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  • 1
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    Multi-messenger Observations of a Binary Neutron Star Merger (2017)
    Institute of Physics
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    Publication Date: 2017-10-16
    Print ISSN: 2041-8205
    Electronic ISSN: 2041-8213
    Topics: Physics
    Published by Institute of Physics
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  • 2
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    Spectral signatures of compact sources in the inverse Compton catastrophe limit (2015)
    Oxford University Press
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    Publication Date: 2015-08-02
    Description: The inverse Compton catastrophe is defined as a dramatic rise in the luminosity of inverse Compton scattered photons. It is described by a non-linear loop of radiative processes that sets in for high values of the electron compactness and is responsible for the efficient transfer of energy from electrons to photons, predominantly through inverse Compton scatterings. We search for the conditions that drive a magnetized non-thermal source to the inverse Compton catastrophe regime and study its multiwavelength (MW) photon spectrum. We develop a generic analytical framework and use numerical calculations as a backup to the analytical predictions. We find that the escaping radiation from a source in the Compton catastrophe regime bears some unique features. The MW photon spectrum is a broken power law with a break at ~ m e c 2 due to the onset of the Klein–Nishina suppression. The spectral index below the break energy depends on the electron and magnetic compactnesses logarithmically, while it is independent of the electron power-law index ( s ). The maximum radiating power emerges typically in the -ray regime, at energies ~ m e c 2 (~ max m e c 2 ) for s  〉 2 ( s   2), where max is the maximum Lorentz factor of the injected electron distribution. We apply the principles of the inverse Compton catastrophe to blazars and -ray bursts using the analytical framework we developed, and show how these can be used to impose robust constraints on the source parameters.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Radio flares of compact binary mergers: the effect of non-trivial outflow geometry (2015)
    Oxford University Press
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    Publication Date: 2015-08-06
    Description: The next generation gravitational waves (GW) detectors are most sensitive to GW emitted by compact (neutron star/black hole) binary mergers. If one of those is a neutron star the merger will also emit electromagnetic radiation via three possible channels: gamma-ray bursts and their (possibly orphan) afterglows, Li-Paczynski Macronovae and radio flares. This accompanying electromagnetic radiation is vitally important in confirming the GW detections. It could also reveal a wealth of information regarding the merger and will open a window towards multimessenger astronomy. Identifying and characterizing these counterparts is therefore of utmost importance. In this work, we explore late time radio flares emitted by the dynamically ejected outflows. We build upon previous work and consider the effect of the outflow's non-trivial geometry. Using an approximate method, we estimate the radio light-curves for several ejected matter distributions obtained in numerical simulations. Our method provides an upper limit to the effect of non-sphericity. Together with the spherical estimates, the resulting light curves bound the actual signal. We find that while non-spherical geometries can in principle lead to an enhanced emission, in most cases they result in an increase in the time-scale compared with a corresponding spherical configuration. This would weaken somewhat these signals and might decrease the detection prospects.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Jet and disc luminosities in tidal disruption events (2015)
    Oxford University Press
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    Publication Date: 2015-08-14
    Description: Tidal disruption events (TDEs) explore the whole range of accretion rates and configurations. A challenging question is what the corresponding light curves of these events are. We explore numerically the disc luminosity and the conditions within the inner region of the disc using a fully general relativistic slim disc model. Those conditions determine the magnitude of the magnetic field that engulfs the black hole and this, in turn, determines the Blandford–Znajek jet power. We estimate this power in two different ways and show that they are self-consistent. We find, as expected earlier from analytic arguments , that neither the disc luminosity nor the jet power follows the accretion rate throughout the disruption event. The disc luminosity varies only logarithmically with the accretion rate at super-Eddington luminosities. The jet power follows initially the accretion rate but remains constant after the transition from super- to sub-Eddington. At lower accretion rates at the end of the magnetically arrested disc (MAD) phase, the disc becomes thin and the jet may stop altogether. These new estimates of the jet power and disc luminosity that do not simply follow the mass fallback rate should be taken into account when searching for TDEs and analysing light curves of TDE candidates. Identification of some of the above-mentioned transitions may enable us to estimate better TDE parameters.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Linear and circular polarization in ultra-relativistic synchrotron sources - implications to GRB afterglows (2015)
    Oxford University Press
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    Publication Date: 2015-11-19
    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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    On the composition of GRBs' Collapsar jets (2015)
    Oxford University Press
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    Publication Date: 2015-04-24
