Superradiant instabilities by accretion disks in scalar-tensor theories

We study the superradiant instability in scalar-tensor theories of gravitation, where matter outside a black hole provides an effective mass to the scalar degree of freedom of the gravitational sector. We discuss this effect for arbitrarily spinning black holes and for realistic models of truncated thin and thick accretion disks (where the perturbation equations are nonseparable), paying particular attention to the role of hot coronal flows in the vicinity of the black hole. The system qualitatively resembles the phenomenology of plasma-driven superradiant instabilities in General Relativity. Nevertheless, we show that the obstacles hampering the efficiency of plasma-driven superradiant instabilities in General Relativity can be circumvented in scalar-tensor theories. We find a wide range of parameter space where superradiant instabilities can be triggered in realistic scenarios, and discuss the constraints on scalar-tensor theories imposed by this effect. In particular, we argue that the existence of highly spinning accreting black holes is in tension with some scalar-tensor alternatives to the dark energy, e.g. symmetron models with screening.Comment: 13 pages, 6 figure

Novel tests of gravity using nano-Hertz stochastic gravitational-wave background signals

Gravity theories that modify General Relativity in the slow-motion regime can introduce nonperturbative corrections to the stochastic gravitational-wave background~(SGWB) from supermassive black-hole binaries in the nano-Hertz band, while remaining perturbative in the highly-relativistic regime and satisfying current post-Newtonian~(PN) constraints. We present a model-agnostic formalism to map such theories into a modified tilt for the SGWB spectrum, showing that negative PN corrections (in particular -2PN) can alleviate the tension in the recent pulsar-timing-array data if the detected SGWB is interpreted as arising from supermassive binaries. Despite being preliminary, current data have already strong constraining power, for example they set a novel (conservative) upper bound on theories with time-varying Newton's constant at least at the level of $\dot{G}/G \lesssim 10^{-5} \text{yr}^{-1}$ for redshift $z=[0.1\div1]$. We also show that NANOGrav data are best fitted by a broken power-law interpolating between a dominant -2PN or -3PN modification at low frequency, and the standard general-relativity scaling at high frequency. Nonetheless, a modified gravity explanation should be confronted with binary eccentricity, environmental effects, nonastrophysical origins of the signal, and scrutinized against statistical uncertainties. These novel tests of gravity will soon become more stringent when combining all pulsar-timing-array facilities and when collecting more data.Comment: 7 pages, 4 figure

Nonlinear photon-plasma interaction and the black hole superradiant instability

Electromagnetic field confinement due to plasma near accreting black holes can trigger superradiant instabilities at the linear level, limiting the spin of black holes and providing novel astrophysical sources of electromagnetic bursts. However, nonlinear effects might jeopardize the efficiency of the confinement, rending superradiance ineffective. Motivated by understanding nonlinear interactions in this scenario, here we study the full $3+1$ nonlinear dynamics of Maxwell equations in the presence of plasma by focusing on regimes that are seldom explored in standard plasma-physics applications, namely a generic electromagnetic wave of very large amplitude but small frequency propagating in an inhomogeneous, overdense plasma. We show that the plasma transparency effect predicted in certain specific scenarios is not the only possible outcome in the nonlinear regime: plasma blow-out due to nonlinear momentum transfer is generically present and allows for significant energy leakage of electromagnetic fields above a certain threshold. We argue that such effect is sufficient to dramatically quench the plasma-driven superradiant instability around black holes even in the most optimistic scenarios.Comment: 17 pages, 9 figures, animations of the simulations available at https://web.uniroma1.it/gmun

Saint Agatha Religious Festival in Catania: Stakeholders’ Functions and Relations

The purpose of the paper is to develop an explorative analysis of the functions and relations among actors involved in the organization and implementation of the Saint Agatha Religious Festival in Catania attracting nearly one million presences during the first week of February. The research is based on the survey of different sources of information, such as literature, news, media, and deep interviews with key informants pertaining to civil and religious institutions. The survey is designed to profile the Festival in terms of history, the character of the stakeholders, size, origin of assets, venues used, decision-making structure, and programs. The Festival’s use of volunteers and sponsors is specifically addressed. Empirical research sketches the network of stakeholders, the relationship between organizations, the importance of local social actors and strategies in enhancing local culture and sustainable tourism, regarding, in particular, the socio-cultural impacts of religious tourism. The local society has historical peculiarities which impose prudential considerations in generalizing about findings, and a comparative study with other Sicilian and/or Italian religious festivals will be important, mainly in order to delineate the actual sustainability of Festivals. The framework developed in this study can be helpful in the application of local social policies and also help comparative festival studies

Plasma-photon interaction in curved spacetime I: formalism and quasibound states around nonspinning black holes

We investigate the linear dynamics of an electromagnetic field propagating in curved spacetime in the presence of plasma. The dynamical equations are generically more involved and richer than the effective Proca equation adopted as a model in previous work. We discuss the general equations and focus on the case of a cold plasma in the background of a spherically-symmetric black hole, showing that the system admits plasma-driven, quasibound electromagnetic states that are prone to become superradiantly unstable when the black hole rotates. The quasibound states are different from those of the Proca equation and have some similarities with the case of a massive scalar field, suggesting that the linear instability can be strongly suppressed compared to previous estimates. Our framework provides the first step towards a full understanding of the plasma-photon interactions around astrophysical black holes.Comment: 11 pages, 4 figure

