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Electron-capture-supernovae (EC-SNe) provide an alternative channel for producing neutron stars (NSs). They play an important role in the formation of double NS (DNS) systems and the chemical evolution of galaxies, and contribute to the NS mass distribution in observations. It is generally believed that EC-SNe originate from e -captures on |$\rm ^{24}Mg$| and |$\rm ^{20}Ne$| in the massive degenerate oxygen–neon (ONe) cores with masses close to the Chandrasekhar limit (M Ch). However, the origin of EC-SNe is still uncertain. In this paper, we systematically studied the EC-SNe in NS + He star systems by considering the explosive oxygen burning that may occur in the near- M Ch ONe core. We provided the initial parameter spaces for producing EC-SNe in the initial orbital period − initial He star mass (log |$P_{\rm orb}^{\rm i}-M_{\rm He}^{\rm i}$|⁠) diagram, and found that both |$M_{\rm He}^{\rm i}$| and minimum |$P_{\rm orb}^{\rm i}$| for EC-SNe increase with metallicity. Then, by considering NS kicks added to the newborn NS, we investigated the properties of the formed DNS systems after the He star companions collapse into NSs, such as the orbital periods, eccentricities, and spin periods of recycle pulsars (P spin), etc. The results show that most of the observed DNS systems can be produced by NS kicks of |$\lesssim$| 50 km s−1. In addition, we found that NSs could accrete more material if the residual H envelope on the He star companions is considered, which can form the mildly recycled pulsars (P spin ∼ 20 ms) in DNS systems. [ABSTRACT FROM AUTHOR]

Type Ia supernovae (SNe Ia) are successful cosmological distance indicators and important element factories in the chemical evolution of galaxies. They are generally thought to originate from thermonuclear explosions of carbon–oxygen white dwarfs in close binaries. However, the observed diversity among SNe Ia implies that they have different progenitor models. In this article, we performed the long-term evolution of NS+He star binaries with different initial He star masses (⁠|$M_{\rm He}^{\rm i}$|⁠) and orbital periods (⁠|$P_{\rm orb}^{\rm i}$|⁠) for the first time, in which the He star companions can explode as SNe Ia eventually. Our simulations indicate that after the He stars develop highly degenerate oxygen–neon (ONe) cores with masses near the Chandrasekhar limit, explosive oxygen burning can be triggered due to the convective Urca process. According to these calculations, we obtained an initial parameter space for the production of SNe Ia in the |$\rm log\,$| |$P^{\rm i}_{\rm orb}{\text {--}}M^{\rm i}_{\rm He}$| plane. Meanwhile, we found that isolated mildly recycled pulsars can be formed after He stars explode as SNe Ia in NS+He star binaries, in which the isolated pulsars have minimum spin periods (⁠|$P_{\rm spin}^{\rm min}$|⁠) of ∼30–110 ms and final orbital velocities of |${\sim} \rm 60{\!-\!}360\, km\, s^{-1}$|⁠ , corresponding to initial orbital periods of 0.07–10 d. Our work suggests that the NS+He star channel may contribute to the formation of isolated mildly recycled pulsars with velocity |$\rm {\lesssim} 360\, km\, s^{-1}$| in observations, and such isolated pulsars should locate in the region of pulsars with massive white dwarf companions in the |$P_{\rm spin} {\!-\!}\dot{P}_{\rm spin}$| diagram. [ABSTRACT FROM AUTHOR]

The disc instability mechanism (DIM) is widely accepted to account for the transient behaviour of dwarf novae (DNe), which experience short outbursts separated by long quiescence. The duty cycle (the ratio between the outburst duration and the recurrence time) determines the amount of accreted mass by the white dwarf (WDs) during outbursts, thus playing an important role in the long-term binary evolution. Employing the code of Modules for Experiments in Stellar Astrophysics, we systemically investigate the influence of the duty cycles on the evolution of DNe and the mass growth of accreting carbon-oxygen (CO) WDs. Our calculations show that, while the DIM can considerably influence the accretion process, efficient WD-mass growth requires a particular range of the duty cycle. For WDs with the initial masses of 0.6, 0.7, and 1.1 M⊙, these duty cycles are 0.006 |$\, \le$| d |$\, \le$| 0.007, d   =  0.005, and , d   = 0.003 and the accumulated mass of the WDs can reach 0.1, 0.13, and 0.21 M⊙, respectively. In all of our simulations, no CO WDs can grow their masses to the explosion mass of Type Ia supernovae of about 1.38 M⊙. Because of a much short time-scale of the outburst state, the final donor-star masses and orbital periods are insensitive to the duty cycles. Therefore, we propose that the DIM in DNe could alleviate the WD mass problem to some extent. [ABSTRACT FROM AUTHOR]

