Microlensing by Compact Objects associated to Gas Clouds

We investigate gravitational microlensing of point-like lenses surrounded by diffuse gas clouds. Besides gravitational bending, one must also consider refraction and absorption phenomena. According to the cloud density, the light curves may suffer small to large deviations from Paczynski curves, up to complete eclipses. Moreover, the presence of the cloud endows this type of microlensing events with a high chromaticity and absorption lines recognizable by spectral analysis. It is possible that these objects populate the halo of our galaxy, giving a conspicuous contribution to the fraction of the baryonic dark matter. The required features for the extension and the mass of the cloud to provide appreciable signatures are also met by several astrophysical objects.Comment: 11 pages with 4 figures. Accepted by A&

A catalogue sample of low mass galaxies observed in X-rays with central candidate black holes

We present a sample of $X$-ray selected candidate black holes in 51 low mass galaxies with $z\le 0.055$ {and mass up to $10^{10}$ M$_{\odot}$} obtained by cross-correlating the NASA-SLOAN Atlas with the 3XMM catalogue. {We have also searched in the available catalogues for radio counterparts of the black hole candidates and find that 19 of the previously selected sources have also a radio counterpart.} Our results show that about $37\%$ of the galaxies of our sample host { an $X$-ray source} (associated to a radio counterpart) spatially coincident with the galaxy center, in agreement with { other recent works}. For these {\it nuclear} sources, the $X$-ray/radio fundamental plane relation allows one to estimate the mass of the (central) candidate black holes which results to be in the range $10^{4}-2\times10^{8}$ M$_{\odot}$ (with median value of $\simeq 3\times 10^7$ M$_{\odot}$ and eight candidates having mass below $10^{7}$ M$_{\odot}$). This result, while suggesting that $X$-ray emitting black holes in low-mass galaxies may have had a key role in the evolution of such systems, makes even more urgent to explain how such massive objects formed in galaxies. {Of course, dedicated follow-up observations both in the $X$-ray and radio bands, as well as in the optical, are necessary in order to confirm our resultsComment: 15 Pages, 2 Figures, 3 Table

XMM-Newton observation of a sample of four close dSph galaxies

We present the results of the analysis of deep archival \sat\ observations towards the dwarf spheroidal galaxies Draco, Leo I, Ursa Major II and Ursa Minor in the Milky Way neighbourhood. The X-ray source population is characterized and cross-correlated with available databases with the aim to infer their nature. We also investigate if intermediate-mass black holes are hosted in the center of these galaxies. In the case of Draco, we detect 96 high-energy sources, two of them being possibly local stars, while no evidence for any X-ray emitting central compact object is found. Towards the Leo I and UMa II field of view we reveal 116 and 49 X-ray sources, respectively. None of them correlates with the putative central black holes and only one is likely associated with a UMa II local source. The study of the UMi dwarf galaxy shows 54 high-energy sources and a possible association {with a source at the dSph center}. We put an upper limit to the central compact object luminosity of 4.02$\times$10$^{33}$ erg/s. Furthermore, via the correlation with a radio source near the galactic center, we get that the putative black hole should have a mass of $\left(2.76^{+32.00}_{-2.54}\right)\times10^6 M_{\odot}$ and be radiatively inefficient. This confirms a previous result obtained by using Chandra data alone.Comment: MNRAS, in press, tables available on lin

Astrometric microlensing

Astrometric microlensing will offer in the next future a new channel for investigating the nature of both lenses and sources involved in a gravitational microlensing event. The effect, corresponding to the shift of the position of the multiple image centroid with respect to the source star location, is expected to occurr on scales from micro-arcoseconds to milli-arcoseconds depending on the characteristics of the lens-source system. Here, we consider different classes of events (single/binary lens acting on a single/binary source) also accounting for additional effects including the finite source size, the blending and orbital motion. This is particularly important in the era of Gaia observations which is making possible astrometric measurements with unprecedent quality.Comment: On IJMP D, 15 pages, 6 Figure

Microlensing by gas filaments

Gas in the interstellar matter is generally organized in filamentary structures, which may be also relevant for a complementary explanation of the dark matter in the Galactic halo. We examine the possibility that such structures may act as gravitational microlenses on background sources. To this purpose, we derive the general properties of a cylindrical lens and compare the light curves produced by such microlensing events with those generated by spherically symmetric clouds. We find that the establishment of the symmetry of the lens through the sole analysis of the light curve may be problematic, while the analysis of the astrometric shift of the centroid of the image can discriminate between the two classes of clouds. On the basis of our analysis, we find that only gas filaments with a very high density could be detectable. Such clouds are unlikely to exist in a long-lived state. Therefore, microlensing cannot discriminate on the existence and the relevance of gas filaments in the Halo, which could well be present and escape detection by ordinary microlensing surveys.Comment: 11 pages, 6 figures, in press on MNRA

