Showing papers by "Ulrich Geppert published in 2000"

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.

Magnetic field decay in neutron stars: Analysis of general relativistic effects

Abstract: An analysis of the role of general relativistic effects on the decay of a neutron star's magnetic field is presented. At first, a generalized induction equation on an arbitrary static background geometry has been derived and, secondly, by a combination of analytical and numerical techniques, a comparison of the time scales for the decay of an initial dipole magnetic field in flat and curved spacetime is discussed. For the case of very simple neutron star models, rotation is not accounted for and, in the absence of cooling effects, we find that the inclusion of general relativistic effects result, on the average, in an enlargement of the decay time of the field in comparison to the flat spacetime case. Via numerical techniques, we show that the enlargement factor depends upon the dimensionless compactness ratio $\ensuremath{\epsilon}{=2GM/c}^{2}R,$ and for \ensuremath{\epsilon} in the range 0.3\char21{}0.5, corresponding to the compactness ratio of realistic neutron star models, this factor is between 1.2 and 1.3. The present analysis shows that general relativistic effects on magnetic field decay ought to be examined more carefully than hitherto. A brief discussion of our findings on the impact of neutron star physics is also presented.

The effect of the neutron-star crust on the evolution of a core magnetic field

Abstract: We consider the expulsion of the magnetic field from the super-conducting core of a neutron star and its subsequent decay in the crust. Particular attention is paid to a strong feedback of the distortion of magnetic field lines in the crust on the expulsion of the flux from the core. This causes a considerable delay in the core flux expulsion if the initial field strength is larger than 1011 G. It is shown that the hypothesis on the magnetic field expulsion induced by the neutron-star spin-down is adequate only for a relatively weak initial magnetic field B≈1011 G. The expulsion time-scale depends not only on the conductivity of the crust, but also on the initial magnetic field strength itself. Our model of the field evolution naturally explains the existence of the residual magnetic field of neutron stars. Its strength is correlated with the impurity concentration in neutron-star crusts and anticorrelated with the initial field strengths.

General relativistic treatment of the thermal, magnetic and rotational evolution of isolated neutron stars with crustal magnetic fields

Abstract: We investigate the thermal, magnetic and rotational evolution of isolated neutron stars assuming that the dipolar magnetic field is confined to the crust. Our treatment, for the first time, uses a fully general relativistic formalism not only for the thermal but also for the magnetic part, and includes par- tial general relativistic effects in the rotational part. Due to the fact that the combined evolution depends crucially upon the compactness of the star, three different equations of state have been employed in the calculations. In the absence of general rel- ativistic effects, while upon increasing compactness a decrease of the crust thickness takes place leading into an accelerating field decay, the inclusion of general relativistic effects intend to "decelerate this acceleration". As a consequence we find that, within the crustal field hypothesis, a given equation of state is compatible with the observed distribution of pulsar periods P and period derivative _ P provided the initial field strength and current location as well as the magnitude of the impurity content are appropriately constrained. Finally, we access the flexibility of the soft, medium and stiff classes of equations of state as candidates in describing the state of the matter in the neutron star interiors. The com- parison of our model calculations with observations, together with the consideration of independent information about neu- tron star evolution, suggests that a not too soft equation of state describes neutron star interiors and its cooling proceeds along the 'standard' scenario.

Wohlbefinden im Alter: Der Einfluss von Selbstwirksamkeit, Kontrollüberzeugungen, Bewältigungsstrategien und persönlichen Zielen. Ergebnisse aus der Münchner GOLD-Studie

