ESA Science & Technology - Publication Archive
Aims. The region of the temperature reversal can be directly observed only in the far infrared and submillimetre spectral regime. We aim at determining the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star alpha Cen A. As a bonus this will also provide a detailed mapping of the spectral energy distribution, i.e. knowledge that is crucial when searching for faint, Kuiper belt-like dust emission around other stars.
Methods. For the nearby binary system alpha Cen, stellar parameters are known with high accuracy from measurements. For the basic model parameters T_eff, log g and [Fe/H], we interpolate stellar model atmospheres in the grid of Gaia/PHOENIX and compute the corresponding model for the G2 V star alpha Cen A. Comparison with photometric measurements shows excellent agreement between observed photospheric data in the optical and infrared. For longer wavelengths, the modelled spectral energy distribution is compared to Spitzer-MIPS, Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry. A specifically tailored Uppsala model based on the MARCS code and extending further in wavelength is used to gauge the emission characteristics of alpha Cen A in the far infared.
[Abstract abbreviated due to character limitations.]
Past exploration of Jupiter's diverse satellite system has forever changed our understanding of the unique environments to be found around gas giants, both in our solar system and beyond. The detailed investigation of three of Jupiter's Galilean satellites (Ganymede, Europa, and Callisto), which are believed to harbour subsurface water oceans, is central to elucidating the conditions for habitability of icy worlds in planetary systems in general. The study of the Jupiter system and the possible existence of habitable environments offer the best opportunity for understanding the origins and formation of the gas giants and their satellite systems. The JUpiter ICy moons Explorer (JUICE) mission, selected by ESA in May 2012 to be the first large mission within the Cosmic Vision Program 2015-2025, will perform detailed investigations of Jupiter and its system in all their inter-relations and complexity with particular emphasis on Ganymede as a planetary body and potential habitat. The investigations of the neighbouring moons, Europa and Callisto, will complete a comparative picture of the Galilean moons and their potential habitability. Here we describe the scientific motivation for this exciting new European-led exploration of the Jupiter system in the context of our current knowledge and future aspirations for exploration, and the paradigm it will bring in the study of giant (exo) planets in general.
A subset of ultraluminous X-ray sources (those with luminosities < 1040 erg/s) are thought to be powered by the accretion of gas onto black holes with masses of ~5-20 MSun, probably via an accretion disc. The X-ray and radio emission are coupled in such Galactic sources, with the radio emission originating in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium precludes determining the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source whose peak luminosity can exceed 1039 erg/s in the nearby galaxy, M31. The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a stellar mass black hole. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.
Aims. The main goal of this study is to understand under which conditions high obscuration could occur.
Methods. We have used an hydrodynamical code to simulate the flow of the stellar wind around the neutron star. A grid of simulations was used to study the dependency of the absorbing column density and of the X-ray light-curves on the model parameters. A comparison between the simulation results and the observations of IGR J17252-3616 provides an estimate on these parameters.
Results. We have constrained the wind terminal velocity to 500-600 km/s and the neutron star mass to 1.75-2.15 MSun.
Conclusions. We have confirmed that the initial hypothesis of a slow wind velocity with a moderate mass loss rate is valid. The mass of the neutron star can be constrained by studying its impact on the accretion flow.
Aims. Our goal is to understand the characteristics of highly absorbed sgHMXB and in particular the companion stellar wind, which is thought to be responsible for the strong absorption.
Methods. We monitored IGR J17252-3616, a highly absorbed system featuring eclipses, with XMM-Newton to study the variability of the column density and the Fe K-alpha emission line along the orbit and during the eclipses. We also compiled a 3D model of the stellar wind to reproduce the observed variability.
Results. We first derive a refined orbital solution based on INTEGRAL, RXTE, and XMM-Newton data. We find that the XMM-Newton monitoring campaign reveals significant variations in the intrinsic absorbing column density along the orbit and the Fe K-alpha line equivalent width around the eclipse. The origin of the soft X-ray absorption is associated with a dense and extended hydrodynamical tail, trailing the neutron star. This structure extends along most of the orbit, indicating that the stellar wind has been strongly disrupted.
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