arXiv subjects

Brian D. Metzger

Publications and source records attributed to Brian D. Metzger.

At least 163 records · Page 9Linked to original sources

arXiv2018

Imprints of r-process heating on fall-back accretion: distinguishing black hole-neutron star from double neutron star mergers

Mergers of compact binaries containing two neutron stars (NS-NS), or a neutron star and a stellar-mass black hole (NS-BH), are likely progenitors of short-duration gamma ray bursts (SGRBs). A fraction >20% of SGRBs are followed by temporally-extended (>~ minute-long), variable X-ray emission, attributed to ongoing activity of the central engine. One source of late-time engine activity is fall-back accretion of bound tidal ejecta; however, observed extended emission light curves do not track the naively-anticipated, uninterrupted ~t^{-5/3} power-law decay, instead showing a lull or gap in emission typically lasting tens of seconds after the burst. Here, we re-examine the impact of heating due to rapid neutron capture (r-process) nucleosynthesis on rate of the fall-back accretion, using ejecta properties obtained from numerical relativity simulations of NS-BH mergers in a toy model for the dynamical influence of nuclear heating. Depending on the mass of the remnant black hole, r-process heating can imprint a variety of fall-back curve shapes, ranging from temporal lulls of up to tens of seconds to complete late-time cut-off in the fall-back rate. This behavior is robust to realistic variations in the nuclear heating experienced by different parts of the ejecta. Central black holes with masses <~ 3 Msun typically experience absolute cut-offs in the fall-back rate, while more massive >6-8 Msun black holes instead show temporal gaps. We thus propose that SGRBs with extended emission arise from NS-BH, rather than NS-NS, mergers. Our model implies a NS-BH merger detection rate by LIGO which, in steady-state, is comparable to or greater than that of NS-NS mergers.

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arXiv2018

A concordance picture of FRB 121102 as a flaring magnetar embedded in a magnetized ion-electron wind nebula

The fast radio burst FRB 121102 has repeated multiple times, enabling the identification of its host galaxy and of a spatially-coincident, compact, steady (`quiescent') radio synchrotron source. It was proposed that FRB 121102 is powered by a young flaring magnetar, embedded within a decades-old supernova remnant. Using a time-dependent one-zone model, we show that a single expanding magnetized electron-ion nebula (powered by the same outbursts likely responsible for the FRBs) can explain all the basic properties of the quiescent source (size, flux, self-absorption constraints) and the large but decreasing rotation measure (RM) of the bursts. The quiescent emission is powered by relativistic thermal electrons heated at the termination shock of the magnetar wind, while the RM originates from non-relativistic electrons injected earlier in the nebula's evolution and cooled through expansion and radiative losses. The model contains few free parameters, which are tightly constrained by observations: the total energy injected into the nebula over its history, $\sim 10^{50}-10^{51}$ erg, agrees with the magnetic energy of a millisecond magnetar; the baryon loading of the magnetar outflow (driven by intermittent flares) is close to the neutron star escape speed; the predicted source age $\sim 10-40$ years is consistent with other constraints on the nebula size. For an energy input rate $\dot{E} \propto t^{-α}$ following the onset of magnetar activity, we predict secular decay of the RM and quiescent source flux, which approximately follow ${\rm RM} \propto t^{-(6+α)/2}$ and $F_ν \propto t^{-(α^2+7α-2)/4}$, respectively.

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arXiv2018

A Search For Pulsations in the Optical Light Curve of the Nova ASASSN-17hx

We present high-speed optical observations of the nova ASASSN-17hx, taken both immediately after its discovery and close to its first peak in brightness, to search for seconds -- minutes pulsations associated with the convective eddy turnover timescale within the nova envelope. We do not detect any periodic signal with greater than $3σ$ significance. Through injection and recovery, we rule out periodic signals of fractional amplitude $>7.08\times10^{-4}$ on timescales of 2 seconds and fractional amplitude $>1.06\times10^{-3}$ on timescales of 10 minutes. Additional observations of novae are planned to further constrain ongoing simulations of the launch and propagation of nova winds.

