arXiv subjects

R. O'Shaughnessy

Publications and source records attributed to R. O'Shaughnessy.

At least 37 records · Page 2Linked to original sources

arXiv2021

LIGO--Virgo correlations between mass ratio and effective inspiral spin: testing the active galactic nuclei channel

Observations by LIGO--Virgo of binary black hole mergers suggest a possible anti-correlation between black hole mass ratio ($q=m_{2}/m_{1}$) and the effective inspiral spin parameter $χ_{\rm eff}$, the mass-weighted spin projection onto the binary orbital angular momentum (Callister et al. 2021). We show that such an anti-correlation can naturally occur for binary black holes assembled in active galactic nuclei (AGN) due to spherical and planar symmetry-breaking effects. We describe a phenomenological model in which: 1) heavier black holes live in the AGN disk and tend to spin up into alignment with the disk; 2) lighter black holes with random spin orientations live in the nuclear spheroid; 3) the AGN disk is dense enough to rapidly capture a fraction of the spheroid component. but small in radial extent to limit the number of bulk disk mergers; 4) migration within the disk is non-uniform, likely disrupted by feedback from migrators or disk turbulence; 5) dynamical encounters in the disk are common and preferentially disrupt binaries that are retrograde around their center of mass, particularly at stalling orbits, or traps. This model may explain trends in LIGO--Virgo data while offering falsifiable predictions. Comparisons of predictions in ($q,χ_{\rm eff}$) parameter space for the different channels may allow us to distinguish their fractional contributions to the observed merger rates.

astro-ph.HE↗

arXiv2021

A Broad Grid of 2D Kilonova Emission Models

Depending upon the properties of their compact remnants and the physics included in the models, simulations of neutron star mergers can produce a broad range of ejecta properties. The characteristics of this ejecta, in turn, define the kilonova emission. To explore the effect of ejecta properties, we present a grid of 2-component 2D axisymmetric kilonova simulations that vary mass, velocity, morphology, and composition. The masses and velocities of each component vary, respectively, from 0.001 to 0.1 M$_{\odot}$ and 0.05 to 0.3$c$, covering much of the range of results from the neutron star merger literature. The set of 900 models is constrained to have a toroidal low electron fraction ($Y_e$) ejecta with a robust r-process composition and either a spherical or lobed high-$Y_e$ ejecta with two possible compositions. We simulate these models with the Monte Carlo radiative transfer code SuperNu using a full suite of lanthanide and 4th row element opacities. We examine the trends of these models with parameter variation, show how it can be used with statistical tools, and compare the model light curves and spectra to those of AT2017gfo, the electromagnetic counterpart of GW170817.

astro-ph.HE↗

arXiv2020

Assessing and marginalizing over compact binary coalescence waveform systematics with RIFT

As Einstein's equations for binary compact object inspiral have only been approximately or intermittently solved by analytic or numerical methods, the models used to infer parameters of gravitational wave (GW) sources are subject to waveform modeling uncertainty. Using a simple scenario, we illustrate these differences, then introduce a very efficient technique to marginalize over waveform uncertainties, relative to a pre-specified sequence of waveform models. Being based on RIFT, a very efficient parameter inference engine, our technique can directly account for any available models, including very accurate but computationally costly waveforms. Our evidence and likelihood-based method works robustly on a point-by-point basis, enabling accurate marginalization for models with strongly disjoint posteriors while simultaneously increasing the reusability and efficiency of our intermediate calculations.

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arXiv2020

Hubble Constant Measurement with GW190521 as an Eccentric Black Hole Merger

Gravitational wave observations can be used to accurately measure the Hubble constant $H_0$ and could help understand the present discrepancy between constraints from Type Ia supernovae and the cosmic microwave background. Neutron star mergers are primarily used for this purpose as their electromagnetic emission can be used to greatly reduce measurement uncertainties. Here we estimate $H_0$ using the recently observed black hole merger GW190521 and its candidate electromagnetic counterpart found by ZTF using a highly eccentric explanation of the properties of GW190521. We find that the reconstructed distance of GW190521 and the redshift of the candidate host galaxy are more consistent with standard cosmology for our eccentric model than if we reconstruct the source parameters assuming no eccentricity. We obtain $H_0=88.6^{+17.1}_{-34.3}$\,km\,s$^{-1}$Mpc$^{-1}$ for GW190521, and $H_0=73.4^{+6.9}_{-10.7}$\,km\,s$^{-1}$Mpc$^{-1}$ in combination with the results of the neutron star merger GW170817. Our results indicate that future $H_0$ computations using black hole mergers will need to account for possible eccentricity. For extreme cases, the orbital velocity of binaries in AGN disks can represent a significant systematic uncertainty.

