Rm Ns Research Articles

A force explosion condition for spherically symmetric core-collapse supernovae

ABSTRACT Understanding which stars explode leaving behind neutron stars and which stars collapse forming black holes remains a fundamental astrophysical problem. We derive an analytic explosion condition for spherically symmetric core-collapse supernovae. The derivation starts with the exact governing equations, considers the balance of integral forces, includes the important dimensionless parameters, and includes an explicit set of self-consistent approximations. The force explosion condition is $\tilde{L}_\nu \tau _g - 0.06 \tilde{\kappa } \gt 0.38$, and only depends upon two dimensionless parameters. The first compares the neutrino power deposited in the gain region with the accretion power, $\tilde{L}_\nu \tau _g = L_{\nu } \tau _g R_{\rm NS}/ (G \dot{M} M_{\rm NS})$. The second, $\tilde{\kappa } = \kappa \dot{M} / \sqrt{G M_{\rm NS} R_{\rm NS}}$, parametrizes the neutrino optical depth in the accreted matter near the neutron star surface. Over the years, many have proposed approximate explosion conditions: the critical neutrino-luminosity, ante-sonic, and time-scale conditions. We are able to derive these other conditions from the force explosion condition, which unifies them all. Using numerical, steady-state and fully hydrodynamic solutions, we test the explosion condition. The success of these tests is promising in two ways. One, the force explosion condition helps to illuminate the underlying physics of explosions. Two, this condition may be a useful explosion diagnostic for more realistic, three-dimensional radiation hydrodynamic core-collapse simulations.

The spins of compact objects born from helium stars in binary systems

ABSTRACT The angular momentum (AM) content of massive stellar cores helps us to determine the natal spin rates of neutron stars and black holes. Asteroseismic measurements of low-mass stars have proven that stellar cores rotate slower than predicted by most prior work, so revised models are necessary. In this work, we apply an updated AM transport model based on the Tayler instability to massive helium stars in close binaries, in which tidal spin-up can greatly increase the star’s AM. Consistent with prior work, these stars can produce highly spinning black holes upon core-collapse if the orbital period is less than $P_{\rm orb} \lesssim \! 1 \, {\rm d}$. For neutron stars, we predict a strong correlation between the pre-explosion mass and the neutron star rotation rate, with millisecond periods ($P_{\rm NS} \lesssim 5 \, {\rm ms}$) only achievable for massive ($M \gtrsim 10 \, M_\odot$) helium stars in tight ($P_{\rm orb} \lesssim 1 \, {\rm d}$) binaries. Finally, we discuss our models in relation to type Ib/c supernovae, superluminous supernove, gamma-ray bursts, and LIGO/Virgo measurements of black hole spins. Our models are roughly consistent with the rates and energetics of these phenomena, with the exception of broad-lined Ic supernovae, whose high rates and ejecta energies are difficult to explain.

Properties of the remnant disk and the dynamical ejecta produced in low-mass black hole-neutron star mergers

We systematically perform numerical-relativity simulations for low-mass black hole-neutron star mergers for the models with seven mass ratios $Q=M_{\rm BH}/M_{\rm NS}$ ranging from 1.5 to 4.4, and three neutron-star equations of state, focusing on properties of matter remaining outside the black hole and ejected dynamically during the merger. We pay particular attention to the dependence on the mass ratio of the binaries. It is found that the rest mass remaining outside the apparent horizon after the merger depends only weakly on the mass ratio for the models with low mass ratios. It is also clarified that the rest mass of the ejecta has a peak at $Q \sim 3$, and decreases steeply as the mass ratio decreases for the low mass-ratio case. We present a novel analysis method for the behavior of matter during the merger, focusing on the matter distribution in the phase space of specific energy and specific angular momentum. Then we model the matter distribution during and after the merger. Using the result of the analysis, we discuss the properties of the ejecta.

