Double Neutron Star Merger Rate Research Articles

Revisiting the Galactic Double Neutron Star merger and LIGO detection rates

ABSTRACT We revisit the merger rate for Galactic double neutron star (DNS) systems in light of recent observational insight into the longitudinal and latitudinal beam shape of the relativistic DNS PSR J1906 + 0746. Due to its young age and its relativistic orbit, the pulsar contributes significantly to the estimate of the joint Galactic merger rate. We follow previous analyses by modelling the underlying pulsar population of nine merging DNS systems and study the impact and resulting uncertainties when replacing simplifying assumptions made in the past with actual knowledge of the beam shape, its extent, and the viewing geometry. We find that the individual contribution of PSR J1906 + 0746 increases to $\mathcal {R} = 6^{+28}_{-5} \, \mathrm{Myr}^{-1}$ although the values are still consistent with previous estimates, given the uncertainties. We also compute contributions to the merger rates from the other DNS systems by applying a generic beam shape derived from that of PSR J1906 + 0746, evaluating the impact of previous assumptions. We derive a joint Galactic DNS merger rate of $\mathcal {R}^{\mathrm{gen}}_{\mathrm{MW}} = 32^{+19}_{-9}\, \mathrm{Myr}^{-1}$, leading to a LIGO detection rate of $\mathcal {R}^{\mathrm{gen}}_{\mathrm{LIGO}} = 3.5^{+2.1}_{-1.0}\, \mathrm{yr}^{-1}$ (90 per cent conf. limit), considering the upcoming O3 sensitivity of LIGO. As these values are in good agreement with previous estimates, we conclude that the method of estimating the DNS merger and LIGO detection rates via the study of the radio pulsar DNS population is less prone to systematic uncertainties than previously thought.

The Role of Strong Gravity and the Nuclear Equation of State on Neutron-star Common-envelope Accretion

Abstract Common-envelope evolution is important in the formation of neutron star binaries within the isolated binary formation channel. As a neutron star inspirals within the envelope of a primary massive star, it accretes and spins up. Because neutron stars are in the strong-gravity regime, they have a substantial relativistic mass deficit, i.e., their gravitational mass is less than their baryonic mass. This effect causes some fraction of the accreted baryonic mass to convert into neutron star binding energy. The relativistic mass deficit also depends on the nuclear equation of state, since more compact neutron stars will have larger binding energies. We model the mass growth and spin-up of neutron stars inspiraling within common-envelope environments and quantify how different initial binary conditions and hadronic equations of state affect the post-common-envelope neutron star’s mass and spin. From these models, we find that neutron star mass growth is suppressed by ≈15%–30%. We also find that for a given amount of accreted baryonic mass, more compact neutron stars will spin-up faster while gaining less gravitational mass, and vice versa. This work demonstrates that a neutron star’s strong gravity and nuclear microphysics plays a role in neutron-star-common-envelope evolution, in addition to the macroscopic astrophysics of the envelope. Strong gravity and the nuclear equation of state may thus affect both the population properties of neutron star binaries and the cosmic double neutron star merger rate.

Imprints of the redshift evolution of double neutron star merger rate on the signal-to-noise ratio distribution

ABSTRACT Proposed third-generation gravitational wave interferometers such as Cosmic Explorer (CE) will have the sensitivity to observe double neutron star mergers (DNS) up to a redshift of ∼5 with good signal-to-noise ratios (SNRs). We argue that the comoving spatial distribution of DNS mergers leaves a unique imprint on the statistical distribution of SNRs of the detected DNS mergers. Hence, the SNR distribution of DNS mergers will facilitate a novel probe of their redshift evolution independent of the luminosity distance measurements. We consider detections of DNS mergers by the CE and study the SNR distribution for different possible redshift evolution models of DNSs and employ Anderson Darling p-value statistic to demonstrate the distinguishability between these different models. We find that a few hundreds of DNS mergers in the CE era will allow us to distinguish between different models of redshift evolution. We further apply the method for various SNR distributions arising due to different merger delay-time and star formation rate (SFR) models and show that for a given SFR model, the SNR distributions are sensitive to the delay-time distributions. Finally, we investigate the effects of sub-threshold events in distinguishing between different merger rate distribution models.

An Updated Galactic Double Neutron Star Merger Rate Based on Radio Pulsar Populations

Following the procedure described in Pol et al., we update the Galactic double neutron star (DNS) merger rate by including the new, highly eccentric DNS system J0509+3801 (Lynch et al.). This leads to a new Galactic DNS merger rate of $\mathcal{R}_{\rm MW} = 37^{+24}_{-11}$ Myr$^{-1}$, where the errors represent 90% confidence intervals. The corresponding DNS merger detection rate for Advanced LIGO is $\mathcal{R} = 1.9^{+1.2}_{-0.6} \times \left( D_{\rm r} / 100 \ \rm Mpc \right)^3 \rm yr^{-1},$ where $D_{\rm r}$ is the range distance. Using the LIGO O3 range distance of 130 Mpc (Abbott et al.), we predict that LIGO will detect anywhere between three and seven DNS mergers per year of observing at O3 sensitivity.

