Tiny Electromagnetic Explosions

This paper considers electromagnetic transients of a modest total energy (${\cal E} \gtrsim 10^{40}$ erg) and small initial size (${\cal R} \gtrsim 10^{-1}$ cm). They could be produced during collisions between relativistic field structures (e.g. macroscopic magnetic dipoles) that formed around, or before, cosmic electroweak symmetry breaking. The outflowing energy has a dominant electromagnetic component; a subdominant thermal component (temperature $> 1$ GeV) supplies inertia in the form of residual $e^\pm$. A thin shell forms that expands subluminally, attaining a Lorentz factor $\sim 10^{6-7}$ before decelerating. Drag is supplied by the reflection of an ambient magnetic field, and by deflection of ambient free electrons. Emission of low-frequency (GHz-THz) superluminal waves takes place through three channels: i) reflection of the ambient magnetic field; ii) direct linear conversion of the embedded magnetic field into a superluminal mode; and iii) excitation outside the shell by corrugation of its surface. The escaping electromagnetic pulse is very narrow (a few wavelengths) and so the width of the detected transient is dominated by propagation effects. GHz radio transients are emitted from i) the dark matter halos of galaxies and ii) the near-horizon regions of supermassive black holes that formed by direct gas collapse and now accrete slowly. Brighter and much narrower 0.01-1 THz pulses are predicted at a rate at least comparable to fast radio bursts, experiencing weaker scattering and absorption. The same explosions also accelerate protons up to $\sim 10^{19}$ eV and heavier nuclei up to $10^{20-21}$ eV.Comment: 25 pages, 16 figures, Astrophysical Journal, in pres

Constrained Evolution of a Radially Magnetized Protoplanetary Disk: Implications for Planetary Migration

We consider the inner $\sim$ AU of a protoplanetary disk (PPD), at a stage where angular momentum transport is driven by the mixing of a radial magnetic field into the disk from a T-Tauri wind. Because the radial profile of the imposed magnetic field is well constrained, a deterministic calculation of the disk mass flow becomes possible. The vertical disk profiles obtained in Paper I imply a stronger magnetization in the inner disk, faster accretion, and a secular depletion of the disk material. Inward transport of solids allows the disk to maintain a broad optical absorption layer even when the grain abundance becomes too small to suppress its ionization. Thus a PPD may show a strong middle-to-near infrared spectral excess even while its mass profile departs radically from the minimum-mass solar nebula. The disk surface density is buffered at $\sim 30$ g cm$^{-2}$: below this, X-rays trigger strong enough magnetorotational turbulence at the midplane to loft mm-cm sized particles high in the disk, followed by catastrophic fragmentation. A sharp density gradient bounds the inner depleted disk, and propagates outward to $\sim 1$-2 AU over a few Myr. Earth-mass planets migrate through the inner disk over a similar timescale, whereas the migration of Jupiters is limited by the supply of gas. Gas-mediated migration must stall outside 0.04 AU, where silicates are sublimated and the disk shifts to a much lower column. A transition disk emerges when the dust/gas ratio in the MRI-active layer falls below $X_d \sim 10^{-6}(a_d/\mu{\rm m})$, where $a_d$ is the grain size.Comment: 22 pp, 18 figures, Astrophysical Journal, in pres