Considerations of high altitude emissions

The status of the Federal Aviation Administration's High Altitude Pollution Program is described which was instituted to develop the detailed quantitative information needed to judge whether or not regulatory action to limit such exhaust emissions would be necessary. The complexities of this question and the nature and magnitude of uncertainties still present in our scientific understanding of the potential interactions between aircraft exhaust emissions and stratospheric ozone and climate are reviewed. The direction and scope of future Federal and international activities are described

Theory of the electronic structure of dilute bismide and bismide-nitride alloys of GaAs: Tight-binding and k.p models

The addition of dilute concentrations of bismuth (Bi) into GaAs to form GaBiAs alloys results in a large reduction of the band gap energy Eg accompanied by a significant increase of the spin-orbit-splitting energy (delta_SO), leading to an Eg < delta_SO regime for ~10% Bi composition which is technologically relevant for the design of highly efficient photonic devices. The quaternary alloy GaBiNAs offers further flexibility for band gap tuning, because both nitrogen and bismuth can independently induce band gap reduction. This work reports sp3s* tight binding and 14-band k.p models for the study of the electronic structure of GaBiAs and GaBiNAs alloys. Our results are in good agreement with the available experimental data.Comment: 2 pages, 1 figur

Theory and design of Inx_{x}Ga1−x_{1-x}As1−y_{1-y}Biy_{y} mid-infrared semiconductor lasers: type-I quantum wells for emission beyond 3 μ\mum on InP substrates

We present a theoretical analysis and optimisation of the properties and performance of mid-infrared semiconductor lasers based on the dilute bismide alloy In$_{x}$Ga$_{1-x}$As$_{1-y}$Bi$_{y}$, grown on conventional (001) InP substrates. The ability to independently vary the epitaxial strain and emission wavelength in this quaternary alloy provides significant scope for band structure engineering. Our calculations demonstrate that structures based on compressively strained In$_{x}$Ga$_{1-x}$As$_{1-y}$Bi$_{y}$ quantum wells (QWs) can readily achieve emission wavelengths in the 3 -- 5 $\mu$m range, and that these QWs have large type-I band offsets. As such, these structures have the potential to overcome a number of limitations commonly associated with this application-rich but technologically challenging wavelength range. By considering structures having (i) fixed QW thickness and variable strain, and (ii) fixed strain and variable QW thickness, we quantify key trends in the properties and performance as functions of the alloy composition, structural properties, and emission wavelength, and on this basis identify routes towards the realisation of optimised devices for practical applications. Our analysis suggests that simple laser structures -- incorporating In$_{x}$Ga$_{1-x}$As$_{1-y}$Bi$_{y}$ QWs and unstrained ternary In$_{0.53}$Ga$_{0.47}$As barriers -- which are compatible with established epitaxial growth, provide a route to realising InP-based mid-infrared diode lasers.Comment: Submitted versio

Magneto-optical Kerr effect in Eu1−xCaxB6Eu_{1-x}Ca_{x}B_{6}

We have measured the magneto-optical Kerr rotation of ferromagnetic $Eu_{1-x}Ca_{x}B_{6}$ with x=0.2 and 0.4, as well as of $YbB_{6}$ serving as the non-magnetic reference material. As previously for $EuB_{6}$, we could identify a feature at 1 $eV$ in the Kerr response which is related with electronic transitions involving the localized 4f electron states. The absence of this feature in the data for $YbB_{6}$ confirms the relevance of the partially occupied 4f states in shaping the magneto-optical features of $Eu$-based hexaborides. Disorder by $Ca$-doping broadens the itinerant charge carrier contribution to the magneto-optical spectra

Inside the Bondi radius of M87

Chandra X-ray observations of the nearby brightest cluster galaxy M87 resolve the hot gas structure across the Bondi accretion radius of the central supermassive black hole, a measurement possible in only a handful of systems but complicated by the bright nucleus and jet emission. By stacking only short frame-time observations to limit pileup, and after subtracting the nuclear PSF, we analysed the X-ray gas properties within the Bondi radius at 0.12-0.22 kpc (1.5-2.8 arcsec), depending on the black hole mass. Within 2 kpc radius, we detect two significant temperature components, which are consistent with constant values of 2 keV and 0.9 keV down to 0.15 kpc radius. No evidence was found for the expected temperature increase within ~0.25 kpc due to the influence of the SMBH. Within the Bondi radius, the density profile is consistent with $\rho\propto r^{-1}$. The lack of a temperature increase inside the Bondi radius suggests that the hot gas structure is not dictated by the SMBH's potential and, together with the shallow density profile, shows that the classical Bondi rate may not reflect the accretion rate onto the SMBH. If this density profile extends in towards the SMBH, the mass accretion rate onto the SMBH could be at least two orders of magnitude less than the Bondi rate, which agrees with Faraday rotation measurements for M87. We discuss the evidence for outflow from the hot gas and the cold gas disk and for cold feedback, where gas cooling rapidly from the hot atmosphere could feed the cirumnuclear disk and fuel the SMBH. At 0.2 kpc radius, the cooler X-ray temperature component represents ~20% of the total X-ray gas mass and, by losing angular momentum to the hot gas component, could provide a fuel source of cold clouds within the Bondi radius.Comment: 14 pages, 8 figures, accepted by MNRA

Limits on the Position Wander of Sgr A*

We present measurements with the VLBA of the variability in the centroid position of Sgr A* relative to a background quasar at 7-mm wavelength. We find an average centroid wander of 71 +/- 45 micro-arcsec for time scales between 50 and 100 min and 113 +/- 50 micro-arcsec for timescales between 100 and 200 min, with no secular trend. These are sufficient to begin constraining the viability of the hot-spot model for the radio variability of Sgr A*. It is possible to rule out hot spots with orbital radii above 15GM_SgrA*/c^2 that contribute more than 30% of the total 7-mm flux. However, closer or less luminous hot spots remain unconstrained. Since the fractional variability of Sgr A* during our observations was ~20% on time scales of hours, the hot-spot model for Sgr A*'s radio variability remains consistent with these limits. Improved monitoring of Sgr A*'s centroid position has the potential to place significant constraints upon the existence and morphology of inhomogeneities in a supermassive black hole accretion flow.Comment: 14 pages, 3 figures submitted to Ap