The slow X-ray pulsar SXP 1062 and associated supernova remnant in the Wing of the Small Magellanic Cloud

SXP 1062 is an exceptional case of a young neutron star in a wind-fed high-mass X-ray binary associated with a supernova remnant. A unique combination of measured spin period, its derivative, luminosity and young age makes this source a key probe for the physics of accretion and neutron star evolution. Theoretical models proposed to explain the properties of SXP 1062 shall be tested with new data.Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and Opportunities after 80 years", J. van Leeuwen (ed.

3D numerical model of the Omega Nebula (M17): simulated thermal X-ray emission

We present 3D hydrodynamical simulations of the superbubble M17, also known as the Omega nebula, carried out with the adaptive grid code yguazu'-a, which includes radiative cooling. The superbubble is modelled considering the winds of 11 individual stars from the open cluster inside the nebula (NGC 6618), for which there are estimates of the mass loss rates and terminal velocities based on their spectral types. These stars are located inside a dense interstellar medium, and they are bounded by two dense molecular clouds. We carried out three numerical models of this scenario, considering different line of sight positions of the stars (the position in the plane of the sky is known, thus fixed). Synthetic thermal X-ray emission maps are calculated from the numerical models and compared with ROSAT observations of this astrophysical object. Our models reproduce successfully both the observed X-ray morphology and the total X-ray luminosity, without taking into account thermal conduction effects.Comment: 8 pages, 6 figures, accepted for publication in MNRA

An extensive study of dynamical friction in dwarf galaxies: the role of stars, dark matter, halo profiles and MOND

We investigate the in-spiraling timescales of globular clusters in dwarf spheroidal (dSph) and dwarf elliptical (dE) galaxies, due to dynamical friction. We address the problem of these timescales having been variously estimated in the literature as much shorter than a Hubble time. Using self-consistent two-component (dark matter and stars) models, we explore mechanisms which may yield extended dynamical friction timescales in such systems in order to explain why dwarf galaxies often show globular cluster systems. As a general rule, dark matter and stars both give a comparable contribution to the dynamical drag. By exploring various possibilities for their gravitational make-up, it is shown that these studies help constrain the parameters of the dark matter haloes in these galaxies, as well as to test alternatives to dark matter. Under the assumption of a dark haloes having a constant density core, dynamical friction timescales are naturally extended upwards of a Hubble time. Cuspy dark haloes yield timescales $\lesssim$ 4.5 Gyr, for any dark halo parameters in accordance with observations of stellar line-of-sight velocity dispersion in dwarf spheroidal galaxies. We find that under the hypothesis of MOND dynamics, due to the enhanced dynamical drag of the stars, the dynamical friction timescales would be extremely short. Taking the well-measured structural parameters of the Fornax dSph and its globular cluster system as a case study, we conclude that requiring dynamical friction timescales comparable to the Hubble time strongly favours dark haloes with a central core.Comment: 18 pages, four figures, final version, accepted in MNRA

Numerical heat conduction in hydrodynamical models of colliding hypersonic flows

Hydrodynamical models of colliding hypersonic flows are presented which explore the dependence of the resulting dynamics and the characteristics of the derived X-ray emission on numerical conduction and viscosity. For the purpose of our investigation we present models of colliding flow with plane-parallel and cylindrical divergence. Numerical conduction causes erroneous heating of gas across the contact discontinuity which has implications for the rate at which the gas cools. We find that the dynamics of the shocked gas and the resulting X-ray emission are strongly dependent on the contrast in the density and temperature either side of the contact discontinuity, these effects being strongest where the postshock gas of one flow behaves quasi-adiabatically while the postshock gas of the other flow is strongly radiative. Introducing additional numerical viscosity into the simulations has the effect of damping the growth of instabilities, which in some cases act to increase the volume of shocked gas and can re-heat gas via sub-shocks as it flows downstream. The resulting reduction in the surface area between adjacent flows, and therefore of the amount of numerical conduction, leads to a commensurate reduction in spurious X-ray emission, though the dynamics of the collision are compromised. The simulation resolution also affects the degree of numerical conduction. A finer resolution better resolves the interfaces of high density and temperature contrast and although numerical conduction still exists the volume of affected gas is considerably reduced. However, since it is not always practical to increase the resolution, it is imperative that the degree of numerical conduction is understood so that inaccurate interpretations can be avoided. This work has implications for the dynamics and emission from astrophysical phenomena which involve high Mach number shocks.Comment: 14 pages, 10 figures, accepted for publication in MNRA

3D hydrodynamic numerical models for nebulae around runaway Wolf-Rayet stars

We present 3D hydrodynamical simulations of the circumstellar bubble from a Wolf-Rayet runaway star. In the models two properties were taken into account: (a) the proper motion of the central star through the interstellar medium (ISM) and (b) the evolution of the stellar wind from the red supergiant (RSG) stage to the Wolf-Rayet (WR) stage. From the hydrodynamic results synthetic X-ray maps in the [0.3 − 1.2] keV energy range were computed. These maps show that the bubble morphology is affected by the stellar motion, producing a bow shock in the RSG stage that can explain the limb-brightened morphology observed. Additionally, these synthetic maps show filamentary and clumpy appearance produced by instabilities triggered from the interaction between the WR wind and the RSG shell. It was found that these types of collisions can explain the origin of the X-ray emission observed in the nebulae of Wolf- Rayet stars.Fil: Reyes Iturbide, J.. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; México. Tecnológico de Estudios Superiores de Tianguistenco; MéxicoFil: Velázquez, Pablo F.. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; MéxicoFil: Rosado, M.. Universidad Nacional Autónoma de México. Instituto de Astronomía; MéxicoFil: Schneiter, Ernesto Matías. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; ArgentinaFil: Ramírez Ballinas, I.. Universidad Nacional Autónoma de México. Instituto de Astronomía; Méxic