Dependence of gravitational wave transient rates on cosmic star formation and metallicity evolution history

ABSTRACT We compare the impacts of uncertainties in both binary population synthesis models and the cosmic star formation history on the predicted rates of gravitational wave (GW) compact binary merger events. These uncertainties cause the predicted rates of GW events to vary by up to an order of magnitude. Varying the volume-averaged star formation rate density history of the Universe causes the weakest change to our predictions, while varying the metallicity evolution has the strongest effect. Double neutron star merger rates are more sensitive to assumed neutron star kick velocity than the cosmic star formation history. Varying certain parameters affects merger rates in different ways depending on the mass of the merging compact objects; thus some of the degeneracy may be broken by looking at all the event rates rather than restricting ourselves to one class of mergers.

Future Prospects for Ground-based Gravitational-wave Detectors: The Galactic Double Neutron Star Merger Rate Revisited

Abstract We present the Galactic merger rate for double neutron star (DNS) binaries using an observed sample of eight DNS systems merging within a Hubble time. This sample includes the recently discovered, highly relativistic DNS systems J1757−1854 and J1946+2052, and is approximately three times the sample size used in previous estimates of the Galactic merger rate by Kim et al. Using this sample, we calculate the vertical scale height for DNS systems in the Galaxy to be z 0 = 0.4 ± 0.1 kpc. We calculate a Galactic DNS merger rate of Myr−1 at the 90% confidence level. The corresponding DNS merger detection rate for Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) is , where D r is the range distance. Using this merger detection rate and the predicted range distance of 120–170 Mpc for the third observing run of LIGO, we predict, accounting for 90% confidence intervals, that LIGO–Virgo will detect anywhere between zero and two DNS mergers. We explore the effects of the underlying pulsar population properties on the merger rate and compare our merger detection rate with those estimated using different formation and evolutionary scenario of DNS systems. As we demonstrate, reconciling the rates is sensitive to assumptions about the DNS population, including its radio pulsar luminosity function. Future constraints from further gravitational wave DNS detections and pulsar surveys anticipated in the near future should permit tighter constraints on these assumptions.

Neutron star kicks – II. Revision and further testing of the conservation of momentum ‘kick’ model

In Bray and Eldridge (2017), we proposed a simple neutron star kick formula, v kick = alpha (M ejecta / M remnant) + beta to explain the observed 2D velocities of young single neutron stars. Using this kick we found that there is no statistically significant preference for a kick orientation nor for any of the three initial mass function (IMF) slopes tested, and that populations including binary stars reproduced the kick distribution better than single star only populations. However, recent analysis by Janka (2017), prompted us to revisit our basic assumptions and our new analysis has led to revised best-fit kick values of alpha=100 km per second and beta = -170 km per second. The reduction of beta to a negative value is due to using the 2D observed kick velocity distribution rather than the modelled 3D velocity distribution for neutron stars (NS). To further test the validity of the new kick, we have created synthetic populations of runaway star and double neutron star (DNS) binaries at solar metallicity (Z=0.02) using our best-fit kick. We find our new kick values create runaway star velocities and DNS period distributions in agreement with the comparable observational distributions with only the DNS eccentricities in tension with the observations. From our DNS and BH-BH datasets we estimate a predicted DNS merger rate at solar metallicity of 3,864 (+1,570/-2,371) per cubic Gpc per yr and a BH-BH merger rate of 5 (+40/-1) per cubic Gpc per yr.

Progenitors of gravitational wave mergers: binary evolution with the stellar grid-based code ComBinE

The first gravitational wave detections of mergers between black holes and neutron stars represent a remarkable new regime of high-energy transient astrophysics. The signals observed with LIGO-Virgo detectors come from mergers of extreme physical objects which are the end products of stellar evolution in close binary systems. To better understand their origin and merger rates, we have performed binary population syntheses at different metallicities using the new grid-based binary population synthesis code ComBinE. Starting from newborn pairs of stars, we follow their evolution including mass loss, mass transfer and accretion, common envelopes and supernova explosions. We apply the binding energies of common envelopes based on dense grids of detailed stellar structure models, make use of improved investigations of the subsequent Case BB Roche-lobe overflow and scale supernova kicks according to the stripping of the exploding stars. We demonstrate that all the double black hole mergers, GW150914, LVT151012, GW151226, GW170104, GW170608 and GW170814, as well as the double neutron star merger GW170817, are accounted for in our models in the appropriate metallicity regime. Our binary interaction parameters are calibrated to match the accurately determined properties of Galactic double neutron star systems, and we discuss their masses and types of supernova origin. Using our default values for the input physics parameters, we find a double neutron star merger rate of about 3.0 Myr^-1 for Milky-Way equivalent galaxies. Our upper limit to the merger-rate density of double neutron stars is R=400 yr^-1 Gpc^-3 in the local Universe (z=0).

