ALBERT

Description: We use 3D hydrodynamic simulations of the long-term evolution of neutron star merger ejecta to predict the light curves of electromagnetic transients that are powered by the decay of freshly produced r -process nuclei. For the dynamic ejecta that are launched by tidal and hydrodynamic interaction, we adopt grey opacities of 10 cm 2  g –1 , as suggested by recent studies. For our reference case of a 1.3–1.4  $\mathrm{{\rm M}}_{\odot }$ merger, we find a broad IR peak 2–4 d after the merger. The peak luminosity is 2 10 40 erg s –1 for an average orientation, but increased by up to a factor of 4 for more favourable binary parameters and viewing angles. These signals are rather weak and hardly detectable within the large error box (~100 deg 2 ) of a gravitational wave trigger. A second electromagnetic transient results from neutrino-driven winds. These winds produce ‘weak’ r -process material with 50 〈  A  〈 130 and abundance patterns that vary substantially between different merger cases. For an adopted opacity of 1 cm 2  g –1 , the resulting transients peak in the UV/optical about 6 h after the merger with a luminosity of 10 41 erg s –1 (for a wind of 0.01  $\mathrm{{\rm M}}_{\odot }$ ) These signals are marginally detectable in deep follow-up searches (e.g. using Hypersuprime camera on Subaru). A subsequent detection of the weaker but longer lasting IR signal would allow an identification of the merger event. We briefly discuss the implications of our results to the recent detection of a near infrared (nIR) transient accompanying GRB 130603B.

Description: We consider the conditions within a Poynting-flux-dominated gamma-ray burst (GRB) emission region. Because of the enormous magnetic energy density, relativistic electrons will cool in such a region extremely rapidly via synchrotron. As there is no known mechanism that can compete in these magnetic environments with synchrotron it must be the source of the prompt sub-MeV emission. This sets strong limits on the size and Lorentz factor of the outflow. Furthermore, synchrotron cooling is too efficient. It overproduces optical and X-ray as compared with the observations. This overproduction of low-energy emission can be avoided if the electrons are re-accelerated many times ( 5 10 4 ) during each pulse (or are continuously heated) or if they escape the emitting region before cooling down. We explore the limitations of both models practically ruling out the later and demonstrating that the former requires two different acceleration mechanisms as well as an extremely large magnetic energy to Baryonic energy ratio. To be viable, any GRB model based on an emission region that is Poynting flux dominated must demonstrate how these conditions are met. We conclude that if GRB jets are launched magnetically dominated they must dissipate somehow most of their magnetic energy before they reach the emission region.

Description: The nature of ultraluminous X-ray sources (ULXs) has long been plagued by an ambiguity about whether the central compact objects are intermediate-mass (IMBH, 10 3 M ) or stellar-mass (a few tens M ) black holes (BHs). The high-luminosity (~=10 39  erg s –1 ) and supersoft spectrum ( T ~= 0.1 keV) during the high state of the ULX source X-1 in the galaxy M101 suggest a large emission radius (10 9  cm), consistent with being an IMBH accreting at a sub-Eddington rate. However, recent kinematic measurement of the binary orbit of this source and identification of the secondary as a Wolf–Rayet star suggest a stellar-mass BH primary with a super-Eddington accretion. If that is the case, a hot, optically thick outflow from the BH can account for the large emission radius and the soft spectrum. By considering the interplay of photons’ absorption and scattering opacities, we determine the radius and mass density of the emission region of the outflow and constrain the outflow mass-loss rate. The analysis presented here can be potentially applied to other ULXs with thermally dominated spectra, and to other super-Eddington accreting sources.

Description: Gamma-ray observations of a stellar tidal disruption event (TDE) detected by the Swift satellite and follow-up observations in radio, millimetre (mm), infrared and X-ray bands have provided a rich data set to study accretion on to massive black holes, production of relativistic jets and their interaction with the surrounding medium. The radio and X-ray data for TDE Swift J1644+57 provide a conflicting picture regarding the energy in relativistic jet produced in this event: X-ray data suggest jet energy declining with time as t –5/3 whereas the nearly flat light curves in radio and mm bands lasting for about 100 d have been interpreted as evidence for the total energy output increasing with time. We show in this work that flat light curves do not require addition of energy to decelerating external shock (which produced radio and mm emission via synchrotron process), instead the flat behaviour is due to inverse-Compton cooling of electrons by X-ray photons streaming through the external shock; the higher X-ray flux at earlier times cools electrons more efficiently thereby reducing the emergent synchrotron flux, and this effect weakens as the X-ray flux declines with time.