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arXiv2018

Constraints on the neutron star equation of state from AT2017gfo using radiative transfer simulations

The detection of the binary neutron star GW170817 together with the observation of electromagnetic counterparts across the entire spectrum inaugurated a new era of multi-messenger astronomy. In this study we incorporate wavelength-dependent opacities and emissivities calculated from atomic-structure data enabling us to model both the measured lightcurves and spectra of the electromagnetic transient AT2017gfo. Best-fits of the observational data are obtained by Gaussian Process Regression, which allows us to present posterior samples for the kilonova and source properties connected to GW170817. Incorporating constraints obtained from the gravitational wave signal obtained by the LIGO-Virgo Scientific Collaboration, we present a $90\%$ upper bound on the mass ratio $q \lesssim 1.38$ and a lower bound on the tidal deformability of $\tildeΛ \gtrsim 197$, which rules out sufficiently soft equations of state. Our analysis is a path-finder for more realistic kilonova models and shows how the combination of gravitational wave and electromagnetic measurements allow for stringent constraints on the source parameters and the supranuclear equation of state.

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arXiv2018

Science with an ngVLA: Tidal disruption events

The tidal disruption and subsequent accretion of a star by a supermassive black hole can be used as a laboratory to study the physics of relativistic jets. The ngVLA is the only planned instrument that can both discover and characterize a large number of these short-lived radio sources. In particular the high-frequency capabilities of the ngVLA enable this important leap forward. Multi-frequency radio follow-up observations (3 - 100 GHz) of tidal disruption events found in optical or X-ray surveys will provide a measurement of the jet efficiency as a function of black hole spin, thus enabling a direct test of the prediction that relativistic jets require high spin. Hundreds of tidal disruption jets will be discovered in a blind ngVLA survey for radio transients. By including VLBI observations with the ngVLA Long Baseline Array, we can resolve some of these sources, obtaining a robust measurement of the jet launch date and the magnetic field strength. From the thermal emission of the tidal disruption flare we can measure the accretion rate at this launch date, thus providing another unique opportunity to identify the conditions that lead to jet production.

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arXiv2018

One thousand days of SN 2015bn: HST imaging shows a light curve flattening consistent with magnetar predictions

We present the first observations of a Type I superluminous supernova (SLSN) at $\gtrsim 1000$ days after maximum light. We observed SN 2015bn using the Hubble Space Telescope Advanced Camera for Surveys in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at $>700$ days of $0.19 \pm 0.03$ mag (100 d)$^{-1}$, much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of $^{56}$Co. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same [O I] and [Ca II] lines as seen at $\sim300-400$ days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates $10^{-2.7} \lesssim \dot{M}/v_{10} \lesssim 10^{-1.1}$ M$_\odot$ yr$^{-1}$ (where $v_{10}$ is the wind velocity in units of 10 km s$^{-1}$). The optical light curve is consistent with $L \propto t^{-4}$, which we show is expected for magnetar spin-down with inefficient trapping; furthermore, the evolution matches predictions from earlier magnetar model fits. The opacity to magnetar radiation is constrained at $\sim 0.01$ cm$^2$ g$^{-1}$, consistent with photon-matter pair-production over a broad $\sim$GeV-TeV range. This suggests the magnetar spectral energy distribution, and hence the 'missing energy' leaking from the ejecta, may peak in this range.

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arXiv2018

Where is the engine hiding its missing energy? Constraints from a deep X-ray non-detection of the Superluminous SN 2015bn

SN 2015bn is a nearby hydrogen-poor superluminous supernova (SLSN-I) that has been intensively observed in X-rays with the goal to detect the spin-down powered emission from a magnetar engine. The early-time UV/optical/infrared (UVOIR) data fit well to the magnetar model, but require leakage of energy at late times of $\lesssim 10^{43}$ erg s$^{-1}$, which is expected to be partially emitted in X-rays. Deep X-ray limits until $\sim$300 days after explosion revealed no X-ray emission. Here, we present the latest deep 0.3--10 keV X-ray limit at 805 days obtained with \textit{XMM-Newton}. We find $L_X < 10^{41}$ erg s$^{-1}$, with no direct evidence for central-engine powered emission. While the late-time optical data still follow the prediction of the magnetar model, the best-fit model to the bolometric light curve predicts that $\sim$97\% of the total input luminosity of the magnetar is escaping outside of the UVOIR bandpass at the time of observation. Our X-ray upper limit is $<$1.5\% of the input luminosity, strongly constraining the high-energy leakage, unless non-radiative losses are important. These deep X-ray observations identify a missing energy problem in SLSNe-I and we suggest future observations in hard X-rays and $γ$-rays for better constraints. Also, independent of the optical data, we constrain the parameter spaces of various X-ray emission scenarios, including ionization breakout by magnetar spin-down, shock interaction between the ejecta and external circumstellar medium, off-axis $γ$-ray burst afterglow, and black hole fallback accretion.

