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We study the thermal Markovian diffusion of tracer particles in a 2D medium with spatially varying diffusivity D(r), mimicking recently measured, heterogeneous maps of the apparent diffusion coefficient in biological cells. For this heterogeneous diffusion process (HDP) we analyse the mean squared displacement (MSD) of the tracer particles, the time averaged MSD, the spatial probability density function, and the first passage time dynamics from the cell boundary to the nucleus. Moreover we examine the non-ergodic properties of this process which are important for the correct physical interpretation of time averages of observables obtained from single particle tracking experiments. From extensive computer simulations of the 2D stochastic Langevin equation we present an in-depth study of this HDP. In particular, we find that the MSDs along the radial and azimuthal directions in a circular domain obey anomalous and Brownian scaling, respectively. We demonstrate that the time averaged MSD stays linear as a function of the lag time and the system thus reveals a weak ergodicity breaking. Our results will enable one to rationalise the diffusive motion of larger tracer particles such as viruses or submicron beads in biological cells.
We study the thermal Markovian diffusion of tracer particles in a 2D medium with spatially varying diffusivity D(r), mimicking recently measured, heterogeneous maps of the apparent diffusion coefficient in biological cells. For this heterogeneous diffusion process (HDP) we analyse the mean squared displacement (MSD) of the tracer particles, the time averaged MSD, the spatial probability density function, and the first passage time dynamics from the cell boundary to the nucleus. Moreover we examine the non-ergodic properties of this process which are important for the correct physical interpretation of time averages of observables obtained from single particle tracking experiments. From extensive computer simulations of the 2D stochastic Langevin equation we present an in-depth study of this HDP. In particular, we find that the MSDs along the radial and azimuthal directions in a circular domain obey anomalous and Brownian scaling, respectively. We demonstrate that the time averaged MSD stays linear as a function of the lag time and the system thus reveals a weak ergodicity breaking. Our results will enable one to rationalise the diffusive motion of larger tracer particles such as viruses or submicron beads in biological cells.
The mystery of the origin of cosmic rays has been tackled for more than hundred years and is still not solved. Cosmic rays are detected with energies spanning more than 10 orders of magnitude and reaching energies up to ~10²¹ eV, far higher than any man-made accelerator can reach. Different theories on the astrophysical objects and processes creating such highly energetic particles have been proposed.
A very prominent explanation for a process producing highly energetic particles is shock acceleration. The observation of high-energy gamma rays from supernova remnants, some of them revealing a shell like structure, is clear evidence that particles are accelerated to ultrarelativistic energies in the shocks of these objects. The environments of supernova remnants are complex and challenge detailed modelling of the processes leading to high-energy gamma-ray emission.
The study of shock acceleration at bow shocks, created by the supersonic movement of individual stars through the interstellar medium, offers a unique possibility to determine the physical properties of shocks in a less complex environment. The shocked medium is heated by the stellar and the shock excited radiation, leading to thermal infrared emission. 28 bow shocks have been discovered through their infrared emission. Nonthermal radiation in radio and X-ray wavelengths has been detected from two bow shocks, pointing to the existence of relativistic particles in these systems. Theoretical models of the emission processes predict high-energy and very high-energy emission at a flux level in reach of current instruments. This work presents the search for gamma-ray emission from bow shocks of runaway stars in the energy regime from 100MeV to ~100TeV.
The search is performed with the large area telescope (LAT) on-board the Fermi satellite and the H.E.S.S. telescopes located in the Khomas Highland in Namibia. The Fermi-LAT was launched in 2008 and is continuously scanning the sky since then. It detects photons with energies from 20MeV to over 300 GeV and has an unprecedented sensitivity. The all-sky coverage allows us to study all 28 bow shocks of runaway stars listed in the E-BOSS catalogue of infrared bow shocks. No significant emission was detected from any of the objects, although predicted by several theoretical models describing the non-thermal emission of bow shocks of runaway stars.
The H.E.S.S. experiment is the most sensitive system of imaging atmospheric Cherenkov telescopes. It detects photons from several tens of GeV to ~100TeV. Seven of the bow shocks have been observed with H.E.S.S. and the data analysis is presented in this thesis. The analyses of the very-high energy data did not reveal significant emission from any of the sources either.
This work presents the first systematic search for gamma-ray emission from bow shocks of runaway stars. For the first time Fermi-LAT data was specifically analysed to reveal emission from bow shocks of runaway stars. In the TeV regime no searches for emission from theses objects have been published so far, the study presented here is the first in this energy regime. The level of the gamma-ray emission from bow shocks of runaway stars is constrained by the calculated upper limits over six orders in magnitude in energy.
The upper limits calculated for the bow shocks of runaway stars in the course of this work, constrain several models. For the best candidate, ζ Ophiuchi, the upper limits in the Fermi-LAT energy range are lower than the predictions by a factor ~5. This challenges the assumptions made in this model and gives valuable input for further modelling approaches.
The analyses were performed with the software packages provided by the H.E.S.S. and Fermi collaborations. The development of a unified analysis framework for gamma-ray data, namely GammaLib/ctools, is rapidly progressing within the CTA consortium. Recent implementations and cross-checks with current software frameworks are presented in the Appendix.
The role of flavonols and anthocyanins in the cold an UV-B acclimation of Arabidopsis thaliana (L.)
(2014)
On the role of fluoro-substituted nucleosides in DNA radiosensitization for tumor radiation therapy
(2014)
Gemcitabine (2′,2′-difluorocytidine) is a well-known radiosensitizer routinely applied in concomitant chemoradiotherapy. During irradiation of biological media with high-energy radiation secondary low-energy (<10 eV) electrons are produced that can directly induce chemical bond breakage in DNA by dissociative electron attachment (DEA). Here, we investigate and compare DEA to the three molecules 2′-deoxycytidine, 2′-deoxy-5-fluorocytidine, and gemcitabine. Fluorination at specific molecular sites, i.e., nucleobase or sugar moiety, is found to control electron attachment and subsequent dissociation pathways. The presence of two fluorine atoms at the sugar ring results in more efficient electron attachment to the sugar moiety and subsequent bond cleavage. For the formation of the dehydrogenated nucleobase anion, we obtain an enhancement factor of 2.8 upon fluorination of the sugar, whereas the enhancement factor is 5.5 when the nucleobase is fluorinated. The observed fragmentation reactions suggest enhanced DNA strand breakage induced by secondary electrons when gemcitabine is incorporated into DNA.