Description: Compact binary mergers are prime sources of gravitational waves (GWs), targeted by current and next generation detectors. The question ‘what is the observable electromagnetic (EM) signature of a compact binary merger?’ is an intriguing one with crucial consequences to the quest for GWs. We present a large set of numerical simulations that focus on the EM signals that emerge from the dynamically ejected subrelativistic material. These outflows produce on a time-scale of a day macronovae – short-lived infrared (IR) to ultraviolet (UV) signals powered by radioactive decay. Like in regular supernovae the interaction of this outflow with the surrounding matter inevitably leads to a long-lasting remnant. We calculate the expected radio signals of these remnants on time-scales longer than a year, when the subrelativistic ejecta dominate the emission. We discuss their detectability in 1.4 GHz and 150 MHz and compare it with an updated estimate of the detectability of short gamma-ray bursts’ orphan afterglows (which are produced by a different component of this outflow). We find that mergers with characteristics similar to those of the Galactic neutron star binary population (similar masses and typical circummerger Galactic disc density of ~1 cm –3 ) that take place at the detection horizon of advanced GW detectors (300 Mpc) yield 1.4 GHz [150 MHz] signals of ~50 [300] μJy, for several years. The signal on time-scales of weeks is dominated by the mildly and/or ultrarelativistic outflow, which is not accounted for by our simulations, and is expected to be even brighter. Upcoming all sky surveys are expected to detect a few dozen, and possibly more, merger remnants at any given time thereby providing robust lower limits to the mergers rate even before the advanced GW detectors become operational. The macronovae signals from the same distance peak in the IR to UV range at an observed magnitude that may be as bright as 22–23 about 10 h after the merger but dimmer, redder and longer if the opacity is larger.

Description: The collapsar model explains the association of long duration gamma-ray bursts (GRBs) with stellar collapse. It involves a relativistic jet that forms at the core of a collapsing massive star. The jet penetrates the stellar envelope and the prompt GRB emission is produced once the jet is well outside the star. Most current models for generation of relativistic jets involve Poynting-flux-dominated outflows. We explore here the propagation of such a jet through a stellar envelope. The jet forms a bow shock in front of it. Energy dissipation at the shock generates an energetic cocoon that surrounds the jet. This cocoon exerts pressure on the jet and collimates it. While this description resembles the propagation of a hydrodynamic jet there are significant qualitative differences. Two strong shocks, the reverse shock that slows down the hydrodynamic jet and the collimation shock that collimates it, cannot form within the Poynting-flux-dominated jet. As a result this jet moves much faster and dissipates much less energy while it crosses the stellar envelope. We construct here a simple analytic model that explores, self consistently, the jet–cocoon interaction and dynamics. Using this model we determine the properties of the jet, including its velocity, propagation time and shape.

