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We investigate nucleosynthesis in the sub-relativistic outflows from black hole (BH) accretion disks formed in failed supernovae from rapidly-rotating Wolf-Rayet stars. These disks reach the neutrino-cooled regime during a portion of their evolution, undergoing significant neutronization and thus having the potential to support the $r$-process. Here, we analyze the formation of heavy elements in the ejecta from global, axisymmetric, long-term, viscous hydrodynamic simulations of these systems that include neutrino emission and absorption, Newtonian self-gravity, a pseudo-Newtonian potential for the BH gravity, and a 19-isotope nuclear network. Tracer particles are used for post-processing with a larger network. In addition to analyzing models from a previous paper, we present new models in which we modify the rotation profile of the progenitor star, to maximize neutrino reprocessing of circularized mass shells. All of our models produce several $M_\odot$ of O, followed by about a solar mass of C, Ne, and Ni, with other alpha elements produced in smaller quantities. Only one of our models, with the lowest viscosity, yields significant amounts of first $r$-process peak elements, with negligible yields at higher nuclear masses. The rest of the set, including models with a modified rotation profile, produces very small or negligible quantities of elements beyond the iron group. Models that produce the heaviest elements (up to $A\sim200$) do so along the proton-rich side of the valley of stability at high entropy ($s/k_B\sim80$), pointing to the $rp$-process as a mechanism that operates in collapsars. The absence of neutron-rich ejecta proves to be insensitive to changes in the rotation profile of the star, suggesting that heavy $r$-process elements are difficult to produce in collapsars if no large-scale poloidal magnetic field is present in the disk to drive outflows during neutronization., Comment: Accepted by PRD, Part 2 of arXiv:2403.08877. Minor changes in the arXiv abstract to fit character limit

We investigate mass ejection from accretion disks formed during the collapse of rapidly-rotating Wolf-Rayet stars. The neutrino-cooled, black hole (BH) accretion disk system that forms at the center of the star -- and the ensuing outflows -- provide the conditions for these systems to be candidate $r$-process element production sites and potential progenitors of broad-lined Type Ic (Ic-BL) supernovae. Here we present global, long-term axisymmetric hydrodynamic simulations of collapsar disks that include angular momentum transport through shear viscosity, neutrino emission and absorption, a 19-isotope nuclear reaction network and nuclear statistical equilibrium solver, a pseudo-Newtonian BH with mass and spin modified by accreted matter, and self-gravity. Starting from a stellar profile collapsed in spherical symmetry, our models capture disk formation self-consistently, and are evolved until after the shock wave -- driven by disk winds -- reaches the surface of the star. None of our models achieve sufficient neutronization to eject significant amounts of $r$-process elements (detailed nucleosynthesis calculations will follow in a companion paper). Sufficient $^{56}$Ni is produced to power a typical type Ic-BL supernova light curve, but the average asymptotic velocity is a factor $\sim 2-3$ times too slow to account for the typical line widths in type Ic-BL supernova spectra. The gap in neutrino emission between BH formation and shocked disk formation, and the magnitude of the subsequent peak in emission, would be observable diagnostics of the internal conditions of the progenitor in a galactic collapsar. Periodic oscillations of the shocked disk prior to its expansion are also a potential observable through their impact on the the neutrino and gravitational wave signals., Comment: Accepted by PRD

In kilonovae, freshly-synthesized $r$-process elements imprint features on optical spectra, as observed in AT2017gfo, the counterpart to the GW170817 binary neutron star merger. However, measuring the $r$-process compositions of the merger ejecta is computationally challenging. Vieira et al. (2023) introduced Spectroscopic $r$-Process Abundance Retrieval for Kilonovae (SPARK), a software tool to infer elemental abundance patterns of the ejecta, and associate spectral features with particular species. Previously, we applied SPARK to the 1.4 day spectrum of AT2017gfo and inferred its abundance pattern for the first time, characterized by electron fraction $Y_e=0.31$, a substantial abundance of strontium, and a dearth of lanthanides and heavier elements. This ejecta is consistent with wind from a remnant hypermassive neutron star and/or accretion disk. We now extend our inference to spectra at 2.4 and 3.4 days, and test the need for multicomponent ejecta, where we stratify the ejecta in composition. The ejecta at 1.4 and 2.4 days is described by the same single blue component. At 3.4 days, a new redder component with lower $Y_e=0.16$ and a significant abundance of lanthanides emerges. This new redder component is consistent with dynamical ejecta and/or neutron-rich ejecta from a magnetized accretion disk. As expected from photometric modelling, this component emerges as the ejecta expands, the photosphere recedes, and the earlier bluer component dims. At 3.4 days, we find an ensemble of lanthanides, with the presence of cerium most concrete. This presence of lanthanides has important implications for the contribution of kilonovae to the $r$-process abundances observed in the Universe., Comment: Accepted to ApJ; 19 pages, 8 figures in body; 10 pages, 7 figures in appendix

