arXiv subjects

Daniel J. D'Orazio

Publications and source records attributed to Daniel J. D'Orazio.

At least 37 records · Page 2Linked to original sources

arXiv2025

Gravitational Wave Decoupling in Retrograde Circumbinary Disks

We present a study of the late-time interaction between supermassive black hole binaries and retrograde circumbinary disks during the period of gravitational wave-driven inspiral. While mergers in prograde disks have received extensive study, retrograde disks offer distinct dynamics that could promote mergers and produce unique observational signatures. Through numerical simulations, we explore the process of binary-disk decoupling, where the binary's orbital decay rate is faster than the disk's viscous response rate. We find the point of decoupling to be comparable in prograde and retrograde disks, suggesting that any associated electromagnetic (EM) signatures will be produced at comparable times preceding merger. However, we find smaller central cavities for retrograde disks, likely leading to higher-frequency EM emissions and shorter post-merger rebrightening timescales compared to their prograde counterparts. Additionally, we identify quasi-periodic flaring due to instabilities unique to low-viscosity retrograde disks, which may produce distinctive EM signatures.

astro-ph.HE↗

arXiv2024

Measuring the Transverse Velocity of Strongly Lensed Gravitational Wave Sources with Ground Based Detectors

Observations of strongly gravitationally lensed gravitational wave (GW) sources provide a unique opportunity for constraining their transverse motion, which otherwise is exceedingly hard for GW mergers in general. Strong lensing makes this possible when two or more images of the lensed GW source are observed, as each image essentially allows the observer to see the GW source from different directional lines-of-sight. If the GW source is moving relative to the lens and observer, the observed GW signal from one image will therefore generally appear blue- or redshifted compared to GW signal from the other image. This velocity induced differential Doppler shift gives rise to an observable GW phase shift between the GW signals from the different images, which provides a rare glimpse into the relative motion of GW sources and their host environment across redshift. We illustrate that detecting such GW phase shifts is within reach of next-generation ground-based detectors such as Einstein Telescope, that is expected to detect $\sim$hundreds of lensed GW mergers per year. This opens up completely new ways of inferring the environment of GW sources, as well as studying cosmological velocity flows across redshift.

astro-ph.HE↗

arXiv2024

Deep Neural Emulation of the Supermassive Black-hole Binary Population

While supermassive black-hole (SMBH)-binaries are not the only viable source for the low-frequency gravitational wave background (GWB) signal evidenced by the most recent pulsar timing array (PTA) data sets, they are expected to be the most likely. Thus, connecting the measured PTA GWB spectrum and the underlying physics governing the demographics and dynamics of SMBH-binaries is extremely important. Previously, Gaussian processes (GPs) and dense neural networks have been used to make such a connection by being built as conditional emulators; their input is some selected evolution or environmental SMBH-binary parameters and their output is the emulated mean and standard deviation of the GWB strain ensemble distribution over many Universes. In this paper, we use a normalizing flow (NF) emulator that is trained on the entirety of the GWB strain ensemble distribution, rather than only mean and standard deviation. As a result, we can predict strain distributions that mirror underlying simulations very closely while also capturing frequency covariances in the strain distributions as well as statistical complexities such as tails, non-Gaussianities, and multimodalities that are otherwise not learnable by existing techniques. In particular, we feature various comparisons between the NF-based emulator and the GP approach used extensively in past efforts. Our analyses conclude that the NF-based emulator not only outperforms GPs in the ease and computational cost of training but also outperforms in the fidelity of the emulated GWB strain ensemble distributions.

astro-ph.IM↗

arXiv2024

A novel category of environmental effect in gravitational waves from binaries perturbed by periodic forces

We study the gravitational wave (GW) emission of sources perturbed by periodic dynamical forces which do not cause secular evolution in the orbital elements. We construct a corresponding post-Newtonian waveform model and provide estimates for the detectability of the resulting GW phase perturbations, for both space-based and future ground-based detectors. We validate our results by performing a set of Bayesian parameter recovery experiments with post-Newtonian waveforms. We find that, in stark contrast to the more commonly studied secular dephasing, periodic phase perturbations do not suffer from degeneracies with any of the tested vacuum binary parameters. We discuss the applications of our findings to a range of possible astrophysical scenarios, finding that such periodic perturbations may be detectable for massive black hole binaries embedded in circum-binary discs, extreme mass-ratio inspirals in accretion discs, as well as stellar-mass compact objects perturbed by tidal fields. We argue that modelling conservative sub-orbital dynamics opens up a promising new avenue to detect environmental effects in binary sources of GWs that should be included in state-of-the-art waveform templates.