Ultracompact X-ray binaries (UCXBs) are low-mass X-ray binaries with hydrogen-deficient mass donors and ultrashort orbital periods. They have been suggested to be the potential Laser Interferometer Space Antenna (LISA) sources in the low-frequency region. Several channels for the formation of UCXBs have been proposed so far. In this paper, we carried out a systematic study on the He star donor channel, in which a neutron star (NS) accretes matter from a He main-sequence (MS) star through Roche lobe overflow, where the mass transfer is driven by the gravitational wave radiation. First, we followed the long-term evolution of the NS+He MS binaries by employing the stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa), and thereby obtained the initial parameter spaces for the production of UCXBs. We then used these results to perform a detailed binary population synthesis approach to obtain the Galactic rates of UCXBs through this channel. We estimate the Galactic rates of UCXBs appearing as LISA sources to be |${\sim} 3.1\!-\!11.9\, \rm Myr^{-1}$| through this channel, and the number of such UCXB- LISA sources in the Galaxy can reach about 1–26 calibrated by observations. This work indicates that the He star donor channel may contribute significantly to the Galactic UCXB formation rate. We found that the evolutionary tracks of UCXBs through this channel can account for the location of the five transient sources with relatively long orbital periods quite well. We also found that such UCXBs can be identified by their locations in the mass-transfer rate versus the orbital period diagram. [ABSTRACT FROM AUTHOR]

PSR J0636+5128 is a 2.87 ms binary millisecond pulsar (MSP) discovered by the Green Bank Northern Celestial Cap Pulsar Survey, and possesses the third shortest orbital period (P  = 1.6 h) among confirmed binary pulsars. Recent observations reported that this source is experiencing a rapid orbital expansion at a rate of |$\dot{P}=(1.89\pm 0.05)\times 10^{-12}\,\rm s\, s^{-1}$|⁠. The evaporation winds of the companion induced by the spin-down luminosity of the MSP may be responsible for such a positive orbital period derivative. However, our calculations show that the winds ejecting from the vicinity of the companion or the inner Lagrangian point cannot account for the observation due to an implausible evaporation efficiency. Assuming that the evaporation winds eject from the vicinity of the MSP in the form of asymmetric disc winds or outflows, the evaporation efficiency can be constrained to be ∼0.1. Therefore, the rapid orbital expansion detected in PSR J0636+5128 provides evidence of outflows and accretion disc around the MSP. [ABSTRACT FROM AUTHOR]

Recently, repeating fast radio bursts (FRBs) with a period of P FRB = 16.35 ± 0.18 d from FRB 180916.J0158+65 were reported. It still remains controversial how such a periodicity might arise for this FRB. In this Letter, based on an assumption of a young pulsar surrounding by a debris disk, we attempt to diagnose whether Lense–Thirring precession of the disk on the emitter can produce the observed periodicity. Our calculations indicate that the Lense–Thirring effect of a tilted disk can result in a precession period of 16 d for a mass inflow rate of 0.5–1.5 × 1018 g s−1, a pulsar spin period of 1–20 ms, and an extremely low viscous parameter α = 10−8 in the disk. The disk mass and the magnetic field of the pulsar are also constrained to be ∼10−3  M ⊙ and <2.5 × 1013 G. In our model, a new-born pulsar with normal magnetic field and millisecond period would successively experience the accretion and propeller phases, and is visible as a strong radio source in the current stage. The rotational energy of such a young neutron star can provide the observed radio bursting luminosity for 400 yr. [ABSTRACT FROM AUTHOR]

The Hubble Space Telescope observation of the double white dwarf (WD) binary SDSS J125733.63+542850.5 reveals that the massive WD has a surface gravity log g1 ~ 8.7 (which implies a mass of M1 ~ 1.06 M⊙) and an effective temperature T1 ~ 13 000 K, while the effective temperature of the low-mass WD (M2 < 0.24 M⊙) is T2 ~ 6400K. Therefore, the massive and the low-mass WDs have a cooling age τ1 ~ 1 Gyr and τ2 ≥ 5 Gyr, respectively. This is in contradiction with traditional binary evolution theory. In this paper, we propose a strange dwarf (SD) scenario to explain the formation of this double WD binary. We assume that the massive WD is a SD originating from a phase transition (PT) in a ~1.1 M⊙ WD, which has experienced accretion and spin-down processes. Its high effective temperature could arise from the heating process during the PT. Our simulations suggest that the progenitor of SDSS J125733.63+542850.5 can be a binary system consisting of a 0.65 M⊙ WD and a 1.5 M⊙ main-sequence star in a 1.492 d orbit. Especially, the secondary star (i.e. the progenitor of the low-mass WD) is likely to have an ultra-low metallicity of Z = 0.0001. [ABSTRACT FROM AUTHOR]

