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Register a new userThe demographics of neutron star - white dwarf mergers: rates, delay-time distributions and progenitors
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The mergers of neutron stars (NSs) and white dwarfs (WDs) could give rise to explosive transients, potentially observable with current and future transient surveys. However, the expected properties and distribution of such events is not well understood. Here we characterize the rates of such events, their delay time distribution, their progenitors and the distribution of their properties. We use binary populations synthesis models and consider a wide range of initial conditions and physical processes. In particular we consider different common-envelope evolution models and different NS natal kick distributions. We provide detailed predictions arising from each of the models considered. We find that the majority of NS-WD mergers are born in systems in which mass-transfer played an important role, and the WD formed before the NS. For the majority of the mergers the WDs have a carbon-oxygen composition (60-80%) and most of the rest are with oxygen-neon WDs. The rates of NS-WD mergers are in the range of 3-15% of the type Ia supernovae (SNe) rate. Their delay time distribution is very similar to that of type Ia SNe, but slightly biased towards earlier times. They typically explode in young 0.1-1Gyr environments, but have a tail distribution extending to long, Gyrs-timescales. Models including significant kicks give rise to relatively wide offset distribution extending to hundreds of kpcs. The demographic and physical properties of NS-WD mergers suggest they are likely to be peculiar type Ic-like SNe, mostly exploding in late type galaxies. Their overall properties could be related to a class of rapidly evolving SNe recently observed, while they are less likely to be related to the class of Ca-rich SNe.
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The gravitational-wave (GW) events, produced by the coalescence of binary neutron-stars (BNS), can be treated as the standard sirens to probe the expansion history of the Universe, if their redshifts could be determined from the electromagnetic observations. For the high-redshift ($zgtrsim 0.1$) events, the short $gamma$-ray bursts (sGRBs) and the afterglows are always considered as the primary electromagnetic counterparts. In this paper, by investigating various models of sGRBs and afterglows, we discuss the rates and distributions of BNS mergers multi-messenger observations with GW detectors in second-generation (2G), 2.5G, 3G era with the detectable sGRBs and the afterglows. For instance, for Cosmic Explorer GW detector, the rate is about (300-3500) per year with GECAM-like detector for $gamma$-ray emissions and LSST/WFST detector for optical afterglows. In addition, we find these events have the redshifts $zlesssim 2$ and the inclination angles $iotalesssim 20^{circ}$. These results justify the rough estimation in previous works. Considering these events as standard sirens to constrain the equation-of-state parameters of dark energy $w_{0}$ and $w_{a}$, we obtain the potential constraints of $Delta w_{0}simeq 0.02-0.05$ and $Delta w_{a}simeq 0.1-0.4$.
Rotating proto-neutron stars can be important sources of gravitational waves to be searched for by present-day and future interferometric detectors. It was demonstrated by Imshennik that in extreme cases the rapid rotation of a collapsing stellar core may lead to fission and formation of a binary proto-neutron star which subsequently merges due to gravitational wave emission. In the present paper, we show that such dynamically unstable collapsing stellar cores may be the product of a former merger process of two stellar cores in a common envelope. We applied population synthesis calculations to assess the expected fraction of such rapidly rotating stellar cores which may lead to fission and formation of a pair of proto-neutron stars. We have used the BSE population synthesis code supplemented with a new treatment of stellar core rotation during the evolution via effective core-envelope coupling, characterized by the coupling time, $tau_c$. The validity of this approach is checked by direct MESA calculations of the evolution of a rotating 15 $M_odot$ star. From comparison of the calculated spin distribution of young neutron stars with the observed one, reported by Popov and Turolla, we infer the value $tau_c simeq 5 times 10^5$ years. We show that merging of stellar cores in common envelopes can lead to collapses with dynamically unstable proto-neutron stars, with their formation rate being $sim 0.1-1%$ of the total core collapses, depending on the common envelope efficiency.
Finite size effects in a neutron star merger are manifested, at leading order, through the tidal deformabilities (Lambdas) of the stars. If strong first-order phase transitions do not exist within neutron stars, both neutron stars are described by the same equation of state, and their Lambdas are highly correlated through their masses even if the equation of state is unknown. If, however, a strong phase transition exists between the central densities of the two stars, so that the more massive star has a phase transition and the least massive star does not, this correlation will be weakened. In all cases, a minimum Lambda for each neutron star mass is imposed by causality, and a less conservative limit is imposed by the unitary gas constraint, both of which we compute. In order to make the best use of gravitational wave data from mergers, it is important to include the correlations relating the Lambdas and the masses as well as lower limits to the Lambdas as a function of mass. Focusing on the case without strong phase transitions, and for mergers where the chirp mass M_chirp<1.4M_sun, which is the case for all observed double neutron star systems where a total mass has been accurately measured, we show that the dimensionless Lambdas satisfy Lambda_1/Lambda_2= q^6, where q=M_2/M_1 is the binary mass ratio; $M$ is mass of each star, respectively. Moreover, they are bounded by q^{n_-}>Lambda_1/Lambda_2> q^{n_{0+}+qn_{1+}}, where n_-<n_{0+}+qn_{1+}; the parameters depend only on M_chirp, which is accurately determined from the gravitational-wave signal. We also provide analytic expressions for the wider bounds that exist in the case of a strong phase transition. We argue that bounded ranges for Lambda_1/Lambda_2, tuned to M_chirp, together with lower bounds to Lambda(M), will be more useful in gravitational waveform modeling than other suggested approaches.
It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star--white dwarf (NSWD) mergers, motivated by FRB 180924-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by cite{mar19}. By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic field via the dynamo mechanism due to differential rotation and convection or possibly via the magnetic flux conservation scenario of a fossil field. If NSWD mergers can indeed create magnetars, then such objects could produce at least a subset of FRB 180924-like FRBs within the framework of flaring magnetars, since the ejecta, local environments, and host galaxies of the final remnants from NSWD mergers resemble those of BNS/BWD/AIC channels. This NSWD channel is also able to well explain both the observational properties of FRB 180924-like and FRB 180916.J0158+65-like FRBs within a large range in local environments and host galaxies.
We report on the results of a 4-year timing campaign of PSR~J2222$-0137$, a 2.44-day binary pulsar with a massive white dwarf (WD) companion, with the Nanc{c}ay, Effelsberg and Lovell radio telescopes. Using the Shapiro delay for this system, we find a pulsar mass $m_{p}=1.76,pm,0.06,M_odot$ and a WD mass $m_{c},=,1.293,pm,0.025, M_odot$. We also measure the rate of advance of periastron for this system, which is marginally consistent with the GR prediction for these masses. The short lifetime of the massive WD progenitor star led to a rapid X-ray binary phase with little ($< , 10^{-2} , M_odot$) mass accretion onto the neutron star (NS); hence, the current pulsar mass is, within uncertainties, its birth mass; the largest measured to date. We discuss the discrepancy with previous mass measurements for this system; we conclude that the measurements presented here are likely to be more accurate. Finally, we highlight the usefulness of this system for testing alternative theories of gravity by tightly constraining the presence of dipolar radiation. This is of particular importance for certain aspects of strong-field gravity, like spontaneous scalarization, since the mass of PSR~J2222$-0137$ puts that system into a poorly tested parameter range.
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