Magnetized hypermassive neutron stars (HMNSs) have been proposed as a way for neutron star (NS) mergers to produce high electron fraction, high velocity ejecta, as required by kilonova models to explain the observed light curve of GW170817. The HMNS drives outflows through neutrino energy deposition and mechanical oscillations, and raises the electron fraction of outflows through neutrino interactions before collapsing to a black hole (BH). Here we perform 3D numerical simulations of HMNS-torus systems in ideal magnetohydrodynamics, using a leakage/absorption scheme for neutrino transport, the nuclear APR equation of state, and Newtonian self-gravity, with a pseudo-Newtonian potential added after BH formation. Due to the uncertainty in the HMNS collapse time, we choose two different parameterized times to induce collapse. We also explore two initial magnetic field geometries in the torus, and evolve the systems until the outflows diminish significantly ($\sim 1 - 2$ $\mathrm{s}$). We find bluer, faster outflows as compared to equivalent BH-torus systems, producing $M\sim 10^{-3} M_\odot$ of ejecta with $Y_e \geq 0.25$ and $v \geq 0.25c$ by the simulation end. Approximately half the outflows are launched in disk winds at times $t\lesssim 500$ $\mathrm{ms}$, with a broad distribution of electron fractions and velocities, depending on the initial condition. The remaining outflows are thermally-driven, characterized by lower velocities and electron fractions. Nucleosynthesis with tracer particles shows patterns resembling solar abundances in all models. Although outflows from our simulations do not match those inferred from two-component modelling of the GW170817 kilonova, self-consistent multidimensional detailed kilonova models are required to determine if our outflows can power the blue kilonova., Comment: 15 pages, 10 figures. Accepted for publication in MNRAS

The accretion disk formed after a neutron star merger is an important contributor to the total ejecta from the merger, and hence to the kilonova and the $r$-process yields of each event. Axisymmetric viscous hydrodynamic simulations of these disks can capture thermal mass ejection due to neutrino absorption and in the advective phase -- after neutrino cooling has subsided -- and are thus likely to provide a lower-limit to the total disk ejecta relative to MHD evolution. Here we present a comparison between two viscous hydrodynamic codes that have been used extensively on this problem over the past decade: ALCAR and FLASH. We choose a representative setup with a black hole at the center, and vary the treatment of viscosity and neutrino transport. We find good overall agreement ($\sim 10\%$ level) in most quantities. The average outflow velocity is sensitive to the treatment of the nuclear binding energy of heavy nuclei, showing a larger variation than other quantities. We post-process trajectories from both codes with the same nuclear network, and explore the effects of code differences on nucleosynthesis yields, heating rates, and kilonova light curves. For the latter, we also assess the effect of varying the number of tracer particles in reconstructing the spatial abundance distribution for kilonova light curve production., Comment: Accepted by PRD. Tracked down small initial difference in degeneracy parameter to differences in the entropy of ions in each EOS. Minor changes otherwise

