4 , 1.5 Z ⊙ ] . Following the potential spin up of the Wolf–Rayet progenitor, the second-born black hole spin is calculated using up-to-date core collapse prescriptions that account for any potential disk formation in the collapsing Wolf–Rayet star. The fits for second-born black hole spin provided in this work can be readily applied to any astrophysical modeling that relies on rapid population synthesis, and will be useful for the interpretation of gravitational-wave sources using such models.

The aftermath of nova Centauri 2013 (V1369 Centauri)

Context.Classical nova progenitors are cataclysmic variables and very old novae are observed to match systems with high mass transfer rates and (relatively) long orbital periods. However, the aftermath of a classical nova has never been studied in detail.Aims.We intend to probe the aftermath of a classical nova explosion in cataclysmic variables and observe as the binary system relaxes to quiescence.Methods.We used multiwavelength time-resolved optical and near-infrared spectroscopy for a bright, well-studied classical nova five years after outburst. We were able to disentangle the contribution of the ejecta at this late epoch using its previous characterization, separating the ejecta emission from that of the binary system.Results.We determined the binary orbital period (P = 3.76 h), the system separation, and the mass ratio (q ≳ 0.17 for an assumed white dwarf mass of 1.2M⊙). We find evidence of an irradiated secondary star and no unambiguous signature of an accretion disk, although we identify a second emission line source tied to the white dwarf with an impact point. The data are consistent with a bloated white dwarf envelope and the presence of unsettled gas within the white dwarf Roche lobe.Conclusions.At more than 5 years after eruption, it appears that this classical nova has not yet relaxed.

The Be/neutron star system Swift J004929.5-733107 in the Small Magellanic Cloud–X-ray characteristics and optical counterpart candidates

ABSTRACT Swift J004929.5-733107 is an X-ray source in the Small Magellanic Cloud (SMC) that has been reported several times, but the optical counterpart has been unclear due to source confusion in a crowded region of the SMC. Previous works proposed [MA93] 302 as the counterpart, however we show here, using data obtained from the S-CUBED project, that the X-ray position is inconsistent with that object. Instead we propose a previously unclassified object which has all the indications of being a newly identified Be star exhibiting strong H α emission. Evidence for the presence of significant I-band variability strongly suggest that this is, in fact, a Be type star with a large circumstellar disc. Over 18 yr worth of optical monitoring by the OGLE project reveal a periodic modulation at a period of 413 d, probably the binary period of the system. A SALT optical spectrum shows strong Balmer emission and supports a proposed spectral classification of B1-3 III-IVe. The X-ray data obtained from the S-CUBED project reveal a time-averaged spectrum well fitted by a photon index Γ = 0.93 ± 0.16. Assuming the known distance to the SMC, the flux corresponds to a luminosity ∼1035 erg s−1. All of these observational facts suggest that this is confirmed as a Be star–neutron star X-ray binary (BeXRB) in the SMC, albeit one with an unusually long binary period at the limits of the Corbet Diagram.

Hard X-Ray Emission Associated with White Dwarfs. IV. Signs of Accretion from Substellar Companions

Abstract With an effective temperature of ≃200,000 K, KPD 0005+5106 is one of the hottest white dwarfs (WDs). ROSAT unexpectedly detected “hard” (∼1 keV) X-rays from this apparently single WD. We have obtained Chandra observations that confirm the spatial coincidence of this hard X-ray source with KPD 0005+5106. We have also obtained XMM-Newton observations of KPD 0005+5106, as well as PG 1159−035 and WD 0121−756, which are also apparently single and whose hard X-rays were detected by ROSAT at 3σ–4σ levels. The XMM-Newton spectra of the three WDs show remarkably similar shapes that can be fitted by models including a blackbody component for the stellar photospheric emission, a thermal plasma emission component, and a power-law component. Their X-ray luminosities in the 0.6–3.0 keV band range from 4 × 1029 to 4 × 1030 erg s−1. The XMM-Newton EPIC-pn soft-band (0.3–0.5 keV) light curve of KPD 0005+5106 is essentially constant, but the hard-band (0.6–3.0 keV) light curve shows periodic variations. An analysis of the generalized Lomb–Scargle periodograms for the XMM-Newton and Chandra hard-band light curves finds a convincing modulation (false-alarm probability of 0.41%) with a period of 4.7 ± 0.3 hr. Assuming that this period corresponds to a binary orbital period, the Roche radii of three viable types of companion have been calculated: M9V star, T brown dwarf, and Jupiter-like planet. Only the planet has a size larger than its Roche radius, although the M9V star and T brown dwarf may be heated by the WD and inflate past the Roche radius. Thus, all three types of companion may be donors to fuel accretion-powered hard X-ray emission.

