Propagation and stability of superluminal waves in pulsar winds

Nonlinear electromagnetic waves with superluminal phase velocity can propagate in the winds around isolated pulsars, and around some pulsars in binary systems. Using a short-wavelength approximation, we find and analyze an integrable system of equations that govern their evolution in spherical geometry. A confined mode is identified that stagnates to finite pressure at large radius and can form a precursor to the termination shock. Using a simplified criterion, we find this mode is stable for most isolated pulsars, but may be unstable if the external pressure is high, such as in the pulsar wind nebulae in starburst galaxies and in W44. Pulsar winds in eccentric binary systems, such as PSR 1259-63, may go through phases with stable and unstable electromagnetic precursors, as well as phases in which the density is too high for these modes to propagate.Comment: published version; correction of fig

Radiative damping and emission signatures of strong superluminal waves in pulsar winds

We analyse the damping by radiation reaction and by Compton drag of strong, superluminal electromagnetic waves in the context of pulsar winds. The associated radiation signature is found by estimating the efficiency and the characteristic radiation frequencies. Applying these estimates to the gamma-ray binary containing PSR B1259-63, we show that the GeV flare observed by Fermi-LAT can be understood as inverse Compton emission by particles scattering photons from the companion star, if the pulsar wind termination shock acquires a precursor of superluminal waves roughly 30 days after periastron. This constrains the mass-loading factor of the wind $\mu=L/\dot{N}mc^2$ (where $L$ is the luminosity and $\dot{N}$ the rate of loss of electrons and positrons) to be roughly $6\times 10^4$.Comment: minor revisions, accepted for publication in Ap

High-energy emission from pulsar binaries

Unpulsed, high-energy emission from pulsar binaries can be attributed to the interaction of a pulsar wind with that of a companion star. At the shock between the outflows, particles carried away from the pulsar magnetosphere are accelerated and radiate both in synchrotron and inverse Compton processes. This emission constitutes a significant fraction of the pulsar spin-down luminosity. It is not clear however, how the highly magnetized pulsar wind could convert its mainly electromagnetic energy into the particles with such high efficiency. Here we investigate a scenario in which a pulsar striped wind converts into a strong electromagnetic wave before reaching the shock. This mode can be thought of as a shock precursor that is able to accelerate particles to ultrarelativistic energies at the expense of the electromagnetic energy it carries. Radiation of the particles leads to damping of the wave. The efficiency of this process depends on the physical conditions imposed by the external medium. Two regimes can be distinguished: a high density one, where the EM wave cannot be launched at all and the current sheets in the striped wind are first compressed by an MHD shock and subsequently dissipate by reconnection, and a low density one, where the wind can first convert into an electromagnetic wave in the shock precursor, which then damps and merges into the surroundings. Shocks in binary systems can transit from one regime to another according to binary phase. We discuss possible observational implications for these objects.Comment: 4 pages, 1 figure. Proceedings of Gamma2012, July 9-13, Heidelberg, German

Superluminal Waves and the Structure of Pulsar Wind Termination Shocks

The termination shock of a pulsar wind is located roughly where the ram pressure matches that of the surrounding medium. Downstream of the shock, MHD models of the diffuse nebular emission suggest the plasma is weakly magnetized. However, the transition from a Poynting-dominated MHD wind to a particle-dominated flow is not well understood. We discuss a solution of this "sigma problem" in which a striped wind converts into a strong, superluminal electromagnetic wave. This mode slows down as it propagates radially, and its ram pressure tends to a constant value at large radius, a property we use to match the solution to the surrounding nebula. The wave thus forms a pre-cursor to the termination shock, which occurs at the point where the wave dissipates. Possible damping and dissipation mechanisms are discussed qualitatively.Comment: 4 pages, 2 figures. Proceedings of the "Electromagnetic Radiation from Pulsars and Magnetars" conference, April 24-27, 2012, Zielona Gora, Polan

