More Like this

1. The impact of r -process heating on the dynamics of neutron star merger accretion disc winds and their electromagnetic radiation

   https://doi.org/10.1093/mnras/stab3583  

   Klion, Hannah ; Tchekhovskoy, Alexander ; Kasen, Daniel ; Kathirgamaraju, Adithan ; Quataert, Eliot ; Fernández, Rodrigo ( December 2021 , Monthly Notices of the Royal Astronomical Society)

   Neutron star merger accretion discs can launch neutron-rich winds of >10−2M⊙. 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 s). Using time-dependent multifrequency multidimensional 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 $\sim 1\, \mathrm{d}$ post-merger. However, the high-velocity tail of themore »ejecta distribution and the early ($\lesssim 1\, \mathrm{d}$) light curves are largely unaffected.

   « less
2. Radiation hydrodynamics modelling of kilonovae with SNEC

   https://doi.org/10.1093/mnras/stac399  

   Wu, Zhenyu ; Ricigliano, Giacomo ; Kashyap, Rahul ; Perego, Albino ; Radice, David ( February 2022 , Monthly Notices of the Royal Astronomical Society)

   We develop a method to compute synthetic kilonova light curves that combine numerical relativity simulations of neutron star mergers and the SNEC radiation–hydrodynamics code. We describe our implementation of initial and boundary conditions, r-process heating, and opacities for kilonova simulations. We validate our approach by carefully checking that energy conservation is satisfied and by comparing the SNEC results with those of two semi-analytic light-curve models. We apply our code to the calculation of colour light curves for three binaries having different mass ratios (equal and unequal mass) and different merger outcome (short-lived and long-lived remnants). We study the sensitivity of our results to hydrodynamic effects, nuclear physics uncertainties in the heating rates, and duration of the merger simulations. We find that hydrodynamics effects are typically negligible and that homologous expansion is a good approximation in most cases. However, pressure forces can amplify the impact of uncertainties in the radioactive heating rates. We also study the impact of shocks possibly launched into the outflows by a relativistic jet. None of our models match AT2017gfo, the kilonova in GW170817. This points to possible deficiencies in our merger simulations and kilonova models that neglect non-LTE effects and possible additional energy injection frommore »the merger remnant and to the need to go beyond the assumption of spherical symmetry adopted in this work.

   « less
3. Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

   https://doi.org/10.3847/2041-8213/ac504a  

   Hajela, A. ; Margutti, R. ; Bright, J. S. ; Alexander, K. D. ; Metzger, B. D. ; Nedora, V. ; Kathirgamaraju, A. ; Margalit, B. ; Radice, D. ; Guidorzi, C. ; et al ( March 2022 , The Astrophysical Journal Letters)

   For the first ∼3 yrs after the binary neutron star merger event GW 170817, the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium withn< 0.01 cm⁻³. We report on observational evidence for an excess of X-ray emission atδt> 900 days after the merger. WithL_x≈ 5 × 10³⁸erg s⁻¹at 1234 days, the recently detected X-ray emission represents a ≥3.2σ(Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context ofJetFitafterglow models, current data represent a departure with statistical significance ≥3.1σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥3.7σ. A lack of detectable 3 GHz radio emission suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e., a kilonova afterglow. In this context, we present a set of ab initio numerical relativity binary neutron star (BNS) merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the mergermore »ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.

   « less
4. Kilonova Detectability with Wide-field Instruments

   https://doi.org/10.3847/1538-4357/ac3d25  

   Chase, Eve A. ; O’Connor, Brendan ; Fryer, Christopher L. ; Troja, Eleonora ; Korobkin, Oleg ; Wollaeger, Ryan T. ; Ristic, Marko ; Fontes, Christopher J. ; Hungerford, Aimee L. ; Herring, Angela M. ( March 2022 , The Astrophysical Journal)

   Kilonovae are ultraviolet, optical, and infrared transients powered by the radioactive decay of heavy elements following a neutron star merger. Joint observations of kilonovae and gravitational waves can offer key constraints on the source of Galacticr-process enrichment, among other astrophysical topics. However, robust constraints on heavy element production require rapid kilonova detection (within ∼1 day of merger) as well as multiwavelength observations across multiple epochs. In this study, we quantify the ability of 13 wide-field-of-view instruments to detect kilonovae, leveraging a large grid of over 900 radiative transfer simulations with 54 viewing angles per simulation. We consider both current and upcoming instruments, collectively spanning the full kilonova spectrum. The Roman Space Telescope has the highest redshift reach of any instrument in the study, observing kilonovae out toz∼ 1 within the first day post-merger. We demonstrate that BlackGEM, DECam, GOTO, the Vera C. Rubin Observatory’s LSST, ULTRASAT, VISTA, and WINTER can observe some kilonovae out toz∼ 0.1 (∼475 Mpc), while DDOTI, MeerLICHT, PRIME, Swift/UVOT, and ZTF are confined to more nearby observations. Furthermore, we provide a framework to infer kilonova ejecta properties following nondetections and explore variation in detectability with these ejecta parameters.
5. A luminosity distribution for kilonovae based on short gamma-ray burst afterglows

   https://doi.org/10.1093/mnras/stz891  

   Ascenzi, Stefano ; Coughlin, Michael W ; Dietrich, Tim ; Foley, Ryan J ; Ramirez-Ruiz, Enrico ; Piranomonte, Silvia ; Mockler, Brenna ; Murguia-Berthier, Ariadna ; Fryer, Chris L ; Lloyd-Ronning, Nicole M ; et al ( June 2019 , Monthly Notices of the Royal Astronomical Society)

   Abstract The combined detection of a gravitational-wave signal, kilonova, and short gamma-ray burst (sGRB) from GW170817 marked a scientific breakthrough in the field of multimessenger astronomy. But even before GW170817, there have been a number of sGRBs with possible associated kilonova detections. In this work, we re-examine these ‘historical’ sGRB afterglows with a combination of state-of-the-art afterglow and kilonova models. This allows us to include optical/near-infrared synchrotron emission produced by the sGRB as well as ultraviolet/optical/near-infrared emission powered by the radioactive decay of r-process elements (i.e. the kilonova). Fitting the light curves, we derive the velocity and the mass distribution as well as the composition of the ejected material. The posteriors on kilonova parameters obtained from the fit were turned into distributions for the peak magnitude of the kilonova emission in different bands and the time at which this peak occurs. From the sGRB with an associated kilonova, we found that the peak magnitude in H bands falls in the range [−16.2, −13.1] ($95{{\ \rm per\ cent}}$ of confidence) and occurs within $0.8\!-\!3.6\, \rm d$ after the sGRB prompt emission. In g band instead we obtain a peak magnitude in range [−16.8, −12.3] occurring within the first 18 h after themore »sGRB prompt. From the luminosity distributions of GW170817/AT2017gfo, kilonova candidates GRB130603B, GRB050709, and GRB060614 (with the possible inclusion of GRB150101B, GRB050724A, GRB061201, GRB080905A, GRB150424A, and GRB160821B) and the upper limits from all the other sGRBs not associated with any kilonova detection we obtain for the first time a kilonova luminosity distribution in different bands.« less