Showing papers on "Radio wave published in 2017"

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta

Abstract: We present Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array ALMA radio observations of GW\,170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter ($13.7$ hours post merger) and millimeter ($2.41$ days post merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 d. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies $\gtrsim 10^{48}$ erg. For fiducial SGRB parameters, our limits require an observer viewer angle of $\gtrsim 20^{\circ}$. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of $\sim 10^{49}-10^{50}$ erg that exploded in a uniform density environment with $n\sim 10^{-4}-10^{-2}$ cm$^{-3}$, viewed at an angle of $\sim 20^{\circ}-40^{\circ}$ from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of $\sim 5-10$ years that will remain detectable for decades with next-generation radio facilities, making GW\,170817 a compelling target for long-term radio monitoring.

Imaging spectroscopy of solar radio burst fine structures.

Abstract: Solar radio observations provide a unique diagnostic of the outer solar atmosphere. However, the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio wave propagation has long been a major challenge in solar physics. Here we report quantitative spatial and frequency characterization of solar radio burst fine structures observed with the Low Frequency Array, an instrument with high-time resolution that also permits imaging at scales much shorter than those corresponding to radio wave propagation in the corona. The observations demonstrate that radio wave propagation effects, and not the properties of the intrinsic emission source, dominate the observed spatial characteristics of radio burst images. These results permit more accurate estimates of source brightness temperatures, and open opportunities for quantitative study of the mechanisms that create the turbulent coronal medium through which the emitted radiation propagates.

Improved Constraints on H 0 from a Combined Analysis of Gravitational-wave and Electromagnetic Emission from GW170817

Abstract: The luminosity distance measurement of GW170817 derived from gravitational-wave analysis in Abbott et al. (2017a, hereafter A17:H0) is highly correlated with the measured inclination of the NS–NS system. To improve the precision of the distance measurement, we attempt to constrain the inclination by modeling the broadband X-ray-to-radio emission from GW170817, which is dominated by the interaction of the jet with the environment. We update our previous analysis and we consider the radio and X-ray data obtained at t 100 days of the X-ray and radio emission will lead to tighter constraints.

The interaction between electromagnetic fields at megahertz, gigahertz and terahertz frequencies with cells, tissues and organisms: risks and potential

Abstract: Since regular radio broadcasts started in the 1920s, the exposure to human-made electromagnetic fields has steadily increased. These days we are not only exposed to radio waves but also other frequencies from a variety of sources, mainly from communication and security devices. Considering that nearly all biological systems interact with electromagnetic fields, understanding the affects is essential for safety and technological progress. This paper systematically reviews the role and effects of static and pulsed radio frequencies (10 0 –10 9 Hz), millimetre waves (MMWs) or gigahertz (10 9 –10 11 Hz), and terahertz (10 11 –10 13 Hz) on various biomolecules, cells and tissues. Electromagnetic fields have been shown to affect the activity in cell membranes (sodium versus potassium ion conductivities) and non-selective channels, transmembrane potentials and even the cell cycle. Particular attention is given to millimetre and terahertz radiation due to their increasing utilization and, hence, increasing human exposure. MMWs are known to alter active transport across cell membranes, and it has been reported that terahertz radiation may interfere with DNA and cause genomic instabilities. These and other phenomena are discussed along with the discrepancies and controversies from published studies.

Electrodynamics of Pulsar Magnetospheres

Abstract: We review electrodynamics of rotating magnetized neutron stars, from the early vacuum model to recent numerical experiments with plasma-filled magnetospheres. Significant progress became possible due to the development of global particle-in-cell simulations which capture particle acceleration, emission of high-energy photons, and electron-positron pair creation. The numerical experiments show from first principles how and where electric gaps form, and promise to explain the observed pulsar activity from radio waves to gamma-rays.

