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DOE OSTI2020

Discovery of a Candidate Binary Supermassive Black Hole in a Periodic Quasar from Circumbinary Accretion Variability

Binary supermassive black holes (BSBHs) are expected to be a generic byproduct from hierarchical galaxy formation. The final coalescence of BSBHs is thought to be the loudest gravitational wave (GW) siren, yet no confirmed BSBH is known in the GW-dominated regime. While periodic quasars have been proposed as BSBH candidates, the physical origin of the periodicity has been largely uncertain. Here we report discovery of a periodicity (P=1607±7 days) at 99.95% significance (with a global p-value of ~10 –3 accounting for the look elsewhere effect) in the optical light curves of a redshift 1.53 quasar, SDSS J025214.67–002813.7. Combining archival Sloan Digital Sky Survey data with new, sensitive imaging from the Dark Energy Survey, the total ~20-yr time baseline spans ~4.6 cycles of the observed 4.4-yr (restframe 1.7-yr) periodicity. The light curves are best fit by a bursty model predicted by hydrodynamic simulations of circumbinary accretion disks. The periodicity is likely caused by accretion rate modulation by a milli-parsec BSBH emitting GWs, dynamically coupled to the circumbinary accretion disk. A bursty hydrodynamic variability model is statistically preferred over a smooth, sinusoidal model expected from relativistic Doppler boost, a kinematic effect proposed for PG1302–102. Furthermore, the frequency dependence of the variability amplitudes disfavors Doppler boost, lending independent support to the circumbinary accretion variability hypothesis. Given our detection rate of one BSBH candidate from circumbinary accretion variability out of 625 quasars, it suggests that future large, sensitive synoptic surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time may be able to detect hundreds to thousands of candidate BSBHs from circumbinary accretion with direct implications for Laser Interferometer Space Antenna.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

Probing the nature of dark matter with accreted globular cluster streams

ABSTRACT The steepness of the central density profiles of dark matter (DM) in low-mass galaxy haloes (e.g. dwarf galaxies) is a powerful probe of the nature of DM. We propose a novel scheme to probe the inner profiles of galaxy subhaloes using stellar streams. We show that the present-day morphological and dynamical properties of accreted globular cluster (GC) streams – those produced from tidal stripping of GCs that initially evolved within satellite galaxies and later merged with the Milky Way (MW) – are sensitive to the central DM density profile and mass of their parent satellites. GCs that accrete within cuspy cold dark matter (CDM) subhaloes produce streams that are physically wider and dynamically hotter than streams that accrete inside cored subhaloes. A first comparison of MW streams ‘GD-1’ and ‘Jhelum’ (likely of accreted GC origin) with our simulations indicates a preference for cored subhaloes. If these results hold up in future data, the implication is that either the DM cusps were erased by baryonic feedback, or their subhaloes naturally possessed cored density profiles implying particle physics models beyond CDM. Moreover, accreted GC streams are highly structured and exhibit complex morphological features (e.g. parallel structures and ‘spurs’). This implies that the accretion scenario can naturally explain the recently observed peculiarities in some of the MW streams. We also propose a novel mechanism for forming ‘gaps’ in stellar streams when the remnant of the parent subhalo (which hosted the GC) later passes through the GC stream. This encounter can last a longer time (and have more of an impact) than the random encounters with DM subhaloes previously considered, because the GC stream and its parent subhalo are on similar orbits with small relative velocities. Current and future surveys of the MW halo will uncover numerous faint stellar streams and provide the data needed to substantiate our preliminary tests with this new probe of DM.

(Galaxy:) globular clusters: individual↗

DOE OSTI2021

No evidence for a strong decrease of planetesimal accretion in old white dwarfs

ABSTRACT A large fraction of white dwarfs are accreting or have recently accreted rocky material from their planetary systems, thereby ‘polluting’ their atmospheres with elements heavier than helium. In recent years, the quest for mechanisms that can deliver planetesimals to the immediate vicinity of their central white dwarfs has stimulated a flurry of modelling efforts. The observed time evolution of the accretion rates of white dwarfs through their multi-Gyr lifetime is a crucial test for dynamical models of evolved planetary systems. Recent studies of cool white dwarf samples have identified a significant decrease of the mass accretion rates of cool, old white dwarfs over Gyr time-scales. Here, we revisit those results using updated white dwarf models and larger samples of old polluted H- and He-atmosphere white dwarfs. We find no compelling evidence for a strong decrease of their time-averaged mass accretion rates for cooling times between 1 and 8 Gyr. Over this period, the mass accretion rates decrease by no more than a factor of the order of 10, which is one order of magnitude smaller than the decay rate found in recent works. Our results require mechanisms that can efficiently and consistently deliver planetesimals inside the Roche radius of white dwarfs over at least 8 Gyr.