gr-qc↗

arXiv2024

Accretion mediated spin-eccentricity correlations in LISA massive black hole binaries

We examine expected effective spin ($χ_{{\rm eff},1\rm yr}$) and orbital eccentricity ($e_{1\rm yr}$) correlations for a population of observable equal-mass massive black hole binaries (MBHBs) with total redshifted mass $M_z\sim[10^{4.5},10^{7.5}]~{\rm M}_\odot$ embedded in a circumbinary disc (CBD), one-year before merging in the LISA band. We find a strong correlation between measurable eccentricity and negative effective spin for MBHBs that are carried to merger by retrograde accretion. This is due to the well-established eccentricity pumping of retrograde accretion and the formation of retrograde CBD-aligned mini-discs, as observed in hydrodynamical simulations. Conversely, prograde accretion channels result in positive $χ_{{\rm eff},1\rm yr}$ and non-measurable $e_{1\rm yr}$. This clear contrast between the two CBD orientations - and particularly the unique signature of retrograde configurations - provides a promising way to unlock the mysteries of MBHB formation channels in the LISA era.

astro-ph.HE↗

arXiv2024

Long-term Evolution of Binary Orbits Induced by Circumbinary Disks

Circumbinary disks are found in a variety of astrophysical scenarios, spanning binary star formation to accreting supermassive black hole binaries. The interaction with a circumbinary disk can yield opposite effects on the binary orbit leading to circularization, or exciting the eccentricity, widening the orbit or shrinking it and facilitating mergers. We present a new formalism for the long-term evolution of the disk-binary interaction based on the results of recent suites of hydrodynamic simulations, which resolve the complex geometry of the gas in the vicinity of the binary and fully account for the gravitational and accretion forces. We release a python package, \texttt{spindler}, that implements our model. We show that, unless the mass reservoir feeding the disk is comparable to the mass of the binary, accretion onto the binary depletes the disk mass before inducing a significant change in orbital separation or mass ratio. This finding implies that, in most scenarios, interaction with a circumbinary disk is not an efficient mechanism to shrink the orbit of the binary. However, as long as the mass of the disk is at least a few percent of the mass of the binary, the interaction can excite the eccentricity up to an equilibrium value, and induce a statistical correlation between mass ratio and eccentricity. We consider the applicability of our model to a variety of astrophysical scenarios: during star formation, in evolved stellar binaries, triples and in supermassive black hole binaries. We discuss the theoretical and observational implications of our predictions.

astro-ph.HE↗

arXiv2024

The Santa Barbara Binary-Disk Code Comparison

We have performed numerical calculations of a binary interacting with a gas disk, using eleven different numerical methods and a standard binary-disk setup. The goal of this study is to determine whether all codes agree on a numerically converged solution, and to determine the necessary resolution for convergence and the number of binary orbits that must be computed to reach an agreed-upon relaxed state of the binary-disk system. We find that all codes can agree on a converged solution (depending on the diagnostic being measured). The zone spacing required for most codes to reach a converged measurement of the torques applied to the binary by the disk is roughly 1% of the binary separation in the vicinity of the binary components. For our disk model to reach a relaxed state, codes must be run for at least 200 binary orbits, corresponding to about a viscous time for our parameters, $0.2 (a^2 Ω_B /ν)$ binary orbits, where $ν$ is the kinematic viscosity. We did not investigate dependence on binary mass ratio, eccentricity, disk temperature, or disk viscosity; therefore, these benchmarks may act as guides towards expanding converged solutions to the wider parameter space but might need to be updated in a future study that investigates dependence on system parameters. We find the most major discrepancies between codes resulted from the dimensionality of the setup (3D vs 2D disks). Beyond this, we find good agreement in the total torque on the binary between codes, although the partition of this torque between the gravitational torque, orbital accretion torque, and spin accretion torque depends sensitively on the sink prescriptions employed. In agreement with previous studies, we find a modest difference in torques and accretion variability between 2D and 3D disk models. We find cavity precession rates to be appreciably faster in 3D than in 2D.