Abstract: Die vorliegende Untersuchung entstammt der Munchner Genetisch Orientierten Lebens-pannenstudie zur Differentiellen Entwicklung (GOLD) und untersucht den Einfluss verschiedener Personlichkeitsmerkmale auf das subjektive Wohlbefinden im Alter. Teilnehmer waren 259 Frauen und 134 Manner im Alter von 63 bis 85 Jahren. Die Stichprobe wurde in drei Altersgruppen von 63 - 68 (n=139), 69 - 72 (n=133) und 73 - 85 Jahren (n=121) unterteilt. Das subjektive Wohlbefinden wurde (a) durch eine Skala zur allgemeinen Lebenszufriedenheit und (b) durch ein Verfahren zur emotionalen Befindlichkeit erfasst. Als Personlichkeitsvariablen wurden erhoben (a) Selbstwirksamkeit, (b) externale Kontrollwahrnehmung (c) Hoffnungslosigkeit, (d) die Bewaltigungsstrategien der assimilativen Persistenz und der akkomoda¬tiven Flexibilitat sowie (e) die volitionalen Paramater personlicher Ziele (Erfolgswahrscheinlichkeit, Entschlossenheit, Realisierbarkeit). Die Ergebnisse zeigen in Ubereinstimmung mit der generellen Befundlage der Wohlbefindensforschung, dass sowohl die Lebenszufriedenheit als auch die emotionale Befindlichkeit auch im Alter deutlich positiv ist und dass Manner ein positiveres Wohlbefinden aufweisen als Frauen. Zwischen den drei Altersgruppen gibt es keine signifikanten Unterschiede; es deutet sich sogar an, dass die Lebenszufriedenheit mit dem Alter steigt. Es gibt demnach keinen Hinweis auf eine Abnahme des Wohlbefindens, wie man es auf Grund (vermuteter) materieller, physischer und sozialer Einbusen im Alter erwarten konnte. In einer Regressionsanalyse erweisen sich Selbstwirksamkeit und (geringe) Hoffnungslosigkeit als wichtigste Personlichkeitsdeterminanten des Wohlbefindens. Personen mit hoher Selbstwirksamkeit sind insgesamt zufriedener und positiver gestimmt. Daruber hinaus wirkt sich eine akkomodative Bewaltigungsstrategie, die die eigenen Aspirationen den im Alter verringerten Ressourcen und vermehrten Hindernissen anzupassen in der Lage ist, positiv auf das Wohlbefinden aus. Die Entschlossenheit, die personlichen Ziele durchzusetzen, steht in keinem Zusammenhang mit dem Wohlbefinden aber je mehr man die personlichen Ziele fur realisierbar halt und je hoher man deren Erfolgswahrscheinlichkeit einschatzt, desto groser ist die Zufriedenheit. Gesonderte Analysen innerhalb der drei Altersgruppen zeigen weiterhin, dass trotz gleichen Niveaus des Wohlbefindens die zugrunde liegenden Determinanten sich erheblich verandern: Bei den jungeren Alten (zwischen 63 und 68 Jahren) spielt neben Selbstwirksamkeit und Hoffnungslosigkeit noch die akkomodative Bewaltigungsstrategie der flexiblen Zielanpassung die groste Rolle, wohingegen die assimilative Bewaltigungsstrategie der hartnackigen Zielverfolgung sich eher negativ auf das Wohlbefinden auswirkt. In der Gruppe der Altesten (zwischen 73 und 85 Jahren) hingegen verlieren alle Personlichkeitsvariablen an Gewicht gegenuber einem Parameter, der in den beiden jungeren Gruppen keine Rolle spielt: die Erfolgswahrscheinlichkeit der eigenen Anliegen. Angesichts sinkender Restlebenszeit gewinnt offenbar zunehmend die Frage an Bedeutung, ob man eigene personliche Ziele noch realisieren kann. Je mehr man glaubt, dass das, was man sich vorgenommen hat, nicht mehr verwirklicht werden kann, desto starker wird das Wohlbefinden beeintrachtigt.

Charting the temperature of the Hot Neutron Star in a Soft X-ray Transient

Abstract: 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. In agreement with Brown, Bildsten and Rutledge (1998) we conclude that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522, and of the recently discovered SAX J1808.4, can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. However, we show that in the case of Cen X-4, despite its long recurrence time, strong neutrino emission from the neutron star inner core is necessary to understand the observed low ratio of quiescent to outburst luminosity. This result implies that the neutron star in Cen X-4 is heavier than the one in the other systems and the pairing critical temperature in its center must be low enough (well below 10^9 K) to avoid a strong suppression of the neutrino emission.

Magneto-rotational and Thermal Evolution of Magnetars with Crustal Magnetic Fields

Abstract: Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are interpreted as young highly magnetized neutron stars (NSs). Their X-ray luminosity in quiescence, exceeding 10^{35} erg s^{-1} cannot be explained as due to cooling of a highly magnetized NS, but requires as an extra heat source the decay of its magnetic field (MF). We study numerically the coupled evolution of the MF, temperature and spin period under the assumption that the currents maintaining the field are confined in the crust of the star. The decay of the field depends on the field strength itself (Hall-drift), on the temperature and injects heat into the star, but is controlled by neutrino emission. Finally we consider the spin down from magnetic dipole braking with this decaying field to track the long term evolution. We find reasonable initial conditions for the MF strength and structure to explain their current observational values both of their rotational period, its time derivative and the X-ray luminosity of AXPs and SGRs.the X-ray luminosity of AXPs and SGRs.

General relativistic treatment of the thermal, magnetic and rotational evolution of neutron stars with crustal magnetic fields

Abstract: We investigate the thermal, magnetic and rotational evolution of isolated neutron stars assuming that the dipolar magnetic field is confined to the crust. Our treatment, for the first time, uses a fully general relativistic formalism not only for the thermal but also for the magnetic part, and includes partial general relativistic effects in the rotational part. Due to the fact that the combined evolution depends crucially upon the compactness of the star, three different equations of state have been employed in the calculations. In the absence of general relativistic effects, while upon increasing compactness a decrease of the crust thickness takes place leading into an accelerating field decay, the inclusion of general relativistic effects intend to ``decelerate this acceleration''. As a consequence we find that within the crustal field hypothesis, a given equation of state is compatible with the observed periods $P$ and period derivative $\dot P$ provided the initial field strength and current location as well as the magnitude of the impurity content are constrained appropriately. Finally, we access the flexibility of the soft, medium and stiff classes of equations of state as candidates in describing the state of the matter in the neutron star interiors. The comparison of our model calculations with observations, together with the consideration of independent information about neutron star evolution, suggests that a not too soft equation of state describes neutron star interiors and its cooling proceeds along the `standard' scenario.

Recycling NSs to ultrashort periods: a statistical analysis of their evolution in the μ – P plane

Abstract: 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 1.558 ms, the minimum detected so far. Crustal magnetic field decay models predict also the existence of massive rapidly spinning neutron stars with very low magnetic moment.