Blouin, Simon (ORCID:0000000296321436)↗

DOE OSTI2023

Black hole superradiance with dark matter accretion

Studies of black hole superradiance often focus on the growth of a cloud in isolation, accompanied by the spin-down of the black hole. Here, in this paper, we consider the additional effect of the accretion of matter and angular momentum from the environment. We show that, in many cases, the black hole evolves by drifting along the superradiance threshold, in which case the evolution of its parameters can be described analytically or semianalytically. We quantify the conditions under which accretion can serve as a mechanism to increase the cloud-to-black hole mass ratio, beyond the standard maximum of about 10%. This occurs by a process we call oversuperradiance, whereby accretion effectively feeds the superradiance cloud, by way of the black hole. We give two explicit examples: accretion from a vortex expected in wave dark matter and accretion from a baryonic disk. In the former case, we estimate the accretion rate by using an analytical fit to the asymptotic behavior of the confluent Heun function. Level transition, whereby one cloud level grows while the other shrinks, can be understood in a similar way.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

The Evolution of Accreting Binaries: From Brown Dwarfs to Supermassive Black Holes

Circumbinary accretion occurs throughout the universe, from the formation of stars and planets to the aftermath of major galactic mergers. We present an extensive investigation of circumbinary accretion disks, studying circular binaries with mass ratios (q ≡ M 2 /M 1 ) from 0.01 to 1 and at each mass ratio probing the effects of disk thickness and viscosity. We study disks with aspect ratios H/r $\in$ {0.1, 0.05, 0.03} and vary both the magnitude and spatial dependence of viscosity. Although thin accretion disks have previously been found to promote rapid inspirals of equal-mass binaries, we find that gravitational torques become weaker at lower mass ratios and most binaries with 0.01 ≤ q ≤ 0.04 outspiral, which may delay the coalescence of black hole binaries formed from minor mergers and cause high-mass exoplanets to migrate outward. However, in a number of cases, the disks accreting onto binaries with mass ratios ~0.07 fail to develop eccentric modes, leading to extremely rapid inspirals. Variability in black hole accretion correlates with disk eccentricity, and we observe variability above the ~10% level even for mass ratios of 0.01. We demonstrate that the spatial dependence of the viscosity (e.g., α vs. constant ν) significantly affects the degree of preferential accretion onto the secondary, resolving discrepancies between previous studies. Colder circumbinary disks remain eccentric even at q ~ 0.01 and sustain deep, asymmetric cavities.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Hydrodynamic Simulations of Oxygen–Neon Classical Novae as Galactic 7 Li Producers and Potential Accretion-induced Collapse Progenitors*

Abstract We report on studies of classical nova (CN) explosions where we follow the evolution of thermonuclear runaways (TNRs) on oxygen–neon (ONe) white dwarfs (WDs). Using NOVA, a 1D hydrodynamic computer code, we accrete solar matter until the TNR is ongoing and then switch to a mixed composition. This approach is guided by the results of multidimensional studies of TNRs in WDs, which find that sufficient mixing with WD core material occurs after the TNR is well underway, and levels of enrichment of the CNONeMg elements are reached that agree with observations of CN ejecta abundances. Because the amount of accreted material is inversely proportional to the oxygen abundance, by first accreting solar matter, the amount of accreted material is larger than in those simulations with an initially enriched composition. We vary the mass of the WD (from 0.6 M ⊙ to 1.35 M ⊙ ) and the composition of the mixed materials. Our results show large enrichments of 7 Be in the ejected gases, implying that ONe CNe and CO CNe may be responsible for a significant fraction (∼100 M ⊙ ) of the Galactic 7 Li (∼1000 M ⊙ ). The production of 22 Na and 26 Al in CN explosions and the γ -ray emission predicted by our simulations are discussed. The WDs in all our simulations eject less material than they accrete and we predict that the WD is growing in mass as a consequence of the CN outburst. ONe CNe, therefore, may be an important channel for accretion-induced collapse events.

Starrfield, Sumner (ORCID:0000000213596312)↗

DOE OSTI2024

Magnetically Arrested Circumbinary Accretion Flows

Abstract Binary systems with comparable masses and a surrounding accretion disk can accrete gas through spiral accretion streams penetrating the central cavity formed by tidal interactions. Using three-dimensional Newtonian magnetohydrodynamics simulations, we investigate the possibility of a magnetically arrested accretion flow through the cavity. Rather than solely continuously feeding the binary through spiral accretion streams, the accretion is regulated by the strong magnetic field inside the cavity. Transport of mass and angular momentum onto the binary then proceeds largely periodically in magnetic flux eruption episodes. The ejected flux tubes carry angular momentum outward and away from the binary, inject hot plasma into the disk, and can launch flares. This likely intermittent scenario could have potential implications for the emission signatures of supermassive black hole binaries and shed light onto the role magnetic fields play in the binary’s orbital evolution.