Description: We explore the multimessenger signatures of encounters between two neutron stars (ns 2 ) and between a neutron star and a stellar mass black hole (nsbh). We focus on the differences between gravitational-wave-driven binary mergers and dynamical collisions that occur, for example, in globular clusters. Our discussion is based on Newtonian hydrodynamics simulations that incorporate a nuclear equation of state and a multiflavour neutrino treatment. For both types of encounters we compare the gravitational wave and neutrino emission properties. We also calculate the rates at which nearly unbound mass is delivered back to the central remnant in a ballistic-fallback-plus-viscous-disc model and we analyse the properties of the dynamically ejected matter. Last but not least we address the electromagnetic transients that accompany each type of encounter. We find that dynamical collisions are at least as promising as binary mergers for producing (short) gamma-ray bursts, but they also share the same possible caveats in terms of baryonic pollution. All encounter remnants produce peak neutrino luminosities of at least ~10 53  erg s –1 , some of the collision cases exceed this value by more than an order of magnitude. The canonical ns 2 merger case ejects more than 1 per cent of a solar mass of extremely neutron-rich ( Y e  ~ 0.03) material, an amount that is consistent with double neutron star mergers being a major source of r-process in the galaxy. nsbh collisions eject very large amounts of matter (~0.15 M ) which seriously constrains their admissible occurrence rates. The compact object collision rate (sum of ns 2 and nsbh) must therefore be less, likely much less, than 10 per cent of the ns 2 merger rate. The radioactively decaying ejecta produce optical–ultraviolet ‘macronova’ which, for the canonical merger case, peak after ~0.4 d with a luminosity of ~5  x 10 41  erg s –1 . ns 2 (nsbh) collisions reach up to two (four) times larger peak luminosities. The dynamic ejecta deposit a kinetic energy comparable to a supernova in the ambient medium. The canonical merger case releases approximately 2  x 10 50  erg, the most extreme (but likely rare) cases deposit kinetic energies of up to 10 52  erg. The deceleration of this mildly relativistic material by the ambient medium produces long lasting radio flares. A canonical ns 2 merger at the detection horizon of advanced LIGO/Virgo produces a radio flare that peaks on a time-scale of 1 yr with a flux of ~0.1 mJy at 1.4 GHz. Collisions eject more material at higher velocities and therefore produce brighter and longer lasting flares.

Description: The binary pulsar J0737–3039 is the only known system having two observable pulsars, thus offering a unique laboratory to test general relativity and explore pulsar physics. Based on the low eccentricity and the position within the galactic plane, two of us have argued that pulsar B had a non-standard formation scenario with little or no mass ejection and predicted that the system would have a very slow proper motion. Pulsar timing measurements confirmed this prediction. The recent observations of the alignment between the spin of pulsar A and the binary orbit is also in agreement with this scenario. Detailed simulations of the formation process of pulsar B show that its progenitor, just before the collapse, was a massive O–Ne–Mg white dwarf surrounded by a tenuous, 0.1–0.16 M , envelope. This envelope was ejected when the white dwarf collapsed to form a neutron star. Pulsar B was born as a slow rotator (spin period ~1 s) and a kick received when the pulsar formed changed its spin direction to the current one. This realization sheds light on the angular momentum evolution of the progenitor star, a process which is strongly affected by interaction with the binary companion. The slow proper motion of the system also implies that the system must have undergone a phase of mass transfer in which star A shed a significant fraction of its mass on to B.

Description: We follow the long-term evolution of the dynamic ejecta of neutron star mergers for up to 100 years and over a density range of roughly 40 orders of magnitude. We include the nuclear energy input from the freshly synthesized, radioactively decaying nuclei in our simulations and study its effects on the remnant dynamics. Although the nuclear heating substantially alters the long-term evolution, we find that running nuclear networks over purely hydrodynamic simulations (i.e. without heating) yields actually acceptable nucleosynthesis results. The main dynamic effect of the radioactive heating is to quickly smooth out inhomogeneities in the initial mass distribution, subsequently the evolution proceeds self-similarly and after 100 years the remnant still carries the memory of the initial binary mass ratio. We also explore the nucleosynthetic yields for two mass ejection channels. The dynamic ejecta very robustly produce ‘strong’ r-process elements with A  〉 130 with a pattern that is essentially independent of the details of the merging system. From a simple model we find that neutrino-driven winds yield ‘weak’ r-process contributions with 50 〈  A  〈 130 whose abundance patterns vary substantially between different merger cases. This is because their electron fraction, set by the ratio of neutrino luminosities, varies considerably from case to case. Such winds do not produce any 56 Ni, but a range of radioactive isotopes that are long-lived enough to produce a second, radioactively powered electromagnetic transient in addition to the ‘macronova’ from the dynamic ejecta. While our wind model is very simple, it nevertheless demonstrates the potential of such neutrino-driven winds for electromagnetic transients and it motivates further, more detailed neutrino-hydrodynamic studies. The properties of the mentioned transients are discussed in more detail in a companion paper.