Ulrich Geppert
Max Planck Society
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Featured researches published by Ulrich Geppert.
Nuclear Physics | 2006
Dany Page; Ulrich Geppert; Fridolin Weber
Abstract The cooling of a compact star depends very sensitively on the state of 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. These results are tested against the body of observed cooling data.
Astronomy and Astrophysics | 2003
Janusz Gil; George I. Melikidze; Ulrich Geppert
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-
The Astrophysical Journal | 2001
Monica Colpi; Ulrich Geppert; Dany Page; Andrea Possenti
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 pycnonuclear reactions in the stellar crust, to the surface photon emission, and to the neutrino cooling. These models allow us to study the neutron stars of the soft X-ray transients. In agreement with recent work of Brown, Bildsten, & Rutledge, we conclude that the soft component of the quiescent luminosity of Aql X-1, of 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 Tc in its center must be low enough (well below 109 K) to avoid a strong suppression of the neutrino emission.
The Astrophysical Journal | 2000
Monica Colpi; Ulrich Geppert; Dany Page
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.
Astronomy and Astrophysics | 2004
Ulrich Geppert
The effect of axisymmetric dipolar fields which are either restricted to the stellar crust, “crustal fields”, or allowed to penetrate the core, “core fields”, on the temperature distribution in neutron star crusts is investigated. While core fields result in practically isothermal crusts unless the surface field exceeds 1015 G, crustal fields with surface strength above a few times 1012 G cause significant deviations from crustal isothermality if the core temperatures is ≤ 108 K. At the stellar surface, the cold equatorial region produced by the suppression of heat transport perpendicular to the field by the Larmor rotation of the electrons in the envelope, present for both core and crustal fields, is significantly extended by that classical suppression at higher densities in the case of crustal fields. Then, two small warm polar regions appear which have observational consequences: the neutron star has a small effective thermally emitting area and the X-ray pulse profiles are expected to have a distinctively different shape compared to the case of a core field. These features, when compared with X-ray data on thermal emission of young cooling neutron stars, provide a first step toward a new way to study the magnetic flux distribution within a neutron star.
Astronomy and Astrophysics | 2006
Ulrich Geppert; M. Rheinhardt
Aims. We study the stability/establishment of dipolar magnetostatic equilibrium configurations in new-born neutron stars (NSs) in dependence on the rotational velocity Ω and on the initial angle α between rotation and magnetic axis. Methods. The NS is modeled as a sphere of a highly magnetized (B ∼ 10 15 G) incompressible fluid of uniform density which rotates rigidly. For the initial dipolar background magnetic field, which defines the magnetic axis, two different configurations are assumed. We solve the 3D non-linear MHD equations by use of a spectral code. The problem in dimensionless form is completely defined by the initial field strength (for a fixed field geometry), the magnetic Prandtl number Pm, and the normalized rotation rate. The evolution of the magnetic and velocity fields is considered for initial magnetic field strengths characterized by the ratio of ohmic diffusion and initial Alfven travel times τOhm/τA,0 ≈ 1000, for Pm = 0.1, 1, 10, and the ratio of rotation period and initial Alfven travel time, P/τA,0 = 0.012, 0.12, 1.2, 12. Results. We find hints for the existence of a unique stable dipolar magnetostatic configuration for any specific α, independent of the initial field geometry. Comparing NSs possessing the same field structure at the end of their proto-NS phase, it turns out that sufficiently fast rotating NSs (P < 6 ms) with α < 45 ◦ retain their magnetar field, while the others lose almost all of their initial magnetic energy by transferring it into magnetic and kinetic energy of relatively small-scaled fields and continue their life as radio pulsars with a dipolar surface field of 10 12...13 G.
Astrophysics and Space Science | 2007
Dany Page; Ulrich Geppert; Manfred Küker
Abstract 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. We apply these models to calculate the thermal evolution of such neutron stars and show that the lowered photon luminosity naturally extends their life-time as detectable thermal X-ray sources.
Astronomy and Astrophysics | 2006
F. Haberl; Ulrich Geppert; B. Aschenbach; G. Hasinger
Context. We report on XMM-Newton observations of SN 1987 A in the Large Magellanic Cloud. Aims. The large collecting area telescopes together with the European Photon Imaging Cameras (EPIC) provide X-ray spectra with unprecedented statistical quality and make it possible to investigate the spectral evolution during the brightening observed since the discovery in X-rays. High resolution spectra from the Reflection Grating Spectrometers yield a complementary view and allow us to perform more detailed investigations of prominent emission lines. Methods. The X-ray spectra were modeled with two-temperature emission components from a hot plasma in collisional ionization equilibrium and in non-equilibrium (NEI). Results. We find a temperature for the equilibrium component of 0.24 ± 0.02 keV in January 2000 and April 2001 which increased to 0.30 ± 0.02 keV in May 2003 and also an indication for a temperature increase in the hot NEI component from ~2 keV to ~3 keV. Emission line ratios inferred from the RGS spectra suggest temperatures as low as 100 eV and an increase in the ionization state of oxygen and neon consistent with the observed temperature increases. The fast readout of the EPIC-pn instrument yields X-ray fluxes free of CCD pile-up effects which we used to normalize pile-up corrections for the published Chandra fluxes. The corrected X-ray light curve of SN 1987 A in the 0.5-2.0 keV energy band is best represented by a linear increase up to about day 4000 after the explosion and an exponential rise afterwards until the last published Chandra observation on day 6716. Modeling the light curve by emission from the inner ring which is approximated by a circular torus a central density
Physical Review D | 2000
Ulrich Geppert; Dany Page; Thomas Zannias
n\sb{\rm{H}}=1.15\times10\sp 4
Astrophysical Journal Supplement Series | 1999
Andrea Possenti; Monica Colpi; Ulrich Geppert; L. Burderi; N. D’Amico
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