Astronomy & Astrophysics↗

NASA NTRS1978

Formation of a rotational accretion column

We consider the problem of the flow produced by the accretion of gas from a rotating cloud onto a point gravitational source. It is shown that, for a cloud initially in uniform rotation, the effect of accretion is to produce a Taylor column in the cloud parallel to the rotation axis. Only gas within the column is accreted; the gas outside the column undergoes oscillatory flow, which is probably subject to shear instability. The column forms in approximately one-half rotation period from the time at which accretion begins. Its initial radius is determined by the accretion rate and the angular velocity of the cloud. Application of the results to the formation of Jupiter by accretion is discussed.

Cassen, P.↗

NASA NTRS1978

Thermal limit for spherical accretion and X-ray bursts

A mechanism is described whereby the rate of spherical accretion on a collapsed object and the resulting X-ray luminosity can be limited by thermal heating and evaporation of the accreting gas. The luminosity limit can be a factor of the order of 100 below the corresponding Eddington limit. Steady accretion flow is not possible within a range above this limit, and X-ray bursts can be produced by accretion surges resulting from the cooling and collapse of a heated gas shell above a critical optical depth. The luminosity and approximate recurrence rate of the bursts from NGC 6624 and other burst sources may then be understood. A general model for bursters is proposed in which massive (about 10 to 100 solar masses) black holes, from massive stars and the cores of disrupted globular clusters, undergo supercritical spherical accretion in interstellar clouds. Both X-ray and gamma-ray bursts are predicted to have similar hard X-ray (about 200 keV) spectra, though high average accretion rates and central optical depths also yield a comparable luminosity at around 5 keV from a Comptonized blackbody spectrum in the X-ray bursts and associated steady sources.

Grindlay, J. E.↗

NASA NTRS1978

Disk accretion by magnetic neutron stars

A model for disk accretion by a rotating magnetic neutron star is proposed which includes a detailed description of matter flow in the transition region between the disk and the magnetosphere. It is shown that the disk plasma cannot be completely screened from the stellar magnetic field and that the resulting magnetic coupling between the star and the disk exerts a significant torque on the star. On the assumption that the distortion of the residual stellar field lines threading the disk is limited by reconnection, the total accretion torque on the star is calculated. The calculated torque gives period changes in agreement with those observed in the pulsating X-ray sources and provides a natural explanation of why a fast rotator like Her X-1 has a spin-up rate much below the conventional estimate for slow rotators. It is shown that for such fast rotators, fluctuations in the mass-accretion rate can produce fluctuations in the accretion torque about 100 times larger. For sufficiently fast rotators or, equivalently, for sufficiently low accretion rates, the star experiences a braking torque even while accretion continues and without any mass ejection from its vicinity.

Ghosh, P.↗

NASA NTRS1979

Gas-rich dwarfs and accretion phenomena in early-type galaxies

An analysis is presented of the combined effects of cloud accretion and galactic winds and coronae. An accretion model is developed wherein gas-rich dwarf galaxies are accreted into galactic halos, which provides an adequate source of H I to account for observations of neutral gas in early-type galaxies. Accretion is found to fuel the wind, thereby regulating the accretion flow and yielding a time-dependent model for star formation, enrichment, and nuclear activity. The permissible parameter range for intergalactic gas clouds and galaxy groups is discussed, along with the frequency of gas-rich dwarfs and their large ratios of gas mass to luminosity. Also considered is the occurrence of gas stripping and the consequent formation of dwarf spheroidal systems that remain in the halo, and gas clouds that dissipate and suffer further infall. A cosmological implication of the model is that, because the characteristic time scale of a gas-rich dwarf galaxy to be accreted and lose its gas is comparable to a Hubble time, there may have been a far more extensive primordial distribution of such systems at earlier epochs.

Silk, J.↗

NASA NTRS1980

X-ray spectra of accreting degenerate stars

X-ray emission from accreting nonmagnetic degenerate dwarfs is discussed. Emphasis is placed on the numerical calculations by Kylafis and Lamb (1979) with regard to the entire range of accretion rates and stellar masses. It is concluded that at moderate and high accretion rates the spectral temperature varies dramatically and the star exhibits a pronounced correlation between X-ray spectral temperature and luminosity. Also considered are the results of Lamb and Masters (1979) of calculations of high-harmonic cyclotron emission from a hot plasma, from which they developed a model of X-ray and UV emission from accreting magnetic degenerate dwarfs. It is found that the position and relative strength of the spectral components change with variations in the accretion rate and differ for different magnetic fields; in addition, magnetic degenerate dwarfs are predicted to be strong UV sources with only a few percent of the total accretion luminosity ordinarily appearing as soft and hard X-rays.

