(shortened) We report the results of an investigation of particle acceleration and electron-positron plasma generation at low altitude in the polar magnetic flux tubes of Rotation Powered Pulsars, when the stellar surface is free to emit whatever cha
rges and currents are demanded by the force-free magnetosphere. We observe novel behavior. a) When the current density is less than the Goldreich-Julian (GJ) value (0<j/j_{GJ}<1), space charge limited acceleration of the current carrying beam is mild, with the full GJ charge density being comprised of the charge density of the beam, co-existing with a cloud of electrically trapped particles with the same sign of charge as the beam. The voltage drops are on the order of mc^2/e, and pair creation is absent. b) When the current density exceeds the GJ value (j/j_{GJ}>1), the system develops high voltage drops, causing emission of gamma rays and intense bursts of pair creation. The bursts exhibit limit cycle behavior, with characteristic time scales somewhat longer than the relativistic fly-by time over distances comparable to the polar cap diameter (microseconds). c) In return current regions, where j/j_{GJ}<0, the system develops similar bursts of pair creation. In cases b) and c), the intermittently generated pairs allow the system to simultaneously carry the magnetospherically prescribed currents and adjust the charge density and average electric field to force-free conditions. We also elucidate the conditions for pair creating beam flow to be steady, finding that such steady flows can occupy only a small fraction of the current density parameter space of the force-free magnetospheric model. The generic polar flow dynamics and pair creation is strongly time dependent. The model has an essential difference from almost all previous quantitative studies, in that we sought the accelerating voltage as a function of the applied current.
We present a new model for the spectral evolution of Pulsar Wind Nebulae inside Supernova Remnants. The model couples the long-term dynamics of these systems, as derived in the 1-D approximation, with a 1-zone description of the spectral evolution of
the emitting plasma. Our goal is to provide a simplified theoretical description that can be used as a tool to put constraints on unknown properties of PWN-SNR systems: a piece of work that is preliminary to any more accurate and sophisticated modeling. In the present paper we apply the newly developed model to a few objects of different ages and luminosities. We find that an injection spectrum in the form of a broken-power law gives a satisfactory description of the emission for all the systems we consider. More surprisingly, we also find that the intrinsic spectral break turns out to be at a similar energy for all sources, in spite of the differences mentioned above. We discuss the implications of our findings on the workings of pulsar magnetospheres, pair multiplicity and on the particle acceleration mechanism(s) that might be at work at the pulsar wind termination shock.
This is a white paper submitted to the Stars and Stellar Evolution (SSE) Science Frontier Panel (SFP) of the NRCs Astronomy and Astrophysics 2010 Decadal Survey. The white paper is endorsed by the American Physical Societys (APS) Topical Group on Plasma Astrophysics (GPAP).
In this White Paper, we emphasize the need for and the important role of plasma astrophysics in the studies of formation, evolution of, and feedback by Active Galaxies. We make three specific recommendations: 1) We need to significantly increase the
resolution of VLA, perhaps by building an EVLA-II at a modest cost. This will provide the angular resolution to study jets at kpc scales, where, for example, detailed Faraday rotation diagnosis can be done at 1GHz transverse to jets; 2) We need to build coordinated programs among NSF, NASA, and DOE to support laboratory plasma experiments (including liquid metal) that are designed to study key astrophysical processes, such as magneto-rotational instability (origin of angular momentum transport), dynamo (origin of magnetic fields), jet launching and stability. Experiments allowing access to relativistic plasma regime (perhaps by intense lasers and magnetic fields) will be very helpful for understanding the stability and dissipation physics of jets from Supermassive Black Holes; 3) Again through the coordinated support among the three Agencies, we need to invest in developing comprehensive theory and advanced simulation tools to study the accretion disks and jets in relativistic plasma physics regime, especially in connecting large scale fluid scale phenomena with relativistic kinetic dissipation physics through which multi-wavelength radiation is produced.
This is a report on the findings of the Galactic compact objects working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper is an APS commissioned document, and the full version has also been released and c
an be found on astro-ph. This section of the white paper discusses the potential of future ground-based gamma-ray experiments to advance our understanding of the physics of Galactic compact objects including pulsars, pulsar wind nebulae, and X-ray binaries.