astro-ph.SR↗

arXiv2024

Eccentric Binaries in Retrograde Disks

Modern numerical hydrodynamics tools have recently enabled detailed examinations of binaries accreting from prograde circumbinary disks. These have re-framed the current understanding of binary-disk interactions and disk driven orbital evolution. We present the first full-domain grid-based hydrodynamics simulations of equal-mass, eccentric binaries accreting from retrograde circumbinary disks. We study binary eccentricities that span $e=0.0$ to $e = 0.8$ continuously, and explore the influence of retrograde accretion on the binary orbital response, disk morphology, and observational properties. We find that, at all eccentricities, retrograde accretion shrinks the binary semi-major axis and pumps its eccentricity leading to the previously identified possibility of highly eccentric mergers. Contrary to past studies and models, we observe gravitational forces to dominate the binary's orbital evolution as opposed to the physical accretion of mass and momentum. Retrograde accretion variability also differs strongly from prograde solutions. Preeminently, binaries with $e > 0.55$ reveal a unique two-period, double-peaked accretion signature that has not previously been identified. We additionally find evidence for the emergence of retrograde Lindblad resonances at large eccentricities in accordance with predictions from linear theory. Our results suggest that some astrophysical binaries for which retrograde accretion is possible will experience factors-of-a-few times faster orbital decay than in prograde disks and will have their eccentricities pumped beyond the limits found from prograde solutions. Such effects could lead to rapid inward migration for some young stellar binaries, the detection of highly-eccentric LISA mergers, and the tentatively observed turnover at the low-frequency end of the gravitational wave background.

astro-ph.GA↗

arXiv2024

Disk induced binary precession: Implications for dynamics and multi-messenger observations of black hole binaries

Many studies have recently documented the orbital response of eccentric binaries accreting from thin circumbinary disks, characterizing the change in binary semi-major axis and eccentricity. We extend these calculations to include the precession of the binary's longitude of periapse induced by the circumbinary disk, and we characterize this precession continuously with binary eccentricity $e_b$ for equal mass components. This disk-induced apsidal precession is prograde with a weak dependence on binary eccentricity when $e_b \lesssim 0.4$ and decreases approximately linearly for $e_b \gtrsim 0.4$; yet at all $e_b$ binary precession is faster than the rates of change to the semi-major axis and eccentricity by an order of magnitude. We estimate that such precession effects are likely most important for sub-parsec separated binaries with masses $\lesssim 10^7 M_\odot$, like LISA precursors. We find that accreting, equal-mass LISA binaries with $M < 10^6 M_\odot$ (and the most massive $M \sim 10^7 M_\odot$ binaries out to $z \sim 3$) may acquire a detectable phase offset due to the disk-induced precession. Moreover, disk-induced precession can compete with General Relativistic precession in vacuum, making it important for observer-dependent electromagnetic searches for accreting massive binaries -- like Doppler boost and binary self-lensing models -- after potentially only a few orbital periods.

astro-ph.HE↗

arXiv2024

NuSTAR Observations of Candidate Subparsec Binary Supermassive Black Holes

We present analysis of NuSTAR X-ray observations of three AGN that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs. We previously observed these AGN with Chandra and found no differences between their low energy X-ray properties and the larger AGN population. However some models predict differences to be more prominent at energies higher than probed by Chandra. We find that even at the higher energies probed by NuSTAR, the spectra of these AGN are indistinguishable from the larger AGN population. This could rule out models predicting large differences in the X-ray spectra in the NuSTAR bands. Alternatively, it might mean that these three AGN are not binary SMBHs.

astro-ph.HE↗

arXiv2024

Gravitational Wave Phase Shifts in Eccentric Black Hole Mergers as a Probe of Dynamical Formation Environments

We quantify for the first time the gravitational wave (GW) phase shift appearing in the waveform of eccentric binary black hole (BBH) mergers formed dynamically in three-body systems. For this, we have developed a novel numerical method where we construct a reference binary, by evolving the post-Newtonian (PN) evolution equations backwards from a point near merger without the inclusion of the third object, that can be compared to the real binary that evolves under the influence from the third BH. From this we quantify how the interplay between dynamical tides, PN-effects, and the time-dependent Doppler shift of the eccentric GW source results in unique observable GW phase shifts that can be mapped to the gravitational dynamics taking place at formation. We further find a new analytical expression for the GW phase shift, which surprisingly has a universal functional form that only depends on the time-evolving BBH eccentricity. The normalization scales with the BH masses and initial separation, which can be linked to the underlying astrophysical environment. GW phase shifts from a chaotic 3-body BH scattering taking place in a cluster, and from a BBH inspiraling in a disk migration trap near a super-massive BH, are also shown for illustration. When current and future GW detectors start to observe eccentric GW sources with high enough signal-to-noise-ratio, we propose this to be among the only ways of directly probing the dynamical origin of individual BBH mergers using GWs alone.

