Large-Scale Regular Morphological Patterns in the Radio Jet of NGC 6251

We report on large-scale, regular morphological patterns found in the radio jet of the nearby radio galaxy NGC 6251. Investigating morphological properties of this radio jet from the nucleus to a radial distance of $\sim$ 300 arcsec ($\approx$ 140 kpc) mapped at 1662 MHz and 4885 MHz by Perley, Bridle, & Willis, we find three chains, each of which consists of five radio knots. We also find that eight radio knots in the first two chains consist of three small sub-knots (the triple-knotty substructures). We discuss the observational properties of these regular morphological patterns.Comment: 8 figures, 15 pages, accepted for publication in A

Effect of isoelectronic doping on honeycomb lattice iridate A_2IrO_3

We have investigated experimentally and theoretically the series (Na$_{1-x}$Li$_{x}$)$_{2}$IrO$_{3}$. Contrary to what has been believed so far, only for $x\leq0.25$ the system forms uniform solid solutions. For larger Li content, as evidenced by powder X-ray diffraction, scanning electron microscopy and density functional theory calculations, the system shows a miscibility gap and a phase separation into an ordered Na$_{3}$LiIr$_2$O$_{6}$ phase with alternating Na$_3$ and LiIr$_2$O$_6$ planes, and a Li-rich phase close to pure Li$_{2}$IrO$_{3}$. For $x\leq 0.25$ we observe (1) an increase of $c/a$ with Li doping up to $x=0.25$, despite the fact that $c/a$ in pure Li$_{2}$IrO$_{3}$ is smaller than in Na$_{2}$IrO$_{3}$, and (2) a gradual reduction of the antiferromagnetic ordering temperature $T_{N}$ and ordered moment. The previously proposed magnetic quantum phase transition at $x\approx 0.7$ may occur in a multiphase region and its nature needs to be re-evaluated.Comment: 8 pages, 9 figures including supplemental informatio

The Ionized Stellar Wind in Vela X-1 During Eclipse

We present a first analysis of a high resolution X-ray spectrum of the ionized stellar wind of Vela X-1 during eclipse. The data were obtained with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. The spectrum is resolved into emission lines with fluxes between 0.02 and 1.04x10^4 ph/cm^2/s. We identify lines from a variety of charge states, including fluorescence lines from cold material, a warm photoionized wind. We can exclude signatures from collisionally ionized plasmas. For the first time we identify fluorescence lines from L-shell ions from lower Z elements. We also detect radiative recombination continua from a kT = 10 eV (1.2 x 10^5 K) photoionized optically thin gas. The fluorescence line fluxes infer the existence of optically thick and clumped matter within or outside the warm photoionized plasma.Comment: 4 pages, 2 figures, accepted by ApJ letter

Structure of the X-ray Emission from the Jet of 3C 273

We present images from five observations of the quasar 3C 273 with the Chandra X-ray Observatory. The jet has at least four distinct features which are not resolved in previous observations. The first knot in the jet (A1) is very bright in X-rays. Its X-ray spectrum is well fitted with a power law with alpha = 0.60 +/- 0.05. Combining this measurement with lower frequency data shows that a pure synchrotron model can fit the spectrum of this knot from 1.647 GHz to 5 keV (over nine decades in energy) with alpha = 0.76 +/- 0.02, similar to the X-ray spectral slope. Thus, we place a lower limit on the total power radiated by this knot of 1.5e43 erg/s; substantially more power may be emitted in the hard X-ray and gamma-ray bands. Knot A2 is also detected and is somewhat blended with knot B1. Synchrotron emission may also explain the X-ray emission but a spectral bend is required near the optical band. For knots A1 and B1, the X-ray flux dominates the emitted energy. For the remaining optical knots (C through H), localized X-ray enhancements that might correspond to the optical features are not clearly resolved. The position angle of the jet ridge line follows the optical shape with distinct, aperiodic excursions of +/-1 deg from a median value of -138.0deg. Finally, we find X-ray emission from the ``inner jet'' between 5 and 10" from the core.Comment: 10 pages, 5 figures; accepted for publication in the Astrophysical Journal Letters. For the color image, see fig1.ps or http://space.mit.edu/~hermanm/papers/3c273/fig1.jp

