Push & Pull: autonomous deployment of mobile sensors for a complete coverage

Mobile sensor networks are important for several strategic applications devoted to monitoring critical areas. In such hostile scenarios, sensors cannot be deployed manually and are either sent from a safe location or dropped from an aircraft. Mobile devices permit a dynamic deployment reconfiguration that improves the coverage in terms of completeness and uniformity. In this paper we propose a distributed algorithm for the autonomous deployment of mobile sensors called Push&Pull. According to our proposal, movement decisions are made by each sensor on the basis of locally available information and do not require any prior knowledge of the operating conditions or any manual tuning of key parameters. We formally prove that, when a sufficient number of sensors are available, our approach guarantees a complete and uniform coverage. Furthermore, we demonstrate that the algorithm execution always terminates preventing movement oscillations. Numerous simulations show that our algorithm reaches a complete coverage within reasonable time with moderate energy consumption, even when the target area has irregular shapes. Performance comparisons between Push&Pull and one of the most acknowledged algorithms show how the former one can efficiently reach a more uniform and complete coverage under a wide range of working scenarios.Comment: Technical Report. This paper has been published on Wireless Networks, Springer. Animations and the complete code of the proposed algorithm are available for download at the address: http://www.dsi.uniroma1.it/~novella/mobile_sensors

Distance and reddening of the Local Group dwarf irregular galaxy NGC 6822

On the basis of a new photometric analysis of the Local Group dwarf irregular galaxy NCG 6822 based on observations obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope, we have obtained a new estimate of the extinction of two fields located in the southeast region of the galaxy. Because of significant differences in the distance estimates to NGC 6822 available in literature, we decided to provide an independent determination of the distance to this galaxy based on an updated and self-consistent theoretical calibration of the tip of the red giant branch brightness. As a result we newly determined the distance to NGC 6822 to be equal to ${\rm(m-M)}_0=23.54\pm 0.05$, and compared our measurement with the most recent determinations of this distance.Comment: 5 pages, 5 figures, Astronomy & Astrophysics (Research Note), in press (proof correction included

A state-of-the-art analysis of the dwarf irregular galaxy NGC 6822

We present a detailed photometric study of the dwarf irregular galaxy NGC 6822 aimed at investigating the properties of its stellar populations and, in particular, the presence of stellar radial gradients. Our goal is to analyse the stellar populations in six fields, which cover the whole bar of this dwarf galaxy. We derived the quantitative star formation history (SFH) of the six fields using the IAC method, involving IAC-pop/MinnIAC codes. The solutions we derived show an enhanced star formation rate (SFR) in Fields 1 and 3 during the past 500 Myr. The SFRs of the other fields are almost extinguished at very recent epochs and. We study the radial gradients of the SFR and consider the total mass converted into stars in two time intervals (between 0 and 0.5 Gyr ago and between 0.5 and 13.5 Gyr ago). We find that the scale lengths of the young and intermediate-to-old populations are perfectly compatible, with the exception of the young populations in Fields 1 and 3. The recent SF in these two fields is greater than in the other ones. This might be an indication that in these two fields we are sampling incipient spiral arms. Further evidence and new observations are required to prove this hypothesis. In addition, we derived the age-metallicity relations. As expected, the metallicity increases with time for all of the fields. We do not observe any radial gradient in the metallicity.Comment: 9 pages, 11 figures, Accepted to A&

INTEGRAL discovery of non-thermal hard X-ray emission from the Ophiuchus cluster

We present the results of deep observations of the Ophiuchus cluster of galaxies with INTEGRAL in the 3-80 keV band. We analyse 3 Ms of INTEGRAL data on the Ophiuchus cluster with the IBIS/ISGRI hard X-ray imager and the JEM-X X-ray monitor. In the X-ray band using JEM-X, we show that the source is extended, and that the morphology is compatible with the results found by previous missions. Above 20 keV, we show that the size of the source is slightly larger than the PSF of the instrument, and is consistent with the soft X-ray morphology found with JEM-X and ASCA. Thanks to the constraints on the temperature provided by JEM-X, we show that the spectrum of the cluster is not well fitted by a single-temperature thermal Bremsstrahlung model, and that another spectral component is needed to explain the high energy data. We detect the high energy tail with a higher detection significance (6.4 sigma) than the BeppoSAX claim (2 sigma). Because of the imaging capabilities of JEM-X and ISGRI, we are able to exclude the possibility that the excess emission comes from very hot regions or absorbed AGN, which proves that the excess emission is indeed of non-thermal origin. Using the available radio data together with the non-thermal hard X-ray flux, we estimate a magnetic field B ~ 0.1-0.2 mu G.Comment: 8 pages, 9 figures, accepted by A&

Correction of distortion for optimal image stacking in Wide Field Adaptive Optics: Application to GeMS data