Constraining Screened Modified Gravity with Spaceborne Gravitational-wave Detectors

Abstract Screened modified gravity (SMG) is a unified theoretical framework that describes scalar–tensor gravity with a screening mechanism. Based on the gravitational-wave (GW) waveform derived in our previous work, in this article we investigate the potential constraints on SMG theory through GW observation by future spaceborne GW detectors, including the Laser Interferometer Space Antenna (LISA), TianQin, and Taiji. We find that, for the extreme-mass-ratio inspirals (EMRIs) consisting of a massive black hole and a neutron star, if the EMRIs are at the Virgo cluster, the GW signals can be detected by the detectors at quite high significance level, and the screened parameter ϵ NS can be constrained at about , which is more than one order of magnitude tighter than the potential constraint given by a ground-based Einstein telescope. However, for the EMRIs consisting of a massive black hole and a white dwarf, it is more difficult to detect them than in the previous case. For the specific SMG models, including chameleon, symmetron, and dilaton, we find these constraints are complementary to that from the Cassini experiment, but weaker than those from lunar laser ranging observations and binary pulsars, due to the strong gravitational potentials on the surface of neutron stars. By analyzing the deviation of the GW waveform in SMG from that in general relativity, as anticipated, we find the dominant contribution of the SMG constraint comes from the correction terms in the GW phases, rather than the extra polarization modes or the correction terms in the GW amplitudes.

Binary neutron star mergers: Effects of spin and post-merger dynamics

Spin can have significant effects on the electromagnetic transients accompanying binary neutron star mergers. The measurement of spin can provide important information about binary formation channels. In the absence of a strong neutron star spin prior, the degeneracy of spin with other parameters leads to significant uncertainties in their estimation, in particular limiting the power of gravitational waves to place tight constraints on the nuclear equation of state. Thus detailed studies of highly spinning neutron star mergers are essential to understand all aspects of multimessenger observation of such events. We perform a systematic investigation of the impact of neutron star spin---considering dimensionless spin values up to $a_{\rm NS}=0.33$---on the merger of equal mass, quasicircular binary neutron stars using fully general-relativistic simulations. We find that the peak frequency of the post-merger gravitational wave signal is only weakly influenced by the neutron star spin, with cases where the spin is aligned (antialigned) with the orbital angular momentum giving slightly lower (higher) values compared to the irrotational case. We find that the one-arm instability arises in a number of cases, with some dependence on spin. Spin has a pronounced impact on the mass, velocity, and angular distribution of the dynamical ejecta, and the mass of the disk that remains outside the merger remnant. We discuss the implications of these findings on anticipated electromagnetic signals, and on constraints that have been placed on the equation of state based on multimessenger observations of GW170817.

Neutron star radius measurement from the ultraviolet and soft X-ray thermal emission of PSR J0437−4715

ABSTRACT We analysed the thermal emission from the entire surface of the millisecond pulsar PSR J0437−4715 observed in the ultraviolet and soft X-ray bands. For this, we calculated non-magnetized, partially ionized atmosphere models of hydrogen, helium, and iron compositions and included plasma frequency effects that may affect the emergent spectrum. This is particularly true for the coldest atmospheres composed of iron (up to a few per cent changes in the soft X-ray flux). Employing a Markov chain Monte Carlo method, we found that the spectral fits favour a hydrogen atmosphere, disfavour a helium composition, and rule out iron atmosphere and blackbody models. By using a Gaussian prior on the dust extinction, based on the latest 3D map of Galactic dust, and accounting for the presence of hot polar caps found in the previous work, we found that the hydrogen atmosphere model results in a well-constrained neutron star radius ${R_{\rm NS}}= 13.6^{+0.9}_{-0.8}{\, {\rm km}}$ and bulk surface temperature ${T_{\rm eff}^{\infty }}=\left(2.3\pm 0.1\right){\times 10^{5}}{\, {\rm K}}$. This relatively large radius favours a stiff equation of state and disfavours a strange quark composition inside neutron stars.

The radius of the quiescent neutron star in the globular cluster M13

X-ray spectra of quiescent low-mass X-ray binaries containing neutron stars can be fit with atmosphere models to constrain the mass and the radius. Mass-radius constraints can be used to place limits on the equation of state of dense matter. We perform fits to the X-ray spectrum of a quiescent neutron star in the globular cluster M13, utilizing data from ROSAT, Chandra and XMM-Newton, and constrain the mass-radius relation. Assuming an atmosphere composed of hydrogen and a 1.4${\rm M}_{\odot}$ neutron star, we find the radius to be $R_{\rm NS}=12.2^{+1.5}_{-1.1}$ km, a significant improvement in precision over previous measurements. Incorporating an uncertainty on the distance to M13 relaxes the radius constraints slightly and we find $R_{\rm NS}=12.3^{+1.9}_{-1.7}$ km (for a 1.4${\rm M}_{\odot}$ neutron star with a hydrogen atmosphere), which is still an improvement in precision over previous measurements, some of which do not consider distance uncertainty. We also discuss how the composition of the atmosphere affects the derived radius, finding that a helium atmosphere implies a significantly larger radius.

