Ultracompact X-ray Binaries Research Articles

A View of the Long-term Spectral Behavior of Ultracompact X-Ray Binary 4U 0614+091

In this study, we examine 51 archival NICER observations and six archival NuSTAR observations of the neutron star ultracompact X-ray binary (UCXB) 4U 0614+091, which span over 5 yr. The source displays persistent reflection features, so we use a reflection model designed for UCXBs, with overabundant carbon and oxygen (xillverCO) to study how various components of the system vary over time. The flux of this source is known to vary quasiperiodically on a timescale of a few days, so we study how the various model components change as the overall flux varies. The flux of most components scales linearly with the overall flux, while the power law, representing coronal emission, is anticorrelated, as expected. This is consistent with previous studies of the source. We also find that during observations of the high-soft state, the disk emissivity profile as a function of radius becomes steeper. We interpret this as the corona receding to be closer to the compact object during these states, at which point the assumed power law illumination of xillverCO may be inadequate to describe the illumination of the disk.

The Influence of the Magnetic Braking Laws on the Evolution of Persistent and Transient Low-mass X-Ray Binaries

Swift J1858.6−0814 (hereafter J1858) is a transient neutron star (NS) low-mass X-ray binary (LMXB). There is controversy regarding its donor mass derived from observations and theoretical calculations. In this paper, we adopt seven magnetic braking (MB) prescriptions suggested in the literature and different metallicity Z to simulate the evolution of the LMXB. Our results show that, employing the MB model proposed by A. Reiners & S. Mohanty (“rm12”), the convection- and rotation-boosted (“carb”) model, and the intermediate (“inter”) and convection-boosted (“cboost”) models in K. X. Van et al. can match (part of) the observational parameters of J1858 well. We then apply our method to other observed LMXBs and find that the “rm12” and “inter” MB laws are most promising in explaining transient LMXBs. In comparison, the simulations with the “cboost” and “carb” MB laws are more inclined to reproduce persistent LMXBs and ultracompact X-ray binaries, respectively. Our results, though subject to computational and/or observational bias, show that it is challenging to find a unified MB law that applies to the NS LMXB subpopulations simultaneously, indicating our lack of understanding of the true MB law. In addition, we explore the influence of various MB laws on the magnitude of the bifurcation periods in LMXBs.

Adiabatic Mass Loss in Binary Stars. IV. Low- and Intermediate-mass Helium Binary Stars

The unstable mass transfer situation in binary systems will asymptotically cause the adiabatic expansion of the donor star and finally lead to the common envelope phase. This process could happen in helium binary systems once the helium donor star fills its Roche-lobe. We have calculated the adiabatic mass-loss model of naked helium stars with a mass range of 0.35 M ⊙–10 M ⊙, and every mass sequence evolved from the helium-zero-age main sequence to the cooling track of white dwarf or carbon ignition. In consideration of the influence of stellar wind, massive helium stars are not considered in this paper. Comparing the stellar radius with the evolution of the Roche-lobe under the assumption of conservative mass transfer, we give the critical mass ratio q crit = M He/M accretor as the binary stability criteria of low- and intermediate-mass helium binary stars. On the helium main sequence, the result shows 1.0 < q crit < 2.6, which is more unstable than the classical result of polytropic model q crit = 3. After the early helium Hertzsprung Gap, the q crit quickly increases even larger than 10 (more stable compared with the widely used result of q crit = 4), which is dominated by the expansion of the radiative envelope. Our result could be useful for these quick mass transfer binary systems such as AM CVns, ultra-compact X-ray binaries, and helium novae, and it could guide the binary population synthesis for the formation of special objects such as type Ia supernova and gravitational wave sources.

