ALBERT

Description: If at least one neutron star (NS) is magnetized in a binary NS merger, then the orbital motion of the conducting companion during the final inspiral induces a strong voltage and current along the magnetic field lines connecting the NSs. If a modest fraction of the extracted electromagnetic power extracted accelerates relativistic particles, the resulting gamma-ray emission a compact volume will result in the formation of an electron–positron pair fireball. Applying a steady-state pair wind model, we quantify the detectability of the precursor fireball with gamma-ray satellites. For  ~ 1 the gamma-ray detection horizon of D max 10( B d /10 14  G) 3/4 Mpc is much closer than the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo horizon of 200 Mpc, unless the NS surface magnetic field strength is very large, $B_{\rm d} \mathrel {{\rlap{{{\sim }}}{ 〉 }}}10^{15}$  G. Given the quasi-isotropic nature of the emission, mergers with weaker NS fields could contribute a nearby population of short gamma-ray bursts. Power not dissipated close to the binary is carried to infinity along the open field lines by a large-scale Poynting flux. Reconnection within this outflow, well outside of the pair photosphere, provides a potential site for non-thermal emission, such as a coherent millisecond radio burst.

Description: We study mass-loss from the outer Lagrange point (L 2 ) in binary stellar mergers and their luminous transients by means of radiative hydrodynamical simulations. Previously, we showed that for binary mass ratios 0.06 q 0.8, synchronous L 2 mass-loss results in a radiatively inefficient, dust-forming unbound equatorial outflow. A similar outflow exists irrespective of q if the ratio of the sound speed to the orbital speed at the injection point is sufficiently large, c T / v orb 0.15. By contrast, for cold L 2 mass-loss ( 0.15) from binaries with q 0.06 or q 0.8, the equatorial outflow instead remains marginally bound and falls back to the binary over tens to hundreds of binary orbits, where it experiences additional tidal torquing and shocking. As the bound gas becomes virialized with the binary, the luminosity of the system increases slowly at approximately constant photosphere radius, causing the temperature to rise. Subsequent evolution depends on the efficiency of radiative cooling. If the bound atmosphere is able to cool efficiently, as quantified by radiative diffusion time being shorter than the advection time ( t diff / t adv 〈〈 1), then the virialized gas collapses to an excretion disc, while for t diff / t adv 1 an isotropic wind is formed. Between these two extremes, an inflated envelope transports the heat generated near the binary to the surface by meridional flows. In all cases, the radiated luminosity reaches a fraction ~10 –2 to 10 –1 of $\skew3\dot{M}v_{\rm orb}^2/2$ , where $\skew3\dot{M}$ is the mass outflow rate. We discuss the implications of our results for transients in the luminosity gap between classical novae and supernovae, such as V1309 Sco and V838 Mon.

Description: Expulsion of neutron-rich matter following the merger of neutron star binaries is crucial to the radioactively powered electromagnetic counterparts of these events and to their relevance as sources of r -process nucleosynthesis. Here we explore the long-term (viscous) evolution of remnant black hole accretion discs formed in such mergers by means of two-dimensional, time-dependent hydrodynamical simulations. The evolution of the electron fraction due to charged-current weak interactions is included, and neutrino self-irradiation is modelled as a lightbulb that accounts for the disc geometry and moderate optical depth effects. Over several viscous times (~1 s), a fraction of ~10 per cent of the initial disc mass is ejected as a moderately neutron-rich wind ( Y e  ~ 0.2) powered by viscous heating and nuclear recombination, with neutrino self-irradiation playing a sub-dominant role. Although the properties of the outflow vary in time and direction, their mean values in the heavy-element production region are relatively robust to variations in the initial conditions of the disc and the magnitude of its viscosity. The outflow is sufficiently neutron-rich that most of the ejecta forms heavy r -process elements with mass number A   130, thus representing a new astrophysical source of r -process nucleosynthesis, distinct from that produced in the dynamical ejecta. Due to its moderately high entropy, disc outflows contain a small residual fraction ~1 per cent of helium, which could produce a unique spectroscopic signature.

