Search

Loop quantum cosmology has achieved great successes, in which the polymerization plays a crucial role. In particular, the phase-space-variable dependent polymerization turns out to be the unique one that leads to consistent quantization of the homogeneous and isotropic universe. However, when applying the same scheme to the quantization of black holes, it meets resistances, when the Kantowski-Sachs (KS) gauge is adopted. In this paper, we continue to study the quantum effects of the polymerization near the location that a classical black hole horizon used to be, from the point of view of effective loop quantum gravity in the KS gauge. In particular, we find a phase-space-variable dependent polymerization scheme that leads to negligible quantum effects near the location of the classical black hole horizon, but significantly alters the spacetime structure near the origin, so that the classical singularity is finally replaced by a finite and regular transition surface. The final geodesically-complete spacetime consists of the regular transition surface that connects a black hole in one side and an anti-trapped region in the other side. In the anti-trapped region, no white hole horizons are found and the spacetime is extended to infinity, at which the geometric radius of the two-spheres becomes infinitely large., Comment: revtex4-2, 17 figures, and no tables; v2: typos corrected

Recently, the first pulsating ultraluminous X-ray source M82 X-2 was reported to be experiencing a rapid orbital decay at a rate of $\dot{P}=-(5.69\pm0.24)\times 10^{-8}~\rm s\,s^{-1}$ based on seven years \emph{NuSTAR} data. To account for the observed orbital-period derivative, it requires a mass transfer rate of $\sim200\dot{M}_{\rm edd}$ ($\dot{M}_{\rm edd}$ is the Eddington accretion rate) from the donor star to the accreting neutron star. However, other potential models cannot be completely excluded. In this work, we propose an anomalous magnetic braking (AMB) model to interpret the detected orbital decay of M82 X-2. If the donor star is an Ap/Bp star with an anomalously strong magnetic field, the magnetic coupling between strong surface magnetic field and irradiation-driven wind from the surface of the donor star could cause an efficient angular-momentum loss, driving a rapid orbital decay observed in M82 X-2. The AMB mechanism of an Ap/Bp star with a mass of $5.0-15.0~M_{\odot}$ and a surface magnetic field of $3000-4500~\rm G$ could produce the observed $\dot{P}$ of M82 X-2. We also discuss the possibility of other alternative models including the companion star expansion and a surrounding circumbinary disk., Comment: 9 pages, 2 figures. ApJ in press

Both the anomalous magnetic braking of Ap/Bp stars and the surrounding circumbinary disk models can account for the formation of black hole (BH) low-mass X-ray binaries (LMXBs), while the simulated effective temperatures of the donor stars are significantly higher than the observed values. Therefore, the formation of BH LMXBs is not still completely understood. In this work, we diagnose whether the dynamical friction between dark matter and the companion stars can drive BH binaries to evolve toward the observed BH LMXBs and alleviate the effective temperature problem. Assuming that there exists a density spike of dark matter around BH, the dynamical friction can produce an efficient angular momentum loss, driving BH binaries with an intermediate-mass companion star to evolve into BH LMXBs for a spike index higher than $\gamma = 1.58$. Our detailed stellar evolution models show that the calculated effective temperatures can match the observed value of most BH LMXBs for a spike index range of $\gamma = 1.7-2.1$. However, the simulated mass-transfer rates when $\gamma = 2.0$ and $2.1$ are too high to be consistent with the observed properties that BH LMXBs appears as soft X-ray transients. Therefore, the dynamical friction of dark matter can only alleviate the effective temperature problem of those BH LMXBs with a relatively short orbital period., Comment: 12 pages, 6 figures, ApJ in press

The fallback disk model is widely accepted to explain long-period neutron stars (NSs) which can't be simulated by magnetic dipole radiation. However, no confirmed detection of disk was found from the newly discovered long period pulsars GLEAM-X 162759.5-523504.3, GPM J1839-10 and the known slowest isolated NSs 1E 161348-5055. This might be that the disks have either been in noninteracting/inactive state where its emission is too weak to be detected or have been disrupted. In this work, we conduct simulations to examine the lifetime of supernova fallback disks around isolated neutron stars. We assume that the disk's mass varies in a self-similar way and its interaction with the NS occurs only in interacting/active state. Our results reveal that nearly all the interacting lifetimes for the disk are shorter than 0.1 Myr while the existence lifetimes are considerably longer., Comment: Accepted in ApJ, comments are welcome

In this paper, we extend the method of calculating the entanglement entropy of Hawking radiation of black holes using the "in" vacuum state, which describes one-sided asymptotically flat neutral black hole formed by gravitational collapse, to dynamic charged black holes. We explore the influence of charge on the position of the boundary of island $\partial I$ and the Page time. Due to their distinct geometric structures, we discuss non-extremal and extremal charged black holes separately. In non-extremal cases, the emergence of island saves the bound of entropy at late times, and the entanglement entropy of Hawking radiation satisfies the Page curve. Moreover, we also find that the position of the boundary of island $\partial I$ depends on the position of the cutoff surface (observers), differing from the behavior in eternal charged black holes. In extremal black holes, when the island exists, the entanglement entropy is approximately equal to the Bekenstein-Hawking entropy, while the entanglement entropy becomes ill-defined when island is absent. Our analysis underscores how different geometric configurations significantly influence the behavior of entropy.

