Ulrich Geppert

Bio: Ulrich Geppert is an academic researcher from University of Zielona Góra. The author has contributed to research in topics: Neutron star & Magnetic field. The author has an hindex of 33, co-authored 128 publications receiving 3540 citations. Previous affiliations of Ulrich Geppert include Leibniz Institute for Astrophysics Potsdam & Leipzig University.

Magneto-thermal evolution of neutron stars

Abstract: Context. The presence of magnetic fields in the crust of neutron stars c auses a non-spherically symmetric temperature distribution. The strong temperature dependence of the magnetic diffusivity and thermal conductivity, together with the heat generated by magnetic dissipation, couple the magnetic and thermal evolution of NSs, that cannot be formulated as separated one‐dimensional problems. Aims. We study the mutual influence of thermal and magnetic evoluti on in a neutron star’s crust in axial symmetry. Taking into account realistic microphysical inputs, we find the heat rel eased by Joule effect consistent with the circulation of currents in the crust , and we incorporate its effects in 2‐dimensional cooling calculations. Methods. We solve the induction equation numerically using a hybrid method (spectral in angles, but a finite‐di fferences scheme in the radial direction), coupled to the thermal diffusion equation. To improve the boundary conditions, we also revisit the envelope stationary solutions updating the well known Tb− Ts‐relations to include the effect of 2‐D heat transfer calculations and new microphysical inputs. Results. We present the first long term 2‐dimensional simulations of t he coupled magneto-thermal evolution of neutron stars. This substantially improves previous works in which a very crude approximation in at least one of the parts (thermal or magnetic diffusion) has been adopted. Our results show that the feedback between Joule heating and magnetic diffusion is strong, resulting in a faster dissipation of the stronger fields during the first 10 5 − 10 6 years of a NS’s life. As a consequence, all neutron stars born with fields larger than a critical value (> 5×10 13 G) reach similar field strengths (≈ 2−3×10 13 G) at late times. Irrespectively of the initial magnetic field strength, after 10 6 years the temperature becomes so low that the magnetic diffusion timescale becomes longer than the typical ages of radio‐pulsars, thus resulting in apparently no diss ipation of the field in old NS. We also confirm the strong correl ation between the magnetic field and the surface temperature of relatively young NSs discussed in preliminary works. The effective temperature of models with strong internal toroidal components are systematically higher than those of models with purely poloidal fie lds, due to the additional energy reservoir stored in the toroidal field tha t is gradually released as the field dissipates.

Magnetic field dissipation in neutron star crusts: From magnetars to isolated neutron stars

Abstract: Context. We study the non-linear evolution of magnetic fields in neutron star crusts with special attention to the influence of the Hall drift. Aims. Our goal is to understand the conditions for fast dissipation due to the Hall term in the induction equation. We study the interplay of Ohmic dissipation and Hall drift in order to find a timescale for the overall crustal field decay. Methods. We solve the Hall induction equation numerically by means of a hybrid method (spectral in angles but finite differences in the radial coordinate). The microphysical input consists of the most modern available crustal equation of state, composition, and electrical conductivities. Results. We present the first long-term simulations of the non-linear magnetic field evolution in realistic neutron star crusts with a stratified electron number density and temperature dependent conductivity. We show that Ohmic dissipation influenced by Hall drift takes place in neutron star crusts on a timescale of 10 6 years. When the initial magnetic field has magnetar strength, the fast Hall drift results in an initial rapid dissipation stage that lasts ∼10 4 years. The interplay of the Hall drift with the temporal variation and spatial gradient of conductivity tends to favor the displacement of toroidal fields toward the inner crust, where stable configurations can last for ∼ 10 6 years. We show that the thermally emitting, isolated neutron stars, such as the Magnificent Seven, are very likely descendants of neutron stars born as magnetars.

Drifting subpulses and inner acceleration regions in radio pulsars

Abstract: The classical vacuum gap model of Ruderman & Sutherland, in which spark-associated subbeams of subpulse emission circulate around the magnetic axis due to the E × B drift of spark plasma filaments, provides a natural and plausible physical mechanism of the subpulse drift phenomenon. Moreover, this is the only model with quantitative predictions that can be compared with observa- tions. 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. Moreover, some pul- sars demonstrate significant time variations of the drift rate, including a change of the apparent drift direction, which is obviously inconsistent with the E × B drift scenario in a pure vacuum gap. We attempted to resolve these discrepancies by considering a partial flow of iron ions from the positively charged polar cap, co- existing with the production of outflowing electron-positron plasmas. The model of such charge-depleted acceleration region is highly sensitive to both the critical ion temperature Ti � 10 6 K (above which ions flow freely with the corotational charge density) and the actual surface temperature Ts of the polar cap, heated by the bombardment of ultra-relativistic charged particles. By fitting the obser- vationally deduced drift-rates to the theoretical values, we managed to estimate polar cap surface temperatures in a number of pulsars. The estimated surface temperatures Ts correspond to a small charge depletion of the order of a few percent of the Goldreich-Julian corotational charge density. Nevertheless, the re-

Period Clustering of the Anomalous X-Ray Pulsars and Magnetic Field Decay in Magnetars

Abstract: We confront theoretical models for the rotational, magnetic, and thermal evolution of an ultramagnetized 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 s (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 timescale of the order of 104 yr.