Galactic orbital effects on pulsar timing

ABSTRACTIn the currently accepted paradigm, dark matter is hypothesized as an explanation of the flat rotation curves of galaxies under the assumption of virialized orbits. The use of millisecond pulsar timing as a probe of Galactic dark matter content is explored as a means of relaxing this assumption. A method of inference of the Galactic potential using the frequency derivative $\dot{\nu }$ is produced, and an estimate for a virialized Galactic rotation curve is given through direct observation of acceleration. The data set used includes 210 pulsars with known $\dot{\nu }$ and astrometric properties, a subset of which also have measured $\ddot{\nu }$. In principle, this enables the exploration of kinematic effects, but in practice, $\ddot{\nu }$ values are found to be too imprecise at present to adequately constrain radial velocities of pulsars. Additionally, surface magnetic field strengths are inferred from $\dot{\nu }$ and the magnetic spin-down contribution to $\ddot{\nu }$ is estimated. For several pulsars, the radial velocity is known, and the kinematic contribution to $\ddot{\nu }$ is estimated accordingly. The binary orbital periods of PSR J1713+0747 and other binary pulsars are also used to constrain Galactic mass density models.

Speckle Observations of TESS Exoplanet Host Stars: Understanding the Binary Exoplanet Host Star Orbital Period Distribution

Abstract We present high-resolution speckle interferometric imaging observations of TESS exoplanet host stars using the NN-EXPLORE Exoplanet and Stellar Speckle Imager instrument at the 3.5 m WIYN telescope. Eight TESS objects of interest that were originally discovered by Kepler were previously observed using the Differential Speckle Survey Instrument. Speckle observations of 186 TESS stars were carried out, and 45 (24%) likely bound companions were detected. This is approximately the number of companions we would expect to observe given the established 46% binarity rate in exoplanet host stars. For the detected binaries, the distribution of stellar mass ratio is consistent with that of the standard Raghavan distribution and may show a decrease in high-q systems as the binary separation increases. The distribution of binary orbital periods, however, is not consistent with the standard Ragahavan model, and our observations support the premise that exoplanet-hosting stars with binary companions have, in general, wider orbital separations than field binaries. We find that exoplanet-hosting binary star systems show a distribution peaking near 100 au, higher than the 40–50 au peak that is observed for field binaries. This fact led to earlier suggestions that planet formation is suppressed in close binaries.

The Wolf–Rayet + Black Hole Binary NGC 300 X-1: What is the Mass of the Black Hole?

Abstract We present new X-ray and UV observations of the Wolf–Rayet + black hole (BH) binary system NGC 300 X-1 with the Chandra X-ray Observatory and the Hubble Space Telescope Cosmic Origins Spectrograph. When combined with archival X-ray observations, our X-ray and UV observations sample the entire binary orbit, providing clues to the system geometry and interaction between the BH accretion disk and the donor star wind. We measure a binary orbital period of 32.7921 ± 0.0003 hr, in agreement with previous studies, and perform phase-resolved spectroscopy using the X-ray data. The X-ray light curve reveals a deep eclipse, consistent with inclination angles of i = 60°–75°, and a pre-eclipse excess consistent with an accretion stream impacting the disk edge. We further measure radial velocity variations for several prominent far-UV spectral lines, most notably H ii λ1640 and C iv λ1550. We find that the He ii emission lines systematically lag the expected Wolf–Rayet star orbital motion by a phase difference of Δϕ ∼ 0.3, while C iv λ1550 matches the phase of the anticipated radial velocity curve of the Wolf–Rayet donor. We assume the C iv λ1550 emission line follows a sinusoidal radial velocity curve (semi-amplitude = 250 km s−1) and infer a BH mass of 17 ± 4 M ⊙. Our observations are consistent with the presence of a wind-Roche lobe overflow accretion disk, where an accretion stream forms from gravitationally focused wind material and impacts the edge of the BH accretion disk.

