Long-term FRII jet evolution in dense environments

We present long-term numerical three-dimensional simulations of a relativistic outflow propagating through a galactic ambient medium and environment, up to distances ∼100 kpc. Our aim is to study the role of dense media in the global dynamics of the radio source. We use a relativistic gas equation of state, and a basic description of thermal cooling terms. In previous work, we showed that a linear perturbation could enhance the jet propagation during the early phases of evolution, by introducing obliquity to the jet reverse shock. Here, we show that this effect is reduced in denser media. We find that the dentist-drill effect acts earlier, due to slower jet propagation and an increased growth of the helical instability. The global morphology of the jet is less elongated, with more prominent lobes. The fundamental physical parameters of the jet generated structure derived from our simulations fall within the estimated values derived for FRII jets in the 3C sample. In agreement with previous axisymmetric and three dimensional simulations in lower density media, we conclude that shock heating of the interstellar and intergalactic media is very efficient in the case of powerful, relativistic jets.

An Accreting Stellar Binary Model for Active Periodic Fast Radio Bursts

In this work, we propose an accreting stellar binary model for understanding the active periodic fast radio bursts (FRBs). The system consists of a stellar compact object (CO) and a donor star (DS) companion in an eccentric orbit, where the DS fills its own Roche lobe near the periastron. The CO accretes the material from the DS and then drives relativistic magnetic blobs. The interaction between the magnetic blobs and the stellar wind of the DS produces a pair of shocks. We find that both the reverse shock and the forward shock are likely to produce FRBs via the synchrotron maser mechanism. We show that this system can in principle sufficiently produce highly active FRBs with a long lifetime, and also can naturally explain the periodicity and the duty cycle of the activity that appeared in FRBs 180916 and 121102. The radio nebula excited by the long-term injection of magnetic blobs into the surrounding environment may account for the associated persistent radio source. In addiction, we discuss the possible multiwavelength counterparts of FRB 180916 in the context of this model. Finally, we encourage the search for FRBs in ultraluminous X-ray sources.

Jet and counter-jet in transonic pulsar wind nebulae

X-ray observations show that a jet and a counter-jet in pulsar wind nebulae often differ one from another. Sometimes one of the jets is not observed at all. We show that the most likely reason for this difference is the relative motion of a pulsar and an ambient matter. Even the slow (subsonic or transonic) ambient matter stream in the pulsar rest frame strongly affects the jets, making the windward jet bright and dynamic, and the leeward jet dim and diffuse. The effect is illustrated using a relativistic MHD model of a double-torus pulsar wind nebula. The model is shown to explain reasonably well the observational appearance of the jets in the Vela nebula - a double-torus object which evolves in a transonic stream initiated by the passage of the reverse shock of the parent supernova.

GRB Polarization: A Unique Probe of GRB Physics

Over half a century from the discovery of gamma-ray bursts (GRBs), the dominant radiation mechanism responsible for their bright and highly variable prompt emission remains poorly understood. Spectral information alone has proven insufficient for understanding the composition and main energy dissipation mechanism in GRB jets. High-sensitivity polarimetric observations from upcoming instruments in this decade may help answer such key questions in GRB physics. This article reviews the current status of prompt GRB polarization measurements and provides comprehensive predictions from theoretical models. A concise overview of the fundamental questions in prompt GRB physics is provided. Important developments in gamma-ray polarimetry including a critical overview of different past instruments are presented. Theoretical predictions for different radiation mechanisms and jet structures are confronted with time-integrated and time-resolved measurements. The current status and capabilities of upcoming instruments regarding the prompt emission are presented. The very complimentary information that can be obtained from polarimetry of X-ray flares as well as reverse-shock and early to late forward-shock (afterglow) emissions are highlighted. Finally, promising directions for overcoming the inherent difficulties in obtaining statistically significant prompt-GRB polarization measurements are discussed, along with prospects for improvements in the theoretical modeling, which may lead to significant advances in the field.

Revisiting the evolution of nonradiative supernova remnants: A hydrodynamical-informed parameterization of the shock positions

Understanding the evolution of a supernova remnant shell in time is fundamental. Such understanding is critical to build reliable models of the dynamics of the supernova remnant shell interaction with any pulsar wind nebula it might contain. Here, we perform a large study of the parameter space for the one-dimensional spherically symmetric evolution of a supernova remnant, accompanying it by analytical analysis. Assuming, as is usual, an ejecta density profile with a power-law core and an envelope, and a uniform ambient medium, we provide a set of highly-accurate approximations for the evolution of the main structural features of supernova remnants, such as the reverse and forward shocks and the contact discontinuity. We compare our results with previously adopted approximations, showing that existing simplified prescriptions can easily lead to large errors. In particular, in the context of pulsar wind nebulae modelling, an accurate description for the supernova remnant reverse shock is required. We also study in depth the self-similar solutions for the initial phase of evolution, when the reverse shock propagates through the envelope of the ejecta. Since these self-similar solutions are exact, but not fully analytical, we here provide highly-accurate approximations as well.

Coherence scale of magnetic fields generated in early time forward shocks of Gamma Ray Bursts

We report the earliest-ever detection of optical polarization from a GRB forward shock (GRB 141220A), measured 129.5 − 204.3 s after the burst using the multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous robotic Liverpool Telescope. The temporal decay gradient of the optical light curves from 86 s to ∼2200 s post-burst is typical of classical forward shocks with α = 1.091 ± 0.008. The low optical polarization $P_{BV} = 2.8 _{- 1.6} ^{+ 2.0} \, \%$ (2σ) at mean time ∼168 s post-burst is compatible with being induced by the host galaxy dust (AV, HG = 0.71 ± 0.15 mag), leaving low polarization intrinsic to the GRB emission itself —as theoretically predicted for forward shocks and consistent with previous detections of low degrees of optical polarization in GRB afterglows observed hours to days after the burst. The current sample of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. $P \sim 1\%-3\%$), (b) contribution from a polarized reverse shock (GRB deceleration time, jet magnetization) or (c) forward shock intrinsic polarization (i.e. $P \le 2\%$), which depends on the magnetic field coherence length scale and the size of the observable emitting region (burst energetics, circumburst density).

Exact solution of one-dimensional relativistic jet with relativistic equation of state

ABSTRACT We study the evolution of one-dimensional relativistic jets, using the exact solution of the Riemann problem for relativistic flows. For this purpose, we solve equations for the ideal special relativistic fluid composed of dissimilar particles in flat space-time and the thermodynamics of fluid is governed by a relativistic equation of state. We obtain the exact solution of jets impinging on denser ambient media. The time variation of the cross-section of the jet-head is modelled and incorporated. We present the initial condition that gives rise to a reverse shock. If the jet-head cross-section increases in time, the jet propagation speed slows down significantly and the reverse-shock may recede opposite to the propagation direction of the jet. We show that the composition of jet and ambient medium can affect the jet solution significantly. For instance, the propagation speed depends on the composition and is maximum for a pair-dominated jet, rather than a pure electron-positron or electron-proton jet. The propagation direction of the reverse-shock may also strongly depend on the composition of the jet.