Freshly-synthesized r-process elements in kilonovae ejecta imprint absorption features on optical spectra, as observed in the GW170817 binary neutron star merger. These spectral features encode insights into the physical conditions of the r-process and the origins of the ejected material, but associating features with particular elements and inferring the resultant abundance pattern is computationally challenging. We introduce Spectroscopic r-Process Abundance Retrieval for Kilonovae (SPARK), a modular framework to perform Bayesian inference on kilonova spectra with the goals of inferring elemental abundance patterns and identifying absorption features at early times. SPARK inputs an atomic line list and abundance patterns from reaction network calculations into the TARDIS radiative transfer code. It then performs fast Bayesian inference on observed kilonova spectra by training a Gaussian process surrogate for the approximate posteriors of kilonova ejecta parameters, via active learning. We use the spectrum of GW170817 at 1.4 days to perform the first inference on a kilonova spectrum, and recover a complete abundance pattern. Our inference shows that this ejecta was generated by an r-process with either (1) high electron fraction Y_e ~ 0.35 and high entropy s/k_B ~ 25, or, (2) a more moderate Y_e ~ 0.30 and s/k_B ~ 14. These parameters are consistent with a shocked, polar dynamical component, and a viscously-driven outflow from a remnant accretion disk, respectively. We also recover previous identifications of strontium absorption at ~8000 AA, and tentatively identify yttrium and/or zirconium at < 4500 AA. Our approach will enable computationally-tractable inference on the spectra of future kilonovae discovered through multi-messenger observations., Comment: Accepted to ApJ; 34 pages, 13 figures

Detectable electromagnetic counterparts to gravitational waves from compact binary mergers can be produced by outflows from the black hole-accretion disk remnant during the first ten seconds after the merger. Two-dimensional axisymmetric simulations with effective viscosity remain an efficient and informative way to model this late-time post-merger evolution. In addition to the inherent approximations of axisymmetry and modeling turbulent angular momentum transport by a viscosity, previous simulations often make other simplifications related to the treatment of the equation of state and turbulent transport effects. In this paper, we test the effect of these modeling choices. By evolving with the same viscosity the exact post-merger initial configuration previously evolved in Newtonian viscous hydrodynamics, we find that the Newtonian treatment provides a good estimate of the disk ejecta mass but underestimates the outflow velocity. We find that the inclusion of heavy nuclei causes a notable increase in ejecta mass. An approximate inclusion of r-process effects has a comparatively smaller effect, except for its designed effect on the composition. Diffusion of composition and entropy, modeling turbulent transport effects, has the overall effect of reducing ejecta mass and giving it a speed with lower average and more tightly-peaked distribution. Also, we find significant acceleration of outflow even at distances beyond 10,000\,km, so that thermal wind velocities only asymptote beyond this radius and at somewhat higher values than previously reported., Comment: 23 pages, 7 figures, version accepted to Classical and Quantum Gravity

We examine the effect of neutrino flavor transformation by the fast flavor instability (FFI) on long-term mass ejection from accretion disks formed after neutron star mergers. Neutrino emission and absorption in the disk set the composition of the disk ejecta, which subsequently undergoes $r$-process nucleosynthesis upon expansion and cooling. Here we perform 28 time-dependent, axisymmetric, viscous-hydrodynamic simulations of accretion disks around hypermassive neutron stars (HMNSs) of variable lifetime, using a 3-species neutrino leakage scheme for emission and an annular-lightbulb scheme for absorption. We include neutrino flavor transformation due the FFI in a parametric way, by modifying the absorbed neutrino fluxes and temperatures, allowing for flavor mixing at various levels of flavor equilibration, and also in a way that aims to respect the lepton-number preserving symmetry of the neutrino self-interaction Hamiltonian. We find that for a promptly-formed black hole (BH), the FFI lowers the average electron fraction of the disk outflow due to a decrease in neutrino absorption, driven primarily by a drop in electron neutrino/antineutrino flux upon flavor mixing. For a long-lived HMNS, the disk emits more heavy lepton neutrinos and reabsorbs more electron neutrinos than for a BH, with a smaller drop in flux compensated by a higher neutrino temperature upon flavor mixing. The resulting outflow has a broader electron fraction distribution, a more proton-rich peak, and undergoes stronger radiative driving. Disks with intermediate HMNS lifetimes show results that fall in between these two limits. In most cases, the impact of the FFI on the outflow is moderate, with changes in mass ejection, average velocity, and average electron fraction of order $\sim 10\%$, and changes in the lanthanide/actinide mass fraction of up to a factor $\sim 2$., Comment: Accepted by PRD. Added appendix with derivation of asymmetric oscillation coefficients, minor changes otherwise