Gravitational waves from hyperbolic encounters

The emission of gravitational waves from a system of massive objects interacting on hyperbolic orbits is studied in the quadrupole approximation. Analytic expressions are derived for the gravitational radiation luminosity, the total energy output and the gravitational radiation amplitude. An estimation of the expected number of events towards different targets (i.e. globular clusters and the center of the Galaxy) is also given. In particular, for a dense stellar cluster at the galactic center, a rate up to one event per year is obtained.Comment: 6 pages, 2 figure

Usability, user experience and mental workload in a mobile Augmented Reality application for digital storytelling in cultural heritage

Augmented Reality (AR) has become an increasingly used technology to support and enhance the enjoyment of cultural heritage. Particularly relevant is its importance for digital storytelling: by framing a portion of a fresco or painting with a smartphone, an AR mobile application can provide contextually relevant information, also in the form of multimedia content, that can help the user to understand the story and meaning behind the images. In this type of application, human factors are of fundamental importance for the effectiveness of the narrative: a mobile AR application must avoid distracting the user’s attention from the content in order to encourage a good level of concentration and immersion. The case study presented in this paper deals with a mobile AR application developed to guide visitors in the interpretation of the frescoes inside the Basilica of Saint Catherina of Alexandria in Galatina. The aim of the study is the analysis of the relations among usability, user experience and mental workload factors in AR-based digital storytelling

Crack of a helicopter main rotor actuator attachment: failure analysis and lessons learned

A Light Utility Helicopter (LUH), in the course of a training flight, leaving the ground during thetaxi to take off, went into an uncontrolled rolling to the right; consequently the helicopter gradually laid downon the right side. The impact with the runway destroyed the rotating blades up to the hubs rotor. The accidentinvestigation focused on main rotor oscillatory plate servo actuators . These components, directly linked to thecloche movements, regulate main rotor blades plane tilt and pitch. Following the preliminary examination, onlyfront servo actuator attachment was found to be broken in two parts. In detail, the present paper deals with thefracture analysis results. The servo actuator attachment material is a 2014 Aluminum alloy extrudate, undergoneto T651 heat treatment. Fracture surfaces were examined by optical and electronic microscopy in order todetermine the main morphological features and consequently to trace the origin of failure mechanism andcauses. The accordance with the specification requirements about alloy composition was verified by quantitativeelementary analysis through inductive coupled plasma spectroscopy (ICP); furthermore, semi-quantitativeelementary analysis was locally verified by Energy dispersion spectroscopy X ray (EDS_RX). Finally, thehydrogen content of the material was evaluated by the total hydrogen analysis. Microstructural andtechnological alloy characteristics were verified as well by using metallographic microscopy and hardness testingof the material.Macroscopic fracture surfaces evidences were characterized by the lack of any significant plastic deformationsand by the presence of symmetry compared to the servo actuator axis. Microscopic fracture features of both theinvestigated surfaces were not coherent to the hypothesis of an impact of the main rotor to the soil. Furtherachieved evidences, such as grain boundary fracture propagation, the presence of corrosion products, were all inaccordance with a Stress Corrosion Cracking (SCC) progressive fracture mechanism.Finite Element Analysis (FEA) located the highest tensile stress value, when the servo actuator is in its nominalworking condition, at the same points where the corrosion products were more concentrated (i.e. in the part ofthe fracture exposed to oxidative air effect for the longest time). The good agreement between FEA andmorphological evidences allowed to determine the progressive fracture origin area, though it was not possible toindividuate the crack initiation point. In fact, in correspondence to the initiation area of both the fracturesurfaces, shining and flat morphology was found;. then there were evidence of plastic deformations, due to thedetachment of a servo actuator part.The ICP analysis and hardness testing results were in accordance with the material specification requirements.However, the hydrogen content was one order of magnitude greater than the required value and many andunexpected globular formations were observed on the fracture surface. Part of these were dendritic formations,while the others looked smooth and shining. Further, FESEM boundary grain observation gave evidences of ahigh presence of precipitates on the investigated surfaces. Hence, observed microstructural characteristics,boundary grain precipitates and globular formations allowed to hypothesize possible overheating/eutecticmelting phenomena, occurred during manufacturing processes.As widely reported in literature, the AA 2014 alloy is one of the aluminum-copper-magnesium-silicon type,employing copper aluminide (CuAl2 ) as the primary precipitation-hardening agent. The need for a maximumCu phase dispersion in solid solution requires a heat treatment range with an upper limit (507°C) that is near tothe melting of the eutectics (510°C). Moreover, since the 1960s, AA2014 has been defined as sensitive to SCC.This condition is mainly related to the presence of coarse-grained and aligned CuAl2 precipitates. Thisarrangement is due to an overheating (more than 507°C) or to a cooling process carried out too slowly.Microstructural analysis was carried out on three items: 1) a large portion of the broken actuator attachment; 2)on a servoactuator coming from the same production batch; 3) on a servo actuator coming from a differentproduction batch.The microstructure from the broken actuator attachment showed a great amount of precipitates (secondphases) lengthwise aligned to the boundary grain, pores, and also cavities and dendritic globular formations.Analysis results, morphology evidences and reference images available on scientific literature were found to bein excellent agreement and validated the embrittlement and subsequent SCC mechanism hypotesis(intergranular failure propagation).In conclusion, flight accident causes are attributable to main rotor actuator attachment failure.Failure mechanism is classifiable as SCC supported by microstructural anomalies of the material. Theinvestigation of the manufacturing process highlighted how one of the servo actuator batches was not properlyproduced due to poor control and accuracy of heat treatment temperature and/or cooling time. This led tohydrogen embrittlement and to a microstructural problem (globular formations and boundary grainprecipitates). The combination of those phenomena caused an increase of the SCC sensitivity and were thebasic progressive failure driving forces.Nevertheless, as above mentioned, alloy composition was found compliant with the material specificationrequirements and this just because none of the scheduled quality control tests is able to determine the peculiarmicrostructural anomalies reported