Superradiance: Axionic Couplings and Plasma Effects

Spinning black holes can transfer a significant fraction of their energy to ultralight bosonic fields via superradiance, condensing them in a co-rotating structure or "cloud". This mechanism turns black holes into powerful particle detectors for bosons with extremely feeble interactions. To explore its full potential, the couplings between such particles and the Maxwell field in the presence of plasma need to be understood. In this work, we study these couplings using numerical relativity. We first focus on the coupled axion-Maxwell system evolving on a black hole background. By taking into account the axionic coupling concurrently with the growth of the cloud, we observe for the first time that a new stage emerges: that of a stationary state where a constant flux of electromagnetic waves is fed by superradiance, for which we find accurate analytical estimates. Moreover, we show that the existence of electromagnetic instabilities in the presence of plasma is entirely controlled by the axionic coupling; even for dense plasmas, an instability is triggered for high enough couplings.Comment: 32 pages, 23 figure

A Distributed Software Platform for Additive Manufacturing

Additive Manufacturing (AM), a cornerstone of Industry 4.0, is expected to revolutionise production in practically all industries. However, multiple production challenges still exist, preventing its diffusion. In recent years, Machine Learning algorithms have been employed to overcome these hurdles. Nonetheless, the usage of these algorithms is constrained by the scarcity of data together with the challenges associated with accessing and integrating the information generated during the AM pipeline. In this work, we present a vendor-agnostic platform for AM that enables collecting, storing, analysing and linking the heterogeneous data of the complete AM process. We conducted an extensive analysis of the different AM datatypes and identified the most suitable technologies for storing them. Furthermore, we performed an in-depth study of the requirements of different AM stakeholders to develop a rich and intuitive Graphical User Interface. We showcased the specific usage of the platform for Powder Bed Fusion, one of the most popular AM processes, in a real industrial scenario, integrating specific existing modules for in-situ monitoring and real-time defect detection

A Comparison Analysis of BLE-Based Algorithms for Localization in Industrial Environments

Proximity beacons are small, low-power devices capable of transmitting information at a limited distance via Bluetooth low energy protocol. These beacons are typically used to broadcast small amounts of location-dependent data (e.g., advertisements) or to detect nearby objects. However, researchers have shown that beacons can also be used for indoor localization converting the received signal strength indication (RSSI) to distance information. In this work, we study the effectiveness of proximity beacons for accurately locating objects within a manufacturing plant by performing extensive experiments in a real industrial environment. To this purpose, we compare localization algorithms based either on trilateration or environment fingerprinting combined with a machine-learning based regressor (k-nearest neighbors, support-vector machines, or multi-layer perceptron). Each algorithm is analyzed in two different types of industrial environments. For each environment, various configurations are explored, where a configuration is characterized by the number of beacons per square meter and the density of fingerprint points. In addition, the fingerprinting approach is based on a preliminary site characterization; it may lead to location errors in the presence of environment variations (e.g., movements of large objects). For this reason, the robustness of fingerprinting algorithms against such variations is also assessed. Our results show that fingerprint solutions outperform trilateration, showing also a good resilience to environmental variations. Given the similar error obtained by all three fingerprint approaches, we conclude that k-NN is the preferable algorithm due to its simple deployment and low number of hyper-parameters

Clinical observations and risk factors for tinnitus in a Sicilian cohort

The aims of this study were to determine the distribution of risk factors associated with tinnitus analysing their role in the development of tinnitus and the effects of their interaction; to evidence the importance of a suitable and adequate clinical and audiologic assessment to avoid those modifiable risk factors responsible for cochlear dysfunction and tinnitus onset. 46 subjects with tinnitus and 74 controls were studied according to: age, sex, Body Mass Index (BMI), neck circumference, tobacco smoking, feeling fatigue or headache, self reporting snoring, hypertension, diabetes, coronary heart disease, and/or hyperlipidemia, and laboratory finding as lipid profile and levels of reactive oxygen metabolites (d-ROM). Audiological assessment was performed by multi-frequency audiometry (PTA0.5–16 kHz) and transient-evoked otoacoustic emissions (TEOAE diagnostic). Univariate analysis was performed to examine the association between determinants and occurrence of tinnitus; Mantel–Haenszel test (G.or) was used to investigate the joint effect of determinants on tinnitus. Tinnitus was more frequent among males with age [50 years; BMI[30 kg/m2, neck circumference[40 cm, headache, hypertension, hypercholesterolemia resulted significant risk factors for tinnitus (P.0001). Tinnitus group had more comorbidity (P.0001) and worse audiometric thresholds (60.87 Vs 21.62 % hearing loss; P.0001) with respect to control group. The interaction between hypertension–BMI C 30 kg/m2 (G.or = 8.45) and smoking–hypercholesterolemia (G.or = 5.08) increases the risk of tinnitus (P.0001). Our results underline that several factors either individually or jointly contribute to tinnitus onset; a comprehensive knowledge about tinnitus risk factors and associated clinical conditions could contribute to minimizing this disorder

Image analytics and machine learning for in-situ defects detection in Additive Manufacturing

In the context of Industry 4.0, metal Additive Manufacturing (AM) is considered a promising technology for medical, aerospace and automotive fields. However, the lack of assurance of the quality of the printed parts can be an obstacle for a larger diffusion in industry. To this date, AM is most of the times a trial-and-error process, where the faulty artefacts are detected only after the end of part production. This impacts on the processing time and overall costs of the process. A possible solution to this problem is the in-situ monitoring and detection of defects, taking advantage of the layer-by-layer nature of the build. In this paper, we describe a system for in-situ defects monitoring and detection for metal Powder Bed Fusion (PBF), that leverages an off-axis camera mounted on top of the machine. A set of fully automated algorithms based on Computer Vision and Machine Learning allow the timely detection of a number of powder bed defects and the monitoring of the object's profile for the entire duration of the build