arXiv subjects

Costantino Sigismondi

Publications and source records attributed to Costantino Sigismondi.

99 records · Page 6Linked to original sources

arXiv2011

Picard satellite for solar astrometry

The Picard solar satellite has been launched on June 15, 2010. This mission is dedicated to the measurement of the solar diameter with an expected accuracy of milliarcseconds of arc. The radiometer onboard is to measure the total solar irradiance. The final goal is the evaluation of the W, the logarithmic ratio of radius and luminosity. This parameter will help the climatologists to recover past values of the solar luminosity when the radius is available from ancient eclipses data.

astro-ph.SR↗

arXiv2005

Identification of Relativistic Effects in X-Ray Astrophysical Source

Precise knowledge of the features of several X-ray sources from compact binary sources to galactic nuclei is gradually unveiling the physical parameters of relativistic neutron stars and black holes therein. Recent developments in theory, astrophysical models and experiments are reviewed.

astro-ph↗

arXiv2005

Solar Oblateness from Archimedes to Dicke

The non-spherical shape of the Sun has been invoked to explain the anomalous precession of Mercury. A brief history of some methods for measuring solar diameter is presented. Archimedes was the first to give upper and lower values for solar diameter in third century before Christ; we also show the method of total eclipses, used after Halley's observative campaign of 1715 eclipse; the variant of partial eclipses useful to measure different chords of the solar disk; the method of Dicke which correlates oblateness with luminous excess in the equatorial zone.

physics.hist-ph↗

arXiv2005

Dark Matter in a One-dimensional Universe

A computer code to simulate temporal evolution of overdensities in a one-dimensional Universe is presented for didactic purposes. The formation of large scale structures in this one-dimensional universe can be studied both in matter or radiation dominated eras. Since large scale structures are already observed at z > 7, primordial dark matter overdensities delta_DM which are 90 times larger than the observed barionic delta_B in the cosmic microwave background are required at z~1000. This makes possible non-linear gravitational collapse at redshift z >7 and the formation of the structures. Primordial perturbations delta_B~10^-5 do not leave the linear regime of growth without the aid of dark matter's potential wells. This code is suitable for commercial worksheets like MSExcel, StarOffice, or OpenOffice.

astro-ph↗

arXiv2005

Astrometry and Relativity

General relativistic effects in astrophyiscal systems have been detected thanks to accurate astrometric measurements. We outline some keystones of astrometry such as stellar aberration (argument development during the years 1727-1872); Mercury's perihelion precession (1845-1916); solar disk oblateness (1966-2001); relativistic light deflection (1916-1919); lunar geodetic precession (1916-1988); Lense-Thirring and Pugh-Schiff precessions (1917-1959), finally presenting the issue of the quest for a guide star for GP-B satellite (1974-2004) as application of all previous topics.

astro-ph↗

arXiv2005

Precessions in Relativity

From Mercury's perihelion precession (open question from 1845 to 1915) to Gravity Probe-B satellite (first proposal in 1959, launch in 2004), General Relativity had to deal with precession phenomena. Perihelion advancement precession (Einstein), geodetic (de Sitter), gravitomagnetic (Lense-Thirring) and spin-spin (Pugh-Schiff) precession are compared with all Newtonian terms in cases of weak and strong gravitational fields. Qualitative models and order of magnitude estimates from classical papers are presented.

astro-ph↗

arXiv2001

Damping time and stability of density fermion perturbations in the expanding universe

The classic problem of the growth of density perturbations in an expanding Newtonian universe is revisited following the work of Bisnovatyi-Kogan and Zel'dovich. We propose a more general analytical approach: a system of free particles satisfying semi-degenerate Fermi-Dirac statistics on the background of an exact expanding solution is examined in the linear approximation. This differs from the corresponding work of Bisnovatyi-Kogan and Zel'dovich where classical particles fulfilling Maxwell-Boltzmann statistics were considered. The solutions of the Boltzmann equation are obtained by the method of characteristics. An expression for the damping time of a decaying solution is discussed and a zone in which free streaming is hampered is found, corresponding to wavelengths less than the Jeans one. In the evolution of the system, due to the decrease of the Jeans length, those perturbations may lead to gravitational collapse. At variance with current opinions, we deduce that perturbations with lambda >=lambda_(J Max)/1.48 are able to generate structures and the lower limit for substructures mass is M = M_(J max)/(1.48)^3 ~ M_(J max)/3, where M_(J max) is the maximum value of the Jeans mass.

astro-ph↗

arXiv2000

On the Fine Structure in the Fireballs Peak of the 1998 Leonids

The Leonids meteor shower of November 1998 has shown a double activity. An unexpected shower of fireballs occurred about 16 hours before the expected maximum of the meteor activity. The activity profile of the fireball shower revealed also a fine structure. The hypotesis of tidal origin of such sub-structures is discussed. The close encounter of the meteoroid stream with the Earth in 1366 and the encounters with Saturn (in 1630) and Jupiter (in 1732), are identified as the cause of the main features of the structure. Such analysis can be applied to the incoming 2000 Leonids' display.

astro-ph↗

arXiv2000

The Observation of Lunar Impacts

The intense activity of cratering on the Moon and in the inner regions of the solar system was accomplished during the first 10^9 years [1]. Occasionally, some impact events occur even nowadays. In Section 1, we treat, from a historical point of view, the Earth-based observation of lunar impacts. In Section 2, we consider the visibility conditions of such events evaluating the luminosity of the background upon which an impact shines. In Section~3, the luminosity of an impact is discussed. The occurrence of lunar impact events outside of meteor shower periods is calculated using the hourly rate of the sporadic meteors and their population index. The evidence of a larger rate of impacts of meteoroids in the past under these hypotheses is presentend in the last section.

astro-ph↗