Gravitational waves × HI intensity mapping: cosmological and astrophysical applications

Two of the most rapidly growing observables in cosmology and astrophysics are gravitational waves (GW) and the neutral hydrogen (HI) distribution. In this work, we investigate the cross-correlation between resolved gravitational wave detections and HI signal from intensity mapping (IM) experiments. By using a tomographic approach with angular power spectra, including all projection effects, we explore possible applications of the combination of the Einstein Telescope and the SKAO intensity mapping surveys. We focus on three main topics: (i) statistical inference of the observed redshift distribution of GWs; (ii) constraints on dynamical dark energy models as an example of cosmological studies; (iii) determination of the nature of the progenitors of merging binary black holes, distinguishing between primordial and astrophysical origin. Our results show that: (i) the GW redshift distribution can be calibrated with good accuracy at low redshifts, without any assumptions on cosmology or astrophysics, potentially providing a way to probe astrophysical and cosmological models; (ii) the constrains on the dynamical dark energy parameters are competitive with IM-only experiments, in a complementary way and potentially with less systematics; (iii) it will be possible to detect a relatively small abundance of primordial black holes within the gravitational waves from resolved mergers. Our results extend towards GW × IM the promising field of multi-tracing cosmology and astrophysics, which has the major advantage of allowing scientific investigations in ways that would not be possible by looking at single observables separately.

A higher-multipole gravitational waveform model for an eccentric binary black holes based on the effective-one-body-numerical-relativity formalism

We have previously constructed a waveform model, SEOBNRE, for spinning binary black hole moving along eccentric orbit based on effective-one-body (EOB) formalism. In the current paper, we update SEOBNRE waveform model in the following three respects. Firstly, we update the EOB dynamics from SEOBNRv1 to SEOBNRv4. Secondly we properly treat the Schott term which has been ignored in previous SEOBNRE. Thirdly, we construct a new factorized waveform including (l,|m|)=(2,2),(2,1),(3,3),(4,4) modes based on effective-one-body (EOB) formalism, which is valid for spinning binary black holes (BBH) in general equatorial orbit. Following our previous SEOBNRE waveform model, we call our new waveform model SEOBNREHM. The (l,|m|)=(2,2) mode waveform of SEOBNREHM can fit the original SEOBNRv4 waveform very well in the case of a quasi-circular orbit. We have validated SEOBNREHM waveform model through comparing the waveform against the Simulating eXtreme Spacetimes (SXS) catalog. The comparison is done for BBH with total mass in (20,200)M_sun using Advanced LIGO designed sensitivity. For the quasi-circular cases we have compared our (2,2) mode waveforms to the 281 numerical relativity (NR) simulations of BBH along quasi-circular orbits. All of the matching factors are bigger than 98\%. For the elliptical cases, 24 numerical relativity simulations of BBH along an elliptic orbit are used. For each elliptical BBH system, we compare our modeled gravitational polarizations against the NR results for different combinations of the inclination angle, the initial orbit phase and the source localization in the sky. We use the minimal matching factor respect to the inclination angle, the initial orbit phase and the source localization to quantify the performance of the higher modes waveform. We found that after introducing the higher modes, the minimum of the minimal matching factor among the 24 tested elliptical BBHs increases from 90\% to 98\%. Our SEOBNREHM waveform model can match all tested 305 SXS waveforms better than 98\% including highly spinning ($\chi=0.99$) BBH, highly eccentric ($e\approx0.6$ at reference frequency $Mf_0=0.002$) BBH and large mass ratio ($q=10$) BBH.

The Eccentric and Accelerating Stellar Binary Black Hole Mergers in Galactic Nuclei: Observing in Ground and Space Gravitational-wave Observatories

We study the stellar binary black holes (BBHs) inspiraling/merging in galactic nuclei based on our numerical method GNC. We find that 3%–40% of all newborn BBHs will finally merge due to various dynamical effects. In a five-year mission, up to 104, 105, and ∼100 of BBHs inspiraling/merging in galactic nuclei can be detected with signal-to-noise ration >8 in Advanced LIGO (aLIGO), Einstein/DECIGO, and TianQin/LISA/TaiJi, respectively. Roughly tens are detectable in both LISA/TaiJi/TianQin and aLIGO. These BBHs have two unique characteristics. (1) Significant eccentricities: 1%–3%, 2%–7%, or 30%–90% of them have e i > 0.1 when they enter into aLIGO, Einstein, or space observatories, respectively. Such high eccentricities provide a possible explanation for that of GW190521. Most highly eccentric BBHs are not detectable in LISA/Tianqin/TaiJi before entering into aLIGO/Einstein, as their strain becomes significant only at f GW ≳ 0.1 Hz. DECIGO becomes an ideal observatory to detect those events, as it can fully cover the rising phase. (2) Up to 2% of BBHs can inspiral/merge at distances ≲103 r SW from the massive black hole, with significant accelerations, such that the Doppler phase drift of ∼10–105 of them can be detected with signal-to-noise ratio >8 in space observatories. The energy density of the gravitational-wave backgrounds (GWBs) contributed by these BBHs deviates from the power-law slope of 2/3 at f GW ≲ 1 mHz. The high eccentricity, significant accelerations, and the different profile of the GWB of these sources make them distinguishable, and thus interesting for future gravitational-wave detections and tests of relativities.