Thorium Energy Futures

The potential for thorium as an alternative or supplement to uranium in fission power generation has long been recognised, and several reactors, of various types, have already operated using thorium-based fuels. Accelerator Driven Subcritical (ADS) systems have benefits and drawbacks when compared to conventional critical thorium reactors, for both solid and molten salt fuels. None of the four options – liquid or solid, with or without an accelerator – can yet be rated as better or worse than the other three, given today's knowledge. We outline the research that will be necessary to lead to an informed choice

Proton acceleration beyond 100 EeV by an oblique shock wave in the jet of 3C 273

We estimate the highest energy of proton diffusively accelerated by shock in knot A1 of the jet in luminous nearby quasar 3C 273. Referring to the recent polarization measurements using very long baseline interferometry (VLBI), we consider the shock propagation across magnetic field lines, namely, configuration of the oblique shock. For larger inclination of the field lines, the effects of particle reflection at the shock front are more pronounced, to significantly increase acceleration efficiency. The quasiperpendicular shock turns out to be needed for safely achieving the proton acceleration to the energy above 100 EeV (10^20 eV) in a parameter domain reflecting conceivable energy restrictions.Comment: 12 pages, 2 figures, accepted for publication in Astrophysical Journal Letter

On the Jet Activity in 3C 273

In this paper we comment on the possibility for intermittent jet activity in quasar 3C 273 on different time-scales. We propose, that striking morphology of the large-scale radio jet in this source, as well as the apparent lack of its counterpart on the opposite side of the active center, may be explained in a framework of a restarting jet model. In particular, we propose that 3C 273 radio source is intrinsically two-sided, and represents an analogue of double-double radio galaxies, but only inclined at a small angle to the line of sight. In this case, the apparent one-sideness of the kiloparsec-scale radio structure may be due to combined Doppler and time-travel effects alone, if the 3C 273 large-scale jet itself is relativistic and matter-dominated. We also propose, that knotty morphology of the discussed jet, which is observed now additionally at optical and X-ray frequencies, indicates modulation in the jet kinetic power. This, together with the variability of the jet at small (parsec) scales, indicates that the jet activity in 3C 273, and possibly in other similar sources, is variable/modulated/intermittent over many different time-scales.Comment: 27 pages, 4 figures included. Modified version, accepted for publication in The Astrophysical Journa

Revision of model parameters for kappa-type charge transfer salts: an ab initio study

Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the kappa-(BEDT-TTF)2X family of organic charge transfer (CT) salts yields a complex phase diagram with magnetic, superconducting, Mott insulating and even spin liquid phases. With extensive density functional theory (DFT) calculations we refresh the link between manybody theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic triangular lattices. The implications of these results on the systems' description are discussed.Comment: Accepted for pupblication in Phys. Rev. Let

Supernova Shock Breakout from a Red Supergiant

Massive stars undergo a violent death when the supply of nuclear fuel in their cores is exhausted, resulting in a catastrophic "core-collapse" supernova. Such events are usually only detected at least a few days after the star has exploded. Observations of the supernova SNLS-04D2dc with the Galaxy Evolution Explorer space telescope reveal a radiative precursor from the supernova shock before the shock reached the surface of the star and show the initial expansion of the star at the beginning of the explosion. Theoretical models of the ultraviolet light curve confirm that the progenitor was a red supergiant, as expected for this type of supernova. These observations provide a way to probe the physics of core-collapse supernovae and the internal structures of their progenitor starsComment: Science, in press. 32 pages, 7 figure

Frustration induced Raman scattering in CuGeO_3

We present experimental data for the Raman intensity in the spin-Peierls compound CuGeO_3 and theoretical calculations from a one-dimensional frustrated spin model. The theory is based on (a) exact diagonalization and (b) a recently developed solitonic mean field theory. We find good agreement between the 1D-theory in the homogeneous phase and evidence for a novel dimerization of the Raman operator in the spin-Peierls state. Finally we present evidence for a coupling between the interchain exchange, the spin-Peierls order parameter and the magnetic excitations along the chains.Comment: Phys. Rev. B, Rapid Comm, in Pres