Recently, it discovered two ultra-long period radio transients GLEAM-X J162759.5-523504.3 (J1627) and GPM J1839$-$10 (J1839) with spin periods longer than 1000 s. The origin of these two ultra-long period radio transients is intriguing in understanding the spin evolution of neutron stars (NSs). In this work, we diagnose whether the interaction between strong magnetized NSs and fallback disks can spin NSs down to the observed ultra-long period. Our simulations found that the magnetar+fallback disk model can account for the observed period, period derivative, and X-ray luminosity of J1627 in the quasi-spin-equilibrium stage. To evolve to the current state of J1627, the initial mass-accretion rate of the fallback disk and the magnetic field of the NS are in the range of $(1.1-30)\times10^{24}~\rm g\,s^{-1}$ and $(2-5)\times10^{14}~\rm G$, respectively. In an active lifetime of fallback disk, J1839 is impossible to achieve the observed upper limit of period derivative. Therefore, we propose that J1839 may be in the second ejector phase after the fallback disk becomes inactive. Those NSs with a magnetic field of $(2-6)\times10^{14}~\rm G$ and a fallback disk with an initial mass-accretion rate of $\sim10^{24}-10^{26}~\rm g\,s^{-1}$ are the possible progenitors of J1839., Comment: 12 pages, 10 figures, ApJ in press

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 ($M_{\rm Ch}$). 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-$M_{\rm Ch}$ 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 ($P_{\rm spin}$), etc. The results show that most of the observed DNS systems can be produced by NS kicks of $\lesssim50\rm\,km\,s^{-1}$. 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 ($P_{\rm spin}\sim20\,$ms) in DNS systems., Comment: 13 pages, 14 figures, 3 tables, accepted for publication in MNRAS, comments welcome!

PSR J1012+5307 is a millisecond pulsar with an extremely low-mass (ELM) white dwarf (WD) companion in an orbit of 14.5 hours. Magnetic braking (MB) plays an important role in influencing the orbital evolution of binary systems with a low-mass ($\lt 1-2~M_{\odot}$) donor star. At present, there exist several different MB descriptions. In this paper, we investigate the formation of PSR J1012+5307 as a probe to test the plausible MB model. Employing a detailed stellar evolution model by the MESA code, we find that the Convection And Rotation Boosted MB and the 'Intermediate' MB models can reproduce the WD mass, WD radius, WD surface gravity, neutron-star mass, and orbital period observed in PSR J1012+5307. However, our simulated WD has higher effective temperature than the observation. Other three MB mechanisms including the standard MB model are too weak to account for the observed orbital period in a Hubble time. A long cooling timescale caused by H-shell flashes of the WD may alleviate the discrepancy between the simulated effective temperature and the observed value., Comment: 12 pages, 4 figures, 1 table; ApJ in press

The source 2A 1822-371 is an eclipsing low-mass X-ray binary (LMXB) consisting of a neutron star (NS) and a $\sim0.5~M_{\odot}$ donor star in an orbit of 5.57 hr. Based on timing of the eclipse arrival times, this source was found to be experiencing a rapid orbital expansion with an orbital-period derivative as $\dot{P}_{\rm orb}=(1.51\pm0.05)\times10^{-10}~\rm s\, s^{-1}$, implying that the mass-transfer rate should be higher than at least three times the Eddington accretion rate. The standard magnetic braking (MB) model cannot produce such a high mass-transfer rate. The modified MB model derived by Van \& Ivanova (2019) can produce a high mass-transfer rate, resulting in a high $\dot{P}_{\rm orb}$. This work proposes an alternative model to account for the anomalously high mass-transfer rate and $\dot{P}_{\rm orb}$ of 2A 1822-371. During the mass transfer, a tiny fraction of the transferred material is thought to form a circumbinary (CB) disk around the LMXB, which can efficiently extract orbital angular momentum from the system by the interaction between the CB disk and the binary. We use the MESA code to model the formation and evolution of 2A 1822-371 for different CB-disk masses. When the CB-disk mass is $2.3\times10^{-8}~ M_{\odot}$, the simulation can reproduce the observed donor-star mass, orbital period, and orbital-period derivative. Such a CB disk can accelerate the evolution of the binary and produce a high mass transfer rate of $1.9\times10^{-7}~ M_\odot\,\rm yr^{-1}$, driving the binary to evolve toward a wide-orbit system. Therefore, we propose that CB disks may be responsible for the rapid orbital changes observed in some LMXBs., Comment: 8 pages, 8 figures, Astronomy and Astrophysics in press