Wireless resonant energy link for pulse generators implanted in the chest

This study investigates the feasibility of using the MedRadio band for wirelessly transmitting power to a rechargeable medical device implanted in the chest. In particular, a wireless energy link operating at 403 MHz and based on an inductive coupling between two planar resonators is considered, and its performance is assessed through simulations and experimental tests. Results show that a maximum value of the measured power transfer efficiency of ~23% is achieved. Furthermore, the possibility of using the link for recharging a lithium-ion battery of a pulse generator implanted in the chest at a depth of 5 mm is discussed, and the compliance with safety regulations is verified. Reported results demonstrate that the proposed link can be used to charge a lithium-ion battery with a capacity up to 7.2 mAh, while inducing a peak of the specific absorption rate averaged over 1 g of tissue < 1.6 W/kg

The puzzling symbiotic X-ray system 4U1700+24

Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems consisting of a neutron star accreting material from a red giant donor star via stellar wind or Roche lobe overflow. Only a few confirmed members are currently known; 4U 1700+24 is a good candidate as it is a relatively bright X-ray object, possibly associated with the late-type star V934 Her. We analysed the archive {\it XMM}-Newton and Swift/XRT observations of 4U 1700+24 in order to have a uniform high-energy ($0.3-10$ keV) view of the source. We confirmed the existence of a red-shifted O VIII Ly-$\alpha$ transition (already observed in the 2002 {\it XMM}-Newton data) in the high-resolution spectra collected via the RGS instruments. The red-shift of the line is found in all the analysed observations and, on average, it was estimated to be $\simeq 0.009$. We also observed a modulation of the centroid energy of the line on short time scales (a few days) and discuss the observations in the framework of different scenarios. If the modulation is due to the gravitational red-shift of the neutron star, it might arise from a sudden re-organization of the emitting $X$-ray matter on the scale of a few hundreds of km. Alternatively, we are witnessing a uni-polar jet of matter (with typical velocity of $1000-4000$ km s$^{-1}$) possibly emitted by the neutron star in an almost face-on system. The second possibility seems to be required by the apparent lack of any modulation in the observed $X$-ray light curve. We also note also that the low-resolution spectra (both {\it XMM}-Newton and Swift/XRT in the $0.3-10$ keV band) show the existence of a black body radiation emitted by a region (possibly associated with the neutron star polar cap) with typical size from a few tens to hundreds of meters. The size of this spot-like region reduces as the overall luminosity of 4U 1700+24 decreases.Comment: In press on A&