On the Magnetic Braking Law in Black Hole Low-mass X-Ray Binaries

Magnetic braking (MB) plays an important role in the evolution of close low-mass X-ray binaries (LMXBs). It is also essential to the formation of ultracompact X-ray binaries (UCXBs). There have been lively investigations on the MB mechanism(s) in both single stars and close binaries including cataclysmic variables and neutron star (NS) LMXBs but with diverse conclusions. In this paper we explore the effect of MB on the black hole (BH) LMXB evolution. We combine binary population synthesis with detailed binary evolution to obtain the expected properties of the Galactic BH LMXB population. The simulated results are compared with the observational data including the BH mass, companion mass, companion temperature, orbital period, and mean accretion rate. Our results reveal that the MB laws with relatively low efficiency (i.e., RM12 and RVJ83) exhibit better agreement with observations, contrary to what was found for NS LMXBs. This raises the interesting question about whether MB really follows the same unified law in different types of binaries. We also predict that only a very small fraction (≲2.5%) of BH LMXBs can evolve to be UCXBs. This explains why no BH UCXB has been discovered so far.

Investigating the Ultracompact X-Ray Binary Candidate SLX 1735-269 with NICER and NuSTAR

We present two simultaneous NICER and NuSTAR observations of the ultracompact X-ray binary (UCXB) candidate SLX 1735−269 while the source was in two different spectral states. Using various reflection modeling techniques, we find that xillverCO, a model used for fitting X-ray spectra of UCXBs with high carbon and oxygen abundances is an improvement over relxill or relxillNS, which instead contains solar-like chemical abundances. This provides indirect evidence in support of the source being ultracompact. We also use this reflection model to get a preliminary measurement of the inclination of the system, i=57−7+23 degrees. This is consistent with our timing analysis, where a lack of eclipses indicates an inclination of i < 80°. The timing analysis is otherwise inconclusive, and we cannot confidently measure the orbital period of the system.

The impact of different magnetic braking prescriptions on the evolution of LMXBs

ABSTRACT We revisit the evolution of low-mass close binary systems under different magnetic braking (MB) prescriptions. We study binaries with a neutron star accretor. During mass transfer episodes, these systems emit X-rays and are known as Low-mass X-ray Binaries (LMXBs). When mass transfer stops, they can be observed as binary pulsars. Additionally, some of these systems can experience mass transfer while having orbital periods of less than 1 h, thus evolving into ultracompact X-ray binaries (UCXBs). The evolution of LMXBs depends on their capability to lose angular momentum and maintain stable mass transfer. Among the angular momentum loss mechanisms, MB is one important and still uncertain phenomenon. The standard MB prescription faces some problems when calculating LMXB evolution, leading to e.g., a fine-tuning problem in the formation of UCXBs. Recent studies proposed new MB prescriptions, yielding diverse outcomes. Here, we investigate the effects of three novel MB prescriptions on the evolution of LMXBs using our stellar code. We found that all MB prescriptions considered allow the formation of binaries with orbital periods spanning from less than 1 h to more than 10 of days. Remarkably, our results enable the occurrence of wide systems even for the MB law that causes the strongest angular momentum losses and very high mass transfer rates. We found that models computed with the strongest MB prescription reach the UCXB state starting from a wider initial orbital period interval. Finally, we discuss and compare our results with observations and previous studies performed on this topic.

Black Hole Ultracompact X-Ray Binaries as Galactic Low-frequency Gravitational Wave Sources: The He Star Channel

Black hole (BH) ultracompact X-ray binaries (UCXBs) are potential Galactic low-frequency gravitational wave (GW) sources. As an alternative channel, BH UCXBs can evolve from BH+He star binaries. In this work, we perform a detailed stellar evolution model for the formation and evolution of BH UCXBs evolving from the He star channel to diagnose their detectability as low-frequency GW sources. Our calculations found that some nascent BH+He star binaries after the common-envelope (CE) phase could evolve into UCXB-LISA sources with a maximum GW frequency of ∼5 mHz, which can be detected in a distance of 10 kpc (or 100 kpc). Once BH+He star systems become UCXBs through mass transfer, they would emit X-ray luminosities of ∼1038 erg s−1, making them ideal multimessenger objects. If the initial He-star masses are ≥0.7 M ⊙, those systems are likely to experience two Roche lobe overflows, and the X-ray luminosity can reach a maximum of 3.5 × 1039 erg s−1 in the second mass-transfer stage. The initial He-star masses and initial orbital periods of progenitors of Galactic BH UCXB-LISA sources are in the range of 0.32–2.9 M ⊙ and 0.02–0.19 days, respectively. Nearly all BH+He star binaries in the above parameter space can evolve into GW sources whose chirp masses can be accurately measured. Employing a population synthesis simulation, we predict the birthrate and detection number of Galactic BH UCXB-LISA sources evolving from the He star channel are R = 2.2 × 10−6 yr−1 and 33 for an optimistic CE parameter, respectively.