We study the long term evolution of magnetic fields generated by a collisionless relativistic $e^+e^-$ shock which is initially unmagnetized. Our 2D particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle
distributions are close to isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially, with the non-propagating magnetic fields forming relatively isolated regions with transverse dimension $sim 10-20$ skin depths. These structures decay in amplitude, with little sign of downstream merging. The fields start with magnetic energy density $sim (0.1-0.2)$ of the upstream kinetic energy within the shock transition, but rapid downstream decay drives the fields to much smaller values, below $10^{-3}$ of equipartition after $10^3$ skin depths. In an attempt to construct a theory that follows field decay to these smaller values, we explore the hypothesis that the observed damping is a variant of Landau damping in an unmagnetized plasma. The model is based on the small value of the downstream magnetic energy density, which suggests that particle orbits are only weakly perturbed from straight line motion, if the turbulence is homogeneous. Using linear kinetic theory applied to electromagnetic fields in an isotropic, relativistic Maxwellian plasma, we find a simple analytic form for the damping rates, $gamma_k$, in two and three dimensions for small amplitude, subluminous electromagnetic fields. We find that magnetic energy does damp due to phase mixing of current carrying particles as $(omega_p t)^{-q}$ with $q sim 1$. (abridged)
We model the interaction between the wind from a newly formed rapidly rotating magnetar and the surrounding progenitor. In the first few seconds after core collapse the magnetar inflates a bubble of plasma and magnetic fields behind the supernova sho
ck, which expands asymmetrically because of the pinching effect of the toroidal magnetic field, as in PWNe, even if the host star is spherically symmetric. The degree of asymmetry depends on the ratio of the magnetic energy to the total energy in the bubble. We assume that the wind by newly formed magnetars inflating these bubbles is more magnetized than for PWNe. We show that for a magnetic to total power supplied by the central magnetar $sim 0.1$ the bubble expands relatively spherically while for values greater than 0.3, most of the pressure in the bubble is exerted close to the rotation axis, driving a collimated outflow out through the host star. This can account for the collimation inferred from observations of long-duration gamma-ray bursts (GRBs). Given that the wind magnetization increases in time, we thus suggest that the magnetar-driven bubble initially expands relatively spherically (enhancing the energy of the associated supernova) while at late times it becomes progressivelymore collimated (producing the GRB). Similar processes may operate in more modestly rotating neutron stars to produce asymmetric supernovae and lower energy transients such as X-ray flashes.
We present time-dependent axisymmetric magnetohydrodynamic simulations of the interaction of a relativistic magnetized wind produced by a proto-magnetar with a surrounding stellar envelope, in the first $sim 10$ seconds after core collapse. We inject
a super-magnetosonic wind with $dot E = 10^{51}$ ergs s$^{-1}$ into a cavity created by an outgoing supernova shock. A strong toroidal magnetic field builds up in the bubble of plasma and magnetic field that is at first inertially confined by the progenitor star. This drives a jet out along the polar axis of the star, even though the star and the magnetar wind are each spherically symmetric. The jet has the properties needed to produce a long-duration gamma-ray burst (GRB). At $sim 5$ s after core bounce, the jet has escaped the host star and the Lorentz factor of the material in the jet at large radii $sim 10^{11}$ cm is similar to that in the magnetar wind near the source. Most of the spindown power of the central magnetar escapes via the relativistic jet. There are fluctuations in the Lorentz factor and energy flux in the jet on $sim 0.01-0.1$ second timescale. These may contribute to variability in GRB emission (e.g., via internal shocks).
We model the interaction between the wind from a newly formed rapidly rotating magnetar and the surrounding supernova shock and host star. The dynamics is modeled using the two-dimensional, axisymmetric thin-shell equations. In the first ~10-100 seco
nds after core collapse the magnetar inflates a bubble of plasma and magnetic fields behind the supernova shock. The bubble expands asymmetrically because of the pinching effect of the toroidal magnetic field, just as in the analogous problem of the evolution of pulsar wind nebulae. The degree of asymmetry depends on E_mag/E_tot. The correct value of E_mag/E_tot is uncertain because of uncertainties in the conversion of magnetic energy into kinetic energy at large radii in relativistic winds; we argue, however, that bubbles inflated by newly formed magnetars are likely to be significantly more magnetized than their pulsar counterparts. We show that for a ratio of magnetic to total power supplied by the central magnetar L_mag/L_tot ~ 0.1 the bubble expands relatively spherically. For L_mag/L_tot ~ 0.3, however, most of the pressure in the bubble is exerted close to the rotation axis, driving a collimated outflow out through the host star. This can account for the collimation inferred from observations of long-duration gamma-ray bursts (GRBs). Outflows from magnetars become increasingly magnetically dominated at late times, due to the decrease in neutrino-driven mass loss as the young neutron star cools. We thus suggest that the magnetar-driven bubble initially expands relatively spherically, enhancing the energy of the associated supernova, while at late times it becomes progressively more collimated, producing the GRB.