In this paper, we consider the B\"ohmer-Vandersloot (BV) model of loop quantum black holes obtained from the improved dynamics approach. We adopt the Saini-Singh gauge, in which it was found analytically that the BV spacetime is geodesically complete. We show that black/white hole horizons do not exist in this geodesically complete spacetime. Instead, there exists only an infinite number of transition surfaces, which always separate trapped regions from anti-trapped ones. Comments on the improved dynamics approach adopted in other models of loop quantum black holes are also given., Comment: 3 figures and no tables. Version published in Commun. Theor. Phys. 76 (2024) 035401. arXiv admin note: text overlap with arXiv:2212.14535

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 $M_\odot$, these duty cycles are 0.006$\,\leq$$d$$\,\leq$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 $M_\odot$, respectively. In all of our simulations, no CO WDs can grow their masses to the explosion mass of Type Ia supernovae of about $1.38~M_\odot$. Because of a much short timescale 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., Comment: 9 pages, 6 figures, 4 tables. Published in MNRAS

The cores of massive neutron stars offer a unique environment for the nuclear matter at intermediate density in the universe. The global characteristics of a neutron star, as well as the gravitational waves emitted from the mergers of two neutron stars, offer valuable insights into dense nuclear matter. In this paper, we investigate the effect of the potential hadron-quark transition on the properties of neutron stars and the signals of the gravitational waves stemming from the merger of binary neutron stars, including waveforms, frequency evolutions as well as the spectrum curves, utilizing the equations of state constructed from the Maxwell ansatz, Gibbs ansatz and, the crossover scenario. We explicitly construct the equations of state in such a way that they converge at low and high densities therefore the differences are only from the scenarios of the transitions and the locations -- or the parameters in the equation of state. Using such constructed equations of state, we simulate the signals of the GW and analyze their differences due to locations of the transition, the scenarios of the transition, and the masses of the component stars. By combining our findings with the expected detection of gravitational waves around $(2-4)$ kHz from binary neutron star mergers and their associated electromagnetic signals, we expect to uncover some key characteristics of dense nuclear matter., Comment: A major revision including more figures, further discussions, and more references was made

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}-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 $\sim 30-110\rm\,ms$ and final orbital velocities of $\sim \rm 60-360\,km\,s^{-1}$, corresponding to initial orbital periods of $0.07-10\rm\,d$. 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 WD companions in the $P_{\rm spin}-\dot P_{\rm spin}$ diagram., Comment: 10 pages, 12 figures, 1 table, accepted for publication in MNRAS, comments welcome!

Recent discoveries of gravitational wave (GW) events most likely originating from black hole (BH) + neutron star (NS) mergers reveal the existence of BH+NS binaries. The formation of BH+NS binaries and their merger rates through isolated binary evolution have been investigated extensively with population synthesis simulations. A detailed stellar evolution modelings of the formation of this population, however, is missing in the literature. In this work, we perform the first complete 1D model of more than 30 BH+NS progenitor systems which are calculated self-consistently until the iron core collapse with infall velocity exceeds 1000 km s^-1. Focusing on the progenitors of BH- NS GW sources, we apply the MESA code starting from a post-common envelope binary with short orbital period (< 1 day) consisting of a BH and a zero-age main-sequence helium star that experiences stable mass transfer. These NS masses could be significantly larger depending on the exact mass cut during the supernova explosion. These BH+NS systems are likely to merge and produce GW events within a Hubble time. System C is a potential progenitor of a GW200115-like event, while Systems A and B are possible candidates for a GW200105-like event and may represent the final destiny of the X-ray binary SS433., Comment: 14pages, 9 figures, Accepted to be published on ApJ

It was proposed recently that the fine-grained entropy of the Hawking radiation can be expressed by the semiclassical island formula, which reproduces the unitary Page curve. In this paper, we choose the ``in'' vacuum state and apply the quantum extremal surface construction to study the Page curve for the step-function Vaidya model of evaporating black holes in four dimensions, which is produced by the spherical null shells. Metrics of the three regions of this spacetimes are obtained. In addition, the entanglement islands for the step-function Vaidya model of evaporating black holes at very late times are studied. When cutoff surface $A$ is located in Minkowski region III with $u_A < u_H$ at very late times, we find that the location of the boundary of island $\partial I$ depends on the value of $8M-v_A+v_I$. Specifically, $\partial I$ is inside, at or outside the horizon when $8M-v_A+v_I$ is less than, equal to or larger than zero respectively. Moreover, when cutoff surface $A$ is located in Minkowski region III with $u_A > u_H$ after the black hole evaporates completely, we find that entanglement island still exists and $\partial I$ is located on an equal-time Cauchy surface of the observer $A$ when $r_{(A)}^2\geq64G_N\kappa c $., Comment: 29 pages, 12 figures, improvements to presentation, references added, and new content added in appendix to comment about the stress-energy tensor of the matter field, published version in JHEP

