Modelo Curricular del Museo de la Universidad de Costa Rica (Museo UCR)

En el marco de la Maestría Profesional en Planificación Curricular se presenta la propuesta de investigación: Modelo curricular del Museo de la Universidad de Costa Rica (Museo UCR), la cual responde a la necesidad de un marco de referencia teórico-práctico en materia curricular, que oriente las acciones educativas del Museo. Esta respuesta se brinda a partir de la identificación de fundamentos y características teórico metodológicas de modelos de museos universitarios en distintos contextos y países, de su análisis e identificación de elementos comunes, posibilidades y limitaciones de aplicación en el Museo UCR; así como la consulta a expertos y personal vinculado. Con lo anterior se determina y diseña el referente curricular adecuado para el Museo UCR, en razón de sus características, funciones, actividades, públicos meta, normativa y políticas a las cuales responde.Summary: Within the framework of the Professional Master's Degree in Curriculum Planning, the research proposal is presented: "Curriculum model for the Museum of the University of Costa Rica (museum + UCR)", which responds to the need for a theoretical-practical reference framework in curricular matters, that guides the education actions of the Museum. This answer is provided from the identification of foundations and theoretical-methodological characteristics of models of university museums in different contexts and countries, from their analysis and identification of common elements, possibilities and limitations of application in the museum + UCR; as well as consulting experts and related personnel. With the above, the appropriate curricular reference for the museum+UCR is determined and designed, due to its characteristics, functions, activities, target audiences, regulations and policies to which it responds.UCR::Vicerrectoría de Investigación::Sistema de Estudios de Posgrado::Ciencias Sociales::Maestría Profesional en Planificación Curricula

Gravitational Waves from Hyper-Accretion onto Nascent Black Holes

We examine the possibility that hyper-accretion onto newly born, black holes occurs in highly intermittent, non-asymmetric fashion favorable to gravitational wave emission in a neutrino cooled disk. This picture of near-hole accretion is motivated by magneto-rotationally induced, ultra-relativistic disk dynamics in the region of the flow bounded from below by the marginally bound geodesic radius. For high spin values, a largely coherent magnetic field in this region has the dynamical implication of compact mass segregation at the displacement nodes of the non-axisymmetric, MRI modes. When neutrino stress competes favorably for the disk dynamical structure, the matter clumps may be rather dense and sufficiently long-lived to excite the Quasi-Normal Ringing (a.k.a. QNR) modes of the Kerr geometry upon their in-fall. We find that such accretion flow may drive bar-like, quadrupole (l,m=2,2) modes in nearly resonant fashion for spin parameters $a \geq .9$. The ensuing build up in strain amplitude of the undamped oscillations warrants a brisk rate of energy deposition into gravitational waves. A detectability assessment for the LIGO interferometers through the match filtering technique is given by integrating the energy flux over a one second epoch of resonant hyper-accretion at 1 \msun \sec^{-1}. Thus, a 15 \msun Kerr black hole spinning at $a \simeq .98$ ($f_{\rm QNR} \simeq 1677$ Hz), and located at 27 Mpc (e.g., GRB980425), will deliver a characteristic strain amplitude, $h_{\rm char} \simeq 2.2_{-21}$, large enough to be detectable by LIGO II. If resonant hyper-accretion were sustainable for a longer period (or at higher rates) possibly associated with a second broad hump in a GRB light-curve, these objects could be detected by LIGO I at very low redshifts.Comment: 12 pages, 4 figures, M.N.R.A.S. submissio

Phase dependent view of Cyclotron lines from model accretion mounds on Neutron Stars

