967 research outputs found
Cut-off Characterisation of Energy Spectra of Bright Fermi Sources: Current instrument limits and future possibilities
In this paper some of the brightest GeV sources observed by the Fermi-LAT
were analysed, focusing on their spectral cut-off region. The sources chosen
for this investigation were the brightest blazar flares of 3C~454.3 and 3C~279
and the Vela pulsar with a reanalysis with the latest Fermi-LAT software. For
the study of the spectral cut-off we first explored the Vela pulsar spectrum,
whose statistics in the time interval of the 3FGL catalog allowed strong
constraints to be obtained on the parameters. We subsequently performed a new
analysis of the flaring blazar SEDs. For these sources we obtained constraints
on the cut-off parameters under the assumption that their underlying spectral
distribution is described by a power-law with a stretched exponential cut-off.
We then highlighted the significant potential improvements on such constraints
by observations with next generation ground based Cherenkov telescopes,
represented in our study by the Cherenkov Telescope Array (CTA). Adopting
currently available simulations for this future observatory, we demonstrate the
considerable improvement in cut-off constraints achievable by observations with
this new instrument when compared with that achievable by satellite
observations.Comment: total number of pages 24, including 6 pages of references. Accepted
by Astroparticle Physic
Experimental characterization of the inner surface in micro-drilling of spray holes: A comparison between ultrashort pulsed laser and EDM
In this research, the inner surface characteristics of micro-drilled holes of fuel injector nozzles were analyzed by Shear Force Microscopy (SHFM). The surface texture was characterized by maximum peak-to-valley distance and periodicity whose dimensions were related to the adopted energy. 180 μm diameter holes were drilled using ultrashort pulsed laser process using pulse energies within the range of 10-50 μJ. Laser ablated surfaces in the tested energy range offer a smooth texture with a peculiar periodic structure with a variation in height between 60 and 90 nm and almost constant periodicity. The Scanning Electron Microscopy (SEM) photograph of the Laser Induced Periodic Surface Structure (LIPSS) showed the co-existence of Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL). A comparative analysis was carried out between the highest laser pulse energy in the tested range energy laser drilling which enables the shortest machining time and micro-Electrical Discharge Machining (μ-EDM). On the contrary, results showed that surfaces obtained by electro-erosion are characterized by a random distribution of craters with a total excursion up to 1.5 μm with a periodicity of 10 μm. The mean-squared surface roughness (Rq) derived from the scanned maps ranges between 220 and 560 nm for μ-EDM, and between 50 and 100 nm for fs-pulses laser drilling
Multi-response Optimization of Laser Welding of Stainless Steels in a Constrained Fillet Joint Configuration Using RSM
This paper presents experimental design approach to process parameter optimization for CW Nd/YAG laser welding of ferritic/austenitic stainless steels in a constrained fillet configuration. To determine the optimal welding parameters, response surface methodology was used to develop a set of mathematical models relating the welding parameters to each of the weld characteristics. The quality criteria considered to determine the optimal settings were the maximization of weld resistance length and shearing force, and the minimization of weld radial penetration. Laser power, welding speed, and incident angle are the factors that affect the weld bead characteristics significantly. A rapid decrease in weld shape factor and increase in shearing force with the line energy input in the range of 15-17 kJ/m depicts the establishment of a keyhole regime. A focused beam with laser power and welding speed respectively in the range of 860-875 W and 3.4-4.0 m/min and an incident angle of around 12° were identified as the optimal set of laser welding parameters to obtain stronger and better welds
Laser beam welding of dissimilar stainless steels in a fillet joint configuration
This paper investigates laser beam welding of dissimilar AISI 304L and AISI 430 stainless steels. Experimental studies were focused on effects of laser power, welding speed, defocus distance, beam incident angle, and line energy on weld bead geometry and shearing force. Metallurgical analysis was conducted on a selected weld only to show various microstructures typically formed at different zones and consequent change in microhardness. Laser power and welding speed were the most significant factors affecting weld geometry and shearing force. All the bead characteristics but radial penetration depth decreased with increased beam incident angle. The focused beam allowed selecting lower laser power and faster welding speed to obtain the same weld geometry. Weld shape factor increased rapidly due to keyhole formation for line energy input ranging from 15 kJ/m to 17 kJ/m. Fusion zone microstructures contained a variety of complex austenite-ferrite structures. Local microhardness of fusion zone was greater than that of both base metals
Fatigue fracture surface investigations with a 3D optical profiler
In this paper a set of specimens, used for the critical distance determination, are investigated with a non-contact 3D optical profiler. The fatigue fracture surfaces of both plain and V-notched specimens, under axial (mode I) and torsional (mode III) loadings are observed, investigating steel 42CrMo4+QT and aluminium alloy 7075-T6. The fatigue fracture profiles are compared to be previously obtained critical distances, both for mode I and mode III. The stage I to stage II transition was found at a smaller size than the axial critical distance, for the steel, while for the torsional load a local plateau at the nucleation was observed. The fracture surface of the axial loading was instead much irregular at the scale of the mode I critical distance, for the aluminium alloy, resembling a not concluded stage I, while again a relatively flat surface was observed for the mode III loading
Measurement of 1323 and 1487 keV resonances in 15N({\alpha}, {\gamma})19F with the recoil separator ERNA
The origin of fluorine is a widely debated issue. Nevertheless, the
^{15}N({\alpha},{\gamma})^{19}F reaction is a common feature among the various
production channels so far proposed. Its reaction rate at relevant temperatures
is determined by a number of narrow resonances together with the DC component
and the tails of the two broad resonances at E_{c.m.} = 1323 and 1487 keV.