Local merger rates of double neutron stars

AbstractThe first detection of gravitational waves from a merging double neutron star (DNS) binary implies a much higher rate of DNS coalescences in the local Universe than typically estimated on theoretical grounds. The recent study by Chruslinska et al. (2018) shows that apart from being particularly sensitive to the common envelope treatment, DNS merger rates appear rather robust against variations of several factors probed in their study (e.g. conservativeness of the mass transfer, angular momentum loss, and natal kicks), unless extreme assumptions are made. Confrontation with the improving observational limits may allow to rule out some of the extreme models. To correctly compare model predictions with observational limits one has to account for the other factors that affect the rates. One of those factors relates to the assumed history of star formation and chemical evolution of the Universe and its impact on the final results needs to be better constrained.

Double neutron stars: merger rates revisited

We revisit double neutron star (DNS) formation in the classical binary evolution scenario in light of the recent LIGO/Virgo DNS detection (GW170817). The observationally estimated Galactic DNS merger rate of $R_{\rm MW}=21^{+28}_{-14}$ Myr$^{-1}$, based on 3 Galactic DNS systems, fully supports our standard input physics model with $R_{\rm MW} =24$ Myr$^{-1}$. This estimate for the Galaxy translates in a non-trivial way (due to cosmological evolution of progenitor stars in chemically evolving Universe) into a local ($z\approx0$) DNS merger rate density of $R_{\rm local}=48$ Gpc$^{-3}$yr$^{-1}$, which {\em is not} consistent with the current LIGO/Virgo DNS merger rate estimate ($1540^{+3200}_{-1220}$ Gpc$^{-3}$yr$^{-1}$). Within our study of the parameter space we find solutions that allow for DNS merger rates as high as $R_{\rm local} \approx 600^{+600}_{-300}$ Gpc$^{-3}$yr$^{-1}$ which are thus consistent with the LIGO/Virgo estimate. However, our corresponding BH-BH merger rates for the models with high DNS merger rates exceed the current LIGO/Virgo estimate of local BH-BH merger rate ($12$-$213$ Gpc$^{-3}$yr$^{-1}$). Apart from being particularly sensitive to the common envelope treatment, DNS merger rates are rather robust against variations of several of the key factors probed in our study (e.g. mass transfer, angular momentum loss, natal kicks). This might suggest that either common envelope development/survival works differently for DNS ($\sim$ 10-20 Msun stars) than for BH-BH ($\sim$ 40-100 Msun stars) progenitors, or high BH natal kicks are needed to meet observational constraints for both types of binaries. Note that our conclusion is based on a limited number of (21) evolutionary models and is valid only within this particular DNS and BH-BH isolated binary formation scenario.

Implications of Binary Black Hole Detections on the Merger Rates of Double Neutron Stars and Neutron Star–Black Holes

Abstract We show that the inferred merger rate and chirp masses of binary black holes (BBHs) detected by advanced LIGO (aLIGO) can be used to constrain the rate of double neutron star (DNS) and neutron star–black hole (NSBH) mergers in the universe. We explicitly demonstrate this by considering a set of publicly available population synthesis models of Dominik et al. and show that if all the BBH mergers, GW150914, LVT151012, GW151226, and GW170104, observed by aLIGO arise from isolated binary evolution, the predicted DNS merger rate may be constrained to be 2.3–471.0 Gpc−3 yr−1 and that of NSBH mergers will be constrained to 0.2–48.5 Gpc−3 yr−1. The DNS merger rates are not constrained much, but the NSBH rates are tightened by a factor of ∼4 as compared to their previous rates. Note that these constrained DNS and NSBH rates are extremely model-dependent and are compared to the unconstrained values 2.3–472.5 Gpc−3 yr−1 and 0.2–218 Gpc−3 yr−1, respectively, using the same models of Dominik et al. (2012a). These rate estimates may have implications for short Gamma Ray Burst progenitor models assuming they are powered (solely) by DNS or NSBH mergers. While these results are based on a set of open access population synthesis models, which may not necessarily be the representative ones, the proposed method is very general and can be applied to any number of models, thereby yielding more realistic constraints on the DNS and NSBH merger rates from the inferred BBH merger rate and chirp mass.

