Spallative ablation of dielectrics by X-ray laser

Short laser pulse in wide range of wavelengths, from infrared to X-ray, disturbs electron-ion equilibrium and rises pressure in a heated layer. The case where pulse duration $\tau_L$ is shorter than acoustic relaxation time $t_s$ is considered in the paper. It is shown that this short pulse may cause thermomechanical phenomena such as spallative ablation regardless to wavelength. While the physics of electron-ion relaxation on wavelength and various electron spectra of substances: there are spectra with an energy gap in semiconductors and dielectrics opposed to gapless continuous spectra in metals. The paper describes entire sequence of thermomechanical processes from expansion, nucleation, foaming, and nanostructuring to spallation with particular attention to spallation by X-ray pulse

High shock release in ultrafast laser irradiated metals: Scenario for material ejection

We present one-dimensional numerical simulations describing the behavior of solid matter exposed to subpicosecond near infrared pulsed laser radiation. We point out to the role of strong isochoric heating as a mechanism for producing highly non-equilibrium thermodynamic states. In the case of metals, the conditions of material ejection from the surface are discussed in a hydrodynamic context, allowing correlation of the thermodynamic features with ablation mechanisms. A convenient synthetic representation of the thermodynamic processes is presented, emphasizing different competitive pathways of material ejection. Based on the study of the relaxation and cooling processes which constrain the system to follow original thermodynamic paths, we establish that the metal surface can exhibit several kinds of phase evolution which can result in phase explosion or fragmentation. An estimation of the amount of material exceeding the specific energy required for melting is reported for copper and aluminum and a theoretical value of the limit-size of the recast material after ultrashort laser irradiation is determined. Ablation by mechanical fragmentation is also analysed and compared to experimental data for aluminum subjected to high tensile pressures and ultrafast loading rates. Spallation is expected to occur at the rear surface of the aluminum foils and a comparison with simulation results can determine a spall strength value related to high strain rates

Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients

We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray transients in the soft and hard states. This model uniquely produces luminosity tracks that are proportional to T^4 for both the accretion disk and boundary layer. The model also indicates low Comptonization levels for the soft state, gaining a similarity to black holes in the relationship between Comptonization level and the strength of integrated rms variability in the power density spectrum. The boundary layer appears small, with a surface area that is roughly constant across soft and hard states. This result may suggestion that the NS radius is smaller than its inner-most stable circular orbit.Comment: 15 pages, 15 figures, accepted for publication in the Ap

Nonlinear dynamics of magnetohydrodynamic flows of heavy fluid over an arbitrary surface in shallow water approximation

The magnetohydrodynamic equations system for heavy fluid over an arbitrary surface in shallow water approximation is studied in the present paper. It is shown that simple wave solutions exist only for underlying surfaces that are slopes of constant inclination. All self-similar discontinuous and continuous solutions are found. The exact explicit solutions of initial discontinuity decay problem over a flat plane and a slope are found. It is shown that the initial discontinuity decay solution is represented by one of five possible wave configurations. For each configuration the necessary and sufficient conditions for its realization are found. The change of dependent and independent variables transforming the initial equations over a slope to those over a flat plane is found.Comment: 43 pages, submitted to Theoretical and Computational Fluid Dynamic

Torsional Magnetic Oscillations in Type I X-Ray Bursts

Thermonuclear burning on the surface of a neutron star causes the expansion of a thin outer layer of the star, $\Delta R(t)$. The layer rotates slower than the star due to angular momentum conservation. The shear between the star and the layer acts to twist the star's dipole magnetic field giving at first a trailing spiral field. The twist of the field acts in turn to `torque up' the layer increasing its specific angular momentum. As the layer cools and contracts, its excess specific angular momentum causes it to {\it rotate faster} than the star which gives a leading spiral magnetic field. The process repeats, giving rise to torsional oscillations. We derive equations for the angular velocity and magnetic field of the layer taking into account the diffusivity and viscosity which are probably due to turbulence. The magnetic field causes a nonuniformity of the star's photosphere (at the top of the heated layer), and this gives rise to the observed X-ray oscillations. The fact that the layer periodically rotates faster than the star means that the X-ray oscillation frequency may ``overshoot'' the star's rotation frequency. Comparison of the theory is made with observations of Chakrabarty et al. (2003) of an X-ray burst of SAX J1808.4-3658.Comment: 7 pages, 6 figures, accepted for publication in the Ap