Dottorato in Fisica

Journal Club Seminari 2021

14/04/2021   ore 15:00
modalita' telematica

ore 15:00	Chloe Helene Martella (DOTTORATO IN FISICA)	Reanalysis of the Active Seismic Experiments performed on the Moon during Apollo 14 and 16 Missions Abstract:  Robotic and human Moon exploration is currently living a new renaissance, as it is considered a benchmark for future human expansion in the Solar System. Many space agencies and private companies are developing strategies to bring the man back on the Moon by the end of this decade. This new international race will be driven by scientific, technological, and economic interests, and will require an in-depth understanding of the Moon subsurface environment. Several new geophysical instruments have been proposed to explore the Moon subsoil at different spatial scales. Amongst these, active seismic methods (reflection and refraction) are two of the most reliable techniques to investigate the lunar shallow stratigraphy. Indeed, refraction seismic technique was already tested during Apollo14, Apollo16 and Apollo 17 missions, providing some information on the thickness and the mechanical properties of the lunar regolith at the three landing sites.  In this work we present a reanalysis of the Active Seismic Experiments (ASE) conducted on the Moon during Apollo 14 and 16 missions. We analyzed all data collected along the seismic lines using the thumper source and we applied modern seismic attributes approach to better perform the first arrival picking. Conversely to previous works that used the LM ascent impacts and grenades, we were able to detect for both sites the direct and the head waves only using the thumper shots and compute the compressional wave velocities for the first and second layer. Finally, we compared our results with those published in previous works.
ore 15:45	Lorenzo Marra (DOTTORATO IN FISICA)	Polarization properties of the X-ray emission of the accretion disk around black holes Abstract: The radiation emitted by accretion disks around stellar mass black holes peaks in the soft X-rays when the source is in the thermal state. For symmetry reasons, the emission is expected to be polarized in a direction parallel or perpendicular to the disk symmetry axis projected onto the plane of the sky as an effect of the electron scattering that photons may undergo before leaving the disk. However, absorption effects in the disk material can influence as well the polarization state of emerging photons. Here we present, for the first time, a study of the polarization properties of the radiation emitted from a typical stellar mass black hole accretion disk, accounting for both scattering and absorption effects, that we have included computing the ionization structure of the disk material. We explore the two cases of a Schwarzschild and an extreme Kerr black hole, focusing our study on a BH with mass M = 10M? and for different values of mass accretion rate and scattering optical depth of the disk. Our results show that absorption plays a crucial role in determining the polarization properties of the X-ray emission. In fact, the polarization degree of radiation tends to be higher in the colder and more absorbed regions of the disk. The polarization angle is found to be perpendicular to the disk symmetry axis. For the time being, in the calculations we did not include any strong gravity effect, so we studied the spectra and the polarization observables as seen by an observer located on the disk surface. An immediate application of this model will be to include all the relevant GR effects to specialize these calculations to the case of stellar mass black holes in binary systems in our Galaxy.
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