We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at t < 160 d after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency ν c is above the X-ray band and the synchrotron frequency ν m is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index p of the distribution of non-thermal relativistic electrons N(γ) ∝ γ −p accelerated by a shock launched by a NS-NS merger to date. We find p = 2.17 ± 0.01, which indicates that radiation from ejecta with Γ ∼ 3 − 10 dominates the observed emission. While constraining the nature of the emission process, these observations do not constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the nonthermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, and emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal SGRB directed away from our line of sight. Observations at t ≤ 200 days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.

We present new observations of the binary neutron star merger GW170817 at ∆t ≈ 220 − 290 days postmerger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at 5σ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until ∼ 1000 days post-merger.

Using numerical hydrodynamics calculations and a novel method for densely sampling parameter space, we measure the accretion and torque on a binary system from a circumbinary disk. In agreement with some earlier studies, we find that the net torque on the binary is positive for mass ratios close to unity, and that accretion always drives the binary toward equal mass. Accretion variability depends sensitively on the numerical sink prescription, but the torque and relative accretion onto each component do not depend on the sink timescale. Positive torque and highly variable accretion occurs only for mass ratios greater than around 0.05. This means that for mass ratios below 0.05, the binary would migrate inward until the secondary accreted sufficient mass, after which it would execute a U-turn and migrate outward. We explore a range of viscosities, from α = 0.03 to α = 0.15, and find that this outward torque is proportional to the viscous torque, so that torque per unit accreted mass is independent of α. Dependence of accretion and torque on mass ratio is explored in detail, densely sampling mass ratios between 0.01 and unity. For mass ratio q > 0.2, accretion variability is found to exhibit a distinct sawtooth pattern, typically with a five-orbit cycle that provides a smoking gun prediction for variable quasars observed over long periods, as a potential means to confirm the presence of a binary.

We present Chandra and VLA observations of GW 170817 at ∼ 521−743 days post merger, and a homogeneous analysis of the entire Chandra dataset. We find that the late-time non-thermal emission follows the expected evolution of an off-axis relativistic jet, with a steep temporal decay F ν ∝ t −1.95±0.15 and power-law spectrum F ν ∝ ν −0.575±0.007 . We present a new method to constrain the merger environment density based on diffuse Xray emission from hot plasma in the host galaxy and find n ≤ 9.6×10 −3 cm −3 . This measurement is independent from inferences based on jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy E 0 = 1.5 +3.6 −1.1 × 10 49 erg (E iso = 2.1 +6.4 −1.5 × 10 52 erg), jet opening angle θ 0 = 5.9 +1.0 −0.7 deg with characteristic Lorentz factor Γ j = 163 +23 −43 , expanding in a low-density medium with n 0 = 2.5 +4.1 −1.9 × 10 −3 cm −3 and viewed θ obs = 30.4 +4.0 −3.4 deg offaxis. The synchrotron emission originates from a power-law distribution of electrons with index p = 2.15 +0.01 −0.02 . The shock microphysics parameters are constrained to e = 0.18 +0.30 −0.13 and B = 2.3 +16.0 −2.2 × 10 −3 . Furthermore, we investigate the presence of X-ray flares and find no statistically significant evidence of ≥ 2.5σ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy E KN k ∝ (Γβ) −α into the environment, finding that shallow stratification indexes α ≤ 6 are disfavored. Future radio and X-ray observations will refine our inferences on the fastest kilonova ejecta properties. arXiv:1909.06393v3 [astro-ph.HE]

The merger of binary neutron stars (BNSs) can lead to large amplifications of the magnetic field due to the development of turbulence and instabilities in the fluid, such as the Kelvin-Helmholtz shear instability, which drive small-scale dynamo activity. In order to properly resolve such instabilities and obtain the correct magnetic field amplification, one would need to employ resolutions that are currently unfeasible in global general relativistic magnetohydrodynamic (GRMHD) simulations of BNS mergers. Here, we present a subgrid model that allows global simulations to take into account the small-scale amplification of the magnetic field which is caused by the development of turbulence during BNS mergers. Assuming dynamo saturation, we show that magnetar-level fields (∼ 10 16 G) can be easily reached, and should therefore be expected from the merger of magnetized BNSs. The total magnetic energy can reach values up to ∼ 10 51 erg and the post-merger remnant can therefore emit strong electromagnetic signals and possibly produce short gamma-ray bursts.