General relativistic magnetohydrodynamics simulations of prompt-collapse neutron star mergers: The absence of jets.

Inspiraling and merging binary neutron stars are not only important source of gravitational waves, but also promising candidates for coincident electromagnetic counterparts. These systems are thought to be progenitors of short gamma-ray bursts (sGRBs). We have shown previously that binary neutron star mergers that undergo delayed collapse to a black hole surrounded by a weighty magnetized accretion disk can drive magnetically powered jets. We now perform magnetohydrodynamic simulations in full general relativity of binary neutron stars mergers that undergo prompt collapse to explore the possibility of jet formation from black hole-light accretion disk remnants. We find that after t - tBH ~26(MNS/1.8 M⊙) ms (MNS is the ADM mass) following prompt black hole formation, there is no evidence of mass outflow or magnetic field collimation. The rapid formation of the black hole following merger prevents magnetic energy from approaching force-free values above the magnetic poles, which is required for the launching of a jet by the usual Blandford-Znajek mechanism. Detection of gravitational waves in coincidence with sGRBs may provide constraints on the nuclear equation of state (EOS): the fate of an NSNS merger-delayed or prompt collapse, and hence the appearance or nonappearance of an sGRB-depends on a critical value of the total mass of the binary, and this value is sensitive to the EOS.

Transcendental Morse inequality and generalised Okounkov bodies

The main goal of this article is to construct arithmetic Okounkov for an arbitrary pseudo-effective (1,1)-class $\alpha$ on a Kahler manifold. Firstly, using Boucksom's divisorial Zariski decompositions for pseudo-effective (1,1)-classes on compact Kahler manifolds, we prove the differentiability of volumes of big classes for Kahler manifolds on which modified nef cones and nef cones coincide; this includes Kahler surfaces. We then apply our differentiability results to prove Demailly's transcendental Morse inequality for these particular classes of Kahler manifolds. In the second part, we construct the convex body $\Delta(\alpha)$ for any big class $\alpha$ with respect to a fixed flag by using positive currents, and prove that this newly defined convex body coincides with the Okounkov body when $\alpha\in {\rm NS}_{\mathbb{R}}(X)$; such convex sets $\Delta(\alpha)$ will be called generalized Okounkov bodies. As an application we prove that any rational point in the interior of Okounkov bodies is valuative. Next we give a complete characterisation of generalized Okounkov bodies on surfaces, and show that the generalized Okounkov bodies behave very similarly to original Okounkov bodies. By the differentiability formula, we can relate the standard Euclidean volume of $\Delta(\alpha)$ in $\mathbb{R}^2$ to the volume of a big class $\alpha$, as defined by Boucksom; this solves a problem raised by Lazarsfeld in the case of surfaces. Finally, we study the behavior of the generalized Okounkov bodies on the boundary of the big cone, which are characterized by numerical dimension.

A NuSTAR OBSERVATION OF THE REFLECTION SPECTRUM OF THE LOW-MASS X-RAY BINARY 4U 1728-34

ABSTRACT We report on a simultaneous NuSTAR and Swift observation of the neutron star low-mass X-ray binary 4U 1728-34. We identified and removed four Type I X-ray bursts during the observation in order to study the persistent emission. The continuum spectrum is hard and described well by a blackbody with kT = 1.5 keV and a cutoff power law with Γ = 1.5, and a cutoff temperature of 25 keV. Residuals between 6 and 8 keV provide strong evidence of a broad Fe Kα line. By modeling the spectrum with a relativistically blurred reflection model, we find an upper limit for the inner disk radius of . Consequently, we find that R NS ≤ 23 km, assuming M = 1.4 M ⊙ and a = 0.15. We also find an upper limit on the magnetic field of B ≤ 2 × 108 G.