Measurement through the direct detection of the 19F recoil ions with the
European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The
reaction was initiated by a 15N beam impinging onto a 4He windowless gas
target. The observed yield of the resonances at Ec.m. = 1323 and 1487 keV is
used to determine their widths in the {\alpha} and {\gamma} channels. We show
that a direct measurement of the cross section of the
^{15}N({\alpha},{\gamma})^{19}F reaction can be successfully obtained with the
Recoil Separator ERNA, and the widths {\Gamma}_{\gamma} and {\Gamma}_{\alpha}
of the two broad resonances have been determined. While a fair agreement is
found with earlier determination of the widths of the 1487 keV resonance, a
significant difference is found for the 1323 keV resonance {\Gamma}_{\alpha} .
The revision of the widths of the two more relevant broad resonances in the
15N({\alpha},{\gamma})19F reaction presented in this work is the first step
toward a more firm determination of the reaction rate. At present, the residual
uncertainty at the temperatures of the ^{19}F stellar nucleosynthesis is
dominated by the uncertainties affecting the Direct Capture component and the
364 keV narrow resonance, both so far investigated only through indirect
experiments.Comment: 8 pages, 11 figures. Accepted for publication in PR
Ultra-high energy Inverse Compton emission from Galactic electron accelerators
It is generally held that >100 TeV emission from astrophysical objects
unambiguously demonstrates the presence of PeV protons or nuclei, due to the
unavoidable Klein-Nishina suppression of inverse Compton emission from
electrons. However, in the presence of inverse Compton dominated cooling, hard
high-energy electron spectra are possible. We show that the environmental
requirements for such spectra can naturally be met in spiral arms, and in
particular in regions of enhanced star formation activity, the natural
locations for the most promising electron accelerators: powerful young pulsars.
Our scenario suggests a population of hard ultra-high energy sources is likely
to be revealed in future searches, and may also provide a natural explanation
for the 100 TeV sources recently reported by HAWC.Comment: Accepted for publication in ApJ
Inferences on Coronal Magnetic Fields from SOHO UVCS Observations
The characteristics of the magnetic field ubiquitously permeating the coronal plasma are still largely unknown. In this paper we analyze some aspects of coronal physics, related to the magnetic field behavior, which forthcoming SOHO UVCS observations can help better understand. To this end, three coronal structures will be examined: streamers, coronal mass ejections (CME's) and coronal holes. As to streamers and CME's, we show, via simulations of the Ly-alpha and white light emission from these objects, calculated on the basis of recent theoretical models, how new data from SOHO can help advancing our knowledge of the streamer/CME magnetic configuration. Our discussion highlights also those observational signatures which might offer clues on reconnection processes in streamers' current sheets. Coronal holes (CH's) are discussed in the last section of the paper. Little is known about CH flux tube geometry, which is closely related to the behavior of the solar wind at small heliocentric distances. Indirect evidence for the flux tube spreading factors, within a few solar radii, is here examined
LESSON LEARNED ON MONITORING CULTURAL HERITAGE AT RISK UNDER CLIMATE CHANGES: STRATEGY, TECHNIQUES AND RESULTS
Rising damp is a recurrent cause of damage, and the climatic changes are going towards the increase of humidity (quantity and spreading distribution) in the historic masonry: at 40/50° latitudes, at continental/Mediterranean climatic conditions, the alternance of dry seasons and almost monsoon seasons dramatically affects the distribution of rising damp in porous materials, as well as the water content. Monitoring the presence and distribution of the water is useful to support the choice of the most appropriate intervention, reducing the risk to apply not effective and expensive products and preventing an oversize intervention.The evaluation of the increase of rising damp is a critical issue for preventing the damages, because the presence of the water can sharply, naturally decrease in the dry seasons, as well as rapidly increases one month or more after the beginning of heavy and constant rain.The study cases will show the interaction between climate changes and the inadequacy of the present plant for collecting and drain rain in archaeological areas and historic buildings and as well as the nearby infrastructures that should prevent stagnant rain close to the bottom of the masonry.</p
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