Nonlinear waves in Poynting-flux dominated outflows

Rotating, compact objects power some of the most spectacular phenomena in astrophysics, e.g., gamma-ray bursts, active galactic nuclei and pulsar winds. The energy is carried by Poynting flux, and the system is usually modelled using relativistic magnetohydrodynamics (MHD). However, in the relatively low density medium expected around some of these objects, the MHD approximation breaks down, allowing new, large-amplitude waves to propagate. We discuss the role of these waves in two astrophysical contexts: In blazar jets, we show that a magnetic shear, launched together with a plasma from the black hole magnetosphere, begins to accelerate particles at a large distance from its source. The resulting non-thermal emission can, nevertheless, be modulated on very short timescales, which can explain the rapid variability of the TeV gamma-ray flux observed from some blazars. In pulsar winds, we analyze the radial propagation of superluminal modes, including their damping by radiation reaction and by interaction with an external photon field. We discuss their effect on the structure of the pulsar wind termination shock, presenting new solutions in which the nonlinear wave is asymptotically matched to the constant pressure surroundings. The observational implications of these solutions are discussed for both isolated pulsars, and pulsars in binary systems

SAMSum Corpus: A Human-annotated Dialogue Dataset for Abstractive Summarization

This paper introduces the SAMSum Corpus, a new dataset with abstractive dialogue summaries. We investigate the challenges it poses for automated summarization by testing several models and comparing their results with those obtained on a corpus of news articles. We show that model-generated summaries of dialogues achieve higher ROUGE scores than the model-generated summaries of news -- in contrast with human evaluators' judgement. This suggests that a challenging task of abstractive dialogue summarization requires dedicated models and non-standard quality measures. To our knowledge, our study is the first attempt to introduce a high-quality chat-dialogues corpus, manually annotated with abstractive summarizations, which can be used by the research community for further studies.Comment: Attachment contains the described dataset archived in 7z format. Please see the attached readme and licence. Update of the previous version: changed formats of train/val/test files in corpus.7

Charge-starved, relativistic jets and blazar variability

High energy emission from blazars is thought to arise in a relativistic jet launched by a supermassive black hole. The emission site must be far from the hole and the jet relativistic, in order to avoid absorption of the photons. In extreme cases, rapid variability of the emission suggests that structures of length-scale smaller than the gravitational radius of the central black hole are imprinted on the jet as it is launched, and modulate the radiation released after it has been accelerated to high Lorentz factor. We propose a mechanism which can account for the acceleration of the jet, and for the rapid variability of the radiation, based on the propagation characteristics of large-amplitude waves in charge-starved, polar jets. Using a two-fluid (electron-positron) description, we find the outflows exhibit a delayed acceleration phase, that starts at roughly 1pc, where the inertia associated with the wave currents becomes important. The time-structure imprinted on the jet at launch modulates photons produced by the accelerating jet provided the pair multiplicity in the black-hole magnetosphere is sufficiently small, suggesting that very rapid variability is confined to sources in which the electromagnetic cascade in the black-hole magnetosphere is not prolific.Comment: 8 pages, 1 figure. Momentum equation corrected. Conclusions unchanged. Erratum submitted to Ap

Pulsar Striped Winds

International audienceAccording to magnetohydrodynamic (MHD) models, the rotational energy of a rapidly spinning neutron star is carried away by a relativistic wind and deposited at a large distance, in the nebula, downstream of the wind termination shock. The energy transport in the outflow is mediated by Poynting flux, but it is not clear how the energy stored in the fields is transferred into the energized population of emitting particles. The most plausible dissipation mechanisms are thought to be related to the "striped" structure of the wind, in particular, to the existence of a current sheet, prone to reconnection events. In this model the current sheet is a natural place for internal dissipation and acceleration of particles responsible for pulsed, high-energy emission. Moreover, reconnection is a promising scenario for explaining annihilation of fields at the shock and conversion of their energy into the kinetic energy of particles. The shock structure, however, is likely to differ in the low-density plasmas, in which non-MHD effects intervene. In this regime, the striped wind can dissipate its energy via an electromagnetic precursor of the shock