astro-ph.HE↗

arXiv2020

Black hole, neutron star and white dwarf merger rates in AGN disks

Advanced LIGO \& Advanced Virgo are detecting a large number of binary stellar origin black hole (BH) mergers. A promising channel for accelerated BH merger lies in active galactic nucleus (AGN) disks of gas around super-masssive black holes. Here we investigate the relative number of compact object mergers in AGN disk models, including BH, neutron stars (NS) and white dwarfs, via Monte Carlo simulations. We find the number of all merger types in the bulk disk grows $\propto t^{1/3}$ which is driven by the Hill sphere of the more massive merger component. Median mass ratios of NS-BH mergers in AGN disks are $\tilde{q}=0.07\pm 0.06(0.14\pm 0.07)$ for mass functions (MF) $M^{-1(-2)}$. If a fraction $f_{AGN}$ of the observed rate of BH-BH mergers (${\cal}{R}_{BH-BH}$) come from AGN, we expect a rate of NS-BH (NS-NS) mergers from the AGN channel of ${\cal}{R}_{BH-NS} \sim f_{AGN}[10,300]\rm{Gpc}^{-3} \rm{yr}^{-1}$,(${\cal}{R}_{NS-NS} \leq f_{AGN}400\rm{Gpc}^{-3} \rm{yr}^{-1}$). Allowing for the ratio of NS-NS/BH-BH LIGO search volumes, from preliminary O3 results we can rule out the AGN channel as a dominant contribution to observed NS-NS mergers. The number of lower mass gap events in this channel is a strong function of the nuclear MF and mass segregation efficiency. Compact object merger ratios derived from LIGO can restrict models of MF, mass segregation and populations embedded in AGN disks. The expected number of EM counterparts to NS-BH mergers in AGN disks at $z<1$ is $\sim [30,900]{\rm{yr}}^{-1}(f_{AGN}/0.1)$. EM searches for flaring due to embedded events in large AGN surveys will complement LIGO constraints on AGN models and the embedded populations that must live in them.

astro-ph.HE↗

arXiv2020

Monte-Carlo simulations of black hole mergers in AGN disks: Low $χ_{\rm eff}$ mergers and predictions for LIGO

Accretion disks around supermassive black holes are promising sites for stellar mass black hole mergers detectable with LIGO. Here we present the results of Monte-Carlo simulations of black hole mergers within 1-d AGN disk models. For the spin distribution in the disk bulk, key findings are: (1) The distribution of $χ_{\rm eff}$ is naturally centered around $\tildeχ_{\rm eff} \approx 0.0$, (2) the width of the $χ_{\rm eff}$ distribution is narrow for low natal spins. For the mass distribution in the disk bulk, key findings are: (3) mass ratios $\tilde{q} \sim 0.5-0.7$, (4) the maximum merger mass in the bulk is $\sim 100-200M_{\odot}$, (5) $\sim 1\%$ of bulk mergers involve BH $>50M_{\odot}$ with (6) $\simeq 80\%$ of bulk mergers are pairs of 1st generation BH. Additionally, mergers at a migration trap grow an IMBH with typical merger mass ratios $\tilde{q}\sim 0.1$. Ongoing LIGO non-detections of black holes $>10^{2}M_{\odot}$ puts strong limits on the presence of migration traps in AGN disks (and therefore AGN disk density and structure) as well as median AGN disk lifetime. The highest merger rate occurs for this channel if AGN disks are relatively short-lived ($\leq 1$Myr) so multiple AGN episodes can happen per Galactic nucleus in a Hubble time.

astro-ph.HE↗

arXiv2020

The evolutionary roads leading to low effective spins, high black hole masses, and O1/O2 rates of LIGO/Virgo binary black holes