The Solar Probe Plus Radio Frequency Spectrometer: Measurement requirements, analog design, and digital signal processing

Abstract: The Radio Frequency Spectrometer (RFS) is a two channel digital receiver and spectrometer, which will make remote sensing observations of radio waves and in situ measurements of electrostatic and electromagnetic fluctuations in the solar wind. A part of the FIELDS suite for Solar Probe Plus (SPP), the RFS is optimized for measurements in the inner heliosphere, where solar radio bursts are more intense and the plasma frequency is higher compared to previous measurements at distances of 1 AU or greater. The inputs to the RFS receiver are the four electric antennas mounted near the front of the SPP spacecraft, and a single axis of the SPP search coil magnetometer (SCM). Each RFS channel selects a monopole or dipole antenna input, or the SCM input, via multiplexers. The primary data products from the RFS are auto and cross spectra from the selected inputs. The spectra are calculated using a polyphase filter bank (PFB), which enables the measurement of low amplitude signals of interest in the presence of high amplitude narrowband noise generated by spacecraft systems. We discuss the science signals of interest driving the RFS measurement objectives, describe the RFS analog design and digital signal processing, and show examples of current performance.

Imaging Spectroscopy of Solar Radio Burst Fine Structures

Abstract: Solar radio observations provide a unique diagnostic of the outer solar atmosphere. However, the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio-wave propagation has long been a major challenge in solar physics. Here we report quantitative spatial and frequency characterization of solar radio burst fine structures observed with the LOw Frequency Array (LOFAR), an instrument with high time resolution that also permits imaging at scales much shorter than those corresponding to radio-wave propagation in the corona. The observations demonstrate that radio-wave propagation effects, and not the properties of the intrinsic emission source, dominate the observed spatial characteristics of radio burst images. These results permit more accurate estimates of source brightness temperatures, and open opportunities for quantitative study of the mechanisms that create the turbulent coronal medium through which the emitted radiation propagates.

How expanded ionospheres of Hot Jupiters can prevent escape of radio emission generated by the cyclotron maser instability

Abstract: We present a study of plasma conditions in the atmospheres of the Hot Jupiters HD 209458b and HD 189733b and for an HD 209458b like planet at orbit locations between 0.2 and 1 au around a Sun-like star. We discuss how these conditions influence the radio emission we expect from their magnetospheres. We find that the environmental conditions are such that the cyclotron maser instability (CMI), the process responsible for the generation of radio waves at magnetic planets in the Solar system, most likely will not operate at Hot Jupiters. Hydrodynamically expanding atmospheres possess extended ionospheres whose plasma densities within the magnetosphere are so large that the plasma frequency is much higher than the cyclotron frequency, which contradicts the condition for the production of radio emission and prevents the escape of radio waves from close-in exoplanets at distances <0.05 au from a Sun-like host star. The upper atmosphere structure of gas giants around stars similar to the Sun changes between 0.2 and 0.5 au from the hydrodynamic to a hydrostatic regime, and this results in conditions similar to Solar system planets with a region of depleted plasma between the exobase and the magnetopause, where the plasma frequency can be lower than the cyclotron frequency. In such an environment, a beam of highly energetic electrons accelerated along the field lines towards the planet can produce radio emission. However, even if the CMI could operate, the extended ionospheres of Hot Jupiters are too dense to allow the radio emission to escape from the planets.

Generation of the Jovian hectometric radiation: First lessons from Juno

Abstract: Using Juno plasma and wave and magnetic observations (JADE and Waves and MAG instruments), the generation mechanism of the Jovian hectometric radio emission is analyzed It is shown that suitable conditions for the cyclotron maser instability (CMI) are observed in the regions of the radio sources Pronounced loss cone in the electron distributions are likely the source of free energy for the instability The theory reveals that sufficient growth rates are obtained from the distribution functions that are measured by the JADE-Electron instrument The CMI would be driven by upgoing electron populations at 5–10 keV and 10–30° pitch angle, the amplified waves propagating at 82°–87° from the B field, a fraction of a percent above the gyrofrequency Typical e-folding times of 10−4 s are obtained, leading to an amplification path of ~1000 km Overall, this scenario for generation of the Jovian hectometric waves differs significantly from the case of the auroral kilometric radiation at Earth

New Evidence for a Coronal Mass Ejection-driven High Frequency Type II Burst near the Sun

Abstract: Corona, the outermost region of the Sun, is believed to be present from a heliocentric distance of ~.01 R ʘ (where R ʘ is the radius of the solar photosphere)to more than 1 AU. Being highly tenuous plasma medium, it harbours large scale structures, as multi-frequency observations reveal. One such powerful solar phenomenon is Coronal Mass Ejections (CMEs), which has radio signatures as Type II bursts. Type II solar radio bursts are considered to originate from plasma waves excited by magnetohydrodynamic(MHD) shocks and converted into radio waves at the local plasma frequency and/or its harmonics. They are the direct diagnostic of MHD shocks in the solar atmosphere.