We examine the long-term evolution of accretion tori around black hole (BH) remnants of compact object mergers involving at least one neutron star, to better understand their contribution to kilonovae and the synthesis of r-process elements. To this end, we modify the unsplit magnetohydrodynamic (MHD) solver in FLASH4.5 to work in non-uniform three-dimensional spherical coordinates, enabling more efficient coverage of a large dynamic range in length scales while exploiting symmetries in the system. This modified code is used to perform BH accretion disk simulations that vary the initial magnetic field geometry and disk compactness, utilizing a physical equation of state, a neutrino leakage scheme for emission and absorption, and modeling the BH's gravity with a pseudo-Newtonian potential. Simulations run for long enough to achieve a radiatively-inefficient state in the disk. We find robust mass ejection with both poloidal and toroidal initial field geometries, and suppressed outflow at high disk compactness. With the included physics, we obtain bimodal velocity distributions that trace back to mass ejection by magnetic stresses at early times, and to thermal processes in the radiatively-inefficient state at late times. The electron fraction distribution of the disk outflow is broad in all models, and the ejecta geometry follows a characteristic hourglass shape. We test the effect of removing neutrino absorption or nuclear recombination with axisymmetric models, finding $\sim 50\%$ less mass ejection and more neutron-rich composition without neutrino absorption, and a subdominant contribution from nuclear recombination. Tests of the MHD and neutrino leakage implementations are included., Comment: Fixed error in timestamps in Figure 4

Over the next year, a new era of observations of compact objects in X-ray polarization will commence. Among the key targets for the upcoming Imaging X-ray Polarimetry Explorer mission, will be the magnetars 4U 0142+61 and 1RXS J170849.0-400910. Here we present the first detailed predictions of the expected polarization from these sources that incorporate realistic models of emission physics at the surface (gaseous or condensed), the temperature distribution on the surface, general relativity, quantum electrodynamics and scattering in the magnetosphere, and also account for the broadband spectral energy distribution of these sources from below 1 keV to nearly 100 keV. We find that either atmospheres or condensed surfaces can account for the emission at a few keV; in both cases either a small hot polar cap or scattering is required to account for the emission at 5-10 keV, and above 10 keV scattering by a hard population of electrons can account for the rising power in the hard X-rays observed in many magnetars in quiescence. Although these different scenarios result in very similar spectral energy distributions, they generate dramatically different polarization signatures from 2-10 keV, which is the range of sensitivity of the Imaging X-ray Polarimetry Explorer. Observations of these sources in X-ray polarization will therefore probe the emission from magnetars in an essentially new way., Comment: 11 pages, 5 figures, 1 table

We examine the effect of spatial resolution on initial mass ejection in grid-based hydrodynamic simulations of binary neutron star mergers. The subset of the dynamical ejecta with velocities greater than $\sim 0.6$c can generate an ultraviolet precursor to the kilonova on $\sim$hr timescales and contribute to a years-long non-thermal afterglow. Previous work has found differing amounts of this fast ejecta, by one- to two orders of magnitude, when using particle-based or grid-based hydrodynamic methods. Here we carry out a numerical experiment that models the merger as an axisymmetric collision in a co-rotating frame, accounting for Newtonian self-gravity, inertial forces, and gravitational wave losses. The lower computational cost allows us to reach spatial resolutions as high as $4$m, or $\sim 3\times 10^{-4}$ of the stellar radius. We find that fast ejecta production converges to within $10\%$ for a cell size of $20$m. This suggests that fast ejecta quantities found in existing grid-based merger simulations are unlikely to increase to the level needed to match particle-based results upon further resolution increases. The resulting neutron-powered precursors are in principle detectable out to distances $\lesssim 200$Mpc with upcoming facilities. We also find that head-on collisions at the free-fall speed, relevant for eccentric mergers, yield fast and slow ejecta quantities of order $10^{-2}M_\odot$, with a kilonova signature distinct from that of quasi-circular mergers., Comment: Accepted by ApJ

