Search results for "Cosmology and Extragalactic Astrophysics"

Gravitational Lensing: the Structure of Quasars and Galaxies

2016

We use gravitational lens systems in which a galaxy produces multiple images of a distant quasar to study the properties of both the unresolved structure of the lensed quasar and the mass distribution in the gravitational lens. First, we estimate the size and the logarithmic slope of the temperature profile in the accretion disk of the lensed quasar Q2237+0305 using a method that is independent of the component velocities, based on six epochs of multi-wavelength narrowband images from the Nordic Optical Telescope. A statistical comparison of the observed microlensing with simulations based on microlensing magnification maps gives Bayesian estimates for the half-light radius of ~8 light-days…

Black-Hole Accretion Discs and Jets at Super-Eddington Luminosity

2004

Super-Eddington accretion discs with 3 and 15 dot M_E around black holes with mass 10 M_sun are examined by two-dimensional radiation hydrodynamical calculations extending from the inner disc edge to 5*10^4 r_g and lasting up to \sim 10^6 r_g/c. The dominant radiation-pressure force in the inner region of the disc accelerates the gas vertically to the disc plane, and jets with 0.2 -- 0.4$c$ are formed along the rotational axis. In the case of the lower accretion rate, the initially anisotropic high-velocity jet expands outward and becomes gradually isotropic flow in the distant region. The mass-outflow rate from the outer boundary is as large as \sim 10^{19} -- 10^{23} g s^{-1}, but it is v…

Multiple accelerated particle populations in the Cygnus Loop with Fermi-LAT

2021

The Cygnus Loop (G74.0-8.5) is a very well-known nearby supernova remnant (SNR) in our Galaxy. Thanks to its large size, brightness, and angular offset from the Galactic plane, it has been studied in detail from radio to $\gamma$-ray emission. The $\gamma$ -rays probe the populations of energetic particles and their acceleration mechanisms at low shock speeds. We present an analysis of the $\gamma$-ray emission detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope over 11 years in the region of the Cygnus Loop. We performed detailed morphological and spectral studies of the $\gamma$-ray emission toward the remnant from 100 MeV to 100 GeV and compared it with X-ra…

Are pulsars born with a hidden magnetic field?

2015

The observation of several neutron stars in the center of supernova remnants and with significantly lower values of the dipolar magnetic field than the average radio-pulsar population has motivated a lively debate about their formation and origin, with controversial interpretations. A possible explanation requires the slow rotation of the proto-neutron star at birth, which is unable to amplify its magnetic field to typical pulsar levels. An alternative possibility, the hidden magnetic field scenario, considers the accretion of the fallback of the supernova debris onto the neutron star as responsible for the submergence (or screening) of the field and its apparently low value. In this paper …

TeV gamma-ray survey of the Northern sky using the ARGO-YBJ detector

2013

The ARGO-YBJ detector is an extensive air shower array that has been used to monitor the northern $\gamma$-ray sky at energies above 0.3 TeV from 2007 November to 2013 January. In this paper, we present the results of a sky survey in the declination band from $-10^{\circ}$ to $70^{\circ}$, using data recorded over the past five years. With an integrated sensitivity ranging from 0.24 to $\sim$1 Crab units depending on the declination, six sources have been detected with a statistical significance greater than 5 standard deviations. Several excesses are also reported as potential $\gamma$-ray emitters. The features of each source are presented and discussed. Additionally, $95\%$ confidence le…

Stochastic gravito-inertial modes discovered by CoRoT in the hot Be star HD 51452

2012

Context.Be stars are rapidly rotating stars with a circumstellar decretion disk. They usually undergo pressure and/or gravity pulsation modes excited by the κ-mechanism, i.e. an effect of the opacity of iron-peak elements in the envelope of the star. In the Milky Way,p-modes are observed in stars that are hotter than or equal to the B3 spectral type, while g-modes are observed at the B2 spectral type and cooler.Aims.We observed a B0IVe star, HD 51452,with the high-precision, high-cadence photometric CoRoT satellite and high-resolution, ground-based HARPS and SOPHIE spectrographs to study its pulsations in great detail. We also used the lower resolution spectra available in the BeSS atabase.…

Equations of State for White Dwarfs

2009

This thesis is about deriving a few equations of state for white dwarfs below the regime of neutron drip. White dwarfs - also called degenerate dwarfs, composed mostly of electron-degenerate matter - are luminous and the color of the light they are emitting is white, hence their name. Because of the relatively enormous density, the gravitational potential of a white dwarf causes a collapse. White dwarfs are classified as compact objects, meaning that their life begins when a star dies, and are therefore considered as one possibility of a final stage of stellar evolution since they are considered static over the lifetime of the Universe. Star death is a point where the most of its nuclear fu…

VISTA Milky Way Public Survey

2009

We propose a public IR variability survey, named \Vista Variables in the Vía Láctea" (V V V ), of the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. This would take 1920 hours, covering ~ 109 point sources within an area of 520 sq deg, including 33 known globular clusters and ~ 350 open clusters. The final products will be a deep IR atlas in 5 passbands and a catalogue of ~ 106 variable point sources. These will produce a 3-D map of the surveyed region (unlike single-epoch surveys that only give 2-D maps) using well-understood primary distance indicators such as RR Lyrae stars. It will yield important information on the ages of the population…

2018

Abstract. Detailed measurements of radiation, atmospheric and aerosol properties were carried out in summer 2013 during the Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region (ADRIMED) campaign in the framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) experiment. This study focusses on the characterization of infrared (IR) optical properties and direct radiative effects of mineral dust, based on three vertical profiles of atmospheric and aerosol properties and IR broadband and narrowband radiation from airborne measurements, made in conjunction with radiosonde and ground-based observations at Lampedusa, in the central Mediterranean. Satell…

Adjusted Normalized Emissivity Method for surface temperature and emissivity retrieval from optical and thermal infrared remote sensing data

2003

[1] A methodology for the retrieval of surface temperatures and emissivities combining visible, near infrared and thermal infrared remote sensing data was applied to Digital Airborne Imaging Spectrometer (DAIS) data and validated with coincident ground measurements acquired in a multiyear experiment held in an agricultural site in Barrax, Spain. The Adjusted Normalized Emissivity Method (ANEM) is based on the use of visible and near infrared data to estimate the vegetation cover and model the maximum emissivity according to the Vegetation Cover Method. The pixel-dependent maximum emissivity is used as the initial guess of the Normalized Emissivity Method to obtain the surface temperature an…