The advent of Wide Field Adaptive Optics (WFAO) systems marks the beginning of a new era in high spatial resolution imaging. The newly commissioned Gemini South Multi-Conjugate Adaptive Optics System (GeMS) combined with the infrared camera Gemini South Adaptive Optics Imager (GSAOI), delivers quasi diffraction-limited images over a field of 2 arc-minutes across. However, despite this excellent performance, some variable residues still limit the quality of the analyses. In particular, distortions severely affect GSAOI and become a critical issue for high-precision astrometry and photometry. In this paper, we investigate an optimal way to correct for the distortion following an inverse problem approach. Formalism as well as applications on GeMS data are presented.Comment: 10 pages, 6 figure

Stationary motion of a self gravitating toroidal incompressible liquid layer

We consider an incompressible fluid contained in a toroidal stratum which is only subjected to Newtonian self-attraction. Under the assumption of infinitesimal tickness of the stratum we show the existence of stationary motions during which the stratum is approximatly a round torus (with radii r, R and R>>r) that rotates around its axis and at the same time rolls on itself. Therefore each particle of the stratum describes an helix-like trajectory around the circumference of radius R that connects the centers of the cross sections of the torus

The role of transformational entrepreneurship in managing a digital platform: the case of Yamamay

Purpose: Transformational entrepreneurship (TE) is a concept referring to the ability of entrepreneurs to face global challenges, such as the economic crisis, to improve the well-being of the community. Considering the current scenario of COVID-19, the way digital platforms support TE in overcoming a crisis, specifically the economic crisis caused by the pandemic, was analysed. Design/methodology/approach: To achieve the goal, the authors used the case study methodology. The interview was compared for the company analysed stands out due to its use of digital platforms as a tool to increase brand value. The authors conducted a semi-structured, open-ended interview with the entrepreneur and founder of Yamamay, a company operating in the retail sector. The results obtained from validity using the open coding method. Findings: The main findings show that the implementation of digital platforms supported the entrepreneur in formulating strategic choices that allowed the company to continue offering its services despite the store closures imposed by the pandemic. The whole concept of traditional retail has been and continues to be revised, rationalising it and integrating it with a more omnichannel logic in which digital platforms play a fundamental role. Practical implications: This paper provides market participants with useful information regarding the ability of this form of technology to support entrepreneurs in a crisis context. The results could also serve as an example for other retail companies regarding how to manage the consequences of the pandemic. Originality/value: This contribution represents an extension of the existing literature that deepens the understanding of the relationship between digital platforms and TE in a particular scenario, such as the COVID-19 pandemic. The effect of business decisions on the adoption of digital platforms to meet increasing and changing customer needs has been examined

An analysis of electron distributions in galaxy clusters by means of the flux ratio of iron lines FeXXV and XXVI

The interpretation of hard X-ray emission from galaxy clusters is still ambiguous and different models proposed can be probed using various observational methods. Here we explore a new method based on Fe line observations. Spectral line emissivities have usually been calculated for a Maxwellian electron distribution. In this paper a generalized approach to calculate the iron line flux for a modified Maxwellian distribution is considered. We have calculated the flux ratio of iron lines for the various possible populations of electrons that have been proposed to account for measurements of hard X-ray excess emission from the clusters A2199 and Coma. We found that the influence of the suprathermal electron population on the flux ratio is more prominent in low temperature clusters (as Abell 2199) than in high temperature clusters (as Coma).Comment: 6 pages, 3 figures, accepted for publication in A&

Nonthermal hard X-ray excess in the cluster Abell 2256 from two epoch observations

After confirmation of the presence of a nonthermal hard X-ray excess with respect to the thermal emission in the Coma cluster from two independent observations, obtained using the Phoswich Detection System onboard BeppoSAX, we present in this Letter also for Abell 2256 the results of two observations performed with a time interval of about 2.5 yr. In both spectra a nonthermal excess is present at a confidence level of ~3.3sigma and ~3.7sigma, respectively. The combined spectrum obtained by adding up the two spectra allows to measure an excess at the level of ~4.8sigma in the 20-80 keV energy range. The nonthermal X-ray flux is in agreement with the published value of the first observation (Fusco-Femiano et al. 2000) and with that measured by a Rossi X-Ray Timing Explorer observation (Rephaeli & Gruber 2003).Comment: 12 pages, 3 figures, 1 table - ApJL, in pres

Stretched exponential relaxation in a diffusive lattice model

We studied the single dimer dynamics in a lattice diffusive model as a function of particle density in the high densification regime. The mean square displacement is found to be subdiffusive both in one and two dimensions. The spatial dependence of the self part of the van Hove correlation function displays as function of $r$ a single peak and signals a dramatic slow down of the system for high density. The self intermediate scattering function is fitted to the Kohlrausch-Williams-Watts law. The exponent $\beta$ extracted from the fits is density independent while the relaxation time $\tau$ follows a scaling law with an exponent 2.5.Comment: 5 pages, 3 figures, to be published in Phys. Rev.