XUV Opacity of Aluminum between the Cold-Solid to Warm-Plasma Transition

We present calculations of the free-free XUV opacity of warm, solid-density aluminum at photon energies between the plasma frequency at 15 eV and the L-edge at 73 eV, using both density functional theory combined with molecular dynamics and a semi-analytical model in the RPA framework with the inclusion of local field corrections. As the temperature is increased from room temperature to 10 eV, with the ion and electron temperatures equal, we calculate an increase in the opacity in the range over which the degree of ionization is constant. The effect is less pronounced if only the electron temperature is allowed to increase. The physical significance of these increases is discussed in terms of intense XUV-laser matter interactions on both femtosecond and picosecond time-scales.Comment: 4 pages, 3 figure

A Coordinated Radio Afterglow Program

We describe a ground-based effort to find and study afterglows at centimeter and millimeter wavelengths. We have observed all well-localized gamma-ray bursts in the Northern and Southern sky since BeppoSAX first started providing rapid positions in early 1997. Of the 23 GRBs for which X-ray afterglows have been detected, 10 have optical afterglows and 9 have radio afterglows. A growing number of GRBs have both X-ray and radio afterglows but lack a corresponding optical afterglow.Comment: To appear in Proc. of the 5th Huntsville Gamma-Ray Burst Symposium, 5 pages, LaTe

Optical and Radio Observations of the Afterglow from GRB990510: Evidence for a Jet

We present multi-color optical and two-frequency radio observations of the bright SAX event, GRB 990510. The well-sampled optical decay, together with the radio observations are inconsistent with simple spherical afterglow models. The achromatic optical steepening and the decay of the radio afterglow both occuring at $t \sim 1$ day are evidence for hydrodynamical evolution of the source, and can be most easily interpreted by models where the GRB ejecta are collimated in a jet. Employing a simple jet model to interpret the observations, we derive a jet opening angle of $\theta_o = 0.08$, reducing the isotropic gamma-ray emission of $2.9 \times 10^{53}$ erg by a factor $\sim 300$. If the jet interpretation is correct, we conclude that GRB observations to-date are consistent with an energy for the central source of E \lsim 10^{52} erg.Comment: 12 pages, 2 figures. Version accepted for publication in ApJ Letter

Optimization of double pulse pumping for Ni-like Sm x-ray lasers

We report a systematic study of double pulse pumping of the Ni-like Sm x-ray laser at 73 Angstrom, currently the shortest wavelength saturated x-ray laser. It is found that the Sm x-ray laser output can change by orders of magnitude when the intensity ratio of the pumping pulses and their relative delay are varied. Optimum pumping conditions are found and interpreted in terms of a simple model. (C) 1999 American Institute of Physics. [S0021-8979(99)07102-9]

Supersonic strain front driven by a dense electron-hole plasma

We study coherent strain in (001) Ge generated by an ultrafast laser-initiated high density electron-hole plasma. The resultant coherent pulse is probed by time-resolved x-ray diffraction through changes in the anomalous transmission. The acoustic pulse front is driven by ambipolar diffusion of the electron-hole plasma and propagates into the crystal at supersonic speeds. Simulations of the strain including electron-phonon coupling, modified by carrier diffusion and Auger recombination, are in good agreement with the observed dynamics.Comment: 4 pages, 6 figure

Atomistic investigation of cavitation and ablation in tantalum foils under irradiation with x-rays approaching 5 keV

The rapid irradiation and heating of matter can lead to material removal via a process known as ablation. While previous investigations have focused on ablation with optical and soft x-ray pulses, the process is not well understood for the high-energy x-rays delivered at current x-ray free electron laser facilities. In this paper, we use hybrid two-temperature model molecular dynamics simulations to determine the damage threshold and dynamics for tantalum foils under irradiation with x-rays in the range 1–5 keV. We report that damage occurs for foils with thickness 300 nm when heated to around 1.25 eV/atom. This damage results from the combined processes of melting and cavitation, finally resulting in the removal of material layers. The predictions of this study, in terms of the cavitation threshold and underlying dynamics, could guide interpretation of experiments as well as applications including development of beamline optics for free-electron lasers. We report consistency between cavitation and ablation behavior in isochoric heating experiments and spall processes in hydrodynamic compression and release experiments, confirming the primary modes of damage are mechanical in nature for the x-ray energies investigated

The γ-ray burst of 980425 and its association with the extraordinary radio emission from a most unusual supernova

Supernova SN 1998bw exploded in the same direction and at the same time as the gamma-ray burst GRB 980425. Here we report radio observations of this type Ic supernova, beginning 4 days after the gamma-ray burst. At its peak the radio source is the most luminous ever seen from a supernova, νL_ν = 4 x 10^(38) erg/s at 5 GHz. More remarkably, the traditional synchrotron interpretation of the radio emission requires that the radio source be expanding at an apparent velocity of at least twice the speed of light, indicating that this supernova was accompanied by a shock wave moving at relativistic speed. The energy U_e associated with the radio-emitting relativistic electrons must lie between 10^(49) erg < U_e < 10^(52) erg, and thus represents a significant fraction of the total kinetic energy ~10^(51) erg associated with supernova explosions. The presence of a relativistic shock in SN 1998bw suggests a physical connection with the gamma-ray burst GRB 980425. We argue that this represents a second class of gamma-ray burst, with much lower emitted energy ~10^(48) erg in gamma-rays than the two powerful ~10^(53) erg high-redshift gamma-ray bursts previously identified

Probing impulsive strain propagation with x-ray pulses

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser-excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes.Comment: 4 pages, 2 figures, LaTeX, eps. Accepted for publication in Phys. Rev. Lett. http://prl.aps.or