In this paper we make a phase dependent study of the effect of the distortion of local magnetic field due to confinement of accreted matter in X-ray pulsars on the cyclotron spectra emitted from the hotspot . We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined at the polar cap of neutron stars. From our solution we model the cyclotron spectra that will be emitted from the region, using a simple prescription and integrating over the entire mound. Radiative transfer through the accretion column overlying the mound may significantly modify the spectra in comparison to those presented here. However we ignore this in the present paper in order to expose the effects directly attributable to the mound itself. We perform a spin phase dependent analysis of the spectra to study the effect of the viewing geometry.Comment: 13 pages, 19 figure

Distorted cyclotron line profile in Cep X-4 as observed by NuSTAR

We present spectral analysis of NuSTAR and Swift observations of Cep X-4 during its outburst in 2014. We observed the source once during the peak of the outburst and once during the decay, finding good agreement in the spectral shape between the observations. We describe the continuum using a powerlaw with a Fermi-Dirac cutoff at high energies. Cep X-4 has a very strong cyclotron resonant scattering feature (CRSF) around 30 keV. A simple absorption-like line with a Gaussian optical depth or a pseudo-Lorentzian profile both fail to describe the shape of the CRSF accurately, leaving significant deviations at the red side of the line. We characterize this asymmetry with a second absorption feature around 19 keV. The line energy of the CRSF, which is not influenced by the addition of this feature, shows a small but significant positive luminosity dependence. With luminosities between (1-6)e36 erg/s, Cep X-4 is below the theoretical limit where such a correlation is expected. This behavior is similar to Vela X-1 and we discuss parallels between the two systems.Comment: 6 pages, 4 figure, accepted for publication in ApJ letter

Study of the accreting pulsar 4U 0115+634 with a bulk and thermal Comptonization model

Highly magnetized pulsars accreting matter in a binary system are bright sources in the X-ray band (0.1-100 keV). Despite the early comprehension of the basic emission mechanism, their spectral energy distribution is generally described by phenomenological or simplified models. We propose a study of the spectral emission from the high mass X-ray binary pulsar 4U 0115+634 by means of thermal and bulk Comptonization models based on the physical properties of such objects. For this purpose, we analyze the BeppoSAX data in the energy range 0.7-100 keV of the 1999 giant outburst, 12 days after the maximum. We model the spectral energy distribution of the system using a two-component continuum. At higher energy, above ~7 keV, the emission is due to thermal and bulk Comptonization of the seed photons produced by cyclotron cooling of the accretion column, and at lower energy, the emission is due to thermal Comptonization of a blackbody source in a diffuse halo close to the stellar surface. From the best fit parameters, we argue that the cyclotron emission is produced ~1.7 km above the stellar surface, and escapes from the column near its base, where the absorption features are generated by the interaction with the magnetic field in a surrounding halo. We find that in 4U 0115+634, the observed spectrum is dominated by reprocessed cyclotron radiation, whereas in other bright sources with stronger magnetic fields such as Her X-1, the spectrum is dominated by reprocessed bremsstrahlung.Comment: 12 pages, accepted for publication on Astronomy and Astrophysic

Confirmation of Two Cyclotron Lines in Vela X-1

We present pulse phase-resolved X-ray spectra of the high mass X-ray binary Vela X-1 using the Rossi X-ray Timing Explorer. We observed Vela X-1 in 1998 and 2000 with a total observation time of ~90 ksec. We find an absorption feature at 23.3 +1.3 -0.6 kev in the main pulse, that we interpret as the fundamental cyclotron resonant scattering feature (CRSF). The feature is deepest in the rise of the main pulse where it has a width of 7.6 +4.4 -2.2 kev and an optical depth of 0.33 +0.06 -0.13. This CRSF is also clearly detected in the secondary pulse, but it is far less significant or undetected during the pulse minima. We conclude that the well known CRSF at 50.9 +0.6 -0.7 kev, which is clearly visible even in phase-averaged spectra, is the first harmonic and not the fundamental. Thus we infer a magnetic field strength of B=2.6 x 10^12 G.Comment: 12 pages, LaTeX, 15 Figures, accepted by A&