On detecting stellar binary black holes via the LISA-Taiji network

The detection of gravitational waves (GWs) by ground-based laser interferometer GW observatories (LIGO/Virgo) reveals a population of stellar binary black holes (sBBHs) with (total) masses up to ∼ 150M ⊙, which are potential sources for space-based GW detectors, such as LISA and Taiji. In this paper, we investigate in details on the possibility of detecting sBBHs by the LISA-Taiji network in future. We adopt the sBBH merger rate density constrained by LIGO/VIRGO observations to randomly generate mock sBBHs samples. Assuming an observation period of 4 years, we find that the LISA-Taiji network may detect several tens (or at least several) sBBHs with signal-to-noise ratio (SNR) > 8 (or > 15), a factor 2 − 3 times larger than that by only using LISA or Taiji observations. Among these sBBHs, no more than a few that can merge during the 4-year observation period. If extending the observation period to 10 years, then the LISA-Taiji network may detect about one hundred (or twenty) sBBHs with SNR> 8 (or > 15), among them about twenty (or at least several) can merge within the observation period. Our results suggest that the LISA-Taiji network may be able to detect at least a handful to twenty or more sBBHs even if assuming a conservative SNR threshold (15) for “detection”, which enables multi-band GW observations by space and ground-based GW detectors. We also further estimate the uncertainties in the parameter estimations of the sBBH systems “detected” by the LISA-Taiji network. We find that the relative errors in the luminosity distance measurements and sky localization are mostly in the range of 0.05 − 0.2 and 1 − 100deg2, respectively, for these sBBHs.

Mass-ratio and Magnetic Flux Dependence of Modulated Accretion from Circumbinary Disks

Accreting supermassive binary black holes (SMBBHs) are potential multimessenger sources because they emit both gravitational-wave and electromagnetic (EM) radiation. Past work has shown that their EM output may be periodically modulated by an asymmetric density distribution in the circumbinary disk, often called an “overdensity” or “lump;” this modulation could possibly be used to identify a source as a binary. We explore the sensitivity of the overdensity to SMBBH mass ratio and magnetic flux through the accretion disk. We find that the relative amplitude of the overdensity and its associated EM periodic signal both degrade with diminishing mass ratio, vanishing altogether somewhere between 1:2 and 1:5. Greater magnetization also weakens the lump and any modulation of the light output. We develop a model to describe how lump formation results from internal stress degrading faster in the lump region than it can be rejuvenated through accretion inflow, and predicts a threshold value in specific internal stress below which lump formation should occur and which all our lump-forming simulations satisfy. Thus, detection of such a modulation would provide a constraint on both mass ratio and magnetic flux piercing the accretion flow.

Binary Black Hole Formation with Detailed Modeling: Stable Mass Transfer Leads to Lower Merger Rates

Rapid binary population synthesis codes are often used to investigate the evolution of compact-object binaries. They typically rely on analytical fits of single-star evolutionary tracks and parameterized models for interactive phases of evolution (e.g., mass transfer on a thermal timescale, determination of dynamical instability, and common envelope) that are crucial to predict the fate of binaries. These processes can be more carefully implemented in stellar structure and evolution codes such as MESA. To assess the impact of such improvements, we compare binary black hole mergers as predicted in models with the rapid binary population synthesis code COSMIC to models ran with MESA simulations through mass transfer and common-envelope treatment. We find that results significantly differ in terms of formation paths, the orbital periods and mass ratios of merging binary black holes, and consequently merger rates. While common-envelope evolution is the dominant formation channel in COSMIC, stable mass transfer dominates in our MESA models. Depending upon the black hole donor mass, and mass-transfer and common-envelope physics, at subsolar metallicity, COSMIC overproduces the number of binary black hole mergers by factors of 2–35 with a significant fraction of them having merger times orders of magnitude shorter than the binary black holes formed when using detailed MESA models. Therefore we find that some binary black hole merger rate predictions from rapid population syntheses of isolated binaries may be overestimated by factors of ∼ 5–500. We conclude that the interpretation of gravitational-wave observations requires the use of detailed treatment of these interactive binary phases.

On Dark Gravitational Wave Standard Sirens as Cosmological Inference and Forecasting the Constraint on Hubble Constant using Binary Black Holes Detected by Deci-Hertz Observator

Gravitational wave (GW) signals from compact binary coalescences can be used as standard sirens to constrain cosmological parameters if its redshift can be measured independently by electromagnetic signals. However, mergers of stellar binary black holes (BBHs) may not have electromagnetic counterparts and thus have no direct redshift measurements. These dark sirens may be still used to statistically constrain cosmological parameters by combining their GW measured luminosity distances and localization with deep redshift surveys of galaxies around it. We investigate this dark siren method to constrain cosmological parameters in details by using mock BBH and galaxy samples. We find that the Hubble constant can be well constrained with an accuracy $\lesssim 1\%$ with a few tens or more BBH mergers at redshift up to $1$ if GW observations can provide accurate estimates of its luminosity distance (with relative error of $\lesssim 0.01$) and localization ($\lesssim 0.1\mathrm{deg}^2$), though the constraint may be significantly biased if the luminosity distance and localization errors are larger. We further generate mock BBH samples, according to current constraints on BBH merger rate and the distributions of BBH properties, and find that Deci-Hertz Observatory (DO) in a half year observation period may detect about one hundred BBHs with signal-to-noise ratio $\varrho \gtrsim 30$, relative luminosity distance error $\lesssim 0.02$, and localization error $\lesssim 0.01\mathrm{deg}^2$. By applying the dark standard siren method, we find that the Hubble constant can be constrained to $\sim 0.1-1\%$ level using these DO BBHs, an accuracy comparable to the constraints obtained by using electromagnetic observations in the near future, thus it may provide insight into the Hubble tension. We also demonstrate that the constraint on the Hubble constant using this dark siren method are robust and do not depend on the choice of the prior for the properties of BBH host galaxies.