Neutron star merger accretion discs can launch neutron-rich winds of $>10^{-2}\,\mathrm{M}_\odot$. This ejecta is a prime site for r-process nucleosynthesis, which will produce a range of radioactive heavy nuclei. The decay of these nuclei releases enough energy to accelerate portions of the wind by ~0.1c. Here, we investigate the effect of r-process heating on the dynamical evolution of disc winds. We extract the wind from a 3D general relativistic magnetohydrodynamic simulation of a disc from a post-merger system. This is used to create inner boundary conditions for 2D hydrodynamic simulations that continue the original 3D simulation. We perform two such simulations: one that includes the r-process heating, and another one that does not. We follow the hydrodynamic simulations until the winds reach homology (60 seconds). Using time-dependent multi-frequency multi-dimensional Monte Carlo radiation transport simulations, we then calculate the kilonova light curves from the winds with and without dynamical r-process heating. We find that the r-process heating can substantially alter the velocity distribution of the wind, shifting the mass-weighted median velocity from 0.06c to 0.12c. The inclusion of the dynamical r-process heating makes the light curve brighter and bluer at ~1 d post-merger. However, the high-velocity tail of the ejecta distribution and the early light curves are largely unaffected., Comment: 12 pages, 10 comments. Submitted to MNRAS

An excess in the X-ray emission from the neutron star merger GW170817 above the predicted afterglow was recently detected t~3.4 years post-merger. One possible origin for the excess is accretion onto the newly unshrouded black hole (BH) remnant. While fall-back of the bound dynamical ejecta is insufficient to generate the excess luminosity, L_X ~ 5e38 erg/s, fall-back from the disk wind ejecta-due to their larger mass and lower velocity-remains a viable possibility. We present hydrodynamic alpha-viscosity simulations of the post-merger disk evolution which extend to an unprecedently long timescale t ~ 35 s post-merger, as necessary to capture the end of photodissociation and the asymptotic evolution into the radiatively inefficient regime. Due to inefficient neutrino cooling, the BH accretion rate decays rapidly at late times (Mdot_BH ~ t^(-\beta_BH), where \beta_BH ~ 2.4-2.8), seemingly incompatible with generating the late-time excess. However, the rate at which matter falls back to the inner disk from the equatorial regions (as inferred by the rate matter is unbound in outflows by viscous heating at higher latitudes) decays more gradually, Mdot_fb ~ t^(-\beta_fb) with \beta_fb ~ 1.43 in our alpha ~ 0.03 simulations. By the present epoch, the fall-back rate has become sub-Eddington and the disk can again accrete efficiently, i.e. Mdot_BH ~ Mdot_fb, this time as a result of photon cooling instead of neutrino cooling. The predicted X-ray accretion luminosity at the present epoch is L_X ~ 0.1 Mdot_BH c^2 ~ (2-70)e38 erg/s for beta_FB ~ 1.43-1.66, thus supporting (with caveats) an accretion-powered origin for the X-ray excess in GW170817. The suppressed BH accretion rate prior to the radiatively efficient (sub-Eddington) transition, weeks to months after the merger, is key to avoid overproducing the kilonova luminosity via reprocessing., Comment: 5 pages, 2 figures, ApJL

In recent years, there have been significant advances in multi-messenger astronomy due to the discovery of the first, and so far only confirmed, gravitational wave event with a simultaneous electromagnetic (EM) counterpart, as well as improvements in numerical simulations, gravitational wave (GW) detectors, and transient astronomy. This has led to the exciting possibility of performing joint analyses of the GW and EM data, providing additional constraints on fundamental properties of the binary progenitor and merger remnant. Here, we present a new Bayesian framework that allows inference of these properties, while taking into account the systematic modeling uncertainties that arise when mapping from GW binary progenitor properties to photometric light curves. We extend the relative binning method presented in Zackay et al. (2018) to include extrinsic GW parameters for fast analysis of the GW signal. The focus of our EM framework is on light curves arising from r-process nucleosynthesis in the ejected material during and after merger, the so called kilonova, and particularly on black hole - neutron star systems. As a case study, we examine the recent detection of GW190425, where the primary object is consistent with being either a black hole (BH) or a neutron star (NS). We show quantitatively how improved mapping between binary progenitor and outflow properties, and/or an increase in EM data quantity and quality are required in order to break degeneracies in the fundamental source parameters., Comment: 22 pages, 12 figures, comments are welcome