Recent timing analysis reveals that the orbital period of the first-discovered accreting millisecond pulsar SAX J1808.4-3658 is increasing at a rate Ṗorb = (3.89 ± 0.15) × 10-12 s s-1, which is at least one order of magnitude higher than the value arising from the conservative mass transfer. An ejection of mass-loss rate of 10-9Mʘ yr-1 from the donor star at the inner Lagrangian point during the quiescence state could interpret the observed orbital-period derivative. However, it is unknown whether this source can offer such a high mass-loss rate. In this work, we attempt to investigate an evolutionary channel towards SAX J1808.4-3658. Once the accretion disc becomes thermally and viscously unstable, the spin-down luminosity of the millisecond pulsar and the X-ray luminosity during outbursts are assumed to evaporate the donor star, and the resulting winds carry away the specific orbital angular momentum at the inner Lagrangian point. Our scenario could yield the observed orbital period, the orbital-period derivative, and the peak X-ray luminosity during outbursts. Low-mass X-ray binaries with a 1.0Mʘ donor star, and an orbital period in the range of 0.8-1.5 d, may be the progenitor of SAX J1808.4-3658. Our numerical calculations propose that the current donor-star mass is 0.044Mʘ, which is approximately in agreement with the minimum mass of the donor star. In addition, our scenario can also account for the formation of black widows or the diamond planets like PSR J1719-1438. [ABSTRACT FROM AUTHOR]

Recently, ultraluminous X-ray source (ULX) M82 X-2 has been identified to be an accreting neutron star, which has a P = 1.37 s spin period, and is spinning up at a rate Ṗ = −2.0 × 10−10 s s−1 −1. Interestingly, its isotropic X-ray luminosity Liso = 1.8 × 1040 erg s−1 during outbursts is 100 times the Eddington limit for a 1.4M⊙ neutron star. In this Letter, based on the standard accretion model we attempt to constrain the dipolar magnetic field of the pulsar in ULX M82 X-2. Our calculations indicate that the accretion rate at the magnetospheric radius must be super-Eddington during outbursts. To support such a super-Eddington accretion, a relatively high multipole field ( ≳ 1013 G) near the surface of the accretor is invoked to produce an accreting gas column. However, our constraint shows that the surface dipolar magnetic field of the pulsar should be in the range of 1.0−3.5 × 1012 G. Therefore, our model supports that the neutron star in ULX M82 X-2 could be a low-magnetic-field magnetar (proposed by Tong) with a normal dipolar field (∼1012 G) and relatively strong multipole field. For the large luminosity variations of this source, our scenario can also present a self-consistency interpretation. [ABSTRACT FROM AUTHOR]

It is difficult for the intermediate-mass X-ray binaries (IMXBs) evolutionary channel to form intermediate-mass binary pulsars (IMBPs) with a short orbital period (less than 3 d) via stable mass transfer. The main reason is that the magnetic braking mechanisms are generally thought not to work for donor stars with a mass of greater than 1.5 M⊙ in the canonical model. However, some intermediate-mass stars have anomalously strong magnetic fields (about 100–10 000 G), i.e. so-called Ap or Bp stars. With the coupling between the magnetic field and the irradiation-driven wind from the surface of Ap/Bp stars, a plausible magnetic braking mechanism should be expected. In this work, we attempt to investigate if IMXBs with Ap/Bp stars can produce IMBPs with a short orbital period (less than 3 d) by such an anomalous magnetic braking mechanism. Using a stellar evolution code, we have simulated the evolution of a large number of IMXBs consisting of a neutron star (NS) and an Ap/Bp star. For the spin evolution of the NS, we consider the accretion torque, the propeller torque and the spin-down torque caused by the interaction between the magnetic field and the accretion disc. The calculated results show that, employing anomalous magnetic braking of Ap/Bp stars, IMXBs can evolve into compact IMBPs with short orbital periods of less than 3 d. However, there exists significant discrepancy between the spin periods of IMBPs in our simulated results and those observed. [ABSTRACT FROM AUTHOR]

