arXiv subjects

Daniel J. D'Orazio

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

At least 73 records · Page 4Linked to original sources

arXiv2019

Stellar multiplicity: an interdisciplinary nexus

Our uncertainties about binary star systems (and triples and so on) limit our capabilities in literally every single one of the Thematic Areas identified for Astro2020. We need to understand the population statistics of stellar multiplicity and their variations with stellar type, chemistry, and dynamical environment: Correct interpretation of any exoplanet experiment depends on proper treatment of resolved and unresolved binaries; stellar multiplicity is a direct outcome of star and companion formation; the most precise constraints on stellar structure come from well-characterized binary systems; stellar populations heavily rely on stellar and binary evolution modeling; high-redshift galaxy radiation and reionization is controlled by binary-dependent stellar physics; compact objects are the outcomes of binary evolution; the interpretation of multi-messenger astronomy from gravitational waves, light, and neutrinos relies on understanding the products of binary star evolution; near-Universe constraints on the Hubble constant with Type Ia supernovae and gravitational-wave mergers are subject to systematics related to their binary star progenitors; local measures of dark-matter substructure masses are distorted by binary populations. In order to realize the scientific goals in each of these themes over the next decade, we therefore need to understand how binary stars and stellar multiplets are formed and distributed in the space of masses, composition, age, and orbital properties, and how the distribution evolves with time. This white paper emphasizes the interdisciplinary importance of binary-star science and advocates that coordinated investment from all astrophysical communities will benefit almost all branches of astrophysics.

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arXiv2019

Multimessenger science opportunities with mHz gravitational waves

LISA will open the mHz band of gravitational waves (GWs) to the astronomy community. The strong gravity which powers the variety of GW sources in this band is also crucial in a number of important astrophysical processes at the current frontiers of astronomy. These range from the beginning of structure formation in the early universe, through the origin and cosmic evolution of massive black holes in concert with their galactic environments, to the evolution of stellar remnant binaries in the Milky Way and in nearby galaxies. These processes and their associated populations also drive current and future observations across the electromagnetic (EM) spectrum. We review opportunities for science breakthroughs, involving either direct coincident EM+GW observations, or indirect multimessenger studies. We argue that for the US community to fully capitalize on the opportunities from the LISA mission, the US efforts should be accompanied by a coordinated and sustained program of multi-disciplinary science investment, following the GW data through to its impact on broad areas of astrophysics. Support for LISA-related multimessenger observers and theorists should be sized appropriately for a flagship observatory and may be coordinated through a dedicated mHz GW research center.

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arXiv2019

Constraining the Stellar Mass Function from the Deficiency of Tidal Disruption Flares in the Nuclei of Massive Galaxies

The rate of tidal disruption flares (TDFs) per mass of the disrupting black hole encodes information on the present-day mass function (PDMF) of stars in the clusters surrounding super massive black holes. We explore how the shape of the TDF rate with black hole mass can constrain the PDMF, with only weak dependence on black hole spin. We show that existing data can marginally constrain the minimum and maximum masses of stars in the cluster, and the high-mass end of the PDMF slope, as well as the overall TDF rate. With of order 100 TDFs expected to be identified with the Zwicky Transient Facility, the overall rate can be highly constrained, but still with only marginal constraints on the PDMF. However, if less than of order 10% of the TDFs expected to be found by LSST over a decade (of order 1000 TDFs) are identified, then precise and accurate estimates can be made for the minimum stellar mass (within a factor of two) and the average slope of the high-mass PDMF (to within ~10%) in nuclear star clusters. This technique could be adapted in the future to probe, in addition to the PDMF, the local black hole mass function and possibly the massive black hole binary population.

