MUSE observations of comet 67P/Churyumov-Gerasimenko:A reference for future comet observations with MUSE

Observations of comet 67P/Churyumov-Gerasimenko were performed with MUSE at large heliocentric distances post-perihelion, between March 3 and 7, 2016. Those observations were part of a simultaneous ground-based campaign aimed at providing large-scale information about comet 67P that complement the ESA/Rosetta mission. We obtained a total of 38 datacubes over 5 nights. We take advantage of the integral field unit (IFU) nature of the instrument to study simultaneously the spectrum of 67P's dust and its spatial distribution in the coma. We also look for evidence of gas emission in the coma. We produce a high quality spectrum of the dust coma over the optical range that could be used as a reference for future comet observations with the instrument. The slope of the dust reflectivity is of 10%$/100$ nm over the 480-900 nm interval, with a shallower slope towards redder wavelengths. We use the $\mathrm{Af\rho}$ to quantify the dust production and measure values of 65$\pm$4 cm, 75$\pm$4 cm, and 82$\pm$4 cm in the V, R, and I bands respectively. We detect several jets in the coma, as well as the dust trail. Finally, using a novel method combining spectral and spatial information, we detect the forbidden oxygen emission line at 630 nm. Using this line we derive a water production rate of $1.5\pm0.6 \times 10^{26} \mathrm{molec./s}$, assuming all oxygen atoms come from the photo-dissociation of water.Comment: Accepted for publication in Astronomy and Astrophysic

Flash-Heating of Circumstellar Clouds by Gamma Ray Bursts

The blast-wave model for gamma-ray bursts (GRBs) has been called into question by observations of spectra from GRBs that are harder than can be produced through optically thin synchrotron emission. If GRBs originate from the collapse of massive stars, then circumstellar clouds near burst sources will be illuminated by intense gamma radiation, and the electrons in these clouds will be rapidly scattered to energies as large as several hundred keV. Low-energy photons that subsequently pass through the hot plasma will be scattered to higher energies, hardening the intrisic spectrum. This effect resolves the "line-of-death" objection to the synchrotron shock model. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electron-positron pairs that can be detected within ~ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy would reveal past GRB explosions.Comment: 9 pages, 1 figure, submitted to ApJ Letter

Self-Organized Criticality in Compact Plasmas

Compact plasmas, that exist near black-hole candidates and in gamma ray burst sources, commonly exhibit self-organized non-linear behavior. A model that simulates the non-linear behavior of compact radiative plasmas is constructed directly from the observed luminosity and variability. The simulation shows that such plasmas self organize, and that the degree of non-linearity as well as the slope of the power density spectrum increase with compactness. The simulation is based on a cellular automaton table that includes the properties of the hot (relativistic) plasmas, and the magnitude of the energy perturbations. The plasmas cool or heat up, depending on whether they release more or less than the energy of a single perturbation. The energy release depends on the plasmas densities and temperatures, and the perturbations energy. Strong perturbations may cool the previously heated plasma through shocks and/or pair creation. New observations of some active galactic nuclei and gamma ray bursters are consistent with the simulationComment: 9 pages, 5 figures, AASTeX, Submitted to ApJ

Tomography of the Alpine region from observations of seismic ambient noise

We use correlations of the ambient seismic noise to study the crust in western Europe. Cross correlation of 1 year of noise recorded at 150 three components broadband stations yields more than 3 000 Rayleigh wave group velocity measurements. These measurements are used to construct Rayleigh group velocity maps of the Alpine region and surrounding area in the 5-80 s period band. In the 5-10 s period band, the seismic noise recorded in Europe is dominated by surface waves originating from the Northern Atlantic ocean. This anisotropy of the noise and the uneven station distribution affect the azimuthal distribution of the paths where we obtain reliable group velocity measurements. As a consequence our group velocity models have better resolution in the northeast direction than in the southwest direction. Finally we invert the resulting Rayleigh wave group velocity maps to determine the Moho depth. Our results are in good agreement with the result of the numerous active experiments in the Alps and provide a continuous image of the Alpine structur

Characterization of the Subsurface of 67P/Churyumov-Gerasimenko's Abydos Site

We investigate the structure of the subsurface of the Abydos site using a cometary nucleus model with parameters adapted to comet 67P/Churyumov-Gerasimenko and the Abydos landing site. We aim to compare the production rates derived from our model with those of the main molecules measured by Ptolemy. This will allow us to retrieve the depths at which the different molecules still exist in solid form