Kylafis, N. D.↗

NASA NTRS1981

Accreting white dwarf models for type 1 supernovae. 1: Presupernova evolution and triggering mechanisms

As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.

Nomoto, K.↗

NASA NTRS1982

X-ray and UV radiation from accreting nonmagnetic degenerate dwarfs. II

Numerical calculations of X-ray and UV emission from accreting nonmagnetic degenerate dwarfs are reported, which span the entire range of accretion rates and stellar masses. Calculations include the effects of bremsstrahlung, Compton cooling, radiation pressure, albedo of the stellar surface, Compton degradation and free-free abscription of the X-ray spectrum by the accreting matter. Maximum X-ray luminosity for degenerate dwarfs undergoing spherical accretion is found to be 2.2 x 10 to the 36th ergs/s, which is little changed if accretion occurs radially over only a fraction of the stellar surface, so that the emitted radiation escapes without significant scattering. The temperature characterizing the X-ray spectra produced by degenerate dwarfs strongly depends on the stellar mass and the accretion rate, and it is suggested that the correlation between spectral temperature and luminosity is an important signature of degenerate X-ray sources.

Kylafis, N. D.↗

NASA NTRS1985

Jet production in super-Eddington accretion disks

A two-dimensional, radiation-coupled, Newtonian hydrodynamic simulation is reported for a super-Eddington, mass accretion rate, M = 4 M(E) disk accretion flow onto a 3-solar mass pseudoblack hole. Near the disk midplane, convection cells effectively block the accretion flow, even though viscous heating maximizes there. Accretion predominantly occurs in a supersonic inflow which follows streamlines of approximately constant angular momentum. The optically thick inflow traps radiation so that 80 percent of the luminosity is absorbed by the black hole; the emergent power is sub-Eddington. An axial jet self consistently forms just outside a conical photosphere which bounds the accretion zone; radiation pressure accelerates the jet to about 10 to the 10th cm/s. The jet's mass efflux is only 0.4 percent of the total mass accretion rate.

Eggum, G. E.↗

NASA NTRS1987

Timescales for planetary accretion and the structure of the protoplanetary disk

Models of planetary accretion which assume the mass of condensable matter in the protoplanetary disk was equal to that present in the planets today predict accretion timescales for the outer planets approximately or less than 10 to the 8th years. Such timescales are inconsistent with observations of star forming regions, which suggest that most of the gas in disks around one solar mass is removed in a few x 10 to the 6th years. A unified scenario was outlined for solar system formation consistent with astrophysical constraints. Jupiter's core could have grown by runaway accretion of planetesimals to a mass sufficient to initiate rapid accretion of gas in times of order of 500,000 to 5,000,000 years, provided the surface density of solids in its accretion zone was at least 5 to 10 times greater than that required by minimum mass models of the protoplanetary disk. The inner planets and the asteroids can be accounted for in this picture if the surface density of the solar nebula was relatively uniform out to Jupiter's orbit. The formation of such a protoplanetary disk requires significant transport of mass and angular momentum, and is consistent with viscous accretion disk models of the solar nebula.

Lissauer, Jack J.↗

NASA NTRS1987

A scaling law for accretion zone sizes

Current theories of runaway planetary accretion require small random velocities of the accreted particles. Two body gravitational accretion cross sections which ignore tidal perturbations of the Sun are not valid for the slow encounters which occur at low relative velocities. Wetherill and Cox have studied accretion cross sections for rocky protoplanets orbiting at 1 AU. Using analytic methods based on Hill's lunar theory, one can scale these results for protoplanets that occupy the same fraction of their Hill sphere as does a rocky body at 1 AU. Generalization to bodies of different sizes is achieved here by numerical integrations of the three-body problem. Starting at initial positions far from the accreting body, test particles are allowed to encounter the body once, and the cross section is computed. A power law is found relating the cross section to the radius of the accreting body (of fixed mass).

Greenzweig, Yuval↗

NASA NTRS1988

Accretional heating of the satellites of Saturn and Uranus

A model developed specifically for accretional heating of small icy satellites of massive planets was used to investigate the effects of various parameters on the accretional heating of the Saturn and Uranus satellites. It was shown that accretional heating is completely dominated by cisplanetary impactors, with substantial deposition of heat occurring well below a satellite's surface. Three parameters that have large uncertainties were found to affect accretional temperature profile: the Safronov parameter for cisplanetary impactors, the fraction of impactor energy deposited below the surface, and the exponent in the impactor size-distribution equation. For most satellites, the depth of maximum heating and the maximum temperature reached have smooth positive dependences on satellite size. Accretional heating of these satellites is insufficient to melt H2O ice, but, if NH3 or CH4 are present, accretional heating may produce a warm buoyant mobile zone tens of kilometers below the surface.

Squyres, Steven W.↗