Ulrich Geppert - Departament de Fisica Aplicada, Universitat d'Alacant

Ulrich Geppert
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Ulrich Geppert
Departament de Fisica Aplicada, Universitat d'Alacant

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Astrophysics (8)
Physics - Space Physics (3)
Solar and Stellar Astrophysics (2)
Physics - Instrumentation and Detectors (2)
High Energy Astrophysical Phenomena (2)
General Relativity and Quantum Cosmology (1)
Physics - Materials Science (1)

Publications Authored By Ulrich Geppert

We constrain the X-ray properties of the nearby $(360\,{\rm pc})$, old ($5\,{\rm Myr}$) pulsar B1133+16 with $\sim 100\,{\rm ks}$ effective exposure time by {\it XMM-Newton}. The observed pulsar flux in the 0.2-3 keV energy range is $\sim 10^{-14} \, {\rm erg \, cm}^{-2} \, {\rm s}^{-1}$, which results in the recording of $\sim 600$ source counts with the EPIC pn and MOS detectors. Read More

A precondition for the radio emission of pulsars is the existence of strong, small-scale magnetic field structures (`magnetic spots') in the polar cap region. Their creation can proceed via crustal Hall drift out of two qualitatively and quantitatively different initial magnetic field configurations: a field confined completely to the crust and another which penetrates the whole star. The aim of this study is to explore whether these magnetic structures in the crust can deform the star sufficiently to make it an observable source of gravitational waves. Read More

The widespread use of metallic structures in space technology brings risk of degradation which occurs under space conditions. New types of materials dedicated for space applications, that have been developed in the last decade, are in majority not well tested for different space mission scenarios. Very little is known how material degradation may affect the stability and functionality of space vehicles and devices during long term space missions. Read More

All material exposed to interplanetary space conditions are subject to degradation processes. For obvious reasons there is a great interest to study these processes for materials that are used in satellite construction. However, also the influence of particle and electromagnetic radiation on the weathering of extraterrestrial rocks and on organic and biological tissues is the research topic of various scientific disciplines. Read More

This paper describes the construction and performance of a VUV-simulator that has been designed to study degradation of materials under space conditions. It is part of the Complex Irradiation Facility at DLR in Bremen, Germany, that has been built for testing of material under irradiation in the complete UV-range as well as under proton and electron irradiation. Presently available UV-sources used for material tests do not allow the irradiation with wavelengths smaller than about $115$ nm where common Deuterium lamps show an intensity cut-off. Read More

Metals are the most common materials used in space technology. Metal structures, while used in space, are subjected to the full spectrum of the electromagnetic radiation together with particle irradiation. Hence, they undergo degradation. Read More

(Abridged) The rotational evolution of isolated neutron stars is dominated by the magnetic field anchored to the solid crust of the star. Assuming that the core field evolves on much longer timescales, the crustal field evolves mainly though Ohmic dissipation and the Hall drift, and it may be subject to relatively rapid changes with remarkable effects on the observed timing properties. We investigate whether changes of the magnetic field structure and strength during the star evolution may have observable consequences in the braking index, which is the most sensitive quantity to reflect small variations of the timing properties that are caused by magnetic field rearrangements. Read More

The non-linear Hall term present in the induction equation in the electron-magneto-hydrodynamics limit is responsible for the Hall drift of the magnetic field and, in some cases, for the so-called Hall instability. We investigate whether or not the growth rates and eigenfunctions found in the linear analysis are consistent with the results of non-linear numerical simulations. Following the linear analysis of Rheinhardt & Geppert, we study the same cases for which the Hall instability was predicted by solving the non-linear Hall induction equation using a two-dimensional conservative and divergence-free finite difference scheme that overcomes intrinsic difficulties of pseudo-spectral methods and can describe situations with arbitrarily high magnetic Reynolds numbers. Read More

Affiliations: 1Instituto de Astronomia, UNAM, 2Departament de Fisica Aplicada, Universitat d'Alacant, 3Astrophysikalisches Institut Potsdam
Category: Astrophysics

We present models of temperature distribution in the crust of a neutron star in the presence of a strong toroidal component superposed to the poloidal component of the magnetic field. The presence of such a toroidal field hinders heat flow toward the surface in a large part of the crust. As a result, the neutron star surface presents two warm regions surrounded by extended cold regions and has a thermal luminosity much lower than in the case the magnetic field is purely poloidal. Read More

Affiliations: 1Inst. de Astronomia, UNAM, Mexico City, 2MPE Garching, 3San Diego State University
Category: Astrophysics

The cooling of a compact star depends very sensitively on the state of dense matter at supranuclear densities, which essentially controls the neutrino emission, as well as on the structure of the stellar outer layers which control the photon emission. Open issues concern the hyperon population, the presence of meson condensates, superfluidity and superconductivity, and the transition of confined hadronic matter to quark matter. This paper describes these issues and presents cooling calculations based on a broad collection of equations of state for neutron star matter and strange matter. Read More

The classical vacuum gap model of Ruderman & Sutherland, in which spark-associated subbeams of subpulse emission circulate around the magnetic axis due to the EB drift, provides a natural and plausible physical mechanism of the subpulse drift phenomenon. Recent progress in the analysis of drifting subpulses in pulsars has provided a strong support to this model by revealing a number of subbeams circulating around the magnetic axis in a manner compatible with theoretical predictions. However, a more detailed analysis revealed that the circulation speed in a pure vacuum gap is too high when compared with observations. Read More

Affiliations: 1Bologna Obs., 2Un. Milano Bicocca, 3UNAM, Mexico D.F., 4AIP, Potsdam
Category: Astrophysics

We explore, using an exact cooling code, the thermal evolution of a neutron star undergoing episodes of intense accretion, alternated by long periods of quiescence (e.g. Soft X-Ray Transients). Read More

Affiliations: 1Un. Milano Bicocca, 2AIP, 3UNAM, 4Obs. Bologna
Category: Astrophysics

We explore the thermal evolution of a neutron star undergoing episodes of intense accretion, separated by long periods of quiescence. By using an exact cooling code we follow in detail the flow of heat in the star due to the time-dependent accretion-induced heating from pycno-nuclear reactions in the stellar crust, to the surface photon emission, and the neutrino cooling. These models allow us to study the neutron stars of the Soft X-Ray Transients. Read More

We confront theoretical models for the rotational, magnetic, and thermal evolution of an ultra-magnetized neutron star, or magnetar, with available data on the Anomalous X-ray Pulsars (AXPs). We argue that, if the AXPs are interpreted as magnetars, their clustering of spin periods between 6 and 12 seconds, observed at present in this class of objects, their period derivatives, their thermal X-ray luminosities, and the association of two of them with young supernova remnants, can only be understood globally if the magnetic field in magnetars decays significantly on a time scale of the order of 10^4 years. Read More

We investigate the statistical evolution of magnetic neutron stars recycled in Low Mass Binary (LMB) systems, simulating synthetic populations. Irrespective to the details of the physical models, we find to be significant the fraction of neutron stars spinning close to their mass shedding limit relative to the millisecond pulsar population. The simulated neutron stars show a tail in their period distribution at periods shorter than 1. Read More

We investigate the statistical evolution of magnetic neutron stars, recycled in binary systems, simulating synthetic populations. To bracket uncertainties, we consider a soft (FP) and a stiff (PS) equation of state (EoS) for nuclear matter and explore the hypothesis that the magnetic field is confined in the stellar crust. We follow the magneto-rotational evolution within a simple recycling scenario, including the possibility of magnetospheric propeller. Read More