Dmitri A. Uzdensky

Fast TeV variability in blazars: jets in a jet

The fast TeV variability of the blazars Mrk 501 and PKS 2155−304 implies a compact emitting region that moves with a bulk Lorentz factor of Γem∼ 100 towards the observer. The Lorentz factor is clearly in excess of the jet Lorentz factors Γj≲ 10 measured on sub-pc scales in these sources. We propose that the TeV emission originates from compact emitting regions that move relativistically within a jet of bulk Γj∼ 10. This can be physically realized in a Poynting flux-dominated jet. We show that if a large fraction of the luminosity of the jet is prone to magnetic dissipation through reconnection, then material outflowing from the reconnection regions can efficiently power the observed TeV flares through synchrotron-self-Compton emission. The model predicts simultaneous far-ultraviolet/soft X-ray flares.

Fast magnetic reconnection in the plasmoid-dominated regime.

A conceptual model of resistive magnetic reconnection via a stochastic plasmoid chain is proposed. The global reconnection rate is shown to be independent of the Lundquist number. The distribution of fluxes in the plasmoids is shown to be an inverse-square law. It is argued that there is a finite probability of emergence of abnormally large plasmoids, which can disrupt the chain (and may be responsible for observable large abrupt events in solar flares and sawtooth crashes). A criterion for the transition from the resistive magnetohydrodynamic to the collisionless regime is provided.

Formation of Plasmoid Chains in Magnetic Reconnection

A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers (10(4) <or= S <or= 10(8)) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfvénically fast formation of plasmoid (magnetic-island) chains. The plasmoid number scales as S(3/8) and the instability growth rate in the linear stage as S(1/4), in agreement with the theory by Loureiro et al. [Phys. Plasmas 14, 100703 (2007)]. In the nonlinear regime, plasmoids continue to grow faster than they are ejected and completely disrupt the reconnection layer. These results suggest that high-Lundquist-number reconnection is inherently time-dependent and hence call for a substantial revision of the standard Sweet-Parker quasistationary picture for S>10(4).

Magnetic reconnection and stochastic plasmoid chains in high-Lundquist-number plasmas

A numerical study of magnetic reconnection in the large-Lundquist-number (S), plasmoid-dominated regime is carried out for S up to 107. The theoretical model of Uzdensky et al. [Phys. Rev. Lett. 105, 235002 (2010)] is confirmed and partially amended. The normalized reconnection rate is Eeff∼0.02 independently of S for S≫104. The plasmoid flux (Ψ) and half-width (wx) distribution functions scale as f(Ψ)∼Ψ-2 and f(wx)∼wx-2. The joint distribution of Ψ and wx shows that plasmoids populate a triangular region wx≳Ψ/B0, where B0 is the reconnecting field. It is argued that this feature is due to plasmoid coalescence. Macroscopic “monster” plasmoids with wx∼10% of the system size are shown to emerge in just a few Alfven times, independently of S, suggesting that large disruptive events are an inevitable feature of large-S reconnection.

Fast TeV variability from misaligned minijets in the jet of M87

The jet of the radio galaxy M87 is misaligned, resulting in a Doppler factor δ~1 for emission of plasma moving parallel to the jet. This makes the observed fast TeV flares on time-scales of t v ~ 5 R g /c harder to understand as emission from the jet. In previous work, we have proposed a jets-in-a-jet model for the ultrafast TeV flares with t v « R g /c seen in Mrk 501 and PKS 2155-304. Here, we show that about half of the minijets beam their emission outside the jet cone. Minijets emitting off the jet axis result in rapidly evolving TeV (and maybe lower energy) flares that can be observed in nearby radio galaxies. The TeV flaring from M87 fits well into this picture, if M87 is a misaligned blazar.

Turbulent Magnetic Reconnection in Two Dimensions

Two-dimensional numerical simulations of the effect of background turbulence on 2D resistive magnetic reconnection are presented. For sufficiently small values of the resistivity (

Stellar Explosions by Magnetic Towers

\eta

Statistical Analysis of Current Sheets in Three-dimensional Magnetohydrodynamic Turbulence

) and moderate values of the turbulent power (

Statistical Description of a Magnetized Corona above a Turbulent Accretion Disk

\epsilon

Reconnection in Marginally Collisionless Accretion Disk Coronae

), the reconnection rate is found to have a much weaker dependence on