A model for cyclotron resonance scattering features

(abbreviated version of the abstract) We study the physics of cyclotron line formation in the high-energy spectra of accreting X-ray pulsars using Monte Carlo methods, assuming that the line-forming region is a low-density electron plasma in a sub-critical magnetic field. We investigate the dependence of the shape of the fundamental line on angle, geometry, optical depth and temperature. We also discuss variations of the line ratios for non-uniform magnetic fields. These numerical predictions for the line profiles are linked to results from observational data analysis using an XSPEC model based on the Monte Carlo simulations. We apply this model to observational data from RXTE and INTEGRAL. The predicted strong emission wings of the fundamental cyclotron feature are not found in observational data, hinting at a bottom illuminated slab geometry for line formation.Comment: 16 pages, 15 figures, Astron. Astrophys. (in press

Cyclotron harmonics in opacities of isolated neutron star atmospheres

Some of X-ray dim isolated neutron stars (XDINS) and central compact objects in supernova remnants (CCO) show absorption features in their thermal soft X-ray spectra. It has been hypothesized that these features could be due to the periodic peaks in free-free absorption opacities, caused by either Landau quantization of electron motion in magnetic fields B<10^{11} G or analogous quantization of ion motion in magnetic fields B>10^{13} G. Here, I review the physics behind cyclotron quantum harmonics in free-free photoabsorption, discuss different approximations for their calculation, and explain why the ion cyclotron harmonics (beyond the fundamental) cannot be observed.Comment: 12 pages, 5 figures. In v.5, a typo (missed sign factor) in Eq.(9) is fixe

Statistical equilibrium and ion cyclotron absorption/emission in strongly magnetized plasmas

We calculate the transition rates between proton Landau levels due to non-radiative and radiative Coulomb collisions in an electron-proton plasma with strong magnetic field B. Both electron-proton collisions and proton-proton collisions are considered. The roles of the first-order cyclotron absorption and second-order free-free absorption and scattering in determining the line strength and shape as well as the continuum are analysed in detail. We solve the statistical balance equation for the populations of proton Landau levels. For temperatures \sim 10^6-10^7 K, the deviations of the proton populations from LTE are appreciable at density \rho < 0.1 B_{14}^{3.5} g cm^{-3}, where B_{14}=B/(10^{14} G). We present general formulae for the plasma emissivity and absorption coefficents under a wide range of physical conditions. Our results are useful for studying the possibility and the conditions of proton/ion cyclotron line formation in the near vicinity of highly magnetized neutron stars.Comment: 17 pages, 8 figures, MNRAS, accepte

Spectral Formation in Accreting X-Ray Pulsars: Bimodal Variation of the Cyclotron Energy with Luminosity

Accretion-powered X-ray pulsars exhibit significant variability of the Cyclotron Resonance Scattering Feature (CRSF) centroid energy on pulse-to-pulse timescales, and also on much longer timescales. Two types of spectral variability are observed. For sources in group 1, the CRSF energy is negatively correlated with the variable source luminosity, and for sources in group 2, the opposite behavior is observed. The physical basis for this bimodal behavior is currently not understood. We explore the hypothesis that the accretion dynamics in the group 1 sources is dominated by radiation pressure near the stellar surface, and that Coulomb interactions decelerate the gas to rest in the group 2 sources. We derive a new expression for the critical luminosity such that radiation pressure decelerates the matter to rest in the supercritical sources. The formula for the critical luminosity is evaluated for 5 sources, using the maximum value of the CRSF centroid energy to estimate the surface magnetic field strength. The results confirm that the group 1 sources are supercritical and the group 2 sources are subcritical, although the situation is less clear for those highly variable sources that cross over the critical line. We also explain the variation of the CRSF energy with luminosity as a consequence of the variation of the characteristic emission height. The sign of the height variation is opposite in the supercritical and subcritical cases, hence creating the observed bimodal behavior.Comment: Accepted for publication in Astronomy & Astrophysic