ABSTRACT The recycling model suggested that low-mass X-ray binaries (LMXBs) could evolve into binary millisecond pulsars (BMSPs). In this work, we attempt to investigate the progenitor properties of BMSPs formed by the recycling evolutionary channel, and if submillisecond pulsars can be produced by this channel. Using Eggleton's stellar evolution code, considering that the dead pulsars can be spun up to a short spin period by the accreting material and angular momentum from the donor star, we have calculated the evolution of close binaries consisting of a neutron star (NS) and a low-mass main-sequence donor star, and the spin evolution of NSs. In the calculation, some physical processes, such as the thermal and viscous instability of an accretion disc, propeller effect and magnetic braking, are included. Our calculated results indicate that all LMXBs with a low-mass donor star of 1.0-2.0 M⊙ and a short orbital period (≲ 3-4 d) can form millisecond pulsars with a spin period less than 10 ms. However, it is difficult to produce submillisecond pulsars by this evolutionary channel. In addition, our evolutionary scenario cannot account for the existence of BMSPs with a long orbital period ( Porb≳ 70-80 d). [ABSTRACT FROM AUTHOR]

ABSTRACT Binary radio pulsars are generally believed to have been spun up to millisecond periods (i.e. recycling) via mass accretion from their donor stars, and they are the descendants of neutron star low-mass X-ray binaries. However, some studies indicate that the formation of pulsars from the accretion-induced collapse (AIC) of accreting white dwarfs (WDs) cannot be excluded. In this work, we use a population-synthesis code to examine if the AIC channel can produce eccentric binary millisecond pulsars (BMSPs) in the Galaxy. Our simulated results indicate that only when the natal MSPs receive a relatively strong kick ( > rsim 100 km s −1), can the AIC channel produce ∼10-180 eccentric ( e > 0.1) BMSPs in the Galaxy, most of which are accompanied by a helium star. Such a kick seems to be highly unlikely in the conventional AIC process, hence the probability of forming eccentric BMSPs via the AIC channel can be ruled out. Even if a high kick is allowed, the AIC channel cannot produce eccentric BMSPs with an orbital period of ≳20 d. Therefore, we propose that the peculiar BMSP PSR J1903+0327 cannot be formed by the AIC channel. However, the AIC evolutionary channel may produce some fraction of isolated MSPs, and even submillisecond pulsars if they really exist. [ABSTRACT FROM AUTHOR]

Justham, Rappaport & Podsiadlowski recently suggested that black hole low-mass X-ray binaries (BHLMXBs) with short orbital periods may have evolved from black hole intermediate-mass X-ray binaries (BHIMXBs). In their model the secondaries in BHIMXBs are assumed to possess anomalously high magnetic fields, so that magnetic braking can lead to substantial loss of angular momentum. In this paper, we propose an alternative mechanism for orbital angular momentum loss in BHIMXBs. We assume that a small fraction δ of the transferred mass from the donor star forms a circumbinary disc surrounding the binary system. The tidal torques exerted by the disc can effectively drain orbital angular momentum from the binary. We have numerically calculated the evolutionary sequences of BHIMXBs, to examine the influence of the circumbinary disc on the binary evolution. Our results indicate when δ≲ 0.01–0.1 (depending on the initial orbital periods), the circumbinary disc can cause secular orbital shrinking, leading to the formation of compact BHLMXBs, otherwise the orbits always expand during the evolution. This scenario also suggests the possible existence of luminous, persistent BHLMXBs, but it suffers the same problem as in Justham et al. that the predicted effective temperatures of the donor stars are significantly higher than those of the observed donor stars in BHLMXBs. [ABSTRACT FROM AUTHOR]

Recent timing observation provides an intriguing result for the braking index of the X-ray pulsar PSR |$\:$| J1640 |$-$| 4631, which has a measured braking index |$n=3.15\pm 0.03$|⁠. The decrease of the inclination angle between the spin axis and the magnetic axis can be responsible for such a high braking index. However, the physical mechanisms causing the change of the magnetic inclination angle have not been fully understood. In this Letter, we apply a two-dipole model given by Hamil, Stone, and Stone (2016, Phys. Rev. D. 94, 063012) to explain the decrease of the magnetic inclination angle of PSR |$\:$| J1640 |$-$| 4631. The rotation effect of a charged sphere and the magnetization of ferromagnetically ordered material produce magnetic moments |$M_{\,1}$| and |$M_{\,2}$|⁠ , respectively. There exists a minimum of the potential energy for the magnetic moment |$M_{\,2}$| in the magnetic field of |$M_{\,1}$|⁠ , hence the |$M_{\,2}$| will freely rotate around the minimum energy position (i.e. equilibrium position), similar to a simple pendulum. Our calculation indicates that the magnetic moment |$M_{\,2}$| would evolve towards alignment with the spin axis for PSR |$\:$| J1640 |$-$| 4631, and cause the magnetic inclination angle to decrease. The single peak in the pulse profile favors a relatively low change rate for the magnetic inclination angle. [ABSTRACT FROM AUTHOR]