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We perform a comparative spectro-temporal analysis on the variability classes of GRS 1915+105 and IGR J17091-3624 to draw inferences regarding the underlying accretion flow mechanism. The ν , as well as C2 class Rossi X-ray Timing Explorer observation, have been considered for analysis. We investigate the intensity variation of the source in different energy domains that correspond to different components of the accretion flow and infer the relative dominance of these flow components during the dip/flare events. We correlate the dependence of the dynamic photon index (Θ) with intensities in different energy bands and comment on the transition of the source to hard/soft phases during soft dips/flares. We also report the presence of sharp QPOs at ∼ 7.1 Hz corresponding to both softer and harder domain in the case of ν variability class of GRS 1915+105 and discuss the possible accretion flow configuration it suggests. Sharp QPO around ∼ 20 mHz is observed in ν and C2 classes of IGR J17091-3624 in low and mid energy band (2.0–6.0 keV and 6.0–15.0 keV), but remains undetected in high energy (15.0–60.0 keV). The 2.5–25.0 keV background-subtracted spectra have also been fitted with TCAF along with a Compton reflection component. A plausible accretion flow mechanism in order to explain the observed variability has been proposed. [ABSTRACT FROM AUTHOR]

A two element multiband multiple-input multiple-output (MIMO) antenna system with low specific absorption rate is proposed for Ka-band space satellite, millimeter-wave (mm-wave), fifth-generation (5G), and future sixth-generation (6G) wireless communication applications. The essential characteristics of both 5G and future 6G communications can be fulfilled by the proposed MIMO antenna design. The proposed MIMO antenna system has significant advantage of covering four relevant and usable frequency bands, i.e., 16 GHz, 25.5 GHz, 28 GHz, and 32 GHz used for the above mentioned applications. The design of the presented antenna looks like a vertical Yagi-Uda antenna with L-shaped ladder. Moreover, two circular slots and a slotted vertical stub are etched in a partially shared ground. The complete electrical dimension of the presented antenna is 6.8 λ 0 × 5. 7 λ 0 × 0. 068 λ 0 , calculated at resonating frequency 25.5 GHz, where λ 0 is wavelength in free space. The geometry of the presented antenna is fabricated using Rogers RO4350TM with dielectric constant (ϵ r) of 3.48, a loss tangent of 0.0025, and the substrate thickness of 0.8 mm. The presented MIMO antenna is excited with a 3 mm wide microstrip feed line having 50 Ω characteristics matched impedance. In terms of performance, the proposed antenna has achieved a reflection coefficient < - 10 dB, and isolation < -15 dB in all the resonating bands, radiation efficiency varies 72%–84%, impedance bandwidth of 4.6 GHz, 1.5 GHz, 1.11 GHz, and 7.5 GHz, and realized gain of 10.18, 12.05, 13.07, & 5.55 dBi is also achieved at frequencies 16 GHz, 25.5 GHz, 28 GHz, and 32 GHz. The achieved values for various MIMO matrices are envelope correlation coefficient > 0.0048, diversity gain of 9.98 dB, channel capacity loss < 0.00023 bits/s/Hz, and mean effective gain < -6 dB. The simulated results obtained by ANSYS I 2022 R2 (HFSS) and the experimental results measured by vector network analyzer (VNA) show a good agreement. [ABSTRACT FROM AUTHOR]

This article investigates a six-port multiple-input multiple-output antenna which operates in mid-bands with a resonating frequency at 4.8 GHz. The overall dimensions of proposed antenna are 0.60 λ 0 × 1. 32 λ 0 × 0. 024 λ 0 , where λ 0 is free space wavelength corresponding to resonating frequency of 4.8 GHz. The proposed geometry consists of six-port electromagnetic wave components along with a plane rectangular patch, slotted partial ring ground, circular cutting ground plane, and inset feeding. For enhancing isolation, I-shaped stubs and slotted ring ground are responsible. Inset feeding, circular cutting ground plane, and partial ground plane are used for enhancing reflection coefficient. The overall −10 dB impedance bandwidth of proposed antenna is ranging from 3.92 GHz to 5.2 GHz. The achieved isolation between all the ports of proposed work is ¿15 dB. The peak values of gain and radiation efficiency provided by this prototype are 6.45 dB & and 86% respectively. The achieved diversity parameters such as envelope correlation coefficients are ¡0.002, diversity gain is 9.98, channel capacity loss is ¡0.014 bits/sec/Hz, and branch power ratio is ¡−1 dB. The proposed antenna can be considered as a suitable candidate for wireless local area network, n77 bands, 5G wireless communications applications, and C-band operations. [ABSTRACT FROM AUTHOR]