Fast Radio Bursts with Extended Gamma-Ray Emission?

Abstract: We consider some general implications of bright gamma-ray counterparts to fast radio bursts (FRBs). We show that even if these manifest in only a fraction of FRBs, gamma-ray detections with current satellites (including Swift) can provide stringent constraints on cosmological FRB models. If the energy is drawn from the magnetic energy of a compact object such as a magnetized neutron star, the sources should be nearby and be very rare. If the intergalactic medium is responsible for the observed dispersion measure, the required gamma-ray energy is comparable to that of the early afterglow or extended emission of short gamma-ray bursts. While this can be reconciled with the rotation energy of compact objects, as expected in many merger scenarios, the prompt outflow that yields the gamma-rays is too dense for radio waves to escape. Highly relativistic winds launched in a precursor phase, and forming a wind bubble, may avoid the scattering and absorption limits and could yield FRB emission. Largely independent of source models, we show that detectable radio afterglow emission from gamma-ray bright FRBs can reasonably be anticipated. Gravitational wave searches can also be expected to provide useful tests.

Global ionosphere maps based on GNSS, satellite altimetry, radio occultation and DORIS

Abstract: Global ionosphere maps (GIMs) provided by the global navigation satellite systems (GNSS) data are essential in ionospheric research as the source of the global vertical total electron content (VTEC). However, conventional GIMs experience lower accuracy and reliability from uneven distribution of GNSS tracking stations, especially in ocean areas with few tracking stations. The orbits of ocean altimetry satellite cover vast ocean areas and can directly provide VTEC at nadir with two different wavelengths of radio waves. Radio occultation observations and the beacons of Doppler orbitography and radio positioning integrated by satellite (DORIS) are evenly distributed globally. Satellite altimetry, radio occultation and DORIS can compensate GNSS data in ocean areas, allowing a more accurate and reliable GIMs to be formed with the integration of these observations. This study builds GIMs with temporal intervals of 2 h by the integration of GNSS, satellite altimetry, radio occultation and DORIS data. We investigate the integration method for multi-source data and used the data in May 2013 to validate the effectiveness of integration. Result shows that VTEC changes by ź11.0 to ź7.0 TECU after the integration of satellite altimetry, radio occultation and DORIS data. The maximum root mean square decreases by 5.5 TECU, and the accuracy of GIMs in ocean areas improves significantly.

Detection of acoustic-gravity waves in lower ionosphere by VLF radio waves

Abstract: We present a new method to study harmonic waves in the low ionosphere (60 - 90 km) by detecting their effects on reflection of very low frequency (VLF) radio waves. Our procedure is based on amplitude analysis of reflected VLF radio waves recorded in real time, which yields an insight into the dynamics of the ionosphere at heights where VLF radio waves are being reflected. The method was applied to perturbations induced by the solar terminator motions at sunrises and sunsets. The obtained results show that typical perturbation frequencies found to exist in higher regions of the atmosphere are also present in the lower ionosphere, which indicates a global nature of the considered oscillations. In our model atmosphere, they turn out to be the acoustic and gravity waves with comparatively short and long periods, respectively.

Synchrotron Emission from Dark Matter Annihilation: Predictions for Constraints from Non-detections of Galaxy Clusters with New Radio Surveys

Abstract: Author(s): Storm, Emma; Jeltema, Tesla E; Splettstoesser, Megan; Profumo, Stefano | Abstract: The annihilation of dark matter particles is expected to yield a broad radiation spectrum via the production of Standard Model particles in astrophysical environments. In particular, electrons and positrons from dark matter annihilation produce synchrotron radiation in the presence of magnetic fields. Galaxy clusters are the most massive collapsed structures in the universe, and are known to host $\sim\mu$G-scale magnetic fields. They are therefore ideal targets to search for, or to constrain the synchrotron signal from dark matter annihilation. In this work we use the expected sensitivities of several planned surveys from the next generation of radio telescopes to predict the constraints on dark matter annihilation models which will be achieved in the case of non-detections of diffuse radio emission from galaxy clusters. Specifically, we consider the Tier 1 survey planned for the Low Frequency Array (LOFAR) at 120 MHz, the EMU survey planned for the Australian Square Kilometre Array Pathfinder (ASKAP) at 1.4 GHz, and planned surveys for APERTIF at 1.4 GHz. We find that, for massive clusters and dark matter masses $\lesssim 100$ GeV, the predicted limits on the annihilation cross section would rule out vanilla thermal relic models for even the shallow LOFAR Tier 1, ASKAP, and APERTIF surveys.