A failed core-collapse supernova from a non-rotating progenitor can eject mass due to a weakening of gravity associated to neutrino emission by the protoneutron star. This mechanism yields observable transients and sets an upper limit to the mass of the black hole (BH) remnant. Previous global simulations of this mechanism have included neutrino losses parametrically, however, with direct implications for the ejecta mass and energy. Here we evolve the inner supernova core with a spherically-symmetric, general-relativistic neutrino radiation-hydrodynamic code until BH formation. We then use the result in a Newtonian code that follows the response of the outer layers of the star to the change in gravity and resolves the surface pressure scale height. We find that the dense-matter equation of state (EOS) can introduce a factor $\sim 2$ variation in gravitational mass lost to neutrinos, with a stiff EOS matching previous parametric results, and a soft EOS yielding lower ejecta masses and energies by a factor of several. This difference is caused primarily by the longer time to BH formation in stiffer EOSs. With a soft EOS, our red and yellow supergiant progenitors fail to unbind mass if hydrogen recombination energy is not included. Using a linear ramp in time for mass-energy lost to neutrinos (with suitable parameters) yields a stellar response within $\sim 10\%$ of that obtained using the detailed history of neutrino losses. Our results imply quantitative but not qualitative modifications to previous predictions for shock breakout, plateau emission, and final BH masses from these events., Comment: Accepted by ApJ with minor corrections. New equation (6) quantifies the difference between baryonic and gravitational mass in the pre-supernova progenitor

Neutron star mergers (NSMs) are promising astrophysical sites for the rapid neutron-capture ("$r$-") process, but can their integrated yields explain the majority of heavy-element material in the Galaxy? One method to address this question has utilized a forward approach that propagates NSM rates and yields along with stellar formation rates, in the end comparing those results with observed chemical abundances of $r$-process-rich, metal-poor stars. In this work, we take the inverse approach by utilizing $r$-process-element abundance ratios of metal-poor stars as input to reconstruct the properties---especially the masses---of the neutron star (NS) binary progenitors of the $r$-process stars. This novel analysis provides an independent avenue for studying the population of the original neutron star binary systems that merged and produced the $r$-process material incorporated in Galactic metal-poor halo stars. We use ratios of elements typically associated with the limited-$r$ process and the actinide region to those in the lanthanide region (i.e., Zr/Dy and Th/Dy) to probe the NS masses of the progenitor merger. We find that NSMs can account for all $r$-process material in metal-poor stars that display $r$-process signatures, while simultaneously reproducing the present-day distribution of double-NS (DNS) systems. However, the most $r$-process enhanced stars (the $r$-II stars) on their own would require progenitor NSMs of very asymmetric systems that are distinctly different from present ones in the Galaxy. As this analysis is model-dependent, we also explore variations in line with future expectation regarding potential theoretical and observational updates, and comment on how these variations impact our results., Comment: Submitted to ApJ, 23 pages, 12 figures

The Cosmic Censorship Conjecture (CCC) states that every singularity (except the cosmological one) must appear "dressed" in the universe. This statement was introduced by Roger Penrose (Penrose, 1969), meaning that every singularity (except the Big Bang) in the universe must be hidden inside an Event Horizon. Mathematically, this is described by the inequality $M^2 \geqslant Q^2 + a^2$ (in geometrized unit system), with $M$ being the mass of the black hole, $Q$ its charge and $a := J/M$ its specific angular momentum. Essentially, this three quantities determines uniquely a black hole, as stated by the no-hair theorem. We study the emission probability of a massive ($m_w$) uncharged scalar wave packet, a semi-classical approximation for a particle, by a static, charged black hole. We show that for a few values of the mass $\mathcal{M} := M+\delta M$ (where $M$ is the fixed value for the mass and $\delta M$ being a small variation to $M$ in the order of $m_w$) with different values for $\delta M$ and fixed charge $Q$ for the black hole, the emission probability tends to zero once the Cosmic Censorship Conjecture is close to be violated, that is, when the emitted packet is such that the new quantity $\mathcal{M}' := \mathcal{M}-m_w$ would violate the inequality $\mathcal{M}' > Q$., Comment: 80 pages, 25 figures, accepted PhD Thesis (Instituto de F\'isica, UFRJ). Chapter 4 originated article published in arXiv:2007.06766