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arXiv2018

Three-dimensional GRMHD simulations of neutrino-cooled accretion disks from neutron star mergers

Merging binaries consisting of two neutron stars (NSs) or an NS and a stellar-mass black hole typically form a massive accretion torus around the remnant black hole or long-lived NS. Outflows from these neutrino-cooled accretion disks represent an important site for $r$-process nucleosynthesis and the generation of kilonovae. We present the first three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulations including weak interactions and a realistic equation of state of such accretion disks over viscous timescales ($380\,\mathrm{ms}$). We witness the emergence of steady-state MHD turbulence, a magnetic dynamo with an $\sim\!20\,\mathrm{ms}$ cycle, and the generation of a `hot' disk corona that launches powerful thermal outflows aided by the energy released as free nucleons recombine into $α$-particles. We identify a self-regulation mechanism that keeps the midplane electron fraction low ($Y_e\sim0.1$) over viscous timescales. This neutron-rich reservoir, in turn, feeds outflows that retain a sufficiently low value of $Y_e\approx 0.2$ to robustly synthesize third-peak $r$-process elements. The quasi-spherical outflows are projected to unbind $40\%$ of the initial disk mass with typical asymptotic escape velocities of $0.1c$, and may thus represent the dominant mass ejection mechanism in NS-NS mergers. Including neutrino absorption, our findings agree with previous hydrodynamical $α-$disk simulations that the entire range of $r$-process nuclei from the first to the third $r$-process peak can be synthesized in the outflows, in good agreement with observed solar system abundances. The asymptotic escape velocities and the quantity of ejecta, when extrapolated to moderately higher disk masses, are consistent with those needed to explain the red kilonova emission following the NS merger GW170817.

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arXiv2018

Unveiling the Engines of Fast Radio Bursts, Super-Luminous Supernovae, and Gamma-Ray Bursts

Young, rapidly spinning magnetars are invoked as central engines behind a diverse set of transient astrophysical phenomena, including gamma-ray bursts (GRB), super-luminous supernovae (SLSNe), fast radio bursts (FRB), and binary neutron star (NS) mergers. However, a barrier to direct confirmation of the magnetar hypothesis is the challenge of directly observing non-thermal emission from the central engine at early times (when it is most powerful and thus detectable) due to the dense surrounding ejecta. We present CLOUDY calculations of the time-dependent evolution of the temperature and ionization structure of expanding supernova or merger ejecta due to photo-ionization by a magnetar engine, in order to study the escape of X-rays (absorbed by neutral gas) and radio waves (absorbed by ionized gas), as well as to assess the evolution of the local dispersion measure due to photo-ionization. We find that ionization breakout does not occur if the engine's ionizing luminosity decays rapidly, and that X-rays typically escape the oxygen-rich ejecta of SLSNe only on $\sim 100 \, {\rm yr}$ timescales, consistent with current X-ray non-detections. We apply these results to constrain engine-driven models for the binary NS merger GW170817 and the luminous transient ASASSN-15lh. In terms of radio transparency and dispersion measure constraints, the repeating FRB 121102 is consistent with originating from a young, $\gtrsim 30-100 \, {\rm yr}$, magnetar similar to those inferred to power SLSNe. We further show that its high rotation measure can be produced within the same nebula that is proposed to power the quiescent radio source observed co-located with FRB 121102. Our results strengthen previous work suggesting that at least some FRBs may be produced by young magnetars, and motivate further study of engine powered transients.

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arXiv2018

The Multi-Dimensional Structure of Radiative Shocks: Suppressed Thermal X-rays and Relativistic Ion Acceleration

