ALBERT

Description: Polarization measurements from relativistic outflows are a valuable tool to probe the geometry of the emission region and the microphysics of the particle distribution. Indeed, the polarization level depends on (i) the local magnetic field orientation, (ii) the geometry of the emitting region with respect to the line of sight and (iii) the electron pitch angle distribution. Here we consider optically thin synchrotron emission and we extend the theory of circular polarization from a point source to an extended radially expanding relativistic jet. We present numerical estimates for both linear and circular polarization in such systems. We consider different configurations of the magnetic field, spherical and jetted outflows, isotropic and anisotropic pitch angle distributions, and outline the difficulty in obtaining the reported high level of circular polarization observed in the afterglow of Gamma Ray Burst (GRB) 121024A. We conclude that the origin of the observed polarization cannot be intrinsic to an optically thin synchrotron process, even when the electron pitch angle distribution is extremely anisotropic.

Description: The prompt emission of low-luminosity gamma-ray bursts ( ll GRBs) indicates that these events originate from a relativistic shock breakout. In this case, we can estimate, based on the properties of the prompt emission, the energy distribution of the ejecta. We develop a general formalism to estimate the afterglow produced by synchrotron emission from the forward shock resulting from the interaction of this ejecta with the circumburst matter. We assess whether this emission can produce the observed radio and X-ray afterglows of the available sample of four ll GRBs. All four radio afterglows can be explained within this model, providing further support for shock breakouts being the origin of ll GRBs. We find that in one of the ll GRBs (GRB 031203), the predicted X-ray emission, using the same parameters that fit the radio, can explain the observed one. In another one (GRB 980425), the observed X-rays can be explained if we allow for a slight modification of the simplest model. For the last two cases (GRBs 060218 and 100316D), we find that, as is the case for previous attempts to model these afterglows, the simplest model that fits the radio emission underpredicts the observed X-ray afterglows. Using general arguments, we show that the most natural location of the X-ray source is, like the radio source, within the ejecta–external medium interaction layer but that emission is due to a different population of electrons or to a different emission process.

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: Recent observation of some luminous transient sources with low colour temperatures suggests that the emission is dominated by optically thick winds driven by super-Eddington accretion. We present a general analytical theory of the dynamics of radiation pressure-driven, optically thick winds. Unlike the classical adiabatic stellar wind solution whose dynamics are solely determined by the sonic radius, here the loss of the radiation pressure due to photon diffusion also plays an important role. We identify two high mass-loss rate regimes ( $\dot{M} 〉 L_{\rm Edd}/c^2$ ). In the large total luminosity regime, the solution resembles an adiabatic wind solution. Both the radiative luminosity, L , and the kinetic luminosity, L k , are super-Eddington with L 〈 L k and $L \propto L_{\rm k}^{1/3}$ . In the lower total luminosity regime, most of the energy is carried out by the radiation with L k 〈 L L Edd . In a third, low mass-loss regime ( $\dot{M} 〈 L_{\rm Edd}/c^2$ ), the wind becomes optically thin early on and, unless gas pressure is important at this stage, the solution is very different from the adiabatic one. The results are independent from the energy generation mechanism at the foot of the wind; therefore, they are applicable to a wide range of mass ejection systems, from black hole accretion, to planetary nebulae, and to classical novae.

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: We study a sample of 23 Type II plateau supernovae (SNe II-P), all observed with the same set of instruments. Analysis of their photometric evolution confirms that their typical plateau duration is 100 d with little scatter, showing a tendency to get shorter for more energetic SNe. We examine the claimed correlation between the luminosity and the rise time from explosion to plateau. We analyse their spectra, measuring typical ejecta velocities, and confirm that they follow a well-behaved power-law decline. We find indications of high-velocity material in the spectra of six of our SNe. We test different dust-extinction correction methods by asking the following – does the uniformity of the sample increase after the application of a given method? A reasonably behaved underlying distribution should become tighter after correction. No method we tested made a significant improvement.

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.