The radiative efficiency of relativistic jet and wind: a case study of GRB 070110

A rapidly spinning, strongly magnetized neutron star is invoked as the central engine for some Gamma-ray bursts (GRBs), especially, the $"$internal plateau$"$ feature of X-ray afterglow. However, for these $"$internal plateau$"$ GRBs, how to produce their prompt emission remains an open question. Two different physical process have been proposed in the literature, (1) a new-born neutron star is surrounded by a hyper-accreting and neutrino cooling disk, the GRB jet can be powered by neutrino annihilation aligning the spin axis; (2) a differentially rotating millisecond pulsar was formed due to different angular velocity between the interior core and outer shell parts of the neutron star, which can power an episodic GRB jet. In this paper, by analyzing the data of one peculiar GRB 070110 (with internal plateau), we try to test which model being favored. By deriving the physical parameters of magnetar with observational data, the parameter regime for initial period ($P_{0\rm }$) and surface polar cap magnetic field ($B_{\rm p}$) of the central NS are $(0.96\sim 1.2 )~\rm ms$ and $(2.4\sim 3.7)\times 10^{14}~\rm G$, respectively. The radiative efficiency of prompt emission is about $\eta_{\gamma} \sim 6\%$. However, the radiative efficiency of internal plateau ($\eta_{\rm X}$) is larger than $31\%$ assuming the $M_{\rm NS}\sim1.4 M_{\odot}$ and $P_{0\rm }\sim1.2 ~\rm ms$. The clear difference between the radiation efficiencies of prompt emission and internal plateau implies that they maybe originated from different components (e.g. prompt emission from the relativistic jet powered by neutrino annihilation, while the internal plateau from the magnetic outflow wind).

BINARY NEUTRON STAR MERGERS: A JET ENGINE FOR SHORT GAMMA-RAY BURSTS.

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of quasi-circular, equal-mass, binary neutron stars that undergo merger. The initial stars are irrotational, n = 1 polytropes and are magnetized. We explore two types of magnetic-field geometries: one where each star is endowed with a dipole magnetic field extending from the interior into the exterior, as in a pulsar, and the other where the dipole field is initially confined to the interior. In both cases the adopted magnetic fields are initially dynamically unimportant. The merger outcome is a hypermassive neutron star that undergoes delayed collapse to a black hole (spin parameter a/MBH ~ 0.74) immersed in a magnetized accretion disk. About 4000M ~ 60(MNS/1.625M⊙) ms following merger, the region above the black hole poles becomes strongly magnetized, and a collimated, mildly relativistic outflow-an incipient jet-is launched. The lifetime of the accretion disk, which likely equals the lifetime of the jet, is Δ t ~ 0.1 (MNS/1.625M⊙) s. In contrast to black hole-neutron star mergers, we find that incipient jets are launched even when the initial magnetic field is confined to the interior of the stars.

Short report on the ectoparasitic fauna from a wild Epinephelus marginatus and osmotic shock efficiency on their removal

Short report on the ectoparasitic fauna from a wild Epinephelus marginatus and osmotic shock efficiency on their removal

Gravitational-wave cutoff frequencies of tidally disruptive neutron star-black hole binary mergers

Tidal disruption has a dramatic impact on the outcome of neutron star-black hole mergers. The phenomenology of these systems can be divided in three classes: nondisruptive, mildly disruptive or disruptive. The cutoff frequency of the gravitational radiation produced during the merger (which is potentially measurable by interferometric detectors) is very different in each regime, and when the merger is disuptive it carries information on the neutron star equation of state. Here we use semianalytical tools to derive a formula for the critical binary mass ratio $Q=M_{\rm BH}/M_{\rm NS}$ below which mergers are disruptive as a function of the stellar compactness $\mathcal{C}=M_{\rm NS}/R_{\rm NS}$ and the dimensionless black hole spin $\chi$. We then employ a new gravitational waveform amplitude model, calibrated to $134$ general relativistic numerical simulations of binaries with black hole spin (anti-)aligned with the orbital angular momentum, to obtain a fit to the gravitational-wave cutoff frequency in the disruptive regime as a function of $\mathcal{C}$, $Q$ and $\chi$. Our findings are important to build gravitational wave template banks, to determine whether neutron star-black hole mergers can emit electromagnetic radiation (thus helping multimessenger searches), and to improve event rate calculations for these systems.

Compactness of Neutron Stars.

Recent progress in the determination of both masses and radii of neutron stars is starting to place stringent constraints on the dense matter equation of state. In particular, new theoretical developments together with improved statistical tools seem to favor stellar radii that are significantly smaller than those predicted by models using purely nucleonic equations of state. Given that the underlying equation of state must also account for the observation of 2M⊙ neutron stars, theoretical approaches to the study of the dense matter equation of state are facing serious challenges. In response to this challenge, we compute the underlying equation of state associated with an assumed mass-radius template similar to the "common radius" assumption used in recent studies. Once such a mass-radius template is adopted, the equation of state follows directly from the implementation of Lindblom's algorithm; assumptions on the nature or composition of the dense stellar core are not required. By analyzing mass-radius profiles with a maximum mass consistent with observation and common radii in the 8-11 km range, a lower limit on the stellar radius of a 1.4M⊙ neutron star of RNS≳10.7 km is required to prevent the equation of state from violating causality.