Radiative shocks, behind which gas cools faster than the dynamical time, play a key role in many astrophysical transients, including classical novae and young supernovae interacting with circumstellar material. The dense layer behind high Mach number $\mathcal{M} \gg 1$ radiative shocks is susceptible to thin-shell instabilities, creating a "corrugated" shock interface. We present two and three-dimensional hydrodynamical simulations of optically-thin radiative shocks to study their thermal radiation and acceleration of non-thermal relativistic ions. We employ a moving-mesh code and a specialized numerical technique to eliminate artificial heat conduction across grid cells. The fraction of the shock's luminosity $L_{\rm tot}$ radiated at X-ray temperatures $kT_{\rm sh} \approx (3/16)μm_p v_{\rm sh}^{2}$ expected from a one-dimensional analysis is suppressed by a factor $L(>T_{\rm sh}/3)/L_{\rm tot} \approx 4.5/\mathcal{M}^{4/3}$ for $\mathcal{M} \approx 4-36$. This suppression results in part from weak shocks driven into under-pressured cold filaments by hot shocked gas, which sap thermal energy from the latter faster than it is radiated. Combining particle-in-cell simulation results for diffusive shock acceleration with the inclination angle distribution across the shock (relative to an upstream magnetic field in the shock plane$-$the expected geometry for transient outflows), we predict the efficiency and energy spectrum of ion acceleration. Though negligible acceleration is predicted for adiabatic shocks, the corrugated shock front enables local regions to satisfy the quasi-parallel magnetic field geometry required for efficient acceleration, resulting in an average acceleration efficiency of $ε_{\rm nth} \sim 0.005-0.02$ for $\mathcal{M} \approx 12-36$, in agreement with modeling of the gamma-ray nova ASASSN-16ma.

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arXiv2018

Effects of Fall-Back Accretion on Proto-Magnetar Outflows in Gamma-Ray Bursts and Superluminous Supernovae

Rapidly spinning, strongly magnetized proto-neutron stars ("millisecond proto-magnetars") are candidate central engines of long-duration gamma-ray bursts (GRB), superluminous supernovae (SLSNe), and binary neutron star mergers. Magnetar birth may be accompanied by the fall-back of stellar debris, lasting for seconds or longer following the explosion. Accretion alters the magnetar evolution by (1) providing an additional source of rotational energy (or a potential sink, if the propeller mechanism operates); (2) enhancing the spin-down luminosity above the dipole rate by compressing the magnetosphere and expanding the polar cap region of open magnetic field lines; (3) supplying an additional accretion-powered neutrino luminosity that sustains the wind baryon-loading, even after the magnetar's internal neutrino luminosity has subsided. The more complex evolution of the jet power and magnetization of an accreting magnetar more readily accounts for the high 56Ni yields GRB SNe and irregular time evolution of some GRB light curves (e.g.~bursts with precursors followed by a long quiescent interval before the main emission episode). Additional baryon-loading from accretion-powered neutrino irradiation of the magnetar polar cap lengthens the timeframe over which the jet magnetization is in the requisite range sigma ~< 1e3 for efficient gamma-ray emission, thereby accommodating GRBs with ultra-long durations. Though accretion does not significantly raise the maximum energy budget from the limit of <~ few 1e52 ergs for an isolated magnetar, it greatly expands the range of magnetic field strengths and birth spin periods capable of powering GRB jets, reducing the differences between the magnetar properties normally invoked to explain GRBs versus SLSNe.

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arXiv2018

A magnetar origin for the kilonova ejecta in GW170817

The neutron star (NS) merger GW170817 was followed over several days by optical-wavelength ("blue") kilonova (KN) emission likely powered by the radioactive decay of light r-process nuclei synthesized by ejecta with a low neutron abundance (electron fraction Ye ~ 0.25-0.35). While the composition and high velocities of the blue KN ejecta are consistent with shock-heated dynamical material, the large quantity is in tension with the results of numerical simulations. We propose an alternative ejecta source: the neutrino-heated, magnetically-accelerated wind from the strongly-magnetized hypermassive NS (HMNS) remnant. A rapidly-spinning HMNS with an ordered surface magnetic field of strength B ~ 1-3e14 G and lifetime t_rem ~ 0.1-1 s can simultaneously explain the velocity, total mass, and electron fraction of the blue KN ejecta. The inferred HMNS lifetime is close to its Alfven crossing time, suggesting global magnetic torques could be responsible for bringing the HMNS into solid body rotation and instigating its gravitational collapse. Different origins for the KN ejecta may be distinguished by their predictions for the emission in the first hours after the merger, when the luminosity is enhanced by heating from internal shocks; the latter are likely generic to any temporally-extended ejecta source (e.g. magnetar or accretion disk wind) and are not unique to the emergence of a relativistic jet. The same shocks could mix and homogenizes the composition to a low but non-zero lanthanide mass fraction, X_La ~ 1e-3, as advocated by some authors, but only if the mixing occurs after neutrons are consumed in the r-process on a timescale >~ 1 s.