We study the tunneling probability of a massive ($m_w$) uncharged scalar packet out from a near-extremal, static charged black hole (with mass $M$ and charge $Q\lesssim M$). We show that there is indeed a net probability that a massive uncharged particle tunnels out from the black hole so that the final state (with new mass $M'\equiv M-m_w < Q$) does violate the cosmic censorship conjecture. Nevertheless, the typical time for such a black hole to discharge (i.e, to absorb charge $-Q$ from its surroundings and then become neutral) is much smaller than the tunneling time; therefore, the violation is never attained in practice. Even for a completely isolated black hole (should it exist), the standard time dilation near the horizon stretches the typical violation time scale to unobservable values., Comment: 9 pages, 8 figures

We investigate mass ejection from accretion disks formed in mergers of black holes (BHs) and neutron stars (NSs). The third observing run of the LIGO/Virgo interferometers provided BH-NS candidate events that yielded no electromagnetic (EM) counterparts. The broad range of disk configurations expected from BH-NS mergers motivates a thorough exploration of parameter space to improve EM signal predictions. Here we conduct 27 high-resolution, axisymmetric, long-term hydrodynamic simulations of the viscous evolution of BH accretion disks that include neutrino emission/absorption effects and post-processing with a nuclear reaction network. In the absence of magnetic fields, these simulations provide a lower-limit to the fraction of the initial disk mass ejected. We find a nearly linear inverse dependence of this fraction on disk compactness (BH mass over initial disk radius). The dependence is related to the fraction of the disk mass accreted before the outflow is launched, which depends on the disk position relative to the innermost stable circular orbit. We also characterize a trend of decreasing ejected fraction and decreasing lanthanide/actinide content with increasing disk mass at fixed BH mass. This trend results from a longer time to reach weak freezout and an increasingly dominant role of neutrino absorption at higher disk masses. We estimate the radioactive luminosity from the disk outflow alone available to power kilonovae over the range of configurations studied, finding a spread of two orders of magnitude. For most of the BH-NS parameter space, the disk outflow contribution is well below the kilonova mass upper limits for GW190814., Comment: Accepted by MNRAS with minor revisions

We present a wide-field optical imaging search for electromagnetic counterparts to the likely neutron star - black hole (NS-BH) merger GW190814/S190814bv. This compact binary merger was detected through gravitational waves by the LIGO/Virgo interferometers, with masses suggestive of a NS-BH merger. We imaged the LIGO/Virgo localization region using the MegaCam instrument on the Canada-France-Hawaii Telescope. We describe our hybrid observing strategy of both tiling and galaxy-targeted observations, as well as our image differencing and transient detection pipeline. Our observing campaign produced some of the deepest multi-band images of the region between 1.7 and 8.7 days post-merger, reaching a 5sigma depth of g > 22.8 (AB mag) at 1.7 days and i > 23.1 and i > 23.9 at 3.7 and 8.7 days, respectively. These observations cover a mean total integrated probability of 67.0% of the localization region. We find no compelling candidate transient counterparts to this merger in our images, which suggests that either the lighter object was tidally disrupted inside of the BH's innermost stable circular orbit, the transient lies outside of the observed sky footprint, or the lighter object is a low-mass BH. We use 5sigma source detection upper limits from our images in the NS-BH interpretation of this merger to constrain the mass of the kilonova ejecta to be Mej < 0.015Msun for a 'blue' (kappa = 0.5 cm^2 g^-1) kilonova, and Mej < 0.04Msun for a 'red' (kappa = 5-10 cm^2 g^-1) kilonova. Our observations emphasize the key role of large-aperture telescopes and wide-field imagers such as CFHT MegaCam in enabling deep searches for electromagnetic counterparts to gravitational wave events., Comment: 23 pages, 13 figures including appendices; accepted to ApJ with minor revisions