The existence of cosmic accelerators able to emit charged particles up to ZeV energies has been confirmed by the observations made in the last years by experiments such as Auger and Telescope Array. The interaction of such energetic cosmic-rays with gas or low energy photons, surrounding the astrophysical sources or present in the intergalactic medium, guarantee an ultra-high-energy neutrino related emission. When these energetic neutrinos interact in a medium produce a thermo-acoustic process where the energy of generated particle cascades can be conveyed in a pressure pulse propagating into the same medium. The kilometric attenuation length as well as the well-defined shape of the expected pulse suggest a large-area-undersea-array of acoustic sensors as an ideal observatory. For this scope, we propose to exploit the existing and no more operative offshore (oil rigs) powered platforms in the Adriatic sea as the main infrastructure to build an acoustic submarine array of dedicated hydrophones covering a surface area up to 10000 km2 and a volume up to 500 km 3. In this work we describe the advantages of this detector concept using a ray tracing technique as well as the scientific goals linked to the challenging purpose of observing for the first time ultra-high-energy cosmic neutrinos. This observatory will be complementary to the dedicated radio array detectors with the advantages of avoiding any possible thermo-acoustic noise from the atmospheric muons. [ABSTRACT FROM AUTHOR]

Variability classes in the enigmatic black hole candidate GRS 1915+105 are known to be correlated with the variation of the Comptonizing Efficiency (CE) which is defined to be the ratio between the number of power-law (hard) photons and seed (soft) photons injected into the Compton cloud. Similarities of light curves of several variability classes of GRS 1915+105 and IGR 17091-3624, some of which are already reported in the literature, motivated us to compute CE for IGR 17091-3624 as well. We find that they are similar to what were reported earlier for GRS 1915+105, even though masses of these objects could be different. The reason is that the both the sizes of the sources of the seed photons and of the Comptonizing corona scale in the same way as the mass of the black hole. This indicates that characterization of variability classes based on CE is likely to be black hole mass independent, in general. [ABSTRACT FROM AUTHOR]

Abstract: The Galactic microquasar GRS 1915+105 exhibits at least seventeen types of variability classes. Intra and inter class transitions are reported to be observed within seconds to hours. Since the observation was not continuous, these classes appeared to be exhibited in a random order. Our goal is to predict a sequence of these classes. In this paper, we compute the ratio of the photon counts obtained from the power-law component and the blackbody component of each class and call this ratio as the ‘Comptonizing efficiency’ (CE) of that class. We sequence the classes in the ascending order of CE and find that this sequence matches with a few class transitions observed by RXTE satellite and IXAE instruments on board IRS-P3. A change in CE corresponds to a change in the optical depth of the Compton cloud. Our result implies that the optical depth of the Compton cloud gradually rises as the variability class becomes harder. [Copyright &y& Elsevier]

Abstract: We present a modified outer gap model to study the phase-resolved spectra of the Crab pulsar. A theoretical double peak profile of the light curve containing the whole phase is shown to be consistent with the observed light curve of the Crab pulsar by shifting the inner boundary of the outer gap inwardly to ∼10 stellar radii above the neutron star surface. In this model, the radial distances of the photons corresponding to different phases can be determined in the numerical calculation. Also the local electrodynamics, such as the accelerating electric field, the curvature radius of the magnetic field line and the soft photon energy, are sensitive to the radial distances to the neutron star. Using a synchrotron self-Compton mechanism, the phase-resolved spectra with the energy range from 100eV to 3GeV of the Crab pulsar can also be explained. [Copyright &y& Elsevier]

