GeV-TeV and X-ray flares from gamma-ray bursts

The recent detection of delayed X-ray flares during the afterglow phase of gamma-ray bursts (GRBs) suggests an inner-engine origin, at radii inside the deceleration radius characterizing the beginning of the forward shock afterglow emission. Given the observed temporal overlapping between the flares and afterglows, there must be inverse Compton (IC) emission arising from such flare photons scattered by forward shock afterglow electrons. We find that this IC emission produces GeV-TeV flares, which may be detected by GLAST and ground-based TeV telescopes. We speculate that this kind of emission may already have been detected by EGRET from a very strong burst--GRB940217. The enhanced cooling of the forward shock electrons by the X-ray flare photons may suppress the synchrotron emission of the afterglows during the flare period. The detection of GeV-TeV flares combined with low energy observations may help to constrain the poorly known magnetic field in afterglow shocks. We also consider the self-IC emission in the context of internal-shock and external-shock models for X-ray flares. The emission above GeV from internal shocks is low, while the external shock model can also produce GeV-TeV flares, but with a different temporal behavior from that caused by IC scattering of flare photons by afterglow electrons. This suggests a useful approach for distinguishing whether X-ray flares originate from late central engine activity or from external shocks.Comment: slightly shortened version, accepted for publication in ApJ Letters, 4 emulateapj pages, no figure

A Multi-Species Model for Hydrogen and Helium Absorbers in Lyman-Alpha Forest Clouds

We have performed a multi-species hydrodynamical simulation of the formation and evolution of Lyman alpha clouds in a flat CDM dominated universe with an external flux of ionizing radiation. We solve the fully coupled non-equilibrium rate equations for the following species: H, H^+, H^-, H_2, H_2^+, He, He^+, He^{++}, and e^-. The statistical properties of the distribution and evolution of both hydrogen and helium absorption lines are extracted and compared to observed data. We find excellent agreement for the following neutral hydrogen data: the distribution of column densities is fit well by a power law with exponent beta=1.55 with a possible deficiency of lines above column density 10^{15} cm^{-2}; the integrated distribution matches observed data over a broad range of column densities 10^{13} to 10^{17} cm^{-2}; a Gaussian statistical fit to the Doppler parameter distribution yields a median of 35.6 km s^{-1}; the evolution of the number of clouds with column densities larger than 10^{14} cm^{-2} follows a power law with exponent gamma=2.22. Analogous calculations are presented for HeII absorption lines and we find the ratio of Doppler parameters b_{HeII}/b_{HI} = 0.87. Our data also suggests that Ly$\alpha$ clouds may belong to two morphologically different groups: small column density clouds which tend to reside in sheets or filamental structures and are very elongated and/or flattened, and the large column density clouds which are typically found at the intersections of these caustic structures and are quasi-spherical.Comment: 14 pages, 4 postscript figure

Collapsed 2-Dimensional Polymers on a Cylinder

Single partially confined collapsed polymers are studied in two dimensions. They are described by self-avoiding random walks with nearest-neighbour attractions below the $\Theta$-point, on the surface of an infinitely long cylinder. For the simulations we employ the pruned-enriched-Rosenbluth method (PERM). The same model had previously been studied for free polymers (infinite lattice, no boundaries) and for polymers on finite lattices with periodic boundary conditions. We verify the previous estimates of bulk densities, bulk free energies, and surface tensions. We find that the free energy of a polymer with fixed length $N$ has, for $N\to \infty$, a minimum at a finite cylinder radius $R^*$ which diverges as $T\to T_\theta$. Furthermore, the surface tension vanishes roughly as $(T_\theta-T)^\alpha$ for $T\to T_\theta$ with $\alpha\approx 1.7$. The density in the interior of a globule scales as $(T_\theta-T)^\beta$ with $\beta \approx 0.32$.Comment: 4 pages, 8 figure