PSR J1953+1844 Probably Being the Descendant of an Ultracompact X-Ray Binary

PSR J1953+1844 (i.e., M71E) is a millisecond pulsar in a 53 minute binary orbit discovered by the Five-hundred-meter Aperture Spherical radio Telescope. The mass function from pulsar timing is 2.3 × 10−7 M ⊙. The possible redback origin of this system has been discussed by Pan et al. We discuss here an alternative evolution track for this binary system, namely that PSR J1953+1844 is a descendant of an ultracompact X-ray binary (UCXB), which has a hydrogen-poor donor accreting onto a neutron star (NS) with an orbital period of ≤1 hr. We noticed that some UCXB systems hold an accreting millisecond X-ray pulsar (AMXP) and a donor with a mass of about 0.01 M ⊙. M71E has a very similar orbit to those of AMXPs, indicating that it might be evolved from a UCXB similar to PSR J1653–0158. The companion star of M71E should be significantly bloated and it most probably has a carbon and oxygen composition, otherwise a low inclination angle of the orbit is required for a helium companion. The discovery of this M71E binary system may shed light on when and how an NS in a UCXB turns into a radio pulsar.

UltraCompCAT: A comprehensive catalogue of ultra-compact and short orbital period X-ray binaries

Ultracompact X-ray binaries (UCXBs) are a distinctive but elusive family of low-mass X-ray binaries (LMXBs) characterised by their tight orbits and degenerate donor stars. Here we present UltraCompCAT, the first online and comprehensive catalogue of UCXBs. The initial version of UltraCompCAT comprises 49 sources, including 20 confirmed UCXBs (those with a measured orbital period shorter than 80 min) and 25 systems that we label ‘candidates’ based on their multi-wavelength phenomenology. For completeness, we also include four LMXBs with orbital periods in the range of 80 to 120 min, since they might be related to (e.g. close progenitors) or even part of the UCXB population that evolved towards longer periods. We discuss the orbital period and Galactic distribution of the catalogue sample. We provide evidence for the presence of at least two separate groups of UCXBs, one formed by persistent systems with orbital periods shorter than 30 min and a second group of transient objects (70 %) with periods in the range of 40 to 60 min. We show that the former group is dominated by sources formed in globular clusters, while the latter accounts for the (known) UCXB population in the Galactic field. We discuss the possible evolutionary channels for the two groups.

Constraining the White-dwarf Mass and Magnetic Field Strength of a New Intermediate Polar through X-Ray Observations

We report timing and broadband spectral analysis of a Galactic X-ray source, CXOGBS J174517.0−321356 (J1745), with a 614 s periodicity. Chandra discovered the source in the direction of the Galactic Bulge. Gong proposed that J1745 was either an intermediate polar (IP) with a mass of ∼1 M ⊙, or an ultracompact X-ray binary (UCXB). To confirm J1745's nature, we jointly fit XMM-Newton and NuSTAR spectra, ruling out a UCXB origin. We have developed a physically realistic model that considers a finite magnetosphere radius, X-ray absorption from the preshock region, and reflection from the white-dwarf (WD) surface to properly determine the IP properties, especially its WD mass. To assess systematic errors on the WD mass measurement, we consider a broad range of specific accretion rates ( g cm−2 s−1) based on the uncertain source distance (d = 3–8 kpc) and fractional accretion area (f = 0.001–0.025). Our model properly implements the fitted accretion column height in the X-ray reflection model and accounts for the underestimated mass accretion rate due to the (unobserved) soft X-ray blackbody and cyclotron cooling emissions. We found that the lowest accretion rate of = 0.6 g cm−2 s−1, which corresponds to the nearest source distance and maximum f value, yields a WD mass of (0.92 ± 0.08)M ⊙. On the other hand, as long as the accretion rate is g cm−2 s−1, the WD mass is robustly measured to be (0.81 ± 0.06)M ⊙, nearly independent of . The derived WD mass range is consistent with the mean WD mass of nearby IPs. Assuming spin equilibrium between the WD and accretion disk, we constrained the WD magnetic field to B ≳ 7 MG, indicating that it could be a highly magnetized IP. Our analysis presents the most comprehensive methodology for constraining the WD mass and B field of an IP by consolidating the effects of cyclotron cooling, finite magnetospheric radius, and accretion column height.

