Time-dependent modelling of PKS 2155-304 in a low state

We apply both leptonic and leptohadronic emission scenarios for modelling the multiwavelength photon spectra and the observed variability in the optical, X-ray, and TeV gamma-ray energy bands of blazar PKS 2155-304 while being in a low state between 25 August and 6 September 2008. We consider three emission models, i.e. a one-component synchrotron self-Compton model (1-SSC), a one-zone proton synchrotron model (LHs), and a two-component SSC model (2-SSC). Only in the first scenario can the emission from the optical up to TeV gamma-rays be attributed to a single particle population from one emission region. Using a time-dependent numerical code that solves the kinetic equations for each particle species, we derived, in all cases, acceptable fits to the time-averaged SED. By imposing variations to one (or more) model parameters according to observed variability pattern in one (or more) frequencies we calculated the respective light curves and compared them with the observations. We show that the 1-SSC model cannot account for the anticorrelation observed between the X-rays and VHE gamma-rays, although it can explain the time-averaged SED. The anticorrelation can be more naturally explained by the two-component emission models. Both of them reproduce satisfactorily the optical, X-ray, and TeV variability but at the cost of additional free parameters, which from four in the 2-SSC model increase to six in the LHs model. Although the results of our time-resolved analysis do not favour one of the aforementioned models, they suggest that a two-component scenario is more adequate for the emission of PKS 2155-304 in the low state of 2008, which agrees with a recent independent analysis. This suggests that the quiescent blazar radiation might result from a superposition of the radiation from different components, while a flare might still be the result of a single component.Comment: 11 pages, 10 figures, 2 tables, 1 appendix, accepted in A&A, shorter abstract than the accepted versio

Competing for Contacts: Network Competition, Trade Intermediation and Fragmented Duopoly

A two-sided, pair-wise matching model is developed to analyse the strategic interaction between two information intermediaries who compete in commission rates and network size, giving rise to a fragmented duopoly market structure. The model suggests that network competition between information intermediaries has a distinctive market structure, where intermediaries are monopolistic service providers to some contacts but duopolists over contacts they share in their network overlap. the intermediaries' inability to price discriminate between the competitive and non-competitive market segments, gives rise to an undercutting game, which has no pure strategy Nash equilibrium. The incentive to randomise commission rates yields a mixed strategy Nash equilibrium. Finally, competition is affected by the technology of network development. The analysis shows that either a monopoly or a fragmented duopoly can prevail in equilibrium, depending on the network-building technology. Under convexity assumptions, both intermediaries invest in a network and compete over common matches, while randomising commission rates. In contrast, linear network development costs can only give rise to a monopolistic outcome.International Trade, Pairwise Matching, Information Cost, Intermediation, Networks

Information Costs, Networks and Intermediation in International Trade

This paper presents a pairwise matching model with two-sided information asymmetry to analyse the impact of information costs on endogenous network building and matching by information intermediaries. The framework innovates by examining the role of information costs on incentives for trade intermediation, thereby endogenising the pattern of direct and indirect trade. Intermediation is shown to unambiguously raise expected trade volume and social welfare by expanding the set of matching technologies available to traders. Moreover, convexity in network-building costs is necessary for both direct and indirect trade to arise in equilibrium while the pattern of trade is shown to depend on the level of information costs as well as the relative effectiveness of direct and indirect matching technologies with changing information costs. The model sheds light on the relationship between information frictions and aggregate trade volume, which may be non-monotonic as a result of conflicting effects of information costs on the incentives for direct and indirect trade.International Trade, Pairwise Matching, Information Cost, Intermediation, Networks

The X-ray dust scattered rings of the black hole low mass binary V404 Cyg

We report on the first detection of X-ray dust scattered rings from the Galactic low mass X-ray binary V404 Cyg. The observation of the system with Swift/XRT on June 30 2015 revealed the presence of five concentric ring-like structures centred at the position of V404 Cyg. Follow-up Swift/XRT observations allowed a time-dependent study of the X-ray rings. Assuming that these are the result of small-angle, single X-ray scattering by dust grains along the line of sight, we find that their angular size scales as $\theta \propto\sqrt{t}$ in agreement with theoretical predictions. The dust grains are concentrated in five dust layers located at about 2.12, 2.05, 1.63, 1.50 and 1.18 kpc from the observer. These coincide roughly with locations of enhanced extinction as determined by infrared photometry. Assuming that the grain size distribution is described by a generalized Mathis-Rumpl-Nordsieck model, we find that the power-law index of the most distant cloud is $q\sim 4.4$, while $q \sim 3.5-3.7$ in all other clouds. We constrain at a $3\sigma$ level the maximum grain size of the intermediate dust layers in the range $0.16-0.20\,\mu$m and set a lower limit of $\sim 0.2\,\mu$m in the other clouds. Hints of an exponential cutoff at the angular intensity profile of the outermost X-ray ring suggest that the smallest grains have sizes $0.01 \mu{\rm m}\le \alpha_{\min} \lesssim 0.03\,\mu$m. Based on the relative ratios of dust column densities we find the highest dust concentration at $\sim 1.6$ kpc. Our results indicate a gradient in the dust properties within 1 kpc from V404 Cyg.Comment: 17 pages, 13 figures, accepted by MNRA