Abstract: It has been recently observed by XMM-Newton that besides the emission line of iron Kα at 6.4 keV there also exists in the Seyfert 1 galaxy NGC 3783 the very intense emission line of iron Kβ at 7.0 keV. Moreover, the ratio of equivalent widths (EWs) of these two lines can hardly be interpreted with the traditional mechanism of “photoelectric absorption — fluorescence emission”. By using the mechanism of Cerenkov line-like radiation proposed by YOU Jun-hian et al., the ratio of EWKα /EWKβ may be reasonably interpreted by means of a detailed model theoretical computation. As shown by the results, the iron K lines in active galactic nuclei (AGNs) are formed not only by the fluorescence mechanism, but also by the Cerenkov line-like radiation. Both coexist in AGNs, with the latter playing the dominant role. [Copyright &y& Elsevier]

Abstract: The nature of the internal thermal X-ray emission seen in “thermal composite” supernova remnants is still uncertain. Chandra observation of the 3C391 shows a southeast–northwest elongated morphology and unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis for the small-scale features reveals normal metal abundance and uniform temperature for the interior gas. The properties of the hot gas comparatively favor the cloudlet evaporation model as a main mechanism for the “thermal composite” X-ray appearance, though radiative rim and thermal conduction may also be effective. A faint protrusion is found in Si and S lines out of the southwest radio border. [Copyright &y& Elsevier]

There is still no complete understanding of the optical radiation of ball lightning. So far no explanation has been found for the variety of its colors, the reasons for the change of color, the generation of light flashes etc. This may be due to incomplete data on the nature of the time dependence of its intensity of its radiation. The analysis of a video film shot in July 2016 near Blagoveshchensk can be of great help in finding the mechanism of ball lightning. For the first time the photo of ball lightning with a normal exposure was obtained, on which the flickering of its radiation and its structure — the core and the shell — are clearly visible. The brightness of ball lightning fluctuates in a chaotic manner, up to a complete extinction for a split second. It is shown that the core is the light emitter, and the shell, while remaining sufficiently transparent, scatters the core's light. It was found that ball lightning retains its size and shape unchanged when the brightness of the glow changes, during a bright flash, and during the final extinction. According to an eyewitness, ball lightning can stop emitting light for several seconds, and then begin to shine again. This may provide a key to explaining the observations of "black" ball lightnings and the cases of its occurrence "as if from nothing". Various options for explaining the process of light emission by ball lightning are considered: thermal radiation, ion recombination, corona discharge. It is shown that the best mechanism for the glow of ball lightning can be explained if we assume that relativistic electrons are the light emitter. • Sometimes the radiation intensity of ball lightning varies randomly. • Ball lightning may stop shining for several seconds and then flash again. • After the emission of a powerful short pulse of light, the shape and size of ball lightning were preserved. • The glow of ball lightning is best explained by synchrotron emission of electrons. • Existence of two types of ball lightning with different structure of core is possible. [ABSTRACT FROM AUTHOR]

• Three peaks exist in the evolution of the special radiation intensity with aluminum spheres hypervelocity impact aluminum thick targets. • Physical mechanism of the first radiation peak is the jetting mechanism considering the shear heating effect. • Physical mechanism of the second radiation peak is the shock-induced gasification. • Main mechanism of the third peak is the ablation of tiny fragments. • The ambient gas pressure has a significant effect on the radiation characteristics. Radiation characteristics of aluminum spheres impacting aluminum thick targets have been obtained through hypervelocity impact tests aluminum spheres. Three peaks exist in the evolution of the special radiation intensity. In this paper, the influences of impact vaporization, jetting process, vaporization product's interaction with ambient gas and fragment's aerodynamic heating are quantitatively analyzed, and the reason for the occurrences of the three peaks are Figured out together with the experimental results. The radiation mechanism of the first peak is not the direct impact vaporization of the projectile, but the jetting considering the shear heating effect; the radiation mechanism of the second peak is the impact vaporization, not the interaction between the jetting vapor and the ambient gas; and the radiation mechanism of the third peak is the ablation of tiny fragments, neither the weakening of the occlusion effect of impact products that results in an increase of the radiation area, nor the shock by the jetting vapor or the shock-induced vapor. The influence of the ambient pressure on radiation characteristics is quite significant. When the ambient pressure is high, the three radiation mechanisms are coupled together in time. Therefore, it is recommended to regard the influence of the ambient pressure when conducting research on the radiation characteristics of hypervelocity impacts, otherwise the reliability of the conclusion would be affected. [ABSTRACT FROM AUTHOR]