Climbing the cosmic ladder with stellar twins

Distances to stars are key to revealing a three-dimensional view of the Milky Way, yet their determination is a major challenge in astronomy. Whilst the brightest nearby stars benefit from direct parallax measurements, fainter stars are subject of indirect determinations with uncertainties exceeding 30%. We present an alternative approach to measuring distances using spectroscopically-identified twin stars. Given a star with known parallax, the distance to its twin is assumed to be directly related to the difference in their apparent magnitudes. We found 175 twin pairs from the ESO public HARPS archives and report excellent agreement with Hipparcos parallaxes within 7.5%. Most importantly, the accuracy of our results does not degrade with increasing stellar distance. With the ongoing collection of high-resolution stellar spectra, our method is well-suited to complement Gaia.Comment: published online on MNRA

Astrometric Effects of Gravitational Wave Backgrounds with non-Luminal Propagation Speeds

A passing gravitational wave causes a deflection in the apparent astrometric positions of distant stars. The effect of the speed of the gravitational wave on this astrometric shift is discussed. A stochastic background of gravitational waves would result in a pattern of astrometric deflections which are correlated on large angular scales. These correlations are quantified and investigated for backgrounds of gravitational waves with sub- and super-luminal group velocities. The statistical properties of the correlations are depicted in two equivalent and related ways: as correlation curves and as angular power spectra. Sub-(super-)luminal gravitational wave backgrounds have the effect of enhancing (suppressing) the power in low-order angular modes. Analytical representations of the redshift-redshift and redshift-astrometry correlations are also derived. The potential for using this effect for constraining the speed of gravity is discussed

Catching GRBs with atmospheric Cherenkov telescopes

Fermi has shown GRBs to be a source of >10 GeV photons. We present an estimate of the detection rate of GRBs with a next generation Cherenkov telescope. Our predictions are based on the observed properties of GRBs detected by Fermi, combined with the spectral properties and redshift determinations for the bursts population by instruments operating at lower energies. While detection of VHE emission from GRBs has eluded ground-based instruments thus far, our results suggest that ground-based detection may be within reach of the proposed Cherenkov Telescope Array (CTA), albeit with a low rate, 0.25 - 0.5/yr. Such a detection would help constrain the emission mechanism of gamma-ray emission from GRBs. Photons at these energies from distant GRBs are affected by the UV-optical background light, and a ground-based detection could also provide a valuable probe of the Extragalactic Background Light (EBL) in place at high redshift.Comment: 4 pages, 3 figures, to appear in the Proceedings of "Gamma Ray Bursts 2010", held Nov. 1-4, 2010 in Annapolis, M

Paramagnon dispersion in β\beta-FeSe observed by Fe LL-edge resonant inelastic x-ray scattering

We report an Fe $L$-edge resonant inelastic x-ray scattering (RIXS) study of the unusual superconductor $\beta$-FeSe. The high energy resolution of this RIXS experiment ($\approx\,$55$\,$meV FWHM) made it possible to resolve low-energy excitations of the Fe $3d$ manifold. These include a broad peak which shows dispersive trends between 100-200$\,$meV along the $(\pi,0)$ and $(\pi,\pi)$ directions of the one-Fe square reciprocal lattice, and which can be attributed to paramagnon excitations. The multi-band valence state of FeSe is among the most metallic in which such excitations have been discerned by soft x-ray RIXS

Constraining the Distribution of L- & T-Dwarfs in the Galaxy

We estimate the thin disk scale height of the Galactic population of L- & T-dwarfs based on star counts from 15 deep parallel fields from the Hubble Space Telescope. From these observations, we have identified 28 candidate L- & T- dwarfs based on their (i'-z') color and morphology. By comparing these star counts to a simple Galactic model, we estimate the scale height to be 350+-50 pc that is consistent with the increase in vertical scale with decreasing stellar mass and is independent of reddening, color-magnitude limits, and other Galactic parameters. With this refined measure, we predict that less than 10^9 M_{sol} of the Milky Way can be in the form L- & T- dwarfs, and confirm that high-latitude, z~6 galaxy surveys which use the i'-band dropout technique are 97-100% free of L- & T- dwarf interlopers.Comment: 4 pages, 4 figures, accepted to ApJ

Generalized coherent states are unique Bell states of quantum systems with Lie group symmetries

We consider quantum systems, whose dynamical symmetry groups are semisimple Lie groups, which can be split or decay into two subsystems of the same symmetry. We prove that the only states of such a system that factorize upon splitting are the generalized coherent states. Since Bell's inequality is never violated by the direct product state, when the system prepared in the generalized coherent state is split, no quantum correlations are created. Therefore, the generalized coherent states are the unique Bell states, i.e., the pure quantum states preserving the fundamental classical property of satisfying Bell's inequality upon splitting.Comment: 4 pages, REVTeX, amssymb style. More information on http://www.technion.ac.il/~brif/science.htm

Multi-chord fiber-coupled interferometer with a long coherence length laser

This paper describes a 561 nm laser heterodyne interferometer that provides time-resolved measurements of line-integrated plasma electron density within the range of 10^15-10^18 cm^(-2). Such plasmas are produced by railguns on the Plasma Liner Experiment (PLX), which aims to produce \mu s-, cm-, and Mbar-scale plasmas through the merging of thirty plasma jets in a spherically convergent geometry. A long coherence length, 320 mW laser allows for a strong, sub-fringe phase-shift signal without the need for closely-matched probe and reference path lengths. Thus only one reference path is required for all eight probe paths, and an individual probe chord can be altered without altering the reference or other probe path lengths. Fiber-optic decoupling of the probe chord optics on the vacuum chamber from the rest of the system allows the probe paths to be easily altered to focus on different spatial regions of the plasma. We demonstrate that sub-fringe resolution capability allows the interferometer to operate down to line-integrated densities of order 10^15 cm^(-2).Comment: submitted to Rev. Sci. Instrum. (2011

Diffuse Extragalactic Background Radiation

Attenuation of high--energy gamma rays by pair--production with UV, optical and IR background photons provides a link between the history of galaxy formation and high--energy astrophysics. We present results from our latest semi-analytic models (SAMs), based upon a $\Lambda$CDM hierarchical structural formation scenario and employing all ingredients thought to be important to galaxy formation and evolution, as well as reprocessing of starlight by dust to mid- and far-IR wavelengths. Our models also use results from recent hydrodynamic galaxy merger simulations. These latest SAMs are successful in reproducing a large variety of observational constraints such as number counts, luminosity and mass functions, and color bimodality. We have created 2 models that bracket the likely ranges of galaxy emissivities, and for each of these we show how the optical depth from pair--production is affected by redshift and gamma-ray energy. We conclude with a discussion of the implications of our work, and how the burgeoning science of gamma-ray astronomy will continue to help constrain cosmology.Comment: 12 pages, 8 figures, to be published in the Proceedings of the 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, held July 2008 in Heidelberg, German

Self-similarities in the frequency-amplitude space of a loss-modulated CO2_2 laser

We show the standard two-level continuous-time model of loss-modulated CO$_2$ lasers to display the same regular network of self-similar stability islands known so far to be typically present only in discrete-time models based on mappings. For class B laser models our results suggest that, more than just convenient surrogates, discrete mappings in fact could be isomorphic to continuous flows.Comment: (5 low-res color figs; for ALL figures high-res PDF: http://www.if.ufrgs.br/~jgallas/jg_papers.html