Search results for "astronomia"
showing 10 items of 673 documents
Multiwavelength study of accretion-powered pulsars
2010
My PhD thesis consists in a multi-frequency approach to High Mass X-ray Binaries (HMXBs), using infrared and X-ray data. On one side, my research project aimed at the identification and characterization of IR counterparts to obscured HMXBs in the Scutum and Norma inner galactic arms. The identification of optical/IR counterparts to HMXBs is a necessary step to undertake detailed studies of these systems. With data limited to the high-energy range, the understanding of their complex structure and dynamics cannot be complete. In the last years, INTEGRAL has revealed the presence of an important population of heavily absorbed HMXBs in the Scutum and Norma regions, virtually unobservable below …
New Insights into Galaxy Clusters: from Simulations to Observations
2011
The work carried out during this Thesis is framed within the field of Numerical Cosmology and focused on several broad lines intimately related which deal with the theoretical and numerical study of galaxy clusters: (i) the halo-finding problem, (ii) new improvements in cosmological simulations, and (iii) the formation and evolution of galaxy clusters. In spite of the achievements reached by Computational Cosmology in the last years, present-day hydrodynamics/N-body simulations still present important discrepancies with the observations, especially in the inner regions of massive galaxy clusters. Among these discrepancies we can cite, for instance, the breaking up of the self-similar scalin…
Asteroseismology and mass loss in Be stars. Study with CoRoT
2010
The interiors of the stars are among the most difficult parts of the Universe to observe. Essentially, asteroseismology tries to make use of the oscillations to prove the stellar interiors, which are not directly observable. The basic principles of asteroseismology are, to a certain extent, similar to those developed and employed by Earth seismologists. Asteroseismology relies on advanced mathematical descriptions of oscillations in a three-dimensional body and numerical modeling. It is therefore a prominent example of interdisciplinary science. The general aim of this work is the study of Be stars with the CoRoT space mission. Classical Be stars are B-type stars that exhibit line emission …
Search for a Stable Six-Quark State at BABAR
2019
Recent investigations have suggested that the six-quark combination uuddss could be a deeply bound state (S) that has eluded detection so far, and a potential dark matter candidate. We report the first search for a stable, doubly strange six-quark state in Upsilon -> S anti-Lambda anti-Lambda decays based on a sample of 90 million Upsilon(2S) and 110 million Upsilon(3S) decays collected by the BABAR experiment. No signal is observed, and 90% confidence level limits on the combined Upsilon(2S,3S) -> S anti-Lambda anti-Lambda branching fraction in the range (1.2-1.4)x10^-7 are derived for m_S < 2.05 GeV. These bounds set stringent limits on the existence of such exotic particles.
Accretion in strong field gravity with eXTP
2019
In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.
On the selection of AGN neutrino source candidates for a source stacking analysis with neutrino telescopes
2006
The sensitivity of a search for sources of TeV neutrinos can be improved by grouping potential sources together into generic classes in a procedure that is known as source stacking. In this paper, we define catalogs of Active Galactic Nuclei (AGN) and use them to perform a source stacking analysis. The grouping of AGN into classes is done in two steps: first, AGN classes are defined, then, sources to be stacked are selected assuming that a potential neutrino flux is linearly correlated with the photon luminosity in a certain energy band (radio, IR, optical, keV, GeV, TeV). Lacking any secure detailed knowledge on neutrino production in AGN, this correlation is motivated by hadronic AGN mode…
Search for neutrino emission from gamma-ray flaring blazars with the ANTARES telescope
2012
The ANTARES telescope observes a full hemisphere of the sky all the time with a duty cycle close to 100%. This makes it well suited for an extensive observation of neutrinos produced in astrophysical transient sources. In the surrounding medium of blazars, i.e. active galactic nuclei with their jets pointing almost directly towards the observer, neutrinos may be produced together with gamma-rays by hadronic interactions, so a strong correlation between neutrinos and gamma-rays emissions is expected. The time variability information of the studied source can be obtained by the gamma-ray light curves measured by the LAT instrument on-board the Fermi satellite. If the expected neutrino flux ob…
X-rays from accretion shocks in classical T Tauri stars: 2D MHD modeling and the role of local absorption
2013
AbstractIn classical T Tauri stars (CTTS) strong shocks are formed where the accretion funnel impacts with the denser stellar chromosphere. Although current models of accretion provide a plausible global picture of this process, some fundamental aspects are still unclear: the observed X-ray luminosity in accretion shocks is order of magnitudes lower than predicted; the observed density and temperature structures of the hot post-shock region are puzzling and still unexplained by models.To address these issues we performed 2D MHD simulations describing an accretion stream impacting onto the chromosphere of a CTTS, exploring different configurations and strengths of the magnetic field. From th…
Pulsating in Unison at Optical and X-Ray Energies: Simultaneous High Time Resolution Observations of the Transitional Millisecond Pulsar PSR J1023+00…
2019
PSR J1023+0038 is the first millisecond pulsar discovered to pulsate in the visible band; such a detection took place when the pulsar was surrounded by an accretion disk and also showed X-ray pulsations. We report on the first high time resolution observational campaign of this transitional pulsar in the disk state, using simultaneous observations in the optical (TNG, NOT, TJO), X-ray (XMM-Newton, NuSTAR, NICER), infrared (GTC) and UV (Swift) bands. Optical and X-ray pulsations were detected simultaneously in the X-ray high intensity mode in which the source spends $\sim$ 70% of the time, and both disappeared in the low mode, indicating a common underlying physical mechanism. In addition, o…
UV variability and accretion dynamics in the young open cluster NGC 2264
2015
We explore UV and optical variability signatures for several hundred members of NGC 2264 (3 Myr). We performed simultaneous u- and r-band monitoring over two full weeks with CFHT/MegaCam. About 750 young stars are probed; 40% of them are accreting. Statistically distinct variability properties are observed for accreting and non-accreting cluster members. The accretors exhibit a significantly higher level of variability than the non-accretors, especially in the UV. The amount of u-band variability correlates statistically with UV excess in disk-bearing objects, which suggests that accretion and star-disk interaction are the main sources of variability. Cool magnetic spots, several hundred de…