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) ultra-compact X-ray binaries (UCXBs) have been investigated extensively. Using the MESA code, we systematically explored the evolution of black hole (BH)-main sequence star (MS) binaries to diagnose whether their descendants can be detected by space-borne GW detectors. Our simulations show 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 detected by LISA at a distance $d=1$ kpc are $\sim10^{33}-10^{35}~\rm erg\,s^{-1}$ ($\sim10^{34}-10^{35}~\rm erg\,s^{-1}$ for $d=10$ kpc), hence it is possible to detect their electromagnetic counterparts. It is worth emphasizing 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 by BH-MS pathway is about 3 mHz, which is smaller than the minimum GW frequency (6.4 mHz) of mass-transferring BH-WD 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 could detect only a few BH UCXB-GW sources forming by the BH-MS channel., Comment: 15 pages, 8 figures, 1 table. ApJ in press

It has been found that the topology effect and the possible emergent scale and hidden local flavor symmetries at high density reveal a novel structure of the compact star matter. The $N_f\geq2$ baryons can be described by the skyrmion in the large $N_c$ limit and there is a robust topology change in the skyrmion matter approach to dense nuclear matter. The hidden scale and local flavor symmetries which are sources introducing the lightest scalar meson -- dilaton -- and lowest lying vector mesons into to nonlinear chiral effective theory are seen to play important roles in understanding the nuclear force. We review in this paper the generalized nuclear effective theory (G$n$EFT), which applicable to nuclear matter from low density to the compact star density, constructed with the robust conclusion from the topology approach to dense matter and emergent scale and hidden local flavor symmetries. The topology change at density larger than two times saturation density $n_0$ encoded in the parameters of the effective field theory is interpreted as the hadron-quark continuity in the sense of Cheshire Cat Principle. A novel feature predicted in this theory that has not been found before is the precocious appearance of the conformal sound velocity in the cores of massive stars, although the trace of the energy-momentum tensor of the system is not zero. That is, in contrast to the usual picture, the cores of massive stars are composed of quasiparticles of fractional baryon charges, neither baryons nor deconfined quarks. Hidden scale and local flavor symmetries emerge and give rise a resolution of the longstanding $g_A$ quench problem in nuclei transition. To illustrate the rationality of the GnEFT, we finally confront the generalized effective field theory to the global properties of neutron star and the data from gravitational wave detections., Comment: Invited contribution to Special Issue of MDPI "Symmetries and Ultra Dense Matter of Compact Stars"

In this paper, we study the quantum geometric effects near the locations where classical black hole horizons used to appear in Einstein's classical theory, within the framework of an improved dynamic approach, in which the internal region of a black hole is modeled by the Kantowski-Sachs (KS) spacetime and the two polymerization parameters are functions of the phase space variables. Our detailed analysis shows that the effects are so strong that black and white hole horizons of the effective quantum theory do not exist at all and instead are replaced by transition surfaces, across which the metric coefficients and their inverses are smooth and remain finite, as are the corresponding curvatures, including the Kretschmann scalar. These surfaces always separate trapped regions from anti-trapped regions. The number of such surfaces is infinite, so the corresponding KS spacetimes become geodesically complete, and no black and white hole-like structures exist in this scheme., Comment: revtex4-2, 6 figures, and two tables, v2: typos corrected, references added, v3: Additional analyses were provided, including new references and figures, while the conclusions remain the same, v4: to appear in SCIENCE CHINA Physics, Mechanics & Astronomy, v5: published version in SCIENCE CHINA Physics, Mechanics & Astronomy

In this paper we analyse and discuss 2D Jackiw-Teitelboim (JT) gravity coupled to primary fermion fields in asymptotically anti-de Sitter (AdS) spacetimes. We get a particular solution of the massless Dirac field outside the extremal black hole horizon and find the solution for the dilaton in JT gravity. Two dimensional JT gravity spacetime is conformally flat, we calculate the two point correlators of primary fermion fields under the Weyl transformations. The key point of this work is to present a standard technique which is called resolvent rather than CFT methods. We redefine the fields in terms of the conformal factor as the fermion fields, and we use the resolvent technique to derive the renormalized entanglement entropy for massless Dirac fields in JT gravity., Comment: 15 pages, 3 figures; improvements to presentation, new content added in appendix to comment about the correlator trace formula of primary fermion fields, published version in CPC