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arXiv2017

The Combined Ultraviolet, Optical, and Near-Infrared Light Curves of the Kilonova Associated with the Binary Neutron Star Merger GW170817: Unified Data Set, Analytic Models, and Physical Implications

We present the first effort to aggregate, homogenize, and uniformly model the combined ultraviolet, optical, and near-infrared dataset for the electromagnetic counterpart of the binary neutron star merger GW170817. By assembling all of the available data from 18 different papers and 46 different instruments, we are able to identify and mitigate systematic offsets between individual datasets, and to identify clear outlying measurements, with the resulting pruned and adjusted dataset offering an opportunity to expand the study of the kilonova. The unified dataset includes 647 individual flux measurements, spanning 0.45 to 29.4 days post-merger, and thus has greater constraining power for physical models than any single dataset. We test a number of semi-analytical models and find that the data are well modeled with a three-component kilonova model: a "blue" lanthanide-poor component with Mej~0.020 Msol and vej~0.27c; an intermediate opacity "purple" component with Mej~0.047 Msol and vej~0.15c; and a "red" lanthanide-rich component with Mej~0.011 Msol and vej~0.14c. We further explore the possibility of ejecta asymmetry and its impact on the estimated parameters. From the inferred parameters we draw conclusions about the physical mechanisms responsible for the various ejecta components, the properties of the neutron stars, and, combined with an up-to-date merger rate, the implications for r-process enrichment via this channel. To facilitate future studies of this keystone event we make the unified dataset and our modeling code public.

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arXiv2017

A Detailed Observational Analysis of V1324 Sco, the Most Gamma-Ray Luminous Classical Nova to Date

It has recently been discovered that some, if not all, classical novae emit GeV gamma rays during outburst, but the mechanisms involved in the production of the gamma rays are still not well understood. We present here a comprehensive multi-wavelength dataset---from radio to X-rays---for the most gamma-ray luminous classical nova to-date, V1324 Sco. Using this dataset, we show that V1324 Sco is a canonical dusty Fe-II type nova, with a maximum ejecta velocity of 2600 km s$^{-1}$ and an ejecta mass of few $\times 10^{-5}$ M$_{\odot}$. There is also evidence for complex shock interactions, including a double-peaked radio light curve which shows high brightness temperatures at early times. To explore why V1324~Sco was so gamma-ray luminous, we present a model of the nova ejecta featuring strong internal shocks, and find that higher gamma-ray luminosities result from higher ejecta velocities and/or mass-loss rates. Comparison of V1324~Sco with other gamma-ray detected novae does not show clear signatures of either, and we conclude that a larger sample of similarly well-observed novae is needed to understand the origin and variation of gamma rays in novae.

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arXiv2017

Three-dimensional GRMHD simulations of the remnant accretion disks from neutron star mergers: outflows and r-process nucleosynthesis

The merger of binary neutron stars, or of a neutron star and a stellar-mass black hole, can result in the formation of a massive rotating torus around a spinning black hole. In addition to providing collimating media for gamma-ray burst jets, unbound outflows from these disks are an important source of mass ejection and rapid neutron capture (r-process) nucleosynthesis. We present the first three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations of neutrino-cooled accretion disks in neutron star mergers, including a realistic equation of state valid at low densities and temperatures, self-consistent evolution of the electron fraction, and neutrino cooling through an approximate leakage scheme. After initial magnetic field amplification by magnetic winding, we witness the vigorous onset of turbulence driven by the magneto-rotational instability (MRI). The disk quickly reaches a balance between heating from MRI-driven turbulence and neutrino cooling, which regulates the midplane electron fraction to a low equilibrium value $Y_\text{e} \approx 0.1$. Over the 380 ms duration of the simulation, we find that a fraction $\approx 20\%$ of the initial torus mass is unbound in powerful outflows with velocities $v \approx 0.03-0.1\,c$ and electron fractions $Y_\text{e} \approx 0.1-0.25$. Post-processing the outflows through a nuclear reaction network shows the production of a robust second and third peak r-process. Though broadly consistent with the results of previous axisymmetric hydrodynamical simulations, extrapolation of our results to late times suggests that the total ejecta mass from GRMHD disks is significantly higher. Our results provide strong evidence that post-merger disk outflows are an important site for the r-process.