Electrodynamics of Amorphous Media at Low Temperatures

Amorphous solids exhibit intrinsic, local structural transitions, that give rise to the well known quantum-mechanical two-level systems at low temperatures. We explain the microscopic origin of the electric dipole moment of these two-level systems: The dipole emerges as a result of polarization fluctuations between near degenerate local configurations, which have nearly frozen in at the glass transition. An estimate of the dipole's magnitude, based on the random first order transition theory, is obtained and is found to be consistent with experiment. The interaction between the dipoles is estimated and is shown to contribute significantly to the Gr\"{u}neisen parameter anomaly in low $T$ glasses. In completely amorphous media, the dipole moments are expected to be modest in size despite their collective origin. In partially crystalline materials, however, very large dipoles may arise, possibly explaining the findings of Bauer and Kador, J. Chem. Phys. {\bf 118}, 9069 (2003).Comment: Submitted for publication; April 27, 2005 versio

8x14Gb/s ring WDM modulator array with integrated tungsten heaters and Ge monitor photodetectors

An 8x14Gb/s wavelength-division multiplexed Si ring modulator array is presented with uniform channel performance. Tungsten heaters and Ge monitor photodetectors at the ring modulator drop ports are co-integrated to track and control the modulation quality

Dipolar superfluidity in electron-hole bilayer systems

Bilayer electron-hole systems, where the electrons and holes are created via doping and confined to separate layers, undergo excitonic condensation when the distance between the layers is smaller than typical distance between particles within a layer. We argue that the excitonic condensate is a novel dipolar superfluid in which the phase of the condensate couples to the {\it gradient} of the vector potential. We predict the existence of dipolar supercurrent which can be tuned by an in-plane magnetic field and detected by independent contacts to the layers. Thus the dipolar superfluid offers an example of excitonic condensate in which the {\it composite} nature of its constituent excitons is manifest in the macroscopic superfluid state. We also discuss various properties of this superfluid including the role of vortices.Comment: 5 pages, 1 figure, minor changes and added few references; final published versio

Nebular Line Emission During the Epoch of Reionization

Nebular emission lines associated with galactic HII regions carry information about both physical properties of the ionised gas and the source of ionising photons as well as providing the opportunity of measuring accurate redshifts and thus distances once a cosmological model is assumed. While nebular line emission has been extensively studied at lower redshift there are currently only few constraints within the epoch of reionisation (EoR, $z>6$), chiefly due to the lack of sensitive near-IR spectrographs. However, this will soon change with the arrival of the Webb Telescope providing sensitive near-IR spectroscopy covering the rest-frame UV and optical emission of galaxies in the EoR. In anticipation of Webb we combine the large cosmological hydrodynamical simulation Bluetides with photoionisation modelling to predict the nebular emission line properties of galaxies at $z=8\to 13$. We find good agreement with the, albeit limited, existing direct and indirect observational constraints on equivalent widths though poorer agreement with luminosity function constraints.Comment: 17 pages, accepted to MNRAS, significant modification from v1.0 data available at https://stephenmwilkins.github.io/BluetidesEmissionLines_Public

Non-substitutional single-atom defects in the Ge_(1-x)Sn_x alloy

Ge_(1-x)Sn_x alloys have proved difficult to form at large x, contrary to what happens with other group IV semiconductor combinations. However, at low x they are typical examples of well-behaved substitutional compounds, which is desirable for harnessing the electronic properties of narrow band semiconductors. In this paper, we propose the appearance of another kind of single-site defect ($\beta-Sn$), consisting of a single Sn atom in the center of a Ge divacancy, that may account for these facts. Accordingly, we examine the electronic and structural properties of these alloys by performing extensive numerical ab-initio calculations around local defects. The results show that the environment of the $\beta$ defect relaxes towards a cubic octahedral configuration, facilitating the nucleation of metallic white tin and its segregation, as found in amorphous samples. Using the information stemming from these local defect calculations, we built a simple statistical model to investigate at which concentration these $\beta$ defects can be formed in thermal equilibrium. These results agree remarkably well with experimental findings, concerning the critical concentration above which the homogeneous alloys cannot be formed at room temperature. Our model also predicts the observed fact that at lower temperature the critical concentration increases. We also performed single site effective-field calculations of the electronic structure, which further support our hypothesis.Comment: 12 pages, 1 table, 16 figure