Modelling and mitigating refractive propagation effects in precision pulsar timing observations

Abstract: To obtain the most accurate pulse arrival times from radio pulsars, it is necessary to correct or mitigate the effects of the propagation of radio waves through the warm and ionised interstellar medium. We examine both the strength of propagation effects associated with large-scale electron-density variations and the methodology used to estimate infinite-frequency arrival times. Using simulations of two-dimensional phase-varying screens, we assess the strength and non-stationarity of timing perturbations associated with large-scale density variations. We identify additional contributions to arrival times that are stochastic in both radio frequency and time and therefore not amenable to correction solely using times of arrival. We attribute this to the frequency dependence of the trajectories of the propagating radio waves. We find that this limits the efficacy of low-frequency (metre-wavelength) observations. Incorporating low-frequency pulsar observations into precision timing campaigns is increasingly problematic for pulsars with larger dispersion measures.

Mode Characteristics of Vortical Radio Wave Generated by Circular Phased Array: Theoretical and Experimental Results

Abstract: This paper investigates the orbital angular momentum (OAM) mode of a vortical radio wave generated by a circular antenna array. Theoretical formulations of the radiation fields are deduced, and factors affecting the mode distribution are identified. Because the array antenna cannot ideally generate a pure mode, two indicators, namely, the fundamental mode energy ratio and the degree of intensity deviation, are defined to evaluate the quality of the radiated vortex beams. The relationship between mode purity and the array parameters, OAM mode number and elevation angle, is shown through both theoretical analysis and numerical experiments. Given a threshold of mode purity, the boundary of the pure mode region is determined, and the minimum number of elements is obtained to ensure that the main lobe is located in the pure region. An X-band experimental setup for vortical radio beams is created, and the radiation field is measured to validate the theoretical results. The theoretical analysis can benefit the array design and application of vortical radio waves.

Exposure to Radiofrequency Electromagnetic Fields From Wi-Fi in Australian Schools

Abstract: The increasing use of Wi-Fi in schools and other places has given rise to public concern that the radiofrequency (RF) electromagnetic fields from Wi-Fi have the potential to adversely affect children. The current study measured typical and peak RF levels from Wi-Fi and other sources in 23 schools in Australia. All of the RF measurements were much lower than the reference levels recommended by international guidelines for protection against established health effects. The typical and peak RF levels from Wi-Fi in locations occupied by children in the classroom were of the order of 10-4 and 10-2% of the exposure guidelines, respectively. Typical RF levels in the classroom were similar between Wi-Fi and radio but higher than other sources. In the schoolyard typical RF levels were higher for radio, TV and mobile phone base stations compared to Wi-Fi. The results of this study showed that the typical RF exposure of children from Wi-Fi at school is very low and comparable or lower to other sources in the environment.

Investigating energetic electron precipitation through combining ground‐based, and balloon observations

Abstract: A detailed comparison is undertaken of the energetic electron spectra and fluxes of two precipitation events that were observed in 18/19 January 2013. A novel but powerful technique of combining simultaneous ground-based subionospheric radio wave data and riometer absorption measurements with X-ray fluxes from a Balloon Array for Relativistic Radiation-belt Electron Losses (BARREL) balloon is used for the first time as an example of the analysis procedure. The two precipitation events are observed by all three instruments, and the relative timing is used to provide information/insight into the spatial extent and evolution of the precipitation regions. The two regions were found to be moving westward with drift periods of 5–11 h and with longitudinal dimensions of ~20° and ~70° (1.5–3.5 h of magnetic local time). The electron precipitation spectra during the events can be best represented by a peaked energy spectrum, with the peak in flux occurring at ~1–1.2 MeV. This suggests that the radiation belt loss mechanism occurring is an energy-selective process, rather than one that precipitates the ambient trapped population. The motion, size, and energy spectra of the patches are consistent with electromagnetic ion cyclotron-induced electron precipitation driven by injected 10–100 keV protons. Radio wave modeling calculations applying the balloon-based fluxes were used for the first time and successfully reproduced the ground-based subionospheric radio wave and riometer observations, thus finding strong agreement between the observations and the BARREL measurements.

