On the identification of a mixture of samples from two multivariate distributions

This paper presents some methods for the classification and the estimation of the mixing ratio of a sample from two populations. These methods can be used at the lowest levels of a decision tree in the analysis of multispectral satellite images. The proposed algorithms have been tested through numerical experiments.     

Viscoelastic behaviour of pumpkin balloons

The lobes of the NASA ULDB pumpkin-shaped super-pressure balloons are made of a thin polymeric film that shows considerable time-dependent behaviour. A nonlinear viscoelastic model based on experimental measurements has been recently established for this film. This paper presents a simulation of the viscoelastic behaviour of ULDB balloons with the finite element software ABAQUS. First, the standard viscoelastic modelling capabilities available in ABAQUS are examined, but are found of limited accuracy even for the case of simple uniaxial creep tests on ULDB films. Then, a nonlinear viscoelastic constitutive model is implemented by means of a user-defined subroutine. This approach is verified by means of biaxial creep experiments on pressurized cylinders and is found to be accurate provided that the film anisotropy is also included in the model. A preliminary set of predictions for a single lobe of a ULDB is presented at the end of the paper. It indicates that time-dependent effects in a balloon structure can lead to significant stress redistribution and large increases in the transverse strains in the lobes.     

Gamma ray astrophysics and signatures of axion-like particles

We propose that axion-like particles (ALPs) with a two-photon vertex, consistent with all astrophysical and laboratory bounds, may lead to effects in the spectra of high-energy gamma-ray sources detectable by satellite or ground-based telescopes. We discuss two kinds of signatures: (i) a peculiar spectral depletion due to gamma rays being converted into ALPs in the magnetic fields of efficient astrophysical accelerators according to the “Hillas criterion”, such as jets of active galactic nuclei or hot spots of radio galaxies; (ii) an appearance of otherwise invisible sources in the GeV or TeV sky due to back-conversion of an ALP flux (associated with gamma-ray emitters suffering some attenuation) in the magnetic field of the Milky Way. These two mechanisms might also provide an exotic way to avoid the exponential cutoff of very high energy gamma-rays expected due to the pair production onto the extragalactic background light.     

Testing the blast wave model with Swift GRBs

The complex structure of the light curves of Swift GRBs (e.g. superimposed flares and shallow decay) has made their interpretation and that of the blast wave caused by the burst, more difficult than in the pre-Swift era. We aim to constrain the blast wave parameters: electron energy distribution, p, density profile of the circumburst medium, k, and the continued energy injection index, q. We do so by comparing the observed multi-wavelength light curves and X-ray spectra of a Swift sample to the predictions of the blast wave model.     

Can a double component outflow explain the X-ray and optical lightcurves of Swift Gamma-Ray Bursts?

An increasing sample of Gamma-Ray Bursts (GRBs) observed by Swift show evidence of ‘chromatic breaks’, i.e. breaks that are present in the X-ray but not in the optical. We find that in a significant fraction of these GRB afterglows the X-ray and the optical emission cannot be produced by the same component. We propose that these afterglow lightcurves are the result of a two-component jet, in which both components undergo energy injection for the whole observation and the X-ray break is due to a jet break in the narrow outflow. Bursts with chromatic breaks also explain another surprising finding, the paucity of late achromatic breaks. We propose a model that may explain the behaviour of GRB emission in both X-ray and optical bands. This model can be a radical and noteworthy alternative to the current interpretation for the ‘canonical’ XRT and UVOT lightcurves, and it bears fundamental implications for GRB physics.     

In Situ Collection of Refractory Dust in the Upper Stratosphere: The DUSTER Facility

DUSTER (Dust from the Upper Stratosphere Tracking Experiment and Retrieval) is an instrument designed to collect nanometer to micrometer scale solid aerosol particles in the upper stratosphere on board balloons. With three DUSTER flights we have demonstrated that: (1) the instrument’s performance was within the design parameters of environmental specifications (?80 ^°C; 3–10?mbar); (2) inertial impact collection of aerosols ～500?nm to 24 microns on holey-carbon thin films mounted on Transmission Electron Microscope mesh grids was achieved; (3) the design of an active collector exposed to the air flux and an identical collector “blank”, not exposed to the air flux, to monitor possible contamination permits unambiguous identification of collected particles; (4) save storage of collected samples and subsequent retrieval in the laboratory was achieved with no measurable contamination; (5)?reduced sample manipulation allowed the chemical and structural characterization of collected dust particles by Field-emission scanning electron microscopy and energy dispersive X-Ray analyses, and infrared and Raman micro-spectroscopy. The main and most ambitious goal is the collection and characterization of solid aerosol particles smaller than 3 microns of solar system debris that are currently not sampled on a routine basis by other instruments in the upper stratosphere. DUSTER will provide a record of the amount of solid aerosols, their size, shapes and chemical properties in the upper stratosphere, including particles less than 3 microns in size. The DUSTER program identified 25 particles as collected during the 2008 flight with sizes in the range of 0.4 to 24 microns.