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We investigate the feasibility of detecting 21 cm absorption features in the afterglow spectra of high redshift long Gamma Ray Bursts (GRBs). This is done employing simulations of cosmic reionization, together with estimates of the GRB radio afterglow flux and the instrumental characteristics of the LOw Frequency ARray (LOFAR). We find that absorption features could be marginally (with a S/N larger than a few) detected by LOFAR at z ? 7 if the GRB is a highly energetic event originating from Pop III stars, while the detection would be easier if the noise were reduced by one order of magnitude, i.e. similar to what is expected for the first phase of the Square Kilometre Array (SKA1-low). On the other hand, more standard GRBs are too dim to be detected even with ten times the sensitivity of SKA1-low, and only in the most optimistic case can a S/N larger than a few be reached at z ? 9.

We investigate the effect of primordial helium on hydrogen reionization using a hydrodynamical simulation combined with the cosmological radiative transfer code crash. The radiative transfer simulations are performed in a 35.12h -1 comoving Mpc box using a variety of assumptions for the amplitude and power-law extreme-ultraviolet (EUV) spectral index of the ionizing emissivity at z > 6. We use an empirically motivated prescription for ionizing sources which, by design, ensures all of the models are consistent with constraints on the Thomson scattering optical depth and the metagalactic hydrogen photoionization rate at z∼ 6. The inclusion of helium slightly delays reionization due to the small number of ionizing photons which reionize neutral helium instead of hydrogen. However, helium has a significant impact on the thermal state of the intergalactic medium (IGM) during hydrogen reionization. Models with a soft EUV spectral index, α= 3, produce IGM temperatures at the mean density at z∼ 6, T 0≃ 10500K, which are ∼20 percent higher compared to models in which helium photoheating is excluded. Harder EUV indices produce even larger IGM temperature boosts by the end of hydrogen reionization. A comparison of these simulations to recent observational estimates of the IGM temperature at z∼ 5-6 suggests that hydrogen reionization was primarily driven by Population II stellar sources with a soft EUV index, We also find that faint, as yet undetected galaxies, characterized by a luminosity function with a steepening faint-end slope (α LF≤-2) and an increasing Lyman continuum escape fraction (f esc∼ 0.5), are required to reproduce the ionizing emissivity used in our simulations at z > 6. Finally, we note there is some tension between recent observational constraints which indicate the IGM is >10percent neutral by volume z∼ 7, and estimates of the ionizing emissivity at z= 6 which indicate only 1-3 ionizing photons are emitted per hydrogen atom over a Hubble time at z= 6. This tension may be alleviated by either a lower neutral fraction at z∼ 7 or an IGM which still remains a few percent neutral by volume at z= 6. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

It has been shown previously that a panel comprising a cellular array can yield a normal incidence transmission loss in a specified low frequency range that is significantly larger than that of a homogeneous panel having the same mass per unit area. The cellular metamaterial considered consists of a periodic arrangement of unit plates held in a grid-like frame. However, when the incident sound field is diffuse, the relative advantage of the metamaterial barrier is reduced or eliminated. Here it will be shown through a sequence of experimental measurements that the relative advantage of the metamaterial barrier can be restored by creating a hybrid system consisting of a layer applied to the front surface of the material that causes sound to approach the barrier at normal incidence, and a layer on the rear surface of the material that compensates for the transmission loss minimum that normally follows the peak in a metamaterial barrier transmission loss. In the implementation considered here, the front layer consists of a lattice structure, and the rear layer consists of high performance glass fiber. The role of each of these components will be illustrated using measurements of transmission loss of a 1:2 m square panel system.

Topologically interlocked material systems are two-dimensional granular crystals created as ordered and adhesion-less assemblies of unit elements of the shape of platonic solids. The assembly resists transverse forces due to the interlocking geometric arrangement of the unit elements. Topologically interlocked material systems yet require an external constraint to provide resistance under the action of external load. Past work considered fixed and passive constraints only. The objective of the present study is to consider active and adaptive external constraints with the goal to achieve variable stiffness and energy absorption characteristics of the topologically interlocked material system through an active control of the in-plane constraint conditions. Experiments and corresponding model analysis are used to demonstrate control of system stiffness over a wide range, including negative stiffness, and energy absorption characteristics. The adaptive characteristics of the topologically interlocked material system are shown to solve conflicting requirements of simultaneously providing energy absorption while keeping loads controlled. Potential applications can be envisioned in smart structure enhanced response characteristics as desired in shock absorption, protective packaging and catching mechanisms.