Light-curve and spectral properties of ultrastripped core-collapse supernovae leading to binary neutron stars

We investigate light-curve and spectral properties of ultra-stripped core-collapse supernovae. Ultra-stripped supernovae are the explosions of heavily stripped massive stars which lost their envelopes via binary interactions with a compact companion star. They eject only ~ 0.1 Msun and may be the main way to form double neutron-star systems which eventually merge emitting strong gravitational waves. We follow the evolution of an ultra-stripped supernova progenitor until iron core collapse and perform explosive nucleosynthesis calculations. We then synthesize light curves and spectra of ultra-stripped supernovae using the nucleosynthesis results and present their expected properties. Ultra-stripped supernovae synthesize ~ 0.01 Msun of radioactive 56Ni, and their typical peak luminosity is around 1e42 erg/s or -16 mag. Their typical rise time is 5 - 10 days. Comparing synthesized and observed spectra, we find that SN 2005ek, some of the so-called calcium-rich gap transients, and SN 2010X may be related to ultra-stripped supernovae. If these supernovae are actually ultra-stripped supernovae, their event rate is expected to be about 1 per cent of core-collapse supernovae. Comparing the double neutron-star merger rate obtained by future gravitational-wave observations and the ultra-stripped supernova rate obtained by optical transient surveys identified with our synthesized light-curve and spectral models, we will be able to judge whether ultra-stripped supernovae are actually a major contributor to the binary neutron star population and provide constraints on binary stellar evolution.

The effect of PSR J0737-3039 on the DNS merger rate and implications for gravity-wave detection

We present the current estimates of the Galactic merger rate of double-neutron-star (DNS) systems. Using a statistical analysis method, we calculate the probability distribution function (PDF) of the rate estimates, which allows us to assign confidence intervals to the rate estimates. We calculate the Galactic DNS merger rate based on the three known systems B1913+16, B1534+12, and J0737-3039. The discovery of J0737-3039 increases the estimated DNS merger rate by a factor ∼6 than what is previously known. The most likely values of DNS merger rate lie in the range 3–190 Myr −1 depending on different pulsar models. Motivated by a strong correlation between the peak rate estimates and a pulsar luminosity function, we calculate a global probability distribution as a single representation of the parameter space covered by different pulsar population models. We compare the global PDF with the observed supernova Ib/c rate, which sets an upper limit on the DNS merger rate. Finally, we remark on implications of new discoveries such as of J1756-2251, the 4th DNS in the Galactic disk, and J1906+0746, a possible DNS system.

Observation time to first detection of double neutron star mergers by gravitational wave observatories

Abstract We constrain the uncertainty in waiting times for detecting the first double neutron star (DNS) mergers by gravitational wave observatories. By accounting for the Poisson fluctuations in the rate density of DNS mergers and galaxy space density inhomogeneity in the local Universe, we define a detection ‘zone’ as a region in a parameter space constrained by the DNS merger rate and two Laser Interferometer Gravitational-wave Observatory (LIGO) operation parameters: an observation horizon distance and a science run duration. Assuming a mean rate of about 80 DNS mergers per Milky Way galaxy Myr−1, we find a 1/20 chance of observing a merger by an enhanced LIGO in only 1 yr of observation. The minimum waiting time and temporal zone width for an advanced LIGO sensitivity are much shorter and imply that there is a 95 per cent probability of detecting a DNS merger in less than 60 d and a 1/20 chance of a first detection in about 1 d. At the 5 per cent probability threshold for a first detection, we find that the effect of galaxy clusters on detection is smoothed out and may only influence detection rates after 5–10 yr observation time.

The gravitational wave 'probability event horizon' for double neutron star mergers

Gravitational waves generated by the final merger of double neutron star (DNS) binary systems are a key target for the gravitational wave (GW) interferometric detectors, such as LIGO (Laser Interferometer Gravitational-wave Observatory), and the next generation detectors, Advanced LIGO. The cumulative GW signal from DNS mergers in interferometric data will manifest as ‘geometrical noise’: a non-continuous stochastic background with a unique statistical signature dominated by the spatial and temporal distribution of the sources. Because geometrical noise is highly non-Gaussian, it could potentially be used to identify the presence of a stochastic GW background from DNS mergers. We demonstrate this by fitting to a simulated distribution of transients using a model for the DNS merger rate and idealized Gaussian detector noise. Using the cosmological ‘probability event horizon’ concept and recent bounds for the Galactic DNS merger rate, we calculate the evolution of the detectability of DNS mergers with observation time. For Advanced LIGO sensitivities and a detection threshold assuming optimal filtering, there is a 95 per cent probability that a minimum of one DNS merger signal will be detectable from the ensemble of events comprising the stochastic background during 12–211 d of observation. For initial LIGO sensitivities, we identify an interesting regime where there is a 95 per cent probability that at least one DNS merger with signal-to-noise ratio greater than unity will occur during 4–68 d of observation. We propose that there exists an intermediate detection regime with pre-filtered signal-to-noise ratio less than unity, where the DNS merger rate is high enough that the geometrical signature could be identified in interferometer data.