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arXiv2018

A Single Progenitor Model for GW150914 and GW170104

The merger of stellar-mass black holes (BHs) is not expected to generate detectable electromagnetic (EM) emission. However, the gravitational wave (GW) events GW150914 and GW170104, detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) to be the result of merging, ~60 solar mass binary black holes (BBHs), each have claimed coincident gamma-ray emission. Motivated by the intriguing possibility of an EM counterpart to BBH mergers, we construct a model that can reproduce the observed EM and GW signals for GW150914- and GW170104-like events, from a single-star progenitor. Following Loeb (2016), we envision a massive, rapidly rotating star within which a rotating bar instability fractures the core into two overdensities that fragment into clumps which merge to form BHs in a tight binary with arbitrary spin-orbit alignment. Once formed, the BBH inspirals due to gas and gravitational-wave drag until tidal forces trigger strong feeding of the BHs with the surrounding stellar-density gas about 10 seconds before merger. The resulting giga-Eddington accretion peak launches a jet that breaks out of the progenitor star and drives a powerful outflow that clears the gas from the orbit of the binary within one second, preserving the vacuum GW waveform in the LIGO band. The single-progenitor scenario predicts the existence of variability of the gamma-ray burst, modulated at the ~0.2 second chirping period of the BBH due to relativistic Doppler boost. The jet breakout should be accompanied by a low-luminosity supernova. Finally, because the BBHs of the single progenitor model do not exist at large separations, they will not be detectable in the low frequency gravitational wave band of the Laser Interferometer Space Antenna (LISA). Hence, the single-progenitor BBHs will be unambiguously discernible from BBHs formed through alternate, double-progenitor evolution scenarios.

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arXiv2018

Black Hole Mergers From Globular Clusters Observable by LISA II: Resolved Eccentric Sources and the Gravitational Wave Background

In paper I of this series we showed that a large percentage of the binary black hole (BBH) mergers that form through dynamical interactions in globular clusters will have significant eccentricity in the ~10^{-3}-10^{-1} Hz LISA band. In this work we quantify the evolution of these highly eccentric binaries through the LISA and LIGO bands, and compute the stochastic gravitational wave background from the merging, eccentric population. We find that the population of BBHs that merge in-between three-body encounters inside their cluster (~50% of all cluster-formed BBH mergers) will have measurable eccentricity for their entire lifetime in the LISA band. The population of BBHs that merge during three-body encounters (~5% of all cluster-formed BBH mergers), will be detectable by LIGO with eccentricities of e~0.1. The gravitational wave background from dynamically assembled BBHs encodes a characteristic bump due to the high initial eccentricities of these systems. The location and amplitude of this bump depends on globular cluster properties.

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arXiv2018

Repeated Imaging of Massive Black Hole Binary Orbits with Millimeter Interferometry: measuring black hole masses and the Hubble constant

Very long baseline interferometry (VLBI) at millimeter (mm) wavelengths is being employed to resolve event-horizon scale structure of the environment surrounding the Milky-Way black hole, at an angular resolution of a few tens of micro-arcseconds. The same approach could also resolve the orbital separation of a population of massive black hole binaries (MBHBs). Modeling the inspiral of binaries due to gravitational wave emission and gas and requiring binary orbital periods of less than 10 years, we estimate that there may exist ~100 resolvable MBHBs that are bright enough to be observed by mm-wavelength VLBI instruments over the entire sky, at redshifts z<0.5. We propose to search for these resolvable MBHBs by identifying binaries with the required orbital separations from periodic quasar light curves identified in optical and near-IR surveys. These periodic-light-curve candidates can be followed up with radio observations to determine their promise for observation with VLBI at mm wavelengths. VLBI observations over the timescale of a binary orbit can allow unprecedented precision in the measurement of the binary mass, to within 30%. In combination with an independent binary mass measurement, VLBI observation would allow a novel (~10%) measurement of the Hubble constant, independent from those currently proposed and employed.

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arXiv2018

Black Hole Mergers From Globular Clusters Observable by LISA I: Eccentric Sources Originating From Relativistic $N$-body Dynamics

We show that nearly half of all binary black hole (BBH) mergers dynamically assembled in globular clusters have measurable eccentricities ($e>0.01$) in the LISA band ($10^{-2}$ Hz), when General Relativistic corrections are properly included in the $N$-body evolution. If only Newtonian gravity is included, the derived fraction of eccentric LISA sources is significantly lower, which explains why recent studies all have greatly underestimated this fraction. Our findings have major implications for how to observationally distinguish between BBH formation channels using eccentricity with LISA, which is one of the key science goals of the mission. We illustrate that the relatively large population of eccentric LISA sources reported here originates from BBHs that merge between hardening binary-single interactions inside their globular cluster. These results indicate a bright future for using LISA to probe the origin of BBH mergers.