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arXiv2017

Welcome to the Multi-Messenger Era! Lessons from a Neutron Star Merger and the Landscape Ahead

The discovery by Advanced LIGO/Virgo of gravitational waves from the binary neutron star (NS) merger GW170817, and subsequently by astronomers of transient counterparts across the electromagnetic (EM) spectrum, has initiated the era of multi-messenger astronomy. Given the slew of papers appearing on this event, I thought it useful to summarize the EM discoveries in the context of theoretical models and present my views on the major take-away lessons from this watershed event. The weak GRB discovered in close time coincidence with GW170817, and potential evidence for a more powerful off-axis relativistic jet (initially beamed away from our line of sight) from the delayed rise of a non-thermal X-ray and radio orphan afterglow, provides the most compelling evidence yet that cosmological short GRBs originate from binary NS mergers. The luminosity and colors of the early optical emission discovered within a day of the merger agrees strikingly well with original predictions (Metzger et al. 2010) for "kilonova" emission powered by the radioactive decay of r-process nuclei, the NS merger origin of which was initially proposed by Lattimer & Schramm 1974. The transition of the spectral energy distribution to NIR wavelengths on timescales of days matches predictions by Barnes & Kasen 2013 and Tanaka & Hotokezaka 2013 if a portion of the ejecta contains heavy r-process nuclei with higher opacities. The "blue" and "red" ejecta components may possess distinct origins (e.g. dynamical ejecta versus accretion disk outflows), with key implications for the merger physics and the properties of neutron stars. I outline the diversity in the EM emission expected from additional mergers-observed with different binary masses and viewing angles-discovered once LIGO/Virgo reach design sensitivity and NS mergers are discovered as frequently as once per week.

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arXiv2017

Constraints on millisecond magnetars as the engines of prompt emission in gamma-ray bursts

We examine millisecond magnetars as central engines of Gamma Ray Bursts' (GRB) prompt emission. Using the proto-magnetar wind model of Metzger et al. 2011, we estimate the temporal evolution of the magnetization and power injection at the base of the GRB jet and apply these to different prompt emission models to make predictions for the GRB energetics, spectra and lightcurves. We investigate both shock and magnetic reconnection models for the particle acceleration, as well as the effects of energy dissipation across optically thick and thin regions of the jet. The magnetization at the base of the jet, $σ_0$, is the main parameter driving the GRB evolution in the magnetar model and the emission is typically released for $100\lesssim σ_0 \lesssim 3000$. Given the rapid increase in $σ_0$ as the proto-magnetar cools and its neutrino-driven mass loss subsides, the GRB duration is typically limited to $\lesssim 100$ s. This low baryon loading at late times challenges magnetar models for ultra-long GRBs, though black hole models likely run into similar difficulties without substantial entrainment from the jet walls. The maximum radiated gamma-ray energy is $\lesssim 5 \times 10^{51}$erg, significantly less than the magnetar's total initial rotational energy and in strong tension with the high end of the observed GRB energy distribution. However, the gradual magnetic dissipation model (Beniamini & Giannios 2017) applied to a magnetar central engine, naturally explains several key observables of typical GRBs, including energetics, durations, stable peak energies, spectral slopes and a hard to soft evolution during the burst.

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arXiv2017

Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disk ejecta from neutron star mergers

We investigate the nucleosynthesis of heavy elements in the winds ejected by accretion disks formed in neutron star mergers. We compute the element formation in disk outflows from hypermassive neutron star (HMNS) remnants of variable lifetime, including the effect of angular momentum transport in the disk evolution. We employ long-term axisymmetric hydrodynamic disk simulations to model the ejecta, and compute r-process nucleosynthesis with tracer particles using a nuclear reaction network containing $\sim 8000$ species. We find that the previously known strong correlation between HMNS lifetime, ejected mass, and average electron fraction in the outflow is directly related to the amount of neutrino irradiation on the disk, which dominates mass ejection at early times in the form of a neutrino-driven wind. Production of lanthanides and actinides saturates at short HMNS lifetimes ($\lesssim 10$ ms), with additional ejecta contributing to a blue optical kilonova component for longer-lived HMNSs. We find good agreement between the abundances from the disk outflow alone and the solar r-process distribution only for short HMNS lifetimes ($\lesssim 10$ ms). For longer lifetimes, the rare-earth and third r-process peaks are significantly under-produced compared to the solar pattern, requiring additional contributions from the dynamical ejecta. The nucleosynthesis signature from a spinning black hole (BH) can only overlap with that from a HMNS of moderate lifetime ($\lesssim 60$ ms). Finally, we show that angular momentum transport not only contributes with a late-time outflow component, but that it also enhances the neutrino-driven component by moving material to shallower regions of the gravitational potential, in addition to providing additional heating.

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