We reconsider $D\geq4$ dimensional asymptotically flat eternal Schwarzschild black hole, and focus on the situation where the inner boundary of the radiation region is chosen to be near the horizon (i.e. $\beta \ll1$). The tension between the near-horizon condition and the short-distance approximation emerges in large dimensions in $[JHEP 06 (2020) 085]$. We remove this tension by introducing a more proper near horizon condition, thus the resulting island solution is well-behaved in any $D\geq4$ dimensional spacetime. Interestingly, a novel constraint is obtained in this situation as required by the existence of the island solution, which directly leads to the constraints on the size of the Schwarzschild black hole, the position of the inner boundary for the radiation region, or the value of $c\cdot\tilde{G}_{N}$ in any $D\geq4$ dimension. When considering the large $D$ limit, the constraint on the size of the Schwarzschild black hole obtained in this situation is in agreement with the result given in $[Phys.Rev.D 102 (2020) 2, 026016]$. We interpret these as the unitary constraints implied by the presence of island in semiclassical gravity., Comment: 21 pages, 4 figures; improvements to the expression; Footnote 2 added to show a new analytical island solution for $n\rightarrow\infty$ while $\alpha\sim \beta/n$; accepted for publication in PRD

We systematically study the top-down model of loop quantum black holes (LQBHs), recently derived by Alesci, Bahrami and Pranzetti (ABP). To understand the structure of the model, we first derive several well-known LQBH solutions by taking proper limits. These include the B\"ohmer-Vandersloot and Ashtekar-Olmedo-Singh models, which were all obtained by the so-called bottom-up polymerizations within the framework of the minisuperspace quantizations. Then, we study the ABP model, and find that the inverse volume corrections become important only when the radius of the two-sphere is of the Planck size. For macroscopic black holes, the minimal radius obtained at the transition surface is always much larger than the Planck scale, and hence these corrections are always sub-leading. The transition surface divides the whole spacetime into two regions, and in one of them the spacetime is asymptotically Schwarzschild-like, while in the other region, the asymptotical behavior sensitively depends on the ratio of two spin numbers involved in the model, and can be divided into three different classes. In one class, the spacetime in the 2-planes orthogonal to the two spheres is asymptotically flat, and in the second one it is not even conformally flat, while in the third one it can be asymptotically conformally flat by properly choosing the free parameters of the model. In the latter, it is asymptotically de Sitter. However, in any of these three classes, sharply in contrast to the models obtained by the bottom-up approach, the spacetime is already geodesically complete, and no additional extensions are needed in both sides of the transition surface. In particular, identical multiple black hole and white hole structures do not exist., Comment: revtex4-2, 18 figures, and four tables, v3: typos corrected, published in PRD

HD49798/RX J0648.0-4418 is a peculiar binary including a hot subdwarf of O spectral type and a compact companion in a 1.55 day orbit. According to the steady spin period derivative $\dot{P}=(-2.17\pm0.01)\times10^{-15} ~\rm s\,s^{-1}$ , the compact object was thought to be a contracting young white dwarf (WD). However, the X-ray luminosity producing by the wind accretion of massive WD is one order of magnitude smaller than the observed value. In this work, we propose an alternative model to account for the observed X-ray luminosity. If the WD was surrounded by a debris disk, the accretion from the debris disk can produce the observed X-ray luminosity and X-ray pulses. Based on the time-varying accretion rate model, the current mass of the debris disk is constrained to be $3.9\times10^{-6}~\rm M_{\odot}$. Comparing with the contraction of the WD, the accretion torque exerting by such a debris disk can only influence the spin evolution of the WD in the early stage. According to the accretion theory, the magnetic field of the WD is constrained to be $\sim (0.7-7)\times10^{4}$ G. The calculated conventional polar cap radius of the WD is larger than the observed emitting-zone radius, which probably originate from the existence of strong and small-scale local magnetic field in the polar cap surface. We expect that further multiband observations on this source can help us to confirm or rule out the existence of a debris disk., Comment: 5 pages, 2 figures, Astronomy and Astrophysics in press

Great breakthrough in solving black hole information paradox took place when semiclassical island rule for entanglement entropy of Hawking radiation was proposed in recent years. Up to now, most papers which discussed island rule of asymptotic flat black hole with $D \ge 4$ focus on eternal black hole. In this paper, we take one more step further by discussing island of "in" vacuum state which describes one-sided asymptotically flat black hole formed by gravitational collapse in $D \ge 4$. We find that island $I$ emerges at late time and saves entropy bound. And boundary of island $\partial I$ depends on the position of cutoff surface. When cutoff surface is far from horizon, $\partial I$ is inside and near horizon. When cutoff surface is set to be near horizon, $\partial I$ is outside and near horizon. This is different from the case of eternal black hole in which $\partial I$ is always outside horizon no matter cutoff surface is far from or near horizon. We will see that different states will manifestly affect $S_{\text{ent}}$ in island formula when cutoff surface is far from horizon and thus have different result for Page time., Comment: v1:22 pages, 2 figures,v2: typos corrected, an appendix on s-wave approximation is added, references added, published version