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arXiv2018

Black Hole Mergers from Globular Clusters Observable by LISA and LIGO: Results from post-Newtonian Binary-Single Scatterings

We study the gravitational wave (GW) frequency and chirp mass distribution of binary black hole (BBH) mergers assembled through three-body interactions in globular clusters (GCs), when GW emission at the 2.5 post-Newtonian (PN) level is included in the $N$-body equation-of-motion (EOM). From performing $\sim 2.5\times10^{6}$ PN binary-single interactions based on GC data from the `MOCCA-Survey Database I' project, and by the use of analytical methods, we find that $5-10\%$ of all the three-body assembled GC BBH mergers have a GW frequency at formation that is $\gtrsim 10^{-1}$ Hz, implying they enter the LIGO band without having drifted through the LISA band first. If PN terms are not included in the EOM, one finds instead that all BBH mergers drifts through both LISA and LIGO. As the fraction of BBH mergers that only show up in LIGO is expected to be $\sim 0\%$ for standard field binary BBH mergers, future joint measurements with LISA and LIGO can be used to gain insight into the formation of BBH mergers.

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arXiv2017

Periodic self-lensing from accreting massive black hole binaries

Nearly 150 massive black hole binary (MBHB) candidates at sub-pc orbital separations have been reported in recent literature. Nevertheless, the definitive detection of even a single such object remains elusive. If at least one of the black holes is accreting, the light emitted from its accretion disc will be lensed by the other black hole for binary orbital inclinations near to the line of sight. This binary self-lensing could provide a unique signature of compact MBHB systems. We show that, for MBHBs with masses in the range 10^6 - 10^10 solar masses and with orbital periods less than ~10 yr, strong lensing events should occur in one to 10s of percent of MBHB systems that are monitored for an entire orbit. Lensing events will last from days for the less massive, shorter period MBHBs up to a year for the most massive ~10 year orbital period MBHBs. At small inclinations of the binary orbit to the line of sight, lensing must occur and will be accompanied by periodicity due to the relativistic Doppler boost. Flares at the same phase as the otherwise average flux of the Doppler modulation would be a smoking gun signature of self-lensing and can be used to constrain binary parameters. For MBHBs with separation >100 Schwarzschild radii, we show that finite-sized source effects could serve as a probe of MBH accretion disc structure. Finally, we stress that our lensing probability estimate implies that ~10 of the known MBHB candidates identified through quasar periodicity should exhibit strong lensing flares.

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arXiv2017

Lighthouse in the Dust: Infrared Echoes of Periodic Emission from Massive Black Hole Binaries

The optical and UV emission from sub-parsec massive black hole binaries (MBHBs) in active galactic nuclei (AGN) is believed to vary periodically, on timescales comparable to the binary's orbital time. If driven by accretion rate fluctuations, the variability could be isotropic. If dominated by relativistic Doppler modulation, the variability should instead be anisotropic, resembling a rotating forward-beamed lighthouse. We consider the infrared (IR) reverberation of either type of periodic emission by pc-scale circumbinary dust tori. We predict the phase and amplitude of IR variability as a function of the ratio of dust light crossing time to the source variability period, and of the torus inclination and opening angle. We enumerate several differences between the isotropic and anisotropic cases. Interestingly, for a nearly face-on binary with an inclined dust torus, the Doppler boost can produce IR variability without any observable optical/UV variability. Such orphan-IR variability would have been missed in optical searches for periodic AGN. We apply our models to time-domain WISE IR data from the MBHB candidate PG 1302-102 and find consistency with dust reverberation by both isotropically emitting and Doppler-boosted sources in the shorter wavelength W1-W2 (2.8-5.3 micrometer) bands. We constrain the dust torus to be thin (aspect ratio ~0.1), with an inner radius at 1-5 pc. More generally, our dust echo models will aid in identifying new MBHB candidates, determining their nature, and constraining the physical properties of MBHBs and their dust tori.

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arXiv2016

Bright transients from strongly-magnetized neutron star-black hole mergers

Direct detection of black hole-neutron star pairs is anticipated with the advent of aLIGO. Electromagnetic counterparts may be crucial for a confident gravitational-wave detection as well as for extraction of astronomical information. Yet black hole-neutron star pairs are notoriously dark and so inaccessible to telescopes. Contrary to this expectation, a bright electromagnetic transient can occur in the final moments before merger as long as the neutron star is highly magnetized. The orbital motion of the neutron star magnet creates a Faraday flux and corresponding power available for luminosity. A spectrum of curvature radiation ramps up until the rapid injection of energy ignites a fireball, which would appear as an energetic blackbody peaking in the x ray to $γ$ rays for neutron star field strengths ranging from $10^{12}$G to $10^{16}$G respectively and a $10M_{\odot} $ black hole. The fireball event may last from a few milliseconds to a few seconds depending on the neutron star magnetic-field strength, and may be observable with the Fermi Gamma-Ray Burst Monitor with a rate up to a few per year for neutron star field strengths $\gtrsim 10^{14}$G. We also discuss a possible decaying post-merger event which could accompany this signal. As an electromagnetic counterpart to these otherwise dark pairs, the black-hole battery should be of great value to the development of multi-messenger astronomy in the era of aLIGO.