All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported from the O1/O2 runs have near zero effective spins. There are only three potential explanations of this fact. If the BH spin magnitudes are large then (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Or, (iii) the BH spin magnitudes are small. We test the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, include revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can match simultaneously the observed BH-BH merger rate density, BH masses, and effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be a key in better reproducing the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely overestimated if the merger GW170729 hosts a BH more massive than 50 Msun. We also estimate rate of BH-NS mergers from recent LIGO/Virgo observations. Our updated models of BH-BH, BH-NS and NS-NS mergers are now publicly available at www.syntheticuniverse.org.

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arXiv2020

The Formation of a 70 Msun Black Hole at High Metallicity

A 70Msun BH was discovered in Milky Way disk in a long period and almost circular detached binary system (LB-1) with a high metallicity 8Msun B star companion. Current consensus on the formation of BHs from high metallicity stars limits the black hole mass to be below 20Msun. Using simple evolutionary model, we show that the formation of a 70Msun BH in high metallicity environment is possible if stellar wind mass loss rates are reduced by factor of 5. As observations indicate, a fraction of massive stars have surface magnetic fields which may quench the wind mass-loss, independently of stellar mass and metallicity. We also computed detailed stellar evolution models and we confirm such a scenario. A non-rotating 85Msun model at Z=0.014 with decreased winds ends up as a 71Msun star prior core-collapse with a 32Msun helium core and a 28Msun CO core. Such star avoids pair-instability pulsation supernova mass loss and may form a 70Msun BH in the direct collapse. Stars that can form such BHs expand to significant size with radius of R>600Rsun, exceeding the size of LB-1 orbit. Therefore, we can explain the formation of BHs upto 70Msun at high metallicity and this result is independent from LB-1. However, if LB-1 hosts a massive BH we are unable to explain how such a binary star system could have formed without invoking some exotic scenarios.

astro-ph.HE↗

arXiv2019

Accelerating parameter inference with graphics processing units

Gravitational wave Bayesian parameter inference involves repeated comparisons of GW data to generic candidate predictions. Even with algorithmically efficient methods like RIFT or reduced-order quadrature, the time needed to perform these calculations and overall computational cost can be significant compared to the minutes to hours needed to achieve the goals of low-latency multimessenger astronomy. By translating some elements of the RIFT algorithm to operate on graphics processing units (GPU), we demonstrate substantial performance improvements, enabling dramatically reduced overall cost and latency.

astro-ph.IM↗

arXiv2019

Gravity and Light: Combining Gravitational Wave and Electromagnetic Observations in the 2020s

As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly something completely unexpected. As we build this sample, we will explore exotic astrophysical topics ranging from nucleosynthesis, stellar evolution, general relativity, high-energy astrophysics, nuclear matter, to cosmology. The discovery potential is extraordinary, and investments in this area will yield major scientific breakthroughs. Here we outline some of the most exciting scientific questions that can be answered by combining GW and EM observations.

astro-ph.HE↗

arXiv2018

Binary neutron star formation and the origin of GW170817

The first neutron star-neutron star merger was detected in by LIGO/Virgo in a galaxy in which the majority of star formation was taking place a long time ago (11 Gyr). LIGO/Virgo estimated that local cosmic NS-NS merger rate is 110-3840 Gpc^-3 yr^-1. Only some extreme evolutionary models can generate NS-NS merger rates in old host galaxies consistent with the LIGO/Virgo estimate. However, we show that these models generate rates exceeding empirical Galactic NS-NS merger rates based on the large population of Milky Way NS-NS binaries. Typically, current evolutionary models produce NS-NS merger rates that are consistent with the Milky Way empirical rates. However, these models generate local cosmic NS-NS merger rate in old host galaxies that are below the LIGO/Virgo estimate. The reason behind this tension is the predicted delay time distribution between star formation and NS-NS mergers that favors short delays. Evolutionary models produce a generic steep power-law NS-NS delay time distribution. This limits NS-NS merger rates in old host galaxies. However, we show that such distribution is consistent with observations of Galactic NS-NS binaries; 50% of which show very long merger times (much longer than Hubble time). Once model distributions are convolved with continuous prolonged (10 Gyr) star formation in the Galactic disk, then 20-70% (depending on a model) of the predicted NS-NS population has very long current Galactic merger times. Although NS-NS binaries are formed predominantly with short delay times, many of short delay time systems merge and do not make it to the present, while long delay time systems survive and contribute to the current Galactic NS-NS population. This study highlights the tension between the current evolutionary predictions and the observation of the first NS-NS merger in an old host galaxy.