Chandra Probes the X-Ray Variability of M51 ULX-7: Evidence of Propeller Transition and X-Ray Dips on Orbital Periods

Abstract We report on the temporal properties of the ultraluminous X-ray (ULX) pulsar M51 ULX-7 inferred from the analysis of the 2018–2020 Swift/X-ray Telescope monitoring data and archival Chandra data obtained over a period of 33 days in 2012. We find an extended low flux state, which might be indicative of propeller transition, lending further support to the interpretation that the neutron star is rotating near equilibrium. Alternatively, this off-state could be related to a variable superorbital period. Moreover, we report the discovery of periodic dips in the X-ray light curve that are associated with the binary orbital period. The presence of the dips implies a configuration where the orbital plane of the binary is closer to an edge-on orientation, and thus demonstrates that favorable geometries are not necessary in order to observe ULX pulsars. These characteristics are similar to those seen in prototypical X-ray pulsars such as Her X-1 and SMC X-1 or other ULX pulsars such as NGC 5907 ULX1.

Building and Calibrating the Binary Star Population Using Kepler Data

Abstract Modeling binary star populations is critical to linking the theories of star formation and stellar evolution with observations. In order to test these theories, we need accurate models of observable binary populations. The Kepler Eclipsing Binary Catalog (KEBC), with its estimated >90% completeness, provides an observational anchor on binary population models. In this work we present the results of a new forward model of the binary star population in the Kepler field. The forward model takes a single star population from a model of the Galaxy and pairs the stars into binaries by applying the constraints on the population from the results of observational binary population surveys such as Raghavan et al. and Duchêne & Kraus. A synthetic binary population is constructed from the initial distributions of orbital parameters. We identify the eclipsing binary sample from the generated binary star population and compare this with the observed sample of eclipsing binaries contained in the KEBC. Finally, we update the distributions of the synthetic population and repeat the process until the synthetic eclipsing binary sample agrees with the KEBC. The end result of this process is a model of the underlying binary star population that has been fit to observations. We find that for fixed flat mass ratio and eccentricity input distributions, the binary period distribution is logarithmically flat above ∼3.2 days. With additional constraints on distributions from observations, we can further adjust the synthetic binary population by relaxing other input constraints, such as mass ratio and eccentricity.

Gravitational Collider Physics via Pulsar–Black Hole Binaries

Abstract We propose to use pulsar–black hole binaries as a probe of gravitational collider physics. Induced by the gravitation of the pulsar, the atomic transitions of the boson cloud around the black hole backreact on the orbital motion. This leads to the deviation of the binary period decrease from that predicted by general relativity, which can be directly probed by the Rømer delay of pulsar times of arrival. The sensitivity and accuracy of this approach are estimated for two typical atomic transitions. It is shown that once the transitions happen within the observable window, the pulsar-timing accuracy is almost always sufficient to capture the resonance phenomenon.

On a new formulation for energy transfer between convection and fast tides with application to giant planets and solar type stars

ABSTRACT All the studies of the interaction between tides and a convective flow assume that the large-scale tides can be described as a mean shear flow that is damped by small-scale fluctuating convective eddies. The convective Reynolds stress is calculated using mixing length theory, accounting for a sharp suppression of dissipation when the turnover time-scale is larger than the tidal period. This yields tidal dissipation rates several orders of magnitude too small to account for the circularization periods of late-type binaries or the tidal dissipation factor of giant planets. Here, we argue that the above description is inconsistent, because fluctuations and mean flow should be identified based on the time-scale, not on the spatial scale, on which they vary. Therefore, the standard picture should be reversed, with the fluctuations being the tidal oscillations and the mean shear flow provided by the largest convective eddies. We assume that energy is locally transferred from the tides to the convective flow. Using this assumption, we obtain values for the tidal Q factor of Jupiter and Saturn and for the circularization periods of pre-main-sequence binaries in good agreement with observations. The time-scales obtained with the equilibrium tide approximation are however still 40 times too large to account for the circularization periods of late-type binaries. For these systems, shear in the tachocline or at the base of the convective zone may be the main cause of tidal dissipation.