Does Nature Allow the Formation of Ultra-compact Black Hole X-Ray Binaries via the Accretion-induced Collapse of Neutron Stars?

The formation path to ultracompact X-ray binaries (UCXBs) with black hole (BH) accretors is still unclear. In the classical formation scenario, it is difficult to eject the massive envelope of the progenitor star of the BH via the common envelope process. Given that some neutron stars (NSs) in binary systems evidently have birth masses close to ∼2.0 M ⊙, we explore here the possibility that BH-UCXBs may form via the accretion-induced collapse (AIC) of accreting NSs, assuming that these previously evolved in low-mass X-ray binaries to masses all the way up to the maximum limit of an NS. We demonstrate this formation path by modeling a few cases of NS-UCXBs with initial NS masses close to the maximum mass of an NS that evolve into BH-UCXBs after the NS accretes material from its He white dwarf (WD) companion. We follow the evolution of the post-AIC BH-UCXB and, based on simple arguments, we anticipate that there is about one BH-UCXB with an AIC origin and a He WD donor within the current sample of known UCXBs and that two to five such BH-UCXBs may be detected in gravitational waves by LISA. In addition, we find that the X-ray luminosity of NS-UCXBs near their orbital period minimum exceeds ∼1039 erg s−1, and thus, such systems may appear as ultraluminous X-ray sources.

Evolving ONe WD+He WD systems to ultra-compact X-ray binaries

ABSTRACTIt has been proposed that accretion-induced collapse (AIC) of massive white-dwarfs (WDs) is an indispensable path for the formation of neutron star (NS) binaries. Although there are still no direct evidence for the existence of AIC events, several kinds of NS systems are suggested to originate from the AIC processes. One of the representative evidence is the detection of the strong magnetic field and slow spin NSs with ultra-light companions (${\le}0.1\, \rm M_{\odot }$) in close orbits. However, previous studies cannot explain the formation of AIC events with such low-mass companions. In the present work, we evolve a series of ONe WD+He WD systems to the formation of AIC events (named as the He WD donor channel), and the NS binaries behave as ultra-compact X-ray binaries when the He WDs refill their Roche lobes. We found that the ONe WD+He WD systems a possible channel for the formation of the newly formed NS+ultra-light companion systems just after the AIC event. Although there are some other inconsistent properties, the detected companion mass and orbital period of 4U 1626–67 (one of the newly formed NS binaries with ultra-light companions) can be reproduced by the He WD donor channel. In addition, combined with previous asteroseismology results, we speculate that an UCXB source (XTE J1751−305) may originate from the He WD donor channel.

Magnetic Inclination Evolution of Accreting Neutron Stars in Intermediate/Low-mass X-Ray Binaries

The magnetic inclination angle χ, namely the angle between the spin and magnetic axes of a neutron star, plays a vital role in its observational characteristics. However, there are few systematic investigations of its long-term evolution, especially for accreting NSs in binary systems. Applying the model of Biryukov & Abolmasov and the binary evolution code MESA, we simultaneously simulate the evolution of the accretion rate, spin period, magnetic field, and magnetic inclination angle of accreting NSs in intermediate/low X-ray binaries. We show that the evolution of χ depends not only on the initial parameters of the binary systems, but also on the mass transfer history and the efficiency of pulsar loss. Based on the calculated results we present the characteristic distribution of χ for various types of systems including ultracompact X-ray binaries, binary millisecond pulsars, and ultraluminous X-ray sources, and discuss their possible observational implications.

