Evaluation of Casimir energies through spectral functions

This is an introductory set of lectures on elliptic differential operators and boundary problems, and their associated spectral functions. The role of zeta functions and traces of heat kernels in the regularization of Casimir energies is emphasized, and the renormalization issue is discussed through simple examples.Comment: Lectures given at the IFSAP, St. Petersburg, Russia, December 2000. LateX, 31 page

Pore formation in fluctuating membranes

We study the nucleation of a single pore in a fluctuating lipid membrane, specifically taking into account the membrane fluctuations, as well as the shape fluctuations of the pore. For large enough pores, the nucleation free energy is well-described by shifts in the effective membrane surface tension and the pore line tension. Using our framework, we derive the stability criteria for the various pore formation regimes. In addition to the well-known large-tension regime from the classical nucleation theory of pores, we also find a low-tension regime in which the effective line and surface tensions can change sign from their bare values. The latter scenario takes place at sufficiently high temperatures, where the opening of a stable pore of finite size is entropically favorable.Comment: 9 pages, 3 figure

CXOU J160103.1-513353: another CCO with a carbon atmosphere?

We report on the analysis of XMM-Newton observations of the central compact object CXOU J160103.1-513353 located in the center of the non-thermally emitting supernova remnant (SNR) G330.2+1.0. The X-ray spectrum of the source is well described with either single-component carbon or two-component hydrogen atmosphere models. In the latter case, the observed spectrum is dominated by the emission from a hot component with a temperature ~3.9MK, corresponding to the emission from a hotspot occupying ~1% of the stellar surface (assuming a neutron star with mass M = 1.5M$_\odot$, radius of 12 km, and distance of ~5 kpc as determined for the SNR). The statistics of the spectra and obtained upper limits on the pulsation amplitude expected for a rotating neutron star with hot spots do not allow us to unambiguously distinguish between these two scenarios. We discuss, however, that while the non-detection of the pulsations can be explained by the unfortunate orientation in CXOU J160103.1-513353, this is not the case when the entire sample of similar objects is considered. We therefore conclude that the carbon atmosphere scenario is more plausible.Comment: accepted in A&

Orbital parameters of V 0332+53 from 2015 giant outburst data

We present the updated orbital solution for the transient Be X-ray binary V 0332+53 comple- menting historical measurements with the data from the gamma-ray burst monitor onboard Fermi obtained during the outburst in June-October 2015. We model the observed changes in the spin- frequency of the pulsar and deduce the orbital parameters of the system. We significantly improve existing constrains and show that contrary to the previous findings no change in orbital parameters is required to explain the spin evolution of the source during the outbursts in 1983, 2005 and 2015. The reconstructed intrinsic spin-up of the neutron star during the latest outburst is found to be comparable with previosly observed values and predictions of the accretion torque theory.Comment: 3 pages, 2 figures, submitted to A&

The Restructuring of Technological Capabilities through Corporate Expansion

This paper analyses the restructuring of technological capabilities following M&A-based growth in large industrial firms with a substantial technological knowledge base. In particular, we focus on the restructuring of those technological capabilities that are of a general purpose kind (namely ICT) or related to the core capabilities of a firm. We develop and test a conceptual framework grounded on a co-evolutionary view, that relates the motivations and environment for corporate expansion to the firm-specific pattern of restructuring in the composition of corporate technological capabilities. We find that distinct patterns of technological capability restructuring are associated with each combination of the motivations and environment for firm growth. In particular, inter-industry contexts reduce technological relatedness in market motivated expansions, while relatedness has also declined in more recent technology-motivated growth in general. The acquisition of ICT is common as well to both technology-motivated inter-industry deals and more recent market-motivated deals. However, we speculate that any similarities in the outcomes of these alternative motives for firm growth arise for quite different purposes.

Searching for coherent pulsations in ultraluminous X-ray sources

Luminosities of ultraluminous X-ray sources (ULXs) are uncomfortably large if compared to the Eddington limit for isotropic accretion onto stellar-mass object. Most often either supercritical accretion onto stellar mass black hole or accretion onto intermediate mass black holes is invoked the high luminosities of ULXs. However, the recent discovery of coherent pulsations from M82 ULX with NuSTAR showed that another scenario implying accretion onto a magnetized neutron star is possible for ULXs. Motivated by this discovery, we re-visited the available XMM-Newton archival observations of several bright ULXs with a targeted search for pulsations to check whether accreting neutron stars might power other ULXs as well. We have found no evidence for significant coherent pulsations in any of the sources including the M82 ULX. We provide upper limits for the amplitude of possibly undetected pulsed signal for the sources in the sample.Comment: 2 pages, 1 figure, submitted to A&

Boundary conditions in the Dirac approach to graphene devices

We study a family of local boundary conditions for the Dirac problem corresponding to the continuum limit of graphene, both for nanoribbons and nanodots. We show that, among the members of such family, MIT bag boundary conditions are the ones which are in closest agreement with available experiments. For nanotubes of arbitrary chirality satisfying these last boundary conditions, we evaluate the Casimir energy via zeta function regularization, in such a way that the limit of nanoribbons is clearly determined.Comment: 10 pages, no figure. Section on Casimir energy adde