### Browsing by Author "Schober, Jennifer"

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Item Magnetohydrodynamical origin of eclipsing time variations in post-common-envelope binaries for solar mass secondaries(OXFORD UNIV PRESS INC, 2020-01) Navarrete, Felipe H.; Schleicher, Dominik R. G.; Kaepylae, Petri J.; Schober, Jennifer; Voelschow, Marcel; Mennickent, Ronald E.; Universidad de Concepción; Centre of Excellence Research on Solar Long-Term Variability and Effects, ReSoLVE; Observatoire de Sauverny; University of Hamburg; Department of Computer ScienceEclipsing time variations have been observed for a wide range of binary systems, including post-common-envelope binaries. A frequently proposed explanation, apart from the possibility of having a third body, is the effect of magnetic activity, which may alter the internal structure of the secondary star, particularly its quadrupole moment, and thereby cause quasi-periodic oscillations. Here, we present two compressible non-ideal magnetohydrodynamical simulations of the magnetic dynamo in a solar mass star, one of them with three times the solar rotation rate ('slow rotator'), and the other one with 20 times the solar rotation rate ('rapid rotator'), to account for the high rotational velocities in close binary systems. For the slow rotator, we find that both the magnetic field and the stellar quadrupole moment change in a quasi-periodic manner, leading to O-C (observed minus corrected times of the eclipse) variations of similar to 0.025 s. For the rapid rotator, the behaviour of the magnetic field as well as the quadrupole moment changes becomes considerably more complex, due to the less coherent dynamo solution. The resulting O-C variations are of the order of 0.13 s. The observed system V471 Tau shows two modes of eclipsing time variations, with amplitudes of 151 and 20 s, respectively. However, the current simulations may not capture all relevant effects due to the neglect of the centrifugal force and self-gravity. Considering the model limitations and that the rotation of V471 Tau is still a factor of 2.5 faster than our rapid rotator, it may be conceivable to reach the observed magnitudes.Item Relic Gravitational Waves from the Chiral Magnetic Effect(IOP Publishing Ltd., 2021-04-20) Brandenburg, Axel; He, Yutong; Kahniashvili, Tina; Rheinhardt, Matthias; Schober, Jennifer; Department of Computer Science; Professorship Korpi-Lagg Maarit; Ilia State University; Stockholm University; Ècole Polytechnique Fédérale de LausanneRelic gravitational waves (GWs) can be produced by primordial magnetic fields. However, not much is known about the resulting GW amplitudes and their dependence on the details of the generation mechanism. Here we treat magnetic field generation through the chiral magnetic effect (CME) as a generic mechanism and explore its dependence on the speed of generation (the product of magnetic diffusivity and characteristic wavenumber) and the speed characterizing the maximum magnetic field strength expected from the CME. When the latter exceeds the former (regime I), which is the regime applicable to the early universe, we obtain an inverse cascade with moderate GW energy that scales with the third power of the magnetic energy. When the generation speed exceeds the CME limit (regime II), the GW energy continues to increase without a corresponding increase of magnetic energy. In the early kinematic phase, the GW energy spectrum (per linear wavenumber interval) has opposite slopes in both regimes and is characterized by an inertial range spectrum in regime I and a white noise spectrum in regime II. The occurrence of these two slopes is shown to be a generic consequence of a nearly monochromatic exponential growth of the magnetic field. The resulting GW energy is found to be proportional to the fifth power of the limiting CME speed and the first power of the generation speed.