astro-ph.HE↗

arXiv2024

Fast Methods for Computing Photometric Variability of Eccentric Binaries: Boosting, Lensing, and Variable Accretion

We analyze accretion-rate time series for equal-mass binaries in co-planar gaseous disks spanning a continuous range of orbital eccentricities up to 0.8, for both prograde and retrograde systems. The dominant variability timescales match that of previous investigations; the binary orbital period is dominant for prograde binaries with $e \gtrsim 0.1$, with a 5 times longer "lump" period taking over for $e\lesssim 0.1$. This lump period fades and drops from 5 times to 4.5 times the binary period as $e$ approaches 0.1, where it vanishes. For retrograde orbits, the binary orbital period dominates at $e \lesssim 0.55$ and is accompanied by a 2 times longer-timescale periodicity at higher eccentricities. The shape of the accretion-rate time series varies with binary eccentricity. For prograde systems, the orientation of an eccentric disk causes periodic trading of accretion between the binary components in a ratio that we report as a function of binary eccentricity. We present a publicly available tool, binlite, that can rapidly ($\lesssim 0.01$~sec) generate templates for the accretion-rate time series, onto either binary component, for choice of binary eccentricity below 0.8. As an example use-case, we build lightcurve models where the accretion rate through the circumbinary disk and onto each binary component sets contributions to the emitted specific flux. We combine these rest-frame, accretion-variability lightcurves with observer-dependent Doppler boosting and binary self-lensing. This allows a flexible approach to generating lightcurves over a wide range of binary and observer parameter space. We envision binlite as the access point to a living database that will be updated with state-of-the-art hydrodynamical calculations as they advance.

astro-ph.HE↗

arXiv2024

Gaseous Dynamical Friction on Elliptical Keplerian Orbits

We compute the Gaseous Dynamical Friction (GDF) force experienced by massive perturbers on elliptical Keplerian orbits. In this paper, we investigate the density wake morphology, dynamical friction force, and secular orbital evolution for massive single perturbers as well as equal mass binaries embedded in an homogenous, static background flow. In all cases, the rate-of-change in semi-major axis is found to be negative (as expected), whereas the rate-of-change in eccentricity is negative for strictly-subsonic trajectories and positive for strictly-supersonic trajectories. Transonic orbits can experience both positive and negative torques during the course of an orbit, with some growing in eccentricity and others circularising. We observe all initial orbits becoming highly supersonic and eccentric (over sufficiently long timescales) due to a relentless semi-major axis decay increasing the Mach number and subsequent eccentricity driving. We compare our findings to previous studies for rectilinear and circular motion, while also making our data for orbital decay available.

astro-ph.HE↗

arXiv2023

Observational Signatures of Supermassive Black Hole Binaries

Despite solid theoretical and observational grounds for the pairing of supermassive black holes (SMBHs) after galaxy mergers, definitive evidence for the existence of close (sub-parsec) separation SMBH binaries (SMBHBs) approaching merger is yet to be found. This chapter reviews techniques aimed at discovering such SMBHBs in galactic nuclei. We motivate the search with a brief overview of SMBHB formation and evolution, and the gaps in our present-day theoretical understanding. We then present existing observational evidence for SMBHBs and discuss ongoing efforts to provide definitive evidence for a population at sub-parsec orbital separations, where many of the aforementioned theoretical gaps lie. We conclude with future prospects for discovery with electromagnetic (primarily time-domain) surveys, high-resolution imaging experiments, and low-frequency gravitational-wave detectors.

astro-ph.HE↗

arXiv2023

The NANOGrav 15-year Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational Wave Background

