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Register a new userInclination Dependence of Kilonova Light Curves from Globally Aspherical Geometries
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The merger of two neutron stars (NSs) or a neutron star and a black hole (BH) produces a radioactively-powered transient known as a kilonova, first observed accompanying the gravitational wave event GW170817. While kilonovae are frequently modeled in spherical symmetry, the dynamical ejecta and disk outflows can be considerably asymmetric. We use Monte Carlo radiative transfer calculations to study the light curves of kilonovae with globally axisymmetric geometries (e.g. an ellipsoid and a torus). We find that the variation in luminosity in these models is most pronounced at early times, and decreases until the light curves become isotropic in the late optically thin phase. The light curve shape and peak time are not significantly modified by the global asymmetry. We show that the projected surface area along the line of sight captures the primary geometric effects, and use this fact to provide a simple analytic estimate of the direction-dependent light curves of the aspherical ejecta. For the kilonova accompanying GW170817, accounting for asymmetry with an oblate (prolate) ellipsoid of axial ratio $2$ ($1/2$) leads to a $sim 40 %$ decrease (increase) in the inferred ejecta mass compared to the spherical case. The pole-to-equator orientation effects are expected to be significantly larger (a factor of $sim 5 - 10$) for the more extreme asymmetries expected for some NS-BH mergers.
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The mergers of binary neutron stars, as well as black hole-neutron star systems, are expected to produce an electromagnetic counterpart that can be analyzed to infer the element synthesis that occurred in these events. We investigate one source of uncertainties pertinent to lanthanide-rich outflows: the nuclear inputs to rapid neutron capture nucleosynthesis calculations. We begin by examining thirty-two different combinations of nuclear inputs: eight mass models, two types of spontaneous fission rates, and two types of fission daughter product distributions. We find that such nuclear physics uncertainties typically generate at least one order of magnitude uncertainty in key quantities such as the nuclear heating (one and a half orders of magnitude at one day post-merger), the bolometric luminosity (one order of magnitude at five days post-merger), and the inferred mass of material from the bolometric luminosity (factor of eight when considering the eight to ten days region). Since particular nuclear processes are critical for determining the electromagnetic signal, we provide tables of key nuclei undergoing $beta$-decay, $alpha$-decay, and spontaneous fission important for heating at different times, identifying decays that are common among the many nuclear input combinations.
Neutron star mergers offer unique conditions for the creation of the heavy elements and additionally provide a testbed for our understanding of this synthesis known as the $r$-process. We have performed dynamical nucleosynthesis calculations and identified a single isotope, $^{254}$Cf, which has a particularly high impact on the brightness of electromagnetic transients associated with mergers on the order of 15 to 250 days. This is due to the anomalously long half-life of this isotope and the efficiency of fission thermalization compared to other nuclear channels. We estimate the fission fragment yield of this nucleus and outline the astrophysical conditions under which $^{254}$Cf has the greatest impact to the light curve. Future observations in the middle-IR which are bright during this regime could indicate the production of actinide nucleosynthesis.
The merger of two neutron stars produces an outflow of radioactive heavy nuclei. Within a second of merger, the central remnant is expected to also launch a relativistic jet, which shock-heats and disrupts a portion of the radioactive ejecta. Within a few hours, emission from the radioactive material gives rise to an ultraviolet, optical, and infrared transient (a kilonova). We use the endstates of a suite of 2D relativistic hydrodynamic simulations of jet-ejecta interaction as initial conditions for multi-dimensional Monte Carlo radiation transport simulations of the resulting viewing angle-dependent light curves and spectra starting at 1.5h after merger. We find that on this timescale, jet shock heating does not affect the kilonova emission. However, the jet disruption to the density structure of the ejecta does change the light curves. The jet carves a channel into the otherwise spheroidal ejecta, revealing the hot, inner regions. As seen from near ($lesssim 30 deg$) the jet axis, the kilonova is brighter by a factor of a few and bluer. The strength of this effect depends on the jet parameters, since the light curves of more heavily disrupted ejecta are more strongly affected. The light curves and spectra are also more heavily modified in the ultraviolet than in the optical.
Compact object mergers can produce a thermal electromagnetic counterpart (a kilonova) powered by the decay of freshly synthesized radioactive isotopes. The luminosity of kilonova light curves depends on the efficiency with which beta-decay electrons are thermalized in the ejecta. Here we derive a simple analytic solution for thermalization by calculating how electrons accumulate in the ejecta and lose energy adiabatically and via plasma losses. We find that the time-dependent thermalization efficiency is well described by $f(t) approx (1 + t/t_e)^{-n}$ where $n approx 1$ and the timescale $t_e$ is a function of the ejecta mass and velocity. For a statistical distribution of r-process isotopes with radioactive power $dot{Q} propto t^{-4/3}$, the late time kilonova luminosity asymptotes to $L propto t^{-7/3}$ and depends super-linearly on the ejecta mass, $L propto M^{5/3}$. If a kilonova is instead powered by a single dominate isotope, we show that the late time luminosity can deviate substantially from the underlying exponential decay and eventually become brighter than the instantaneous radioactivity due to the accumulation of trapped electrons. Applied to the kilonova associated with the gravitational wave source GW170817, these results imply that a possible steepening of the observed light curve at $gtrsim 7$ days is unrelated to thermalization effects and instead could mark the onset of translucency in a high opacity component of ejecta. The analytic results should be convenient for estimating the properties of observed kilonovae and assessing the potential late time detectability of future events.
The detection of AT2017gfo proved that binary neutron star mergers are progenitors of kilonovae. Using a combination of numerical-relativity and radiative-transfer simulations, the community has developed sophisticated models for these transients for a wide portion of the expected parameter space. Using these simulations and surrogate models made from them, it has been possible to perform Bayesian inference of the observed signals to infer properties of the ejected matter. It has been pointed out that combining inclination constraints derived from the kilonova with gravitational-wave measurements increases the accuracy with which binary parameters can be measured and allows a more accurate inference of the Hubble Constant. In order to not introduce biases, constraints on the inclination angle for AT2017gfo should be insensitive to the employed models. In this work, we compare different assumptions about the ejecta and radiative reprocesses used by the community and we investigate their impact on the parameter inference. While most inferred parameters agree, we find disagreement between posteriors for the inclination angle for different geometries that have been used in the literature. According to our study, the inclusion of reprocessing of the photons between different ejecta types improves the modeling fits to AT2017gfo and in some cases affects the inferred constraints. Our study motivates the inclusion of large $sim$ 1 mag uncertainties in the kilonova models employed for Bayesian analysis to capture yet unknown systematics, especially when inferring inclination angles, although smaller uncertainties seem appropriate to capture model systematics for other parameters. We also use this method to impose soft constraints on the ejecta geometry of the kilonova AT2017gfo.
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