astro-ph.HE↗

arXiv2018

Explaining LIGO's observations via isolated binary evolution with natal kicks

We compare binary evolution models with different assumptions about black-hole natal kicks to the first gravitational-wave observations performed by the LIGO detectors. Our comparisons attempt to reconcile merger rate, masses, spins, and spin-orbit misalignments of all current observations with state-of-the-art formation scenarios of binary black holes formed in isolation. We estimate that black holes (BHs) should receive natal kicks at birth of the order of $σ\simeq 200$ (50) km/s if tidal processes do (not) realign stellar spins. Our estimate is driven by two simple factors. The natal kick dispersion $σ$ is bounded from above because large kicks disrupt too many binaries (reducing the merger rate below the observed value). Conversely, the natal kick distribution is bounded from below because modest kicks are needed to produce a range of spin-orbit misalignments. A distribution of misalignments increases our models' compatibility with LIGO's observations, if all BHs are likely to have natal spins. Unlike related work which adopts a concrete BH natal spin prescription, we explore a range of possible BH natal spin distributions. Within the context of our models, for all of the choices of $σ$ used here and within the context of one simple fiducial parameterized spin distribution, observations favor low BH natal spin.

astro-ph.HE↗

arXiv2017

Host galaxies and relativistic ejecta of compact binary mergers in the ngVLA era

We present the results of a community study aimed at exploring some of the scientific opportunities that the next generation Very Large Array (ngVLA) could open in the field of multi-messenger time-domain astronomy. We focus on compact binary mergers, golden astrophysical targets of ground-based gravitational wave (GW) detectors such as advanced LIGO. A decade from now, a large number of these mergers is likely to be discovered by a world-wide network of GW detectors. This will enable the identification of host galaxies, either directly through detection of electromagnetic (EM) counterparts, or indirectly by probing potential anisotropies in the spatial distribution of mergers. Identifying the host galaxy population of GW mergers would provide a way to constrain the efficiency of various binary neutron star (NS) or black hole (BH) formation scenarios, and the merger timescale distributions as linked to merger rates in early- and late-type galaxies. We discuss how a radio array with ~10x the sensitivity of the current Karl G. Jansky VLA and ~10x the resolution, would enable resolved radio continuum studies of binary merger hosts, probing regions of the galaxy undergoing star formation (which can be heavily obscured by dust and gas), AGN components, and mapping the offset distribution of the mergers with respect to the host galaxy light. For compact binary mergers containing at least one NS and with associated EM counterparts, we show how the ngVLA would enable direct size measurements of the relativistic merger ejecta and probe, for the first time directly, their dynamics.

astro-ph.IM↗

arXiv2017

Systematic challenges for future gravitational wave measurements of precessing binary black holes

The properties of precessing, coalescing binary black holes are presently inferred through comparison with two approximate models of compact binary coalescence. In this work we show these two models often disagree substantially when binaries have modestly large spins ($a\gtrsim 0.4$) and modest mass ratios ($q\gtrsim 2$). We demonstrate these disagreements using standard figures of merit and the parameters inferred for recent detections of binary black holes. By comparing to numerical relativity, we confirm these disagreements reflect systematic errors. We provide concrete examples to demonstrate that these systematic errors can significantly impact inferences about astrophysically significant binary parameters. For the immediate future, parameter inference for binary black holes should be performed with multiple models (including numerical relativity), and carefully validated by performing inference under controlled circumstances with similar synthetic events.

gr-qc↗

arXiv2016

Validating gravitational-wave detections: The Advanced LIGO hardware injection system