Black Hole Ultracompact X-Ray Binaries: Galactic Low-frequency Gravitational Wave Sources

In the Galaxy, close binaries with compact objects are important low-frequency gravitational wave (GW) sources. As potential low-frequency GW sources, neutron star/white dwarf (WD) ultracompact X-ray binaries (UCXBs) have been investigated extensively. Using the Modules for Experiments in Stellar Astrophysics code, we systematically explored the evolution of black hole (BH)-main-sequence star (MS) binaries to determine whether their descendants can be detected by space-borne GW detectors. Our simulations showed that BH-MS binaries with an initial orbital period less than the bifurcation period can evolve into BH UCXBs that can be detected by LISA. Such an evolutionary channel would form compact mass-transferring BH-WD systems rather than detached BH-WD systems. The calculated X-ray luminosities of BH UCXBs that can be detected by LISA at a distance d = 1 kpc are ∼1033–1035 erg s−1 (∼1034–1035 erg s−1 for d = 10 kpc); hence, it is possible to detect their electromagnetic counterparts. It is worth emphasizing that only some BH-MS systems with an initial orbital period very close to the bifurcation period can evolve toward low-frequency GW sources whose chirp masses can be measured. The maximum GW frequency of BH UCXBs forming via the BH-MS pathway is about 3 mHz, which is smaller than the minimum GW frequency (6.4 mHz) of mass-transferring BH-WDs originating from a dynamic process. Furthermore, we obtain an initial parameter space (donor-star masses and orbital periods) of progenitors of BH UCXB-GW sources, which can be applied to future population synthesis simulations. By a rough estimation, we predict that LISA would only be able to detect a few BH UCXB-GW sources formed by the BH-MS channel.

Evolution of AM CVn Binaries with White Dwarf Donors

The evolution and the stability of mass transfer of CO+He white dwarf (WD) binaries are not well understood. Observationally they may emerge as AM CVn binaries and are important gravitational wave (GW) emitters. In this work, we have modeled the evolution of double WD binaries with accretor masses of 0.50–1.30 M ⊙ and donor masses of 0.17–0.45 M ⊙ using the detailed stellar evolution code mesa. We find that the evolution of binaries with same donor masses but different accretor masses is very similar and binaries with same accretor masses but larger He donor masses have larger maximum mass transfer rates and smaller minimum orbital periods. We also demonstrate that the GW signal from AM CVn binaries can be detected by spaceborne GW observatories, such as LISA and TianQin. There is a linear relation between the donor mass and gravitational wave frequency during the mass transfer phase. In our calculation, all binaries can have dynamically stable mass transfer, which is very different from previous studies. The threshold donor mass of Eddington-limited mass transfer for a given accretor WD mass is lower than previous studies. Assuming that a binary may enter a common envelope if the mass transfer rate exceeds the maximum stable burning rate of He, we provide a new criterion for double WDs surviving mass transfer, which is below the threshold of the Eddington limit. Finally, we find that some systems with oxygen–neon (ONe) WDs in our calculation may evolve into detached binaries consisting of neutron stars and extremely low-mass He WDs, and further ultracompact X-ray binaries.

Formation of black widows through ultracompact X-ray binaries with He star companions

ABSTRACT Black widows (BWs) are a type of eclipsing millisecond pulsars (MSPs) with companion masses $M_2\lesssim 0.05\, \rm M_\odot$, which can be used to study the accretion history and the radiation of pulsars, as well as the origin of isolated MSPs. Recent observations indicate that there are two subtypes of BWs. One is the BWs with $M_2 \sim 0.01\!-\!0.05\, \rm M_\odot$, whereas another with $M_2 \lesssim 0.01\, \rm M_\odot$. However, the origin of the latter is still highly uncertain. In this paper, we investigated the formation of BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ through ultracompact X-ray binaries (UCXBs) with He star companions, in which a neutron star (NS) accretes material from a He star through Roche lobe overflow. By considering different He star masses and evaporation efficiencies with the stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa ), we evolved a series of NS+He star systems that can undergo UCXB stage. This channel can explain the formation of the BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ within the Hubble time, especially three widely studied BWs, i.e. PSRs J1719−1438, J2322−2650, and J1311−3430. We found that X-ray irradiation feedback does not affect the evolutionary tracks of evaporation process. The simulations indicate that the UCXBs with He star companions are the potential progenitors of isolated MSPs, and that the origin of BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ is different with another subtype of BWs. In addition, this work suggests that the BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ may not be produced by redback systems.