ANGULAR MOMENTUM ROLE IN THE HYPERCRITICAL ACCRETION OF BINARY-DRIVEN HYPERNOVAE

The induced gravitational collapse (IGC) paradigm explains a class of energetic, $E_{\rm iso}\gtrsim 10^{52}$~erg, long-duration gamma-ray bursts (GRBs) associated with Ic supernovae, recently named binary-driven hypernovae (BdHNe). The progenitor is a tight binary system formed of a carbon-oxygen (CO) core and a neutron star companion. The supernova ejecta of the exploding CO core triggers a hypercritical accretion process onto the neutron star, which reaches in a few seconds the critical mass, and gravitationally collapses to a black hole emitting a GRB. In our previous simulations of this process we adopted a spherically symmetric approximation to compute the features of the hypercritical accretion process. We here present the first estimates of the angular momentum transported by the supernova ejecta, $L_{\rm acc}$, and perform numerical simulations of the angular momentum transfer to the neutron star during the hyperaccretion process in full general relativity. We show that the neutron star: i) reaches in a few seconds either mass-shedding limit or the secular axisymmetric instability depending on its initial mass; ii) reaches a maximum dimensionless angular momentum value, $[c J/(G M^2)]_{\rm max}\approx 0.7$; iii) can support less angular momentum than the one transported by supernova ejecta, $L_{\rm acc} > J_{\rm NS,max}$, hence there is an angular momentum excess which necessarily leads to jetted emission.

Non-singlet spin structure function $g_{1}^{\rm NS}(x,t)$ in the DGLAP approach

An analytical solution of the non-singlet polarized parton distribution Δq NS(x,Q 2) = (Δu(x,Q 2) − Δd(x,Q 2)) is obtained by solving the DGLAP (Gribov and Lipatov, Sov. J. Nucl. Phys. 15, 438 (1972); Lipatov, ibid, 20, 94 (1975); Dokshitzer, Sov. Phys. JETP 46, 641 (1997); Altarelli and Parisi, Nucl. Phys. B126, 298 (1977)) equation by the method of characteristics. We then evaluate the non-singlet spin-dependent structure function $g_1^{\rm NS}$ . The result is compared with the data from HERMES (Airapetian et al, Phys. Rev. D75, 012007 (2007)).

Rotating strings in AdS4× CP3 with BNS holonomy

We study solutions for rigidly rotating strings on AdS$_4 \times$ CP$^3$ in the presence of $B_{\rm NS}$ holonomy turned on over $CP^1 \subset CP^3$. We construct general solutions for rotating strings with two and three angular momenta in $CP^3$ and discuss various limits corresponding to giant magnon and spike like solutions.

Slowly balding black holes

The "no hair" theorem, a key result in General Relativity, states that an isolated black hole is defined by only three parameters: mass, angular momentum, and electric charge; this asymptotic state is reached on a light-crossing time scale. We find that the "no hair" theorem is not formally applicable for black holes formed from collapse of a rotating neutron star. Rotating neutron stars can self-produce particles via vacuum breakdown forming a highly conducting plasma magnetosphere such that magnetic field lines are effectively "frozen-in" the star both before and during collapse. In the limit of no resistivity, this introduces a topological constraint which prohibits the magnetic field from sliding off the newly-formed event horizon. As a result, during collapse of a neutron star into a black hole, the latter conserves the number of magnetic flux tubes $N_B = e \Phi_\infty /(\pi c \hbar)$, where $\Phi_\infty \approx 2 \pi^2 B_{NS} R_{NS}^3 /(P_{\rm NS} c)$ is the initial magnetic flux through the hemispheres of the progenitor and out to infinity. We test this theoretical result via three-dimensional general relativistic plasma simulations of rotating black holes that start with a neutron star dipole magnetic field with no currents initially present outside the event horizon. The black hole's magnetosphere subsequently relaxes to the split monopole magnetic field geometry with self-generated currents outside the event horizon. The dissipation of the resulting equatorial current sheet leads to a slow loss of the anchored flux tubes, a process that balds the black hole on long resistive time scales rather than the short light-crossing time scales expected from the vacuum "no-hair" theorem.