Huirong Yan

The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts

The recent Fermi observation of GRB 080916C shows that the bright photosphere emission associated with a putative fireball is missing, which suggests that the central engine likely launches a Poynting-flux-dominated (PFD) outflow. We propose a model of gamma-ray burst (GRB) prompt emission in the PFD regime, namely, the InternalCollision-induced MAgnetic Reconnection and Turbulence (ICMART) model. It is envisaged that the GRB central engine launches an intermittent, magnetically dominated wind, and that in the GRB emission region, the ejecta is still moderately magnetized (e.g., 1 σ 100). Similar to the internal shock (IS) model, the mini-shells interact internally at the radius RIS ∼ Γ 2 cΔt. Most of these early collisions, however, have little energy dissipation, but serve to distort the ordered magnetic field lines entrained in the ejecta. At a certain point, the distortion of magnetic field configuration reaches the critical condition to allow fast reconnection seeds to occur, which induce relativistic MHD turbulence in the interaction regions. The turbulence further distorts field lines easing additional magnetic reconnections, resulting in a runway release of the stored magnetic field energy (an ICMART event). Particles are accelerated either directly in the reconnection zone, or stochastically in the turbulent regions, which radiate synchrotron photons that power the observed gamma rays. Each ICMART event corresponds to a broad pulse in the GRB light curve, and a GRB is composed of multiple ICMART events. This model retains the merits of IS and other models, but may overcome several difficulties/issues faced by the IS model (e.g., low efficiency, fast cooling, electron number excess, Amati/Yonetoku relation inconsistency, and missing bright photosphere). Within this model, the observed GRB variability timescales could have two components, one slow component associated with the central engine time history, and another fast component associated with relativistic magnetic turbulence in the emission region. The model predicts a decrease of gamma-ray polarization degree and Ep in each ICMART event (broad pulse) during the prompt GRB phase, as well as a moderately magnetized external reverse shock. The model may be applied to the GRBs that have time-resolved, featureless Band-function spectra, such as GRB 080916C and most GRBs detected by Fermi LAT.

Cosmic-Ray Scattering and Streaming in Compressible Magnetohydrodynamic Turbulence

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for revisions in the picture of cosmic-ray transport. In this paper we use recently obtained scaling laws for MHD modes to obtain the scattering frequency for cosmic rays. Using quasi-linear theory, we calculate gyroresonance with MHD modes (Alfvenic, slow, and fast) and transit-time damping (TTD) by fast modes. We provide calculations of cosmic-ray scattering for various phases of interstellar medium with realistic interstellar turbulence driving that is consistent with the velocity dispersions observed in diffuse gas. We account for the turbulence cutoff arising from both collisional and collisionless damping. We obtain analytical expressions for diffusion coefficients that enter the Fokker-Planck equation describing cosmic-ray evolution. We obtain the scattering rate and show that fast modes provide the dominant contribution to cosmic-ray scattering for the typical interstellar conditions in spite of the fact that fast modes are subjected to damping. We determine how the efficiency of the scattering depends on the characteristics of ionized media, e.g., plasma β. We calculate the range of energies for which the streaming instability is suppressed by the ambient MHD turbulence.

Dust Dynamics in Compressible Magnetohydrodynamic Turbulence

We calculate the relative grain-grain motions arising from interstellar magnetohydrodynamic (MHD) turbulence. The MHD turbulence includes both fluid motions and magnetic fluctuations. While the fluid motions accelerate grains through hydrodrag, the electromagnetic fluctuations accelerate grains through resonant interactions. We consider both incompressive (Alfv?n) and compressive (fast and slow) MHD modes and use descriptions of MHD turbulence obtained by Cho and Lazarian in 2002. Calculations of grain relative motion are made for realistic grain charging and interstellar turbulence that are consistent with the velocity dispersions observed in diffuse gas, including cutoff of the turbulence from various damping processes. We show that fast modes dominate grain acceleration and can drive grains to supersonic velocities. Grains are also scattered by gyroresonance interactions, but the scattering is less important than acceleration for grains moving with sub-Alfv?nic velocities. Since the grains are preferentially accelerated with large pitch angles, the supersonic grains will be aligned with long axes perpendicular to the magnetic field. We compare grain velocities arising from MHD turbulence with those arising from photoelectric emission, radiation pressure, and H2 thrust. We show that for typical interstellar conditions, turbulence should prevent these mechanisms from segregating small and large grains. Finally, gyroresonant acceleration is bound to preaccelerate grains that are further accelerated in shocks. Grain-grain collisions in the shock may then contribute to the overabundance of refractory elements in the composition of Galactic cosmic rays.

SCATTERING OF COSMIC RAYS BY MAGNETOHYDRODYNAMIC INTERSTELLAR TURBULENCE

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for substantial revisions in our understanding of cosmic ray transport. We use recently obtained scalings of MHD modes to calculate the scattering frequency for cosmic rays. We consider gyroresonance with MHD modes (Alfvénic, slow, and fast) and transit-time damping by fast modes. We conclude that the gyroresonance with fast modes is the dominant contribution to cosmic ray scattering for the typical interstellar conditions. In contrast to earlier studies, we find that Alfvénic and slow modes are inefficient because they are far from the isotropy usually assumed.

Magnetically Limited X-Ray Filaments in Young Supernova Remnants

We discuss the damping of strong magnetic turbulence downstream of the forward shock of young supernova remnants (SNRs). We find that strong magnetic fields that have been produced by the streaming instability in the upstream region of the shock or by other kinetic instabilities at the shock may be efficiently damped, so the region of enhanced magnetic field strength would typically have a thickness of the order ld = 1016-1017?cm. The nonthermal X-ray filaments observed in young SNRs are thus possibly limited by the magnetic field and not by the energy losses of the radiating electrons. Therefore, the thickness of the filaments would not be a measure of the magnetic field strength, and claims of efficient cosmic-ray acceleration on account of a runaway streaming instability would appear premature.