The NANOGrav 15-year data set shows evidence for the presence of a low-frequency gravitational-wave background (GWB). While many physical processes can source such low-frequency gravitational waves, here we analyze the signal as coming from a population of supermassive black hole (SMBH) binaries distributed throughout the Universe. We show that astrophysically motivated models of SMBH binary populations are able to reproduce both the amplitude and shape of the observed low-frequency gravitational-wave spectrum. While multiple model variations are able to reproduce the GWB spectrum at our current measurement precision, our results highlight the importance of accurately modeling binary evolution for producing realistic GWB spectra. Additionally, while reasonable parameters are able to reproduce the 15-year observations, the implied GWB amplitude necessitates either a large number of parameters to be at the edges of expected values, or a small number of parameters to be notably different from standard expectations. While we are not yet able to definitively establish the origin of the inferred GWB signal, the consistency of the signal with astrophysical expectations offers a tantalizing prospect for confirming that SMBH binaries are able to form, reach sub-parsec separations, and eventually coalesce. As the significance grows over time, higher-order features of the GWB spectrum will definitively determine the nature of the GWB and allow for novel constraints on SMBH populations.

astro-ph.HE↗

arXiv2023

Massive Black Hole Binaries as LISA Precursors in the Roman High Latitude Time Domain Survey

With its capacity to observe $\sim 10^{5-6}$ faint active galactic nuclei (AGN) out to redshift $z\approx 6$, Roman is poised to reveal a population of $10^{4-6}\, {\rm M_\odot}$ black holes during an epoch of vigorous galaxy assembly. By measuring the light curves of a subset of these AGN and looking for periodicity, Roman can identify several hundred massive black hole binaries (MBHBs) with 5-12 day orbital periods, which emit copious gravitational radiation and will inevitably merge on timescales of $10^{3-5}$ years. During the last few months of their merger, such binaries are observable with the Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave mission set to launch in the mid-2030s. Roman can thus find LISA precursors, provide uniquely robust constraints on the LISA source population, help identify the host galaxies of LISA mergers, and unlock the potential of multi-messenger astrophysics with massive black hole binaries.

astro-ph.HE↗

arXiv2023

Unusual Hard X-ray Flares Caught in NICER Monitoring of the Binary Supermassive Black Hole Candidate AT2019cuk/Tick Tock/SDSS J1430+2303

The nuclear transient AT2019cuk/Tick Tock/SDSS J1430+2303 has been suggested to harbor a supermassive black hole (SMBH) binary near coalescence. We report results from high-cadence NICER X-ray monitoring with multiple visits per day from January-August 2022, as well as continued optical monitoring during the same time period. We find no evidence of periodic/quasi-periodic modulation in the X-ray, UV, or optical bands, however we do observe exotic hard X-ray variability that is unusual for a typical AGN. The most striking feature of the NICER light curve is repetitive hard (2-4 keV) X-ray flares that result in distinctly harder X-ray spectra compared to the non-flaring data. In its non-flaring state, AT2019cuk looks like a relatively standard AGN, but it presents the first case of day-long, hard X-ray flares in a changing-look AGN. We consider a few different models for the driving mechanism of these hard X-ray flares, including: (1) corona/jet variability driven by increased magnetic activity, (2) variable obscuration, and (3) self-lensing from the potential secondary SMBH. We prefer the variable corona model, as the obscuration model requires rather contrived timescales and the self-lensing model is difficult to reconcile with a lack of clear periodicity in the flares. These findings illustrate how important high-cadence X-ray monitoring is to our understanding of the rapid variability of the X-ray corona and necessitate further high-cadence, multi-wavelength monitoring of changing-look AGN like AT2019cuk to probe the corona-jet connection.

astro-ph.HE↗

arXiv2022

Uncovering a hidden black hole binary from secular eccentricity variations of a tertiary star

We study the dynamics of a solar-type star orbiting around a black hole binary (BHB) in a nearly coplanar system. We present a novel effect that can prompt a growth and significant oscillations of the eccentricity of the stellar orbit when the system encounters an "apsidal precession resonance", where the apsidal precession rate of the outer stellar orbit matches that of the inner BHB. The eccentricity excitation requires the inner binary to have a non-zero eccentricity and unequal masses, and can be created even in non-coplanar triples. We show that the secular variability of the stellar orbit's apocenter, induced by the changing eccentricity, could be potentially detectable by \textit{Gaia}. Detection is favorable for BHBs emitting gravitational waves in the frequency band of the Laser Interferometer Space Antenna (LISA), hence providing a distinctive, multi-messenger probe on the existence of stellar-mass BHBs in the Milky Way.

astro-ph.HE↗