X-ray Rich GRB, Photospheres and Variability

We investigate the relationship between the quasi-thermal baryon-related photosphere in relativistic outflows, and the internal shocks arising outside them, which out to a limiting radius may be able to create enough pairs to extend the optically thick region. Variable gamma-ray light curves are likely to arise outside this limiting pair-forming shock radius, while X-ray excess bursts may arise from shocks occurring below it; a possible relation to X-ray flashes is discussed. This model leads to a simple physical interpretation of the observational gamma-ray variability-luminosity relation.Comment: accepted in Ap.J. 6/20/02; subm 5/8/02; aaspp4, 12 pages, 1 fi

Mapping Microstructural Dynamics up to the Nanosecond of the Conjugated Polymer P3HT in the Solid State

We present a detailed study of the structure-dynamics relationship of regio-regular and regio-random PEHT using different neutron scattering techniques. Deuteration is employed to modulate the coherent and incoherent cross-sections, allowing particularly to access both self-motions and collective dynamics of the materials. The measurements are underpinned by extensive quantitative calculations using classical MD, as well as first principles quantum chemistry. MD reproduced well the main structural features and slow motions, and shed light on differences in collective dynamics between Q-values linked with the $\pi-\pi$ stacking and the lamellar stacking, with the crystalline phase being the most impacted. On the other hand MD led to a limited description of molecular vibrations. In this context, first principles molecular calculations described well the high-energy vibrational features ( $>$ 900 cm$^{-1}$ ), while periodic calculations allowed to better describe the low- and mid-energy vibrational ranges ( 200-900 cm$^{-1}$ ). The mid-energy range is predominantly associated with both intra-molecular and inter-molecular mode coupling, which encloses information about both the polymer conformation and the polymer packing at short range. One of the outcomes of this study is the validation of the common assumption made that RRa-P3HT is a good approximation for the amorphous phase of RR-P3HT at the macroscopic level. The present work helps to clarify unambiguously the latter point which has been largely overlooked in the literature. We highlight the importance to complement optical spectroscopy techniques with inelastic neutron scattering. The latter offering the advantage of being insensitive to the delocalized $\pi$-electron system, and thus enabling to infer relevant quantities like conjugation lengths, for instance, impacting properties of conjugated polymer.Comment: Featured as ACS Editors' Choice. Featured on the Cover of the December 10, 2019 issue of Chemistry of Material

Polaron states in fullerene adducts modeled by coarse-grained molecular dynamics and tight binding

Strong electron–phonon coupling leads to polaron localization in molecular semiconductor materials and influences charge transport, but it is expensive to calculate atomistically. Here, we propose a simple and efficient model to determine the energy and spatial extent of polaron states within a coarse-grained representation of a disordered molecular film. We calculate the electronic structure of the molecular assembly using a tight-binding Hamiltonian and determine the polaron state self-consistently by perturbing the site energies by the dielectric response of the surrounding medium to the charge. When applied to fullerene derivatives, the method shows that polarons extend over multiple molecules in C60 but localize on single molecules in higher adducts of phenyl-C61-butyric-acid-methyl-ester (PCBM) because of packing disorder and the polar side chains. In PCBM, polarons localize on single molecules only when energetic disorder is included or when the fullerene is dispersed in a blend. The method helps to establish the conditions under which a hopping transport model is justified

A Medium Survey of the Hard X-Ray Sky with ASCA. II.: The Source's Broad Band X-Ray Spectral Properties

A complete sample of 60 serendipitous hard X-ray sources with flux in the range $\sim 1 \times 10^{-13}$ \ecs to $\sim 4 \times 10^{-12}$ \ecs (2 - 10 keV), detected in 87 ASCA GIS2 images, was recently presented in literature. Using this sample it was possible to extend the description of the 2-10 keV LogN(>S)-LogS down to a flux limit of $\sim 6\times 10^{-14}$ \ecs (the faintest detectable flux), resolving about a quarter of the Cosmic X-ray Background. In this paper we have combined the ASCA GIS2 and GIS3 data of these sources to investigate their X-ray spectral properties using the "hardness" ratios and the "stacked" spectra method. Because of the sample statistical representativeness, the results presented here, that refer to the faintest hard X-ray sources that can be studied with the current instrumentation, are relevant to the understanding of the CXB and of the AGN unification scheme.Comment: 28 pages plus 6 figures, LaTex manuscript, Accepted for publication in the Astrophysical Journal, Figure 5 can retrieved via anonymous ftp at ftp://ftp.brera.mi.astro.it/pub/ASCA/paper2/fig5.ps.g