The Segregated Lambda-coalescent

We construct an extension of the Lambda-coalescent to a spatial continuum and analyse its behaviour. Like the Lambda-coalescent, the individuals in our model can be separated into (i) a dust component and (ii) large blocks of coalesced individuals. We identify a five phase system, where our phases are defined according to changes in the qualitative behaviour of the dust and large blocks. We completely classify the phase behaviour, including necessary and sufficient conditions for the model to come down from infinity. We believe that two of our phases are new to Lambda-coalescent theory and directly reflect the incorporation of space into our model. Firstly, our semicritical phase sees a null but non-empty set of dust. In this phase the dust becomes a random fractal, of a type which is closely related to iterated function systems. Secondly, our model has a critical phase in which the coalescent comes down from infinity gradually during a bounded, deterministic time interval.Comment: Updated to accepted article - to appear in the Annals of Probability. 36 pages, 2 figure

Big Data in Critical Infrastructures Security Monitoring: Challenges and Opportunities

Critical Infrastructures (CIs), such as smart power grids, transport systems, and financial infrastructures, are more and more vulnerable to cyber threats, due to the adoption of commodity computing facilities. Despite the use of several monitoring tools, recent attacks have proven that current defensive mechanisms for CIs are not effective enough against most advanced threats. In this paper we explore the idea of a framework leveraging multiple data sources to improve protection capabilities of CIs. Challenges and opportunities are discussed along three main research directions: i) use of distinct and heterogeneous data sources, ii) monitoring with adaptive granularity, and iii) attack modeling and runtime combination of multiple data analysis techniques.Comment: EDCC-2014, BIG4CIP-201

Multimessenger astronomy with pulsar timing and X-ray observations of massive black hole binaries

We demonstrate that very massive (>10^8\msun), cosmologically nearby (z<1) black hole binaries (MBHBs), which are primary targets for ongoing and upcoming pulsar timing arrays (PTAs), are particularly appealing multimessenger carriers. According to current models for massive black hole formation and evolution, the planned Square Kilometer Array (SKA) will collect gravitational wave signals from thousands of such massive systems, being able to individually resolve and locate in the sky several of them (maybe up to a hundred). By employing a standard model for the evolution of MBHBs in circumbinary discs, with the aid of dedicated numerical simulations, we characterize the gas-binary interplay, identifying possible electromagnetic signatures of the PTA sources. We concentrate our investigation on two particularly promising scenarios in the high energy domain, namely, the detection of X-ray periodic variability and of double broad K\alpha iron lines. Up to several hundreds of periodic X-ray sources with a flux >10^-13 erg s^-1 cm^-2 will be in the reach of upcoming X-ray observatories. Double relativistic K\alpha lines may be observable in a handful of low redshift (z<0.3) sources by proposed deep X-ray probes, such as Athena. (Abridged)Comment: 19 pages, 11 figures, submitted to MNRAS, minor revision of the reference lis

Astrophysical science metrics for next-generation gravitational-wave detectors

The second generation of gravitational-wave detectors are being built and tuned all over the world. The detection of signals from binary black holes is beginning to fulfill the promise of gravitational-wave astronomy. In this work, we examine several possible configurations for third-generation laser interferometers in existing km-scale facilities. We propose a set of astrophysically motivated metrics to evaluate detector performance. We measure the impact of detector design choices against these metrics, providing a quantitative cost-benefit analyses of the resulting scientific payoffs

Evolution of the double neutron star merging rate and the cosmological origin of gamma-ray burst sources

Evolution of the coalescence rate of double neutron stars (NS) and neutron star -- black hole (BH) binaries are computed for model galaxies with different star formation rates. Assuming gamma-ray bursts (GRB) to originate from NS+NS or NS+BH merging in distant galaxies, theoretical logN--logS distributions and tests of gamma-ray bursts (GRB) are calculated for the first time taking the computed merging rates into account. We use a flat cosmological model (Omega=1) with different values of the cosmological constant Lambda and under various assumptions about the star formation history in galaxies. The calculated source evolution predicts a 5-10 times increase of the source statistics at count rates 3-10 times lower than the exising BATSE sensitivity limit. The most important parameter in fitting the 2nd BATSE catalogue is the initial redshift of star formation, which is found to be z_*=2-5 depending on a poorly determined average spectral index of GRB.Comment: 13 pages, compressed uuencoded postscript, 6 figures. This paper has been accepted for publication in the Astrophysical Journal part