Dense Multipath Component Characteristics in 11-GHz-Band Indoor Environments

Abstract: In the next-generation mobile communication system, the higher frequency bands from C-band to V-band are expected to be utilized because it has the potential to improve network capacity drastically by the available wideband spectrum. Since the characteristics of reflected and scattered radio waves from surrounding environments in those bands are thought to be quite different than at lower frequencies, the clarification of its influence on the multiple-input multiple-output (MIMO) transmission performance is a critical issue. In this paper, we focused on the characteristics of diffuse scattering in X-band, and conducted the MIMO channel measurements in indoor environments in the 11-GHz band. The frequency, angular, and polarization domain dense multipath component (DMC) propagation parameters were jointly estimated by using the RiMAX-based estimator. The measurement result showed the existence of significant DMC, which is thought to have originated from the floor, the ceiling as well as the walls. The angular spreads of the DMC tended to increase, and their decay factor tended to decrease as the room size decreased. It is also shown that the existence of DMC significantly affected the eigenvalue structure of the MIMO channel, which defines the MIMO transmission performance. The result is expected to be utilized for novel MIMO channel modeling in X-band that includes the DMC contribution.

Identification of scintillation signatures on GPS signals originating from plasma structures detected with EISCAT incoherent scatter radar along the same line of sight

Abstract: Ionospheric scintillation originates from the scattering of electromagnetic waves through spatial gradients in the plasma density distribution, drifting across a given propagation direction. Ionospheric scintillation represents a disruptive manifestation of adverse space weather conditions through degradation of the reliability and continuity of satellite telecommunication and navigation systems and services (e.g., European Geostationary Navigation Overlay Service, EGNOS). The purpose of the experiment presented here was to determine the contribution of auroral ionization structures to GPS scintillation. European Incoherent Scatter (EISCAT) measurements were obtained along the same line of sight of a given GPS satellite observed from Tromso and followed by means of the EISCAT UHF radar to causally identify plasma structures that give rise to scintillation on the co-aligned GPS radio link. Large-scale structures associated with the poleward edge of the ionospheric trough, with auroral arcs in the nightside auroral oval and with particle precipitation at the onset of a substorm were indeed identified as responsible for enhanced phase scintillation at L band. For the first time it was observed that the observed large-scale structures did not cascade into smaller-scale structures, leading to enhanced phase scintillation without amplitude scintillation. More measurements and theory are necessary to understand the mechanism responsible for the inhibition of large-scale to small-scale energy cascade and to reproduce the observations. This aspect is fundamental to model the scattering of radio waves propagating through these ionization structures. New insights from this experiment allow a better characterization of the impact that space weather can have on satellite telecommunications and navigation services.

Photonic probing of radio waves for k-space tomography.

Abstract: We harness coherent optical processing to simultaneously sense the angle of arrival and the frequency of radio waves. Signals captured by a distributed antenna array are up-converted to optical domain using electro-optic modulators coupled to individual antennas. Employing a common laser source to feed all the modulators ensures spatially coherent up-conversion of radio-frequency (RF) waves to optical beams carried by optical fibers. Fiber-length dispersion extends the spatial aperture of the distributed antenna array into the temporal dimension. The interference of beams emanating from the fibers is captured by a CCD and used to computationally reconstruct RF waves in k-space.

Let's Do the Twist!: Radiators, Experiments, and Techniques to Generate Twisted Waves at Radio Frequencies

Abstract: With the development of fifth-generation (5G) network solutions and high-data communication systems, millimeter-wave frequencies are being proposed for their licensing and bandwidth benefits. E-band radios at 71-76 and 81-86 GHz have recently been developed for backhaul and fixed point-to-point applications. Typically highgain, waveguide-based antennas are used in these deployments. By utilizing polarization, data rates can be increased two-fold. Another way to increase data rates for communication systems on the hardware side is through introducing orbital angular momentum (OAM). By twisting beams, multiple orthogonal modes can travel along the same wireless channel on the same frequency. This article highlights radiators, experiments, and techniques to generate twisted waves at radio frequency (RF).

Descattering of Giant Pulses in PSR B1957+20

Abstract: The interstellar medium scatters radio waves, which causes pulsars to scintillate. For intrinsically short bursts of emission, the observed signal should be a direct measurement of the