Recent discoveries of double neutron star (DNS) mergers and ultra-stripped supernovae (SNe) raise the questions of their origin and connection. We present the first 1D~model of a DNS progenitor system which is calculated self-consistently until an ultra-stripped iron core collapse. We apply the \texttt{MESA} code starting from a post-common envelope binary consisting of a $1.35\;M_\odot$ NS and a $3.20\;M_\odot$ zero-age main-sequence helium star and continue the modelling via Case~BB Roche-lobe overflow until the infall velocity of the collapsing iron core exceeds $1000\;{\rm km\,s}^{-1}$. The exploding star has a total mass of $\sim 1.90\;M_\odot$, consisting of a $\sim 0.29\;M_\odot$ He-rich envelope embedding a CO core of $\sim 1.61\;M_\odot$ and an iron-rich core of $\sim 1.50\;M_\odot$. The resulting second-born NS has an estimated mass of $\sim 1.44\;M_\odot$ and we discuss the fate of the post-SN system, as well as the mild recycling of the first-born NS. Depending on the initial conditions, this family of systems is anticipated to reproduce the DNS mergers detected by the LIGO-network., Comment: 8 pages, 5 figures, ApJL in press

PSR J$1946+3417$ is a millisecond pulsar (MSP) with a spin period $P\simeq3.17\rm~ms$. Harbored in a binary with an orbital period $P_{\rm b}\simeq27$ days, the MSP is accompanied by a white dwarf (WD). The masses of the MSP and the WD were determined to be $1.83\rm~M_\odot$ and $0.266\rm~M_\odot$, respectively. Specially, its orbital eccentricity is $e\simeq0.134$, which is challenging the recycling model of MSPs. Assuming that the neutron star in a binary may collapse to a strange star when its mass reaches a critical limit, we propose a phase transition (PT) scenario to account for the origin of the system. The sudden mass loss and the kick induced by asymmetric collapse during the PT may result in the orbital eccentricity. If the PT event takes place after the mass transfer ceases, the eccentric orbit can not be re-circularized in the Hubble time. Aiming at the masses of both components, we simulate the evolution of the progenitor of PSR J$1946+3417$ via \texttt{MESA}. The simulations show that a NS / main sequence star binary with initial masses of $1.4+1.6\rm~M_\odot$ in an initial orbit of 2.59 days will evolve into a binary consisting of a $2.0\rm~M_\odot$ MSP and a $0.27\rm~M_\odot$ WD in an orbit of $\sim21.5$ days. Assuming that the gravitational mass loss fraction during PT is $10\%$, we simulate the effect of PT via the kick program of \texttt{BSE} with a velocity of $\sigma_{\rm PT}=60~{\rm km~s}^{-1}$. The results show that the PT scenario can reproduce the observed orbital period and eccentricity with higher probability then other values., Comment: accepted to be published on RAA

In young dense clusters, an intermediate-mass black hole (IMBH) may get a companion star via exchange encounters or tidal capture, and then evolves toward IMBH X-ray binary by the Roche lobe overflow. It is generally thought that IMBH X-ray binaries are potential ultra-luminous X-ray sources (ULXs), hence their evolution is very significant. However, the irradiation-driven winds by the strong X-ray flux from the accretion disks around the IMBHs play an important role in determining the evolution of IMBH X-ray binaries, and should be considered in the detailed binary evolution simulation. Employing the models with the MESA code, we focus on the influence of irradiation-driven winds on the evolution of IMBH X-ray binaries. Our simulations indicate that a high wind-driving efficiency ($f=0.01$ for $Z=0.02$, and $f=0.002$ for $Z=0.001$) substantially shorten the duration in the ULX stage of IMBH X-ray binaries with an intermediate-mass ($5~M_{\odot}$) donor star. However, this effect can be ignored for high-mass ($10~M_{\odot}$) donor stars. The irradiation effect ($f=0.01$ or $0.002$) markedly shrink the initial parameter space of IMBH binaries evolving toward ULXs with high luminosity ($L_{\rm X}>10^{40}~\rm erg\,s^{-1}$) and hyperluminous X-ray sources in the donor-star mass versus orbital period diagram. Furthermore, the irradiation effect results in an efficient angular momentum loss, yielding to IMBH X-ray binaries with relatively close orbits. In our simulated parameter space, about 1\% of IMBH binaries would evolve toward compact X-ray sources owing to short initial orbital periods, some of which might be detected as low-frequency gravitational wave sources., Comment: 10 pages, 5 figures, 1 table, ApJ in press