Human MotivationThe Mind in Action: A Study of Motives and Values. A. Campbell Garnett

Abstract: In his Preface the author' says that he started out to review all the more important theories upon the topics ordinarily discussed under human motivation but soon found himself more and more limited to the presentation of his own point of view. This very well characterizes the book. It is a very personal product. It is an outline with some defense of the author's own thinking about instincts and appetites and sentiments and how they function in human behavior. And as the author draws so heavily upon James and McDougall, especially the latter, the book may well be looked upon as a sort of sequel to their efforts. There is a thought-provoking distinction presented between instinct and appetite. An instinct is said to be aroused always by something in the external situation; and, correspondingly, an appetite is said to be aroused by sensations from within the body itself. This places, of course, a heavy emphasis upon the cognitive factor in all instinctive behaviors; and the author prefers to use the cognitive factor, especially the knowledge of that end-experience which will satisfy, as a means of differentiating one instinct from another. In this there is a recognized difference from McDougall who placed more emphasis for differentiation upon the emotional accompaniment. The list of instincts arrived at by this procedure is much like that of McDougall, although the author is forced by his criteria to present the possibility of food-seeking and sex and sleep operating both in the manner of an appetite and also as an instinct. The Shand-McDougall concept of sentiment is taken over and used in the explanation of moral motivation. There is the development within each personality of a sentiment for some moral principle. But this sentiment is not a very powerful motivating factor. It is reinforced by social pressures and by religion, which is treated as an effort of finite man to live in harmony with the infinite reality. Those whose psychological thinking is largely in terms of McDougall will doubtless find this volume a very satisfying expansion; but those who are at all inclined to support their psychological thinking by reference to experimental studies will not be so well pleased. The James-Lange theory, for example, is discussed without mention of the many experimental studies which it has provoked. Theoretical sources appear in general to be preferred to experimental investigations.

Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques

Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

A life-span theory of control.

Abstract: A life-span theory of development is presented that is based on the concepts of primary and secondary control. Primary control refers to behaviors directed at the external environment and involves attempts to change the world to fit the needs and desires of the individual. Secondary control is targeted at internal processes and serves to minimize losses in, maintain, and expand existing levels of primary control. Secondary control helps the individual to cope with failure and fosters primary control by channeling motivational resources toward selected action goals throughout the life course. Primary control has functional primacy over secondary control. An analysis of extensive and diverse literatures spanning infancy through old age shows that trade-offs between primary and secondary control undergo systematic shifts across the life course in response to the opportunities and constraints encountered. In this article, we present a life-span theory of development based on the concept of control. Research on life-span development has become an increasingly active area of inquiry in the last two decades and has the potential of yielding fundamental theories about the emergence and transformation of human behavior over the life course (e.g., P. B. Baltes, 1987). However, as noted by Birren and Bengtson (1988), the field of life-course development and aging is at present best characterized as data rich but theory poor. Most existing developmental theories embrace limited temporal and functioning domains. With the exception of personality theorists such as Erikson, Loevinger, Gould, and Levinson (see Cavanaugh, 1990; Schulz & Ewen, 1993), psychologists have made few attempts to develop lifespan theories of development, in part because such efforts require the integration of information from multiple domains, including the behavioral, social, and biological sciences. A second formidable challenge posed by such an undertaking is the problem of level of analysis. Ideally, such a theory should explain both macro-level behaviors such as major life-course decisions

Young Massive Star Clusters

Abstract: Young massive clusters are dense aggregates of young stars that form the fundamental building blocks of galaxies. Several examples exist in the Milky Way Galaxy and the Local Group, but they are particularly abundant in starburst and interacting galaxies. The few young massive clusters that are close enough to resolve are of prime interest for studying the stellar mass function and the ecological interplay between stellar evolution and stellar dynamics. The distant unresolved clusters may be effectively used to study the star-cluster mass function, and they provide excellent constraints on the formation mechanisms of young cluster populations. Young massive clusters are expected to be the nurseries for many unusual objects, including a wide range of exotic stars and binaries. So far only a few such objects have been found in young massive clusters, although their older cousins, the globular clusters, are unusually rich in stellar exotica. In this review we focus on star clusters younger than $\sim100$ Myr, more than a few current crossing times old, and more massive than $\sim10^4$ \Msun, irrespective of cluster size or environment. We describe the global properties of the currently known young massive star clusters in the Local Group and beyond, and discuss the state of the art in observations and dynamical modeling of these systems. In order to make this review readable by observers, theorists, and computational astrophysicists, we also review the cross-disciplinary terminology.