Luminous X-ray Flares from Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697

We report results of the first search specifically targeting short-timescale X-ray flares from low-mass X-ray binaries in an early-type galaxy. A new method for flare detection is presented. In NGC 4697, the nearest, optically luminous, X-ray faint elliptical galaxy, 3 out of 157 sources are found to display flares at >99.95% probability, and all show more than one flare. Two sources are coincident with globular clusters and show flare durations and luminosities similar to (but larger than) Type-I X-ray superbursts found in Galactic neutron star (NS) X-ray binaries (XRBs). The third source shows more extreme flares. Its flare luminosity (~6E39 erg/s) is very super-Eddington for an NS and is similar to the peak luminosities of the brightest Galactic black hole (BH) XRBs. However, the flare duration (~70 s) is much shorter than are typically seen for outbursts reaching those luminosities in Galactic BH sources. Alternative models for the flares are considered.Comment: Astrophysical Journal Letters, accepted: 4 page

Thermodynamic properties of Pb determined from pressure-dependent critical-field measurements

We have carried out extensive low-temperature (1.5 to 10 K) measurements of the critical field, $H_c$, for the element Pb up to a pressure of $P=1.2$ GPa. From this data the electronic entropy, specific heat, thermal expansion coefficient and compressibility is calculated as a function of temperature, pressure and magnetic field. The zero-field data is consistent with direct thermodynamic measurements and the $P$-dependence of $T_c$ and specific heat coefficient, $\gamma(T,P)$ allows the determination of the $P$-dependence of the pairing interaction.Comment: 5 pages, 6 figures, in press Phys. Rev.

Are stealth scalar fields stable?

Non-gravitating (stealth) scalar fields associated with Minkowski space in scalar-tensor gravity are examined. Analytical solutions for both non-minimally coupled scalar field theory and for Brans-Dicke gravity are studied and their stability with respect to tensor perturbations is assessed using a covariant and gauge-invariant formalism developed for alternative gravity. For Brans-Dicke solutions, the stability with respect to homogeneous perturbations is also studied. There are regions of parameter space corresponding to stability and other regions corresponding to instability.Comment: 10 pages, 1 table, no figures, to appear in Phys. Rev,

Extending invariant complex structures

We study the problem of extending a complex structure to a given Lie algebra g, which is firstly defined on an ideal h of g. We consider the next situations: h is either complex or it is totally real. The next question is to equip g with an additional structure, such as a (non)-definite metric or a symplectic structure and to ask either h is non-degenerate, isotropic, etc. with respect to this structure, by imposing a compatibility assumption. We show that this implies certain constraints on the algebraic structure of g. Constructive examples illustrating this situation are shown, in particular computations in dimension six are given.Comment: 22 pages, plus an Addendu

Strong covalent bonding between two graphene layers

We show that two graphene layers stacked directly on top of each other (AA stacking) form strong chemical bonds when the distance between planes is 0.156 nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial double-bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of graphene bilayer is meta-stable w.r.t. the one bound by van der Waals forces at a larger separation (0.335 nm) with an activation energy of 0.16 eV/cell. Similarly to the structure found in hexaprismane, C forms four single bonds in a geometry mixing 90^{0} and 120^{0} angles. Intermediate separations between layers can be stabilized under external anisotropic stresses showing a rich electronic structure changing from semimetal at van der Waals distance, to metal when compressed, to wide gap semiconductor at the meta-stable minimum.Comment: tar gzip latex 4 pages 4 figure

Ab-initio molecular dynamics simulation of hydrogen diffusion in α\alpha-iron

First-principles atomistic molecular dynamics simulation in the micro-canonical and canonical ensembles has been used to study the diffusion of interstitial hydrogen in $\alpha$-iron. Hydrogen to Iron ratios between $\theta=1/16 and 1/2 have been considered by locating interstitial hydrogen atoms at random positions in a$2 \times 2 \times 2$ supercell. We find that the average optimum absorption site and the barrier for diffusion depend on the concentration of interestitials. Iron Debye temperature decreases monotonically for increasing concentration of interstitial hydrogen, proving that iron-iron interatomic potential is significantly weakened in the presence of a large number of diffusing hydrogen atoms

Shuttle Mechanism for Charge Transfer in Coulomb Blockade Nanostructures

Room-temperature Coulomb blockade of charge transport through composite nanostructures containing organic inter-links has recently been observed. A pronounced charging effect in combination with the softness of the molecular links implies that charge transfer gives rise to a significant deformation of these structures. For a simple model system containing one nanoscale metallic cluster connected by molecular links to two bulk metallic electrodes we show that self-excitation of periodic cluster oscillations in conjunction with sequential processes of cluster charging and decharging appears for a sufficiently large bias voltage. This new `electron shuttle' mechanism of discrete charge transfer gives rise to a current through the nanostructure, which is proportional to the cluster vibration frequency.Comment: 4 pages, 4 figure

Training telescope operators and support astronomers at Paranal

The operations model of the Paranal Observatory relies on the work of efficient staff to carry out all the daytime and nighttime tasks. This is highly dependent on adequate training. The Paranal Science Operations department (PSO) has a training group that devises a well-defined and continuously evolving training plan for new staff, in addition to broadening and reinforcing courses for the whole department. This paper presents the training activities for and by PSO, including recent astronomical and quality control training for operators, as well as adaptive optics and interferometry training of all staff. We also present some future plans.Comment: Paper 9910-123 presented at SPIE 201