Hardware injections are simulated gravitational-wave signals added to the Laser Interferometer Gravitational-wave Observatory (LIGO). The detectors' test masses are physically displaced by an actuator in order to simulate the effects of a gravitational wave. The simulated signal initiates a control-system response which mimics that of a true gravitational wave. This provides an end-to-end test of LIGO's ability to observe gravitational waves. The gravitational-wave analyses used to detect and characterize signals are exercised with hardware injections. By looking for discrepancies between the injected and recovered signals, we are able to characterize the performance of analyses and the coupling of instrumental subsystems to the detectors' output channels. This paper describes the hardware injection system and the recovery of injected signals representing binary black hole mergers, a stochastic gravitational wave background, spinning neutron stars, and sine-Gaussians.

astro-ph.IM↗

arXiv2015

A semianalytic Fisher matrix for precessing binaries with a single significant spin

Gravitational waves from a binary with a single dynamically significant spin, notably including precessing black hole-neutron star (BH-NS) binaries, let us constrain that binary's properties: the two masses and the dominant black hole spin. Based on a straightforward fourier transform of $h(t)$ enabled by the corotating frame, we show the Fisher matrix for precessing binaries can be well-approximated by an extremely simple semianalytic approximation. This approximation can be easily understood as a weighted average of independent information channels, each associated with one gravitational wave harmonic. Generalizing previous studies of nonprecessing binaries to include critical symmetry-breaking precession effects required to understand plausible astrophysical sources, our ansatz can be applied to address how well gravitational wave measurements can address a wide range of astrophysical and fundamental questions. Our approach provides a simple method to assess what parameters gravitational wave detectors can measure, how well, and why.

gr-qc↗

arXiv2015

Double Compact Objects III: Gravitational Wave Detection Rates

The unprecedented range of second-generation gravitational-wave (GW) observatories calls for refining the predictions of potential sources and detection rates. The coalescence of double compact objects (DCOs)---i.e., neutron star-neutron star (NS-NS), black hole-neutron star (BH-NS), and black hole-black hole (BH-BH) binary systems---is the most promising source of GWs for these detectors. We compute detection rates of coalescing DCOs in second-generation GW detectors using the latest models for their cosmological evolution, and implementing inspiral-merger-ringdown (IMR) gravitational waveform models in our signal-to-noise ratio calculations. We find that: (1) the inclusion of the merger/ringdown portion of the signal does not significantly affect rates for NS-NS and BH-NS systems, but it boosts rates by a factor $\sim 1.5$ for BH-BH systems; (2) in almost all of our models BH-BH systems yield by far the largest rates, followed by NS-NS and BH-NS systems, respectively, and (3) a majority of the detectable BH-BH systems were formed in the early Universe in low-metallicity environments. We make predictions for the distributions of detected binaries and discuss what the first GW detections will teach us about the astrophysics underlying binary formation and evolution.

astro-ph.HE↗

arXiv2015

A novel scheme for rapid parallel parameter estimation of gravitational waves from compact binary coalescences

We introduce a highly-parallelizable architecture for estimating parameters of compact binary coalescence using gravitational-wave data and waveform models. Using a spherical harmonic mode decomposition, the waveform is expressed as a sum over modes that depend on the intrinsic parameters (e.g. masses) with coefficients that depend on the observer dependent extrinsic parameters (e.g. distance, sky position). The data is then prefiltered against those modes, at fixed intrinsic parameters, enabling efficiently evaluation of the likelihood for generic source positions and orientations, independent of waveform length or generation time. We efficiently parallelize our intrinsic space calculation by integrating over all extrinsic parameters using a Monte Carlo integration strategy. Since the waveform generation and prefiltering happens only once, the cost of integration dominates the procedure. Also, we operate hierarchically, using information from existing gravitational-wave searches to identify the regions of parameter space to emphasize in our sampling. As proof of concept and verification of the result, we have implemented this algorithm using standard time-domain waveforms, processing each event in less than one hour on recent computing hardware. For most events we evaluate the marginalized likelihood (evidence) with statistical errors of less than about 5%, and even smaller in many cases. With a bounded runtime independent of the waveform model starting frequency, a nearly-unchanged strategy could estimate NS-NS parameters in the 2018 advanced LIGO era. Our algorithm is usable with any noise curve and existing time-domain model at any mass, including some waveforms which are computationally costly to evolve.

gr-qc↗