Description: We study the radioactively powered transients produced by accretion disc winds following a compact object merger. Based on the outflows found in two-dimensional hydrodynamical disc models, we use wavelength-dependent radiative transfer calculations to generate synthetic light curves and spectra. We show that resulting kilonova transients generally produce both optical and infrared emission, with the brightness and colour carrying information about the merger physics. In those regions of the wind subject to high neutrino irradiation, r-process nucleosynthesis may halt before producing high-opacity, complex ions (the lanthanides). The kilonova light curves thus typically has two distinct components: a brief (~2 d) blue optical transient produced in the outer lanthanide-free ejecta, and a longer (~10 d) infrared transient produced in the inner, lanthanide line-blanketed region. Mergers producing a longer lived neutron star, or a more rapidly spinning black hole, have stronger neutrino irradiation, generate more lanthanide-free ejecta and are optically brighter and bluer. At least some optical emission is produced in all disc wind models, which should enhance the detectability of electromagnetic counterparts to gravitational wave sources. However, the presence of even a small amount (10 –4 M ) of overlying, neutron-rich dynamical ejecta will act as a ‘lanthanide-curtain’, obscuring the optical wind emission from certain viewing angles. Because the disc outflows have moderate velocities (~10 000 km s –1 ), numerous resolved line features are discernible in the spectra, distinguishing disc winds from fast-moving dynamical ejecta, and offering a potential diagnostic of the detailed composition of freshly produced r-process material.

Description: The Fermi Large Area Telescope (LAT) discovery that classical novae produce 100 MeV gamma-rays establishes that shocks and relativistic particle acceleration are key features of these events. These shocks are likely to be radiative due to the high densities of the nova ejecta at early times coincident with the gamma-ray emission. Thermal X-rays radiated behind the shock are absorbed by neutral gas and reprocessed into optical emission, similar to Type IIn (interacting) supernovae. Gamma-rays are produced by collisions between relativistic protons with the nova ejecta (hadronic scenario) or inverse Compton/bremsstrahlung emission from relativistic electrons (leptonic scenario), where in both scenarios the efficiency for converting relativistic particle energy into LAT gamma-rays is at most a few tens of per cent. The measured ratio of gamma-ray and optical luminosities, L / L opt , thus sets a lower limit on the fraction of the shock power used to accelerate relativistic particles, nth . The measured value of L / L opt for two classical novae, V1324 Sco and V339 Del, constrains nth 10 –2 and 10 –3 , respectively. Leptonic models for the gamma-ray emission are disfavoured given the low electron acceleration efficiency, nth ~ 10 –4 –10 –3 , inferred from observations of Galactic cosmic rays and particle-in-cell numerical simulations. A fraction f sh 100( nth /0.01) –1 and 10( nth /0.01) –1  per cent of the optical luminosity is powered by shocks in V1324 Sco and V339 Del, respectively. Such high fractions challenge standard models that instead attribute all nova optical emission to the direct outwards transport of thermal energy released near the white dwarf surface. We predict hard ~10–100 keV X-ray emission coincident with the LAT emission, which should be detectable by NuSTAR or ASTRO-H , even at times when softer 10 keV emission is absorbed by neutral gas ahead of the shocks.

