Probing Helical Magnetic Fields in AGN by Rotation Measure Gradients Studies

One of the tools that can provide evidence about the existence of helical magnetic fields in AGN is the observation of rotation measure gradients across the jet. Such observations have been previously made successfully, proving that such gradients are far from being rare, but common and typically persistent over several years, although some of them may show a reversal in the direction along the jet. Further studies of rotation measure gradients can help us in our understanding of the magnetic field properties and structure in the base of the jets. We studied Very Long Baseline Array (VLBA) polarimetric observations of 8 sources consistent of some quasars and BL Lacs at 12, 15, 22, 24 and 43 GHz and we find that all but two sources show indications of rotation measure gradients, either parallel or perpendicular to the jet. We interpret gradients perpendicular to the jet as indications of the change of the line of sight of the magnetic field due to its helicity, and gradients parallel to the jet as the decrease of magnetic field strength and/or electron density as we move along the jet. When comparing our results with the literature, we find tentative evidence of a rotation measure gradient flip, which can be explained as a change of the pitch angle or jet bending.Comment: 4 pages, 2 figures. Contribution for the Proceedings of HEPRO3 (Barcelona, 27 June - 1 July 2011). To be published on the International Journal of Modern Physics Conference Series, edited by J.M. Paredes, M. Rib\'o, F.A. Aharonian and G.E. Romer

Analysis of stationary roving mass effect for damage detection in beams using wavelet analysis of mode shapes

Publicado en Journal of Physics: Conference Series 628 (2015), 012014- . Ghent, Belgium : 24 – 26 August 2015One of the main challenges in damage detection techniques is sensitivity to damage. During the last years, a large number of papers have used wavelet analysis as a sensitive mathematical tool for identifying changes in mode shapes induced by damage. This paper analyzes the e ect of adding a mass to the structure at di erent positions. Depending on the location and severity of damage, the presence of the mass a ects the natural frequencies and mode shapes in a di erent way. The paper applies a damage detection methodology proposed by the authors, although it has been modi ed in order to consider the addition of the mas. This methodology is based on a wavelet analysis of the di erence of mode shapes of a damaged and a reference state. The singular behavior of a normalized weighted addition of wavelet coe cients is used as an indicator of damage. The presence of damage is detected by combining all the information provided by mode shapes and natural frequencies for di erent positions of the roving mass. A continuous wavelet transform is used to detect the di erence between the response of a healthy state and a damaged one. The paper shows the results obtained for a beam with di erent cracks. The paper analyzes the sensitivity to damage of the proposed methodology by considering some practical issues such as the size of the crack, the number of measuring points and the e ect of experimental noise