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arXiv2016

A transition in circumbinary accretion discs at a binary mass ratio of 1:25

We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio $q \equiv M_s/M_p \sim 0.04$. For mass ratios above $q=0.04$, solutions for the hydrodynamic flow transition from steady, to strongly-fluctuating; a narrow annular gap in the surface density around the secondary's orbit changes to a hollow central cavity; and a spatial symmetry is lost, resulting in a lopsided disc. This phase transition is coincident with the mass ratio above which stable orbits do not exist around the L4 and L5 equilibrium points of the R3B problem. Using the DISCO code, we find that for thin discs, for which a gap or cavity can remain open, the mass ratio of the transition is relatively insensitive to disc viscosity and pressure. The $q=0.04$ transition has relevance for the evolution of massive black hole binary+disc systems at the centers of galactic nuclei, as well as for young stellar binaries and possibly planets around brown dwarfs.

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arXiv2015

Relativistic boost as the cause of periodicity in a massive black-hole binary candidate

Because most large galaxies contain a central black hole, and galaxies often merge, black-hole binaries are expected to be common in galactic nuclei. Although they cannot be imaged, periodicities in the light curves of quasars have been interpreted as evidence for binaries, most recently in PG~1302-102, with a short rest-frame optical period of 4 years. If the orbital period matches this value, then for the range of estimated black hole masses the components would be separated by 0.007-0.017 pc, implying relativistic orbital speeds. There has been much debate over whether black hole orbits could be smaller than 1 pc. Here we show that the amplitude and the sinusoid-like shape of the variability of PG~1302-102 can be fit by relativistic Doppler boosting of emission from a compact, steadily accreting, unequal-mass binary. We predict that brightness variations in the ultraviolet light curve track those in the optical, but with a 2-3 times larger amplitude. This prediction is relatively insensitive to the details of the emission process, and is consistent with archival UV data. Follow-up UV and optical observations in the next few years can test this prediction and confirm the existence of a binary black hole in the relativistic regime.

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arXiv2015

A reduced orbital period for the supermassive black hole binary candidate in the quasar PG 1302-102?

Graham et al. (2015) have detected a 5.2 year periodic optical variability of the quasar PG 1302-102 at redshift $z=0.3$, which they interpret as the redshifted orbital period $(1+z)t_{\rm bin}$ of a putative supermassive black hole binary (SMBHB). Here we consider the implications of a $3-8$ times shorter orbital period, suggested by hydrodynamical simulations of circumbinary discs (CBDs) with nearly equal--mass SMBHBs ($q\equiv M_2/M_1\gtrsim 0.3$). With the corresponding $2-4$ times tighter binary separation, PG 1302 would be undergoing gravitational wave dominated inspiral, and serve as a proof that the BHs can be fueled and produce bright emission even in this late stage of the merger. The expected fraction of binaries with the shorter $t_{\rm bin}$, among bright quasars, would be reduced by 1-2 orders of magnitude, compared to the 5.2 year period, in better agreement with the rarity of candidates reported by Graham et al. (2015). Finally, shorter periods would imply higher binary speeds, possibly imprinting periodicity on the light curves from relativistic beaming, as well as measurable relativistic effects on the Fe K$α$ line. The CBD model predicts additional periodic variability on timescales of $t_{\rm bin}$ and $\approx 0.5 t_{\rm bin}$, as well as periodic variation of broad line widths and offsets relative to the narrow lines, which are consistent with the observations. Future observations will be able to test these predictions and hence the binary+CBD hypothesis for PG 1302.