Investigating the Stability of Mass Transfer in Neutron Star–helium White Dwarf Binaries

Neutron star–helium white dwarf (NS+He WD) binaries are important evolutionary products of close-orbit binary star systems. They are often observed as millisecond pulsars and may continue evolving into ultracompact X-ray binaries (UCXBs) and continuous gravitational wave (GW) sources that will be detected by space-borne GW observatories, such as LISA, TianQin, and Taiji. Nevertheless, the stability of NS+He WD binaries undergoing mass transfer has not been well studied and is still under debate. In this paper, we model the evolution of NS+He WD binaries with WD masses ranging from 0.17–0.45 M ⊙, applying the detailed stellar evolution code mesa. Contrary to previous studies based on hydrodynamics, we find that apparently all NS+He WD binaries undergo stable mass transfer. We find for such UCXBs that the larger the WD mass, the larger the maximum mass-transfer rate and the smaller the minimum orbital period during their evolution. Finally, we demonstrate numerically and analytically that there is a tight correlation between WD mass and GW frequency for UCXBs, independent of NS mass.

A Spectroscopic Angle on Central Engine Size Scales in Accreting Neutron Stars

Abstract Analyses of absorption from disk winds and atmospheres in accreting compact objects typically treat the central emitting regions in these systems as point sources relative to the absorber. This assumption breaks down if the absorbing gas is located within a few × 1000 GM/c 2, in which case a small component of the absorber’s Keplerian motion contributes to the velocity width of absorption lines. Here, we demonstrate how this velocity-broadening effect can be used to constrain the sizes of central engines in accreting compact objects via a simple geometric relationship, and develop a method for modeling this effect. We apply this method to the Chandra/HETG spectra of three ultracompact and short-period neutron star X-ray binaries in which evidence of gravitationally redshifted absorption, owing to an inner-disk atmosphere, has recently been reported. The significance of the redshift is above 5σ for XTE J1710−281 (this work) and 4U 1916−053, and is inconsistent with various estimates of the relative radial velocity of each binary. For our most sensitive spectrum (XTE J1710−281), we obtain a 1σ upper bound of 310 km s−1 on the magnitude of this geometric effect and a central engine of size R CE &lt; 60 GM/c 2 (or &lt; 90 GM/c 2 at the 3σ level). These initial constraints compare favorably to those obtained via microlensing in quasars and approach the sensitivity of constraints via relativistic reflection in neutron stars. This sensitivity will increase with further exposures, as well as the launch of future microcalorimeter and grating missions.

Chandra/LETGS Studies of the Collisional Plasma in 4U 1626–67

Abstract We present an analysis of Chandra/LETGS observations of the ultracompact X-ray binary (UCXB)4U 1626–67, continuing our project to analyze the existing Chandra gratings data of this interesting source. The extremely-low-mass, hydrogen-depleted donor star provides a unique opportunity to study the properties and structure of the metal-rich accreted plasma. There are strong, double-peaked emission features of O vii–VIII and Ne ix–X, but no other identified emission lines are detected. Our spectral fit simultaneously models the emission-line profiles and the plasma parameters, using a two-temperature collisionally-ionized plasma. Based on our line-profile fitting, we constrain the inclination of the system to 25–60° and the inner disk radius to ∼1500 gravitational radii, in turn constraining the donor mass to ≲0.026 M ⊙, while our plasma modeling confirms previous reports of high neon abundance in the source, establishing a Ne/O ratio in the system of 0.47 ± 0.04, while simultaneously estimating a very low Fe/O ratio of 0.0042 ± 0.0008 and limiting the Mg/O ratio to less than 1% by number. We discuss these results in light of previous work.

Erratum: Formation of millisecond pulsars with helium white dwarfs, ultra-compact X-ray binaries, and gravitational wave sources

Erratum: Formation of millisecond pulsars with helium white dwarfs, ultra-compact X-ray binaries, and gravitational wave sources