Galactic 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.

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.

SYSTEMATICS OF DYNAMICAL MASS EJECTION, NUCLEOSYNTHESIS, AND RADIOACTIVELY POWERED ELECTROMAGNETIC SIGNALS FROM NEUTRON-STAR MERGERS

We investigate systematically the dynamical mass ejection, r-process nucleosynthesis, and properties of electromagnetic counterparts of neutron-star (NS) mergers in dependence on the uncertain properties of the nuclear equation of state (EoS) by employing 40 representative, microphysical high-density EoSs in relativistic, hydrodynamical simulations. The crucial parameter determining the ejecta mass is the radius R_1.35 of a 1.35 M_sun NS. NSs with smaller R_1.35 ("soft" EoS) eject systematically higher masses. These range from ~10^-3 M_sun to ~10^-2 M_sun for 1.35-1.35 M_sun binaries and from ~5*10^-3 M_sun to ~2*10^-2 M_sun for 1.2-1.5 M_sun systems (with kinetic energies between ~5*10^49 erg and 10^51 erg). Correspondingly, the bolometric peak luminosities of the optical transients of symmetric (asymmetric) mergers vary between 3*10^41 erg/s and 14*10^41 erg/s (9*10^41 erg/s and 14.5*10^41 erg/s) on timescales between ~2 h and ~12 h. If these signals with absolute bolometric magnitudes from -15.0 to -16.7 are measured, the tight correlation of their properties with those of the merging NSs might provide valuable constraints on the high-density EoS. The r-process nucleosynthesis exhibits a remarkable robustness independent of the EoS, producing a nearly solar abundance pattern above mass number 130. By the r-process content of the Galaxy and the average production per event the Galactic merger rate is limited to 4*10^-5/yr (4*10^-4/yr) for a soft (stiff) NS EoS, if NS mergers are the main source of heavy r-nuclei. The production ratio of radioactive 232Th to 238U attains a stable value of 1.64-1.67, which does not exclude NS mergers as potential sources of heavy r-material in the most metal-poor stars.

DOUBLE COMPACT OBJECTS. I. THE SIGNIFICANCE OF THE COMMON ENVELOPE ON MERGER RATES

The last decade of observational and theoretical developments in stellar and binary evolution provides an opportunity to incorporate major improvements to the predictions from populations synthesis models. We compute the Galactic merger rates for NS-NS, BH-NS, and BH-BH mergers with the StarTrack code. The most important revisions include: updated wind mass loss rates (allowing for stellar mass black holes up to $80 \msun$), a realistic treatment of the common envelope phase (a process that can affect merger rates by 2--3 orders of magnitude), and a qualitatively new neutron star/black hole mass distribution (consistent with the observed "mass gap"). Our findings include: (i) The binding energy of the envelope plays a pivotal role in determining whether a binary merges within a Hubble time. (ii) Our description of natal kicks from supernovae plays an important role, especially for the formation of BH-BH systems. (iii) The masses of BH-BH systems can be substantially increased in the case of low metallicities or weak winds. (iv) Certain combinations of parameters underpredict the Galactic NS-NS merger rate, and can be ruled out. {\em (v)} Models incorporating delayed supernovae do not agree with the observed NS/BH "mass gap", in accordance with our previous work. This is the first in a series of three papers. The second paper will study the merger rates of double compact objects as a function of redshift, star formation rate, and metallicity. In the third paper we will present the detection rates for gravitational wave observatories, using up-to-date signal waveforms and sensitivity curves.