    Description: The duration distribution of long gamma-ray bursts (GRBs) reveals a plateau at durations shorter than ~20 s (in the observer frame) and a power-law decline at longer durations. Such a plateau arises naturally in the Collapsar model. In this model, the engine has to operate long enough to push the jet out of the stellar envelope and the observed duration of the burst is the difference between the engine's operation time and the jet breakout time. The jet breakout time inferred from the duration distribution (~10 s in the burst's frame) is comparable to the breakout time of both analytic estimates and numerical simulations (both 2D and 3D) of a hydrodynamic jet (~10 s for typical parameters). Recently, we have estimated analytically the breakout time of a Poynting-flux-dominated jet and have shown that it is consistent with 2D numerical simulations. We find that such a jet with the same overall energy breaks out much faster (1 s). If this result holds for 3D simulations it implies that only hydrodynamic jets are compatible with the duration of the plateau in the GRB duration distribution and hence the jet should be hydrodynamic during most of the time that its head is within the envelope of the progenitor star and around the time when it emerges from the star. This would naturally arise if the jet forms as a hydrodynamic jet in the first place or if it forms Poynting flux dominated but dissipates most of its magnetic energy early on within the progenitor star and emerges as a hydrodynamic jet.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Mass ejection from neutron star mergers: different components and expected radio signals (2015)
    Oxford University Press
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    Publication Date: 2015-04-28
    Description: In addition to producing a strong gravitational signal, a short gamma-ray burst (GRB), and a compact remnant, neutron star mergers eject significant masses (up to a few per cent of M ) at significant kinetic energies. The different components of the ejected mass include a dynamical ejected mass, a GRB jet and also a shock-breakout material, a cocoon resulting from the interaction of the jet with other ejecta, and viscous- and neutrino-driven winds. The interaction of these ejecta with the surrounding interstellar medium will produce a long-lasting radio flare. We estimate here the expected radio flares arising from these outflows. The flares are rather weak and uncertainties in the kinetic energy, the velocity, and the external density make exact estimates of these signals difficult. The relative strength of the different signals depends strongly on the viewing angle. An observer along the jet axis or close to it will detect a strong signal at a few dozen days from the radio afterglow (or the orphan radio afterglow) produced by the highly relativistic GRB jet. A generic observer at larger viewing angles will generally observe the dynamical ejecta, whose contribution peaks a year or so after the event. Depending on the observed frequency and the external density, other components may also give rise to a significant contribution. If the short GRB 130603B was a merger event, its radio flare from the dynamical ejecta might be detectable with the EVLA and the LOFAR for the higher range of external densities n   0.5 cm –3
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    A revised analysis of gamma-ray bursts prompt efficiencies (2016)
    Oxford University Press
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    Publication Date: 2016-06-30
    Description: The prompt gamma-ray bursts’ (GRBs) efficiency is an important clue on the emission mechanism producing the -rays. Previous estimates of the kinetic energy of the blast waves, based on the X-ray afterglow luminosity L X , suggested that this efficiency is large, with values above 90 per cent in some cases. This poses a problem to emission mechanisms and in particular to the internal shocks model. These estimates are based, however, on the assumption that the X-ray emitting electrons are fast cooling and that their Inverse Compton (IC) losses are negligible. The observed correlations between L X (and hence the blast wave energy) and E , iso , the isotropic equivalent energy in the prompt emission, has been considered as observational evidence supporting this analysis. It is reasonable that the prompt gamma-ray energy and the blast wave kinetic energy are correlated and the observed correlation corroborates, therefore, the notion L X is indeed a valid proxy for the latter. Recent findings suggest that the magnetic field in the afterglow shocks is significantly weaker than was earlier thought and its equipartition fraction, B , could be as low as 10 –4 or even lower. Motivated by these findings we reconsider the problem, taking now IC cooling into account. We find that the observed L X – E , iso correlation is recovered also when IC losses are significant. For small B values the blast wave must be more energetic and we find that the corresponding prompt efficiency is significantly smaller than previously thought. For example, for B ~ 10 –4 we infer a typical prompt efficiency of ~15 per cent.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Particle acceleration, magnetization and radiation in relativistic shocks (2016)
    Oxford University Press
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    Publication Date: 2016-06-09
    Description: The mechanisms of particle acceleration and radiation, as well as magnetic field build-up and decay in relativistic collisionless shocks, are open questions with important implications to various phenomena in high-energy astrophysics. While the Weibel instability is possibly responsible for magnetic field build-up and diffusive shock acceleration is a model for acceleration, both have problems and current particle-in-cell simulations show that particles are accelerated only under special conditions and the magnetic field decays on a very short length-scale. We present here a novel model for the structure and the emission of highly relativistic collisionless shocks. The model takes into account (and is based on) non-local energy and momentum transport across the shock front via emission and absorption of high-energy photons. This leads to a pre-acceleration of the fluid and pre-amplification of the magnetic fields in the upstream region. Both have drastic implications on the shock structure. The model explains the persistence of the shock-generated magnetic field at large distances from the shock front. The dissipation of this magnetic field results in a continuous particle acceleration within the downstream region. A unique feature of the model is the existence of an ‘attractor’, towards which any shock will evolve. The model is applicable to any relativistic shock, but its distinctive features show up only for sufficiently large compactness. We demonstrate that prompt and afterglow gamma-ray bursts’ shocks satisfy the relevant conditions, and we compare their observations with the predictions of the model.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    The afterglow of a relativistic shock breakout and low-luminosity GRBs (2015)
    Oxford University Press
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    Publication Date: 2015-02-07
    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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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