Binary millisecond pulsars (MSPs) are detached binary systems consisting of a MSP and a He white dwarf. If the initial orbital periods of binary MSPs are less than 0.3 day, they would evolve toward ultra-compact binary pulsars due to the rapid orbital shrinkage by the gravitational wave (GW) radiation. During the orbital decay, the MSP with an ellipticity would spin down by the GW radiation and the magnetic dipole radiation. Our calculations indicate that the angular momentum loss is dominated by the GW radiation when the ellipticities of the neutron stars (NSs) are in the range of $(1-50)\times 10^{-7}$, and the frequencies of high-frequency GW signals from the rotating NSs are $10-100$ Hz when the binary pulsars can be visible as low-frequency GW sources. These high-frequency GW signals are possible to be detected by the aLIGO and the third-generation GW detectors such as Einstein Telescope, depending on the frequencies and the distances. Therefore, some ultra-compact binary pulsars have an opportunity to become intriguing dual-line GW sources. By detecting the low-frequency GW signals, the NS mass can be accurately derived. A dual-line detection of two band GW signals could provide a constraint on the moment of inertia and the ellipticity of the NS. Thus the dual-line GW sources can be potentially applied to constrain the equation of state of the NS., Comment: 7 pages, 3 figures, Phys. Rev. D in press

The space gravitational wave (GW) detector Laser Interferometer Space Antenna (LISA) that was planed to launch in the early 2030s is to detect the low-frequency GW signals in the Galaxy. AM CVn stars were generally thought to be important low-frequency GW sources. Employing the MESA code, in this work we calculate the evolution of a great number of binary systems consisting of a white dwarf (WD) and a main sequence (MS) star, and diagnose whether their descendants-AM CVn stars can be visible by the LISA. The simulated results show that the progenitors of these LISA sources within a distance of 1 kpc are WD-MS binaries with a donor star of $1.0-1.4~M_\odot$ (for initial WD mass of $0.5~M_\odot$) or $1.0-2.0~M_\odot$ (for initial WD mass of $0.7~M_\odot$), and an initial orbital period slightly smaller than the bifurcation period. Our simulations also indicate ten verification AM CVn sources can be reproduced by the standard magnetic braking model, and are potential LISA sources. Based on the birthrate of AM CVns simulated by the population synthesis, the birthrate of AM CVn-LISA sources evolving from the evolved donor star channel within a distance of 1 kpc can be estimated to be $(0.6-1.4)\times10^{-6}~\rm yr^{-1}$, and the predicted number of AM CVn-LISA sources is about $340-810$. Therefore, the evolved donor star channel play an important role in forming AM CVn-LISA sources in the Galaxy., Comment: 8 pages, 5 figures, 2 tables, Accepted by ApJ

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 ($P=1.6$ hour) among confirmed binary pulsars. Recent observations reported that this source is experiencing a rapid orbital expansion at a rate of $\dot{P}=(1.89\pm0.05)\times 10^{-12}~\rm s\,s^{-1}$. 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 can not 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 asymmetry disc winds or outflow, the evaporation efficiency can be constrained to be $\sim 0.1$. Therefore, the rapidly orbital expansion detected in PSR J0636+5128 provides evidence of outflows and accretion disc around the MSP., Comment: 5 pages, 1 table, MNRAS in press

Topological structure has been extensively studied and confirmed in highly correlated condensed matter physics. We explore the gravitational waves emitted from binary neutron star mergers using the pseudoconformal model for dense nuclear matter for compact stars. This model considers the topology change and the possible emergent scale symmetry and satisfies all the constraints from astrophysics. We find that the location of the topology change affects gravitational waves dramatically owing to its effect on the equation of state. In addition, the effect of this location on the waveforms of the gravitational waves is within the ability of the on-going and up-coming facilities for detecting gravitational waves, thus suggesting a possible way to measure the topology structure in nuclear physics., Comment: Version published in Sci. China-Phys. Mech. Astron