Cosmic-Ray Propagation: Nonlinear Diffusion Parallel and Perpendicular to Mean Magnetic Field

We consider the propagation of cosmic rays in turbulent magnetic fields. We use the models of magnetohydrodynamic turbulence that were tested in numerical simulations, in which the turbulence is injected on large scales and cascades to small scales. Our attention is focused on the models of the strong turbulence, but we also briefly discuss the effects that the weak turbulence and the slab Alfvenic perturbations can have. The latter are likely to emerge as a result of instabilities within the cosmic-ray fluid itself, e.g., beaming and gyroresonance instabilities of cosmic rays. To describe the interaction of cosmic rays with magnetic perturbations we develop a nonlinear formalism that extends the ordinary quasi-linear theory that is routinely used for the purpose. This allows us to avoid the usual problem of 90° scattering and enables our computation of the mean free path of cosmic rays. We apply the formalism to the cosmic-ray propagation in the Galactic halo and in the warm ionized medium. In addition, we address the issue of the transport of cosmic rays perpendicular to the mean magnetic field and show that the issue of cosmic-ray subdiffusion (i.e., propagation with retracing the trajectories backward, which slows down the diffusion) is only important for restricted cases when the ambient turbulence is far from what numerical simulations suggest to us. As a result, this work provides formalism that can be applied for calculating cosmic-ray propagation in a wide variety of circumstances.

Shattering and coagulation of dust grains in interstellar turbulence

We investigate shattering and coagulation of dust grains in turbulent interstellar medium (ISM). The typical velocity of dust grain as a function of grain size has been calculated for various ISM phases based on a theory of grain dynamics in compressible magnetohydrodynamic turbulence. In this paper, we develop a scheme of grain shattering and coagulation and apply it to turbulent ISM by using the grain velocities predicted by the above turbulence theory. Since large grains tend to acquire large velocity dispersions as shown by earlier studies, large grains tend to be shattered. Large shattering effects are indeed seen in warm ionized medium within a few Myr for grains with radius a 10 −6 cm. We also show that shattering in warm neutral medium can limit the largest grain size in ISM (a ∼ 2 × 10 −5 cm). On the other hand, coagulation tends to modify small grains since it only occurs when the grain velocity is small enough. Coagulation significantly modifies the grain size distribution in dense clouds (DC), where a large fraction of the grains with a < 10 −6 cm coagulate in 10 Myr. In fact, the correlation among RV , the carbon bump strength and the ultraviolet slope in the observed Milky Way extinction curves can be explained by the coagulation in DC. It is possible that the grain size distribution in the Milky Way is determined by a combination of all the above effects of shattering and coagulation. Considering that shattering and coagulation in turbulence are effective if dust-to-gas ratio is typically more than ∼1/10 of the Galactic value, the regulation mechanism of grain size distribution should be different between metal-poor and metal-rich

Magnetically limited x-ray filaments in young SNR

We discuss the damping of strong magnetic turbulence downstream of the forward shock of young supernova remnants (SNR). We find that strong magnetic fields, that have been produced by the streaming instability in the upstream region of the shock, or by other kinetic instabilities at the shock, will be efficiently reduced, so the region of enhanced magnetic field strength would typically have a thickness of the order ld = (10 16 − 10 17 ) cm. The non-thermal X-ray filaments observed in young SNR are thus likely limited by the magnetic field and not by the energy losses of the radiating electrons. Consequently the thickness of the filaments would not be a measure of the magnetic field strength and claims of efficient cosmic-ray acceleration on account of a run-away streaming instability appear premature.

SUPERDIFFUSION OF COSMIC RAYS: IMPLICATIONS FOR COSMIC RAY ACCELERATION

Diffusion of cosmic rays (CRs) is the key process for understanding their propagation and acceleration. We employ the description of spatial separation of magnetic field lines in magnetohydrodynamic turbulence in Lazarian & Vishniac to quantify the divergence of the magnetic field on scales less than the injection scale of turbulence and show that this divergence induces superdiffusion of CR in the direction perpendicular to the mean magnetic field. The perpendicular displacement squared increases, not as the distance x along the magnetic field, which is the case for a regular diffusion, but as the x 3 for freely streaming CRs. The dependence changes to x 3/2 for the CRs propagating diffusively along the magnetic field. In the latter case, we show that it is important to distinguish the perpendicular displacement with respect to the mean field and to the local magnetic field. We consider how superdiffusion changes the acceleration of CRs in shocks and show how it decreases efficiency of the CRs acceleration in perpendicular shocks. We also demonstrate that in the case when the small-scale magnetic field is generated in the pre-shock region, an efficient acceleration can take place for the CRs streaming without collisions along the magnetic loops.

Grain Acceleration by Magnetohydrodynamic Turbulence: Gyroresonance Mechanism

We discuss a new mechanism of dust acceleration that acts in a turbulent magnetized medium. The magnetohydrodynamic (MHD) turbulence includes both fluid motions and magnetic fluctuations. We show that while the fluid motions bring about grain motions through the drag, the electromagnetic fluctuations can accelerate grains through resonant interactions. In this Letter, we calculate the grain acceleration by the gyroresonance in the cold neutral medium. We consider both incompressible and compressible MHD modes. We show that fast modes dominate the grain acceleration. For the parameters chosen, fast modes render to grains supersonic velocities that may shatter the grains and enable the efficient absorption of heavy elements. Since the grains are preferentially accelerated with large pitch angles, the supersonic grains get aligned with long axes perpendicular to the magnetic field.