The origin of many elements of the periodic table remains an unsolved problem. While many nucleosynthetic channels are broadly understood, significant uncertainties remain regarding certain groups of elements such as the intermediate and rapid neutron-capture processes, the p-process, or the origin of odd-Z elements in the most metal-poor stars. Canada has a long tradition of leadership in nuclear astrophysics, dating back to the work of Alastair Cameron in the 1950s. Recent faculty hires have further boosted activity in the field, including transient observation and theory, survey science on galactic nucleosynthesis, and nuclear experiments. This white paper contains a brief overview of recent activity in the community, highlighting strengths in each sub-field, and provides recommendations to improve interdisciplinary collaboration. Sustaining Canadian leadership in the next decade will require, on the observational side, access to transient and non-transient surveys like LSST, SKA, or MSE, support for target-of-opportunity observing in current and future Canadian telescopes, and participation in next-generation X-ray telescopes such as ATHENA. State-of-the-art theoretical predictions will require an ambitious succession plan for the Niagara supercomputer to support large parallel jobs. We propose a funding instrument for postdoctoral training that reflects the interdisciplinary nature of nuclear astrophysics research, and the creation of a national collaborative funding program that allows for joint projects and workshop organization., Comment: White paper submitted to the Canadian Long Range Plan 2020. Minor formatting changes relative to submitted version

We study mass ejection from accretion disks formed in the merger of a white dwarf with a neutron star or black hole. These disks are mostly radiatively-inefficient and support nuclear fusion reactions, with ensuing outflows and electromagnetic transients. Here we perform time-dependent, axisymmetric hydrodynamic simulations of these disks including a physical equation of state, viscous angular momentum transport, a coupled $19$-isotope nuclear network, and self-gravity. We find no detonations in any of the configurations studied. Our global models extend from the central object to radii much larger than the disk. We evolve these global models for several orbits, as well as alternate versions with an excised inner boundary to much longer times. We obtain robust outflows, with a broad velocity distribution in the range $10^2-10^4$ km s$^{-1}$. The outflow composition is mostly that of the initial white dwarf, with burning products mixed in at the $\lesssim 10-30\%$ level by mass, including up to $\sim 10^{-2}M_\odot$ of ${}^{56}$Ni. These heavier elements (plus ${}^{4}$He) are ejected within $\lesssim 40^\circ$ of the rotation axis, and should have higher average velocities than the lighter elements that make up the white dwarf. These results are in broad agreement with previous one- and two-dimensional studies, and point to these systems as progenitors of rapidly-rising ($\sim $ few day) transients. If accretion onto the central BH/NS powers a relativistic jet, these events could be accompanied by high energy transients with peak luminosities $\sim 10^{47}-10^{50}$ erg s$^{-1}$ and peak durations of up to several minutes, possibly accounting for events like CDF-S XT2., Comment: Accepted by MNRAS. Updated Fig 10 (velocity distribution for NS or BH) and added new Fig 15 (comparison with exploding model from Paper I). Minor changes otherwise

We study mass ejection from accretion disks around newly-formed hypermassive neutron stars (HMNS). Standard kilonova model fits to GW170817 require at least a lanthanide-poor ('blue') and lanthanide-rich ('red') component. The existence of a blue component has been used as evidence for a HMNS remnant of finite lifetime, but average disk outflow velocities from existing long-term HMNS simulations fall short of the inferred value ($\sim 0.25c$) by a factor of $\sim 2$. Here we use time-dependent, axisymmetric hydrodynamic simulations of HMNS disks to explore the limits of the model and its ability to account for observations. For physically plausible parameter choices compatible with GW170817, we find that hydrodynamic models that use shear viscosity to transport angular momentum cannot eject matter with mass-averaged velocities larger than $\sim 0.15c$. While outflow velocities in our simulations can exceed the asymptotic value for a steady-state neutrino-driven wind, the increase in the average velocity due to viscosity is not sufficient. Therefore, viscous HMNS disk winds cannot reproduce by themselves the ejecta properties inferred from multi-component fits to kilonova light curves from GW170817. Three possible resolutions remain feasible within standard merger ejecta channels: more sophisticated radiative transfer models that allow for photon reprocessing between ejecta components, inclusion of magnetic stresses, or enhancement of the dynamical ejecta. We provide fits to our disk outflow models once they reach homologous expansion., Comment: Accepted by ApJ letters