Double white dwarfs and LISA

Close pairs of white dwarfs are potential progenitors of type Ia supernovae and they are common, with the order of 100–300 million in the Galaxy. As such they will be significant, probably dominant, sources of the gravitational waves detectable by LISA. In the context of LISA's goals for fundamental physics, double white dwarfs are a source of noise, but from an astrophysical perspective, they are of considerable interest in their own right. In this paper I discuss our current knowledge of double white dwarfs and their close relatives (and possible descendants) the AM CVn stars. LISA will add to our knowledge of these systems by providing the following unique constraints: (i) an almost direct measurement of the galactic merger rate of DWDs from the detection of short period systems and their period evolution, (ii) an accurate and precise normalization of binary evolution models at shortest periods, (iii) a determination of the evolutionary pathways to the formation of AM CVn stars, (iv) measurements of the influence of tidal coupling in white dwarfs and its significance for stabilizing mass transfer, and (v) discovery of numerous examples of eclipsing white dwarfs with the potential for optical follow-up to test models of white dwarfs.

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.

Pulsar Binaries as Gravitational-Wave Sources: Rate predictions

Pulsar binaries are important targets for ground-based and future space-borne gravitational-wave (GW) detectors. In order for improving detector design and assessing detector performances, it is a prerequisite to understand the astrophysics of GW sources such as the population size or merger rates. Here, we summarize recent results for Galactic merger rates of two known types of pulsar binaries: (a) double-neutron star-system (DNS) and (b) neutron star-white dwarf (NS-WD) binaries. Based on the merger rate estimates, we discuss prospects for the ground-based interferometers considering LIGO (Laser Interferometer Gravitational-wave Observatory) and space mission LISA (Laser Interferometer Space Antenna).

Observations of the high-redshift galaxy B2 0902+343 at 92 cm

We present 92 cm observations of the high-redshift galaxy B2 0902+343 (z = 3.40) with the Westerbork Synthesis Radio Telescope (WSRT). Twenty-one cm H 1 is detected in absorption, at a depth of 15.9 mJy (=6σ) and rest frame velocity width 120 km s - 1 . We also report a null detection of OH emission or absorption at the 1665/1667 MHz rest-frame transitions. Based on our spectral sensitivity, we derive an upper limit on OH maser luminosity of log(L/L O .) < 5.2. In addition, we consider the possibility of absorption, and estimate a maximum column depth N(OH) of 1 x 10 1 5 cm - 2 , corresponding to at most 2 × 10 5 - 9 × 10 6 M O . of OH in the galaxy. The implications of these results are discussed in the context of current efforts to extend the OH megamaser luminosity function and galactic merger rate to high redshift.

Galaxy pairs in redshift catalogs as a probe of dark matter

A combined northern and southern galaxy redshift catalog is analyzed. Examining galaxy pairs in terms of two observables: the line-of-sight velocity difference delta(v) and the projected spatial separation. (1) In low-density regions there is an excess of pairs with radial velocity differences of less than 150 km/s, even for pairs with projected separations r(p) as large as 1 Mpc; thus, any initial correlations would have been disrupted before now, and these pairs must be bound. (2) No Kepler relationship is seen between the pair properties, even for pairs that are isolated. (3) The distribution of r(p) for low-density pairs with delta(v) less than 150 km/s is flat and featureless from very small separations out to 2 Mpc. These three facts indicate that large amounts of dark matter surround a typical galaxy. The third point suggests a model where pairs at small r(p) merge rapidly by dynamical friction but are replenished by pairs that begin at larger separation and are heading toward merger, and it allows an estimate of the galactic merger rate. 34 refs.

Mergers and bias in a cold dark matter cosmology

A dissipative N-body code is used to follow the evolution of a cold dark matter spectrum in an Omega = 1, h = 0.5 background. The star and galaxy formation in boxes of three different sizes is shown, and the relative degrees of clustering of dark matter, halos, and galaxies is discussed and used to normalize to the current epoch. The streaming velocities of the galaxies and dark matter are measured and compared with linear theory. The pairwise velocities of the galaxies and dark matter are studied and used with estimates of galactic merger rates to discuss the role that mergers play in galaxy formation. The number density-redshift relation of galaxies is shown to be well described by the statistical description of density perturbation peaks by Bardeen et al. (1986), whereas halos are better fitted by the volume-averaging, hierarchical model of Press and Schechter (1974). The model galaxies exhibit only a modest enhancement of their two-point correlation function over that of the dark matter. 44 refs.