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arXiv2014

The Migration of Gap-Opening Planets is not Locked to Viscous Disk Evolution

Most standard descriptions of Type II migration state that massive, gap-opening planets must migrate at the viscous drift rate. This is based on the idea that the disk is separated into an inner and outer region and gas is considered unable to cross the gap. In fact, gas easily crosses the gap on horseshoe orbits, nullifying this necessary premise which would set the migration rate. In this work, it is demonstrated using highly accurate numerical calculations that the actual migration rate is dependent on disk and planet parameters, and can be significantly larger or smaller than the viscous drift rate. In the limiting case of a disk much more massive than the secondary, the migration rate saturates to a constant which is sensitive to disk parameters and is not necessarily of order viscous rate. In the opposite limit of a low-mass disk, the migration rate decreases linearly with disk mass. Steady-state solutions in the low disk mass limit show no pile-up outside the secondary's orbit, and no corresponding drainage of the inner disk.

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arXiv2013

Big Black Hole, Little Neutron Star: Magnetic Dipole Fields in the Rindler Spacetime

As a black hole and neutron star approach during inspiral, the field lines of a magnetized neutron star eventually thread the black hole event horizon and a short-lived electromagnetic circuit is established. The black hole acts as a battery that provides power to the circuit, thereby lighting up the pair just before merger. Although originally suggested as a promising electromagnetic counterpart to gravitational-wave detection, the luminous signals are promising more generally as potentially detectable phenomena, such as short gamma-ray bursts. To aid in the theoretical understanding, we present analytic solutions for the electromagnetic fields of a magnetic dipole in the presence of an event horizon. In the limit that the neutron star is very close to a Schwarzschild horizon, the Rindler limit, we can solve Maxwell's equations exactly for a magnetic dipole on an arbitrary worldline. We present these solutions here and investigate a proxy for a small segment of the neutron star orbit around a big black hole. We find that the voltage the black hole battery can provide is in the range ~10^16 statvolts with a projected luminosity of 10^42 ergs/s for an M=10M_sun black hole, a neutron star with a B-field of 10^12 G, and an orbital velocity ~0.5c at a distance of 3M from the horizon. Larger black holes provide less power for binary separations at a fixed number of gravitational radii. The black hole/neutron star system therefore has a significant power supply to light up various elements in the circuit possibly powering jets, beamed radiation, or even a hot spot on the neutron star crust.

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arXiv2013

Accretion into the Central Cavity of a Circumbinary Disk

A near-equal mass binary black hole can clear a central cavity in a circumbinary accretion disk; however, previous works have revealed accretion streams entering this cavity. Here we use 2D hydrodynamical simulations to study the accretion streams and their periodic behavior. In particular, we perform a suite of simulations, covering different binary mass ratios $q=M_2/M_1$ in the range $0.01 \leq q \leq 1$. In each case, we follow the system for several thousand binary orbits, until it relaxes to a stable accretion pattern. We find the following results: (i) while the binary is efficient in maintaining a low-density cavity, the time-averaged mass accretion rate into the cavity, through narrow coherent accretion streams, is suppressed by at most a factor of $\sim 5$, compared to a disk with a single BH with the same mass; (ii) the largest suppression occurs for $q\approx 0.05$; binaries whose mass ratios are either lower or higher both suppress accretion less significantly; (iii) for $q \gtrsim 0.05$, the accretion rate is strongly modulated by the binary, and depending on the precise value of $q$, the power spectrum of the accretion rate shows either one, two, or three distinct periods; and (v) for $q \lesssim 0.05$, the accretion rate becomes steady, with no time-variations. Most binaries produced in galactic mergers are expected to have $q\gtrsim 0.05$. If the luminosity of these binaries tracks their accretion rate, then a periodogram of their light-curve could help in their identification, and to constrain their mass ratio and disk properties.

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arXiv2010

An analytic solution for weak-field Schwarzschild geodesics

It is well known that the classical gravitational two body problem can be transformed into a spherical harmonic oscillator by regularization. We find that a modification of the regularization transformation has a similar result to leading order in general relativity. In the resulting harmonic oscillator, the leading-order relativistic perturbation is formally a negative centrifugal force. The net centrifugal force changes sign at three Schwarzschild radii, which interestingly mimics the innermost stable circular orbit (ISCO) of the full Schwarzschild problem. Transforming the harmonic-oscillator solution back to spatial coordinates yields, for both timelike and null weak-field Schwarzschild geodesics, a solution for $t,r,ϕ$ in terms of elementary functions of a variable that can be interpreted as a generalized eccentric anomaly. The textbook expressions for relativistic precession and light deflection are easily recovered. We suggest how this solution could be combined with additional perturbations into numerical methods suitable for applications such as relativistic accretion or dynamics of the Galactic-centre stars.

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