Positive-Negative Birefringence in Multiferroic Layered Metasurfaces

We uncover and identify the regime for a magnetically and ferroelectrically controllable negative refraction of light traversing multiferroic, oxide-based metastructure consisting of alternating nanoscopic ferroelectric (SrTiO$_2$) and ferromagnetic (Y$_3$Fe$_2$(FeO$_4$)$_3$, YIG) layers. We perform analytical and numerical simulations based on discretized, coupled equations for the self-consistent Maxwell/ferroelectric/ferromagnetic dynamics and obtain a biquadratic relation for the refractive index. Various scenarios of ordinary and negative refraction in different frequency ranges are analyzed and quantified by simple analytical formula that are confirmed by full-fledge numerical simulations. Electromagnetic-waves injected at the edges of the sample are propagated exactly numerically. We discovered that for particular GHz frequencies, waves with different polarizations are characterized by different signs of the refractive index giving rise to novel types of phenomena such as a positive-negative birefringence effect, and magnetically controlled light trapping and accelerations

The ultimate outcome of black hole - neutron star mergers

We present a simple, semi--analytical description for the final stages of mergers of black hole (BH) -- neutron star (NS) systems. Such systems are of much interest as gravitational wave sources and gamma--ray burst progenitors. Numerical studies show that in general the neutron star is not disrupted at the first phase of mass transfer. Instead, what remains of the neutron star is left on a wider, eccentric, orbit. We consider the evolution of such systems as they lose angular momentum via gravitational radiation and come into contact for further phases of mass transfer. During each mass transfer event the neutron star mass is reduced until a critical value where mass loss leads to a rapid increase in the stellar radius. At this point Roche lobe overflow shreds what remains of the neutron star, most of the mass forming a disc around the black hole. Such a disc may be massive enough to power a gamma--ray burst. The mass of the neutron star at the time of disruption (and therefore the disc mass) is largely independent of the initial masses of the black hole and neutron star, indicating that BH--NS star mergers may be standard candles.Comment: MNRAS, in pres

Neutron star binaries and long duration gamma-ray bursts

Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate from the core collapse to black holes of stripped massive stars. Those with sufficient rotation form a centrifugally-supported torus whose collapse powers the GRB. We investigate the role of tidal locking within a tight binary as a source of the necessary angular momentum. We find that the binary orbit must be no wider than a few solar radii for a torus to form upon core collapse. Comparing this criterion to the observed population of binaries containing two compact objects suggests that rotation may have been important in the formation of up to 50% of the observed systems. As these systems created a neutron star and not a black hole they presumably did not produce highly luminous GRBs. We suggest instead that they make the subset of GRBs in the relatively local universe which have much lower luminosity.Comment: 7 pages, accepted for publication in MNRA

Radio Observations of GRB Host Galaxies

We present 5.5 and 9.0 GHz observations of a sample of seventeen GRB host galaxies at 0.5<z<1.4, using the radio continuum to explore their star formation properties in the context of the small but growing sample of galaxies with similar observations. Four sources are detected, one of those (GRB 100418A) likely due to lingering afterglow emission. We suggest that the previously-reported radio afterglow of GRB 100621A may instead be due to host galaxy flux. We see no strong evidence for redshift evolution in the typical star formation rate of GRB hosts, but note that the fraction of `dark' bursts with detections is higher than would be expected given constraints on the more typical long GRB population. We also determine the average radio-derived star formation rates of core collapse supernovae at comparable redshift, and show that these are still well below the limits obtained for GRB hosts, and show evidence for a rise in typical star formation rate with redshift in supernova hosts.Comment: 15 pages, MNRAS accepte

Creep fatigue life prediction for engine hot section materials (isotropic)

A series of high temperature strain controlled fatigue tests have been completed to study the effects of thermomechanical fatigue, multiaxial loading, reactive environments, and imposed mean stresses. The baseline alloy used in these tests was cast B1900+Hf (with and without coatings); a small number of tests of wrought INCO 718 are also included. A strong path dependence was demonstrated during the thermomechanical fatigue testing, using in-phase, out-phase, and non-proportional (elliptical and 'dogleg') strain-temperature cycles. The multiaxial tests also demonstrated cycle path to be a significant variable, using both proportional and non-proportional tension-torsion loading. Environmental screening tests were conducted in moderate pressure oxygen and purified argon; the oxygen reduced the specimen lives by two, while the argon testing produced ambiguous data. Both NiCoCrAlY overlay and diffusion aluminide coatings were evaluated under isothermal and TMF conditions; in general, the lives of the coated specimens were higher that those of uncoated specimens. Controlled mean stress TMF tests showed that small mean stress changes could change initiation lives by orders of magnitude; these results are not conservatively predicted using traditional linear damage summation rules. Microstructures were evaluated using optical, SEM and TEM methods

Swift J1112.2-8238: A Candidate Relativistic Tidal Disruption Flare

We present observations of Swift J1112.2-8238, and identify it as a candidate relativistic tidal disruption flare (rTDF). The outburst was first detected by Swift/BAT in June 2011 as an unknown, long-lived (order of days) $\gamma$-ray transient source. We show that its position is consistent with the nucleus of a faint galaxy for which we establish a likely redshift of $z=0.89$ based on a single emission line that we interpret as the blended [OII]$\lambda3727$ doublet. At this redshift, the peak X/$\gamma$-ray luminosity exceeded $10^{47}$ ergs s$^{-1}$, while a spatially coincident optical transient source had $i^{\prime} \sim 22$ (M$_g \sim -21.4$ at $z=0.89$) during early observations, $\sim 20$ days after the Swift trigger. These properties place Swift J1112.2-8238 in a very similar region of parameter space to the two previously identified members of this class, Swift J1644+57 and Swift J2058+0516. As with those events the high-energy emission shows evidence for variability over the first few days, while late time observations, almost 3 years post-outburst, demonstrate that it has now switched off. Swift J1112.2-8238 brings the total number of such events observed by Swift to three, interestingly all detected by Swift over a $\sim$3 month period ($<3\%$ of its total lifetime as of March 2015). While this suggests the possibility that further examples may be uncovered by detailed searches of the BAT archives, the lack of any prime candidates in the years since 2011 means these events are undoubtedly rare.Comment: 11 pages, 5 figures, accepted for publication by MNRA