Description: We model the non-thermal transient Swift  J1644+57 as resulting from a relativistic jet powered by the accretion of a tidally disrupted star on to a supermassive black hole. Accompanying synchrotron radio emission is produced by the shock interaction between the jet and the dense circumnuclear medium, similar to a gamma-ray burst afterglow. An open mystery, however, is the origin of the late-time radio re-brightening, which occurred well after the peak of the jetted X-ray emission. Here, we systematically explore several proposed explanations for this behaviour by means of multidimensional hydrodynamic simulations coupled to a self-consistent radiative transfer calculation of the synchrotron emission. Our main conclusion is that the radio afterglow of Swift  J1644+57 is not naturally explained by a jet with a one-dimensional top-hat angular structure. However, a more complex angular structure comprised of an ultrarelativistic core (Lorentz factor  ~ 10) surrounded by a slower ( ~ 2) sheath provides a reasonable fit to the data. Such a geometry could result from the radial structure of the super-Eddington accretion flow or as the result of jet precession. The total kinetic energy of the ejecta that we infer of ~ few 10 53 erg requires a highly efficient jet launching mechanism. Our jet model providing the best fit to the light curve of the on-axis event Swift  J1644+57 is used to predict the radio light curves for off-axis viewing angles. Implications for the presence of relativistic jets from tidal disruption events (TDEs) detected via their thermal disc emission, as well as the prospects for detecting orphan TDE afterglows with upcoming wide-field radio surveys and resolving the jet structure with long baseline interferometry, are discussed.

Description: When a star is tidally disrupted by a supermassive black hole (SMBH), roughly half of its mass falls back to the SMBH at super-Eddington rates. As this gas is tenuously gravitationally bound and unable to cool radiatively, only a small fraction f in  〈〈 1 may accrete, with the majority instead becoming unbound in an outflow of velocity ~10 4 km s –1 . The outflow spreads laterally as it expands to large radii, encasing the SMBH and blocking the inner disc's EUV/X-ray radiation, which becomes trapped in a radiation-dominated nebula. Ionizing nebular radiation heats the inner edge of the ejecta, converting the emission to optical/near-UV wavelengths where photons more readily escape due to the lower opacity. This can explain the unexpectedly low and temporally constant effective temperatures of optically discovered tidal disruption event (TDE) flares. For high-mass SMBHs, M • 10 7 M , the ejecta can become fully ionized at an earlier stage, or for a wider range of viewing angles, producing a TDE flare accompanied by thermal X-ray emission. The peak optical luminosity is suppressed as the result of adiabatic losses in the inner disc wind when M •  〈〈 10 7 M , possibly contributing to the unexpected dearth of optical TDEs in galaxies with low-mass SMBHs. In the classical picture, where f in 1, TDEs de-spin supermassive SMBHs and cap their maximum spins well below theoretical accretion physics limits. This cap is relaxed in our model, and existing Fe Kα spin measurements provide preliminary evidence that f in  〈 1.

Description: We construct time-dependent one-dimensional (vertically averaged) models of accretion discs produced by the tidal disruption of a white dwarf (WD) by a binary neutron star (NS) companion. Nuclear reactions in the disc mid-plane burn the WD matter to increasingly heavier elements at sequentially smaller radii, releasing substantial energy which can impact the disc dynamics. A model for disc outflows is employed, by which cooling from the outflow balances other sources of heating (viscous, nuclear) in regulating the Bernoulli parameter of the mid-plane to a fixed value 0. We perform a comprehensive parameter study of the compositional yields and velocity distributions of the disc outflows for WDs of different initial compositions. For C/O WDs, the radial composition profile of the disc evolves self-similarly in a quasi-steady-state manner, and is remarkably robust to model parameters. The nucleosynthesis in helium WD discs does not exhibit this behaviour, which instead depends sensitively on factors controlling the disc mid-plane density (e.g. the strength of the viscosity, α). By the end of the simulation, a substantial fraction of the WD mass is unbound in outflows at characteristic velocities of ~10 9 cm s –1 . The outflows from WD-NS merger discs contain 10 –4 –3 x 10 –3 M of radioactive 56 Ni, resulting in fast (~ week long) dim (~10 40 erg s –1 ) optical transients; shock heating of the ejecta by late-time outflows may increase the peak luminosity to ~10 43 erg s –1 . The accreted mass on to the NS is probably not sufficient to induce gravitational collapse, but may be capable of spinning up the NS to periods of ~10 ms, illustrating the feasibility of this channel in forming isolated millisecond pulsars.