In this paper we systematically study a five-parameters class of spherically symmetric polymer black/white hole solutions, and find that only three independent combinations are physical and uniquely determine the spacetime properties. After exploring the whole 3-dimensional (3D) phase space, we find that the model has very rich physics, and depending on the choice of these parameters, various possibilities exist, including: (i) spacetimes that have the standard black/white hole structures, that is, spacetimes that are free of spacetime curvature singularities and possess two asymptotically flat regions, which are connected by a transition surface (throat) with a finite and non-zero geometric radius. The black/white hole masses measured by observers in the two asymptotically flat regions are all positive, and the surface gravity of the black (white) hole is positive (negative). (ii) Spacetimes that have wormhole-like structures, in which the two masses are all positive, but no horizons exist.(iii) Spacetimes that still possess curvature singularities, which can be either hidden inside trapped regions or naked. However, such spacetimes correspond to only some limit cases, and the necessary (but not sufficient) condition is that at least one of the two "polymerization" parameters vanishes. In addition, even for solar mass black/white holes, quantum gravitational effects can be still very large at the black/white hole horizons, again depending on the choice of the parameters., Comment: 37 pages, 18 figures, 4 tables, v2: title changed, typos corrected, paragraphs and references added, published version

Ultra-compact X-ray binaries (UCXBs) are low-mass X-ray binaries with ultra-short orbital periods (usually less than 1 hour) and hydrogen-poor donor stars, which are proposed to be the potential LISA sources. In this work, we firstly employ the MESA code to examine the parameter space of the progenitors of UCXBs that LISA will detect. Our simulations indicate that the initial binaries with a neutron star and a $0.4-3.3~M_{\odot}$ companion star in an orbit of initial orbital period smaller than the bifurcation period could evolve into UCXBs, some of which will emit gravitational wave signals that can be detectable by the LISA. However, the initial orbital periods of the binaries that will evolve into UCXB-LISA sources in a distance of 10 kpc are located in a very narrow range, i.e. the formation of these LISA source requires an extremely fine-tuning of initial parameter. According to the characteristic stains and the derived maximum detectable distances, four sources among eight UCXBs with the observed distances are expected to be detected by the LISA. Based on the parameter space given by the detailed binary evolution models and the rapid binary star evolution code, the birthrate of UCXBs appearing as LISA sources in the Galaxy is estimated to be $(2-2.6)\times10^{\rm -6}~\rm yr^{-1}$. Considering the contribution of UCXBs in the globular clusters, the number of UCXB-LISA sources can reach $240 - 320$. Although the formation condition is severe, the detectability of UCXBs by the LISA is still optimistic and significant because they provide an opportunity to pursue full multi-messenger investigations., Comment: 10 pages, 7 figures, ApJ Letters in press

A spinning neutron star (NS) that is asymmetric with respect to its spin axis can emit continuous gravitational wave (GW) signals. The spin frequencies and their distribution of radio millisecond pulsars (MSPs) and accreting MSPs provide some evidences of GW radiation, and MSPs are ideal probes detecting high frequency GW signals. It is generally thought that MSPs originate from the recycled process, in which the NS accretes the material and angular momentum from the donor star. The accreted matter would be confined at the polar cap zone by an equatorial belt of compressed magnetic field fixed in the deep crust of the NS, and yields "magnetic mountain". Based on an assumption that the spin-down rates of three transitional MSPs including PSR J1023+0038 are the combinational contribution of the accretion torque, the propeller torque, and the GW radiation torque, in this work we attempt to constrain the ellipticities of MSPs with observed spin-down rates. Assuming some canonical parameters of NSs, the ellipticities of three transitional MSPs and ten redbacks are estimated to be $\epsilon=(0.9-23.4)\times 10^{-9}$. The electrical resistivities of three transitional MSPs are also derived to be in the range $\eta=(1.2-15.3)\times 10^{-31}~\rm s$, which display an ideal power law relation with the accretion rate. The characteristic strains ($h_{\rm c}=(0.6-2.5)\times10^{-27}$) of GW signals emitting by these sources are obviously beyond the sensitivity scope of the aLIGO. We expect that the third-generation GW detectors like the Einstein Telescope can seize the GW signals from these sources in the future., Comment: 7 pages, 2 figures. PRD in press

Recently, repeating fast radio bursts (FRBs) with a period of $P_{\rm FRB}=16.35\pm0.18$ days from FRB 180916.J0158+65 had been reported. It still remains controversial how to give rise to such a periodicity of this FRB. In this Letter, based on an assumption of a young pulsar surrounding by a debris disk, we attempt to diagnose whether the 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 days for a mass inflow rate of $0.5-1.5\times10^{18}~\rm g\,s^{-1}$, a spin period of 1-20 ms of the pulsar, and an extremely low viscous parameter $\alpha=10^{-8}$ in the disk. The disk mass and the magnetic field of the pulsar are also constrained to be $\sim10^{-3}~\rm M_{\odot}$ and $< 2.5\times 10^{13}~\rm G$. In our model, a new born pulsar with normal magnetic field and millisecond period would successively experience accretion phase, propeller phase, and is visible as a strong radio source in the current stage. The rotational energy of such a young NS can provide the observed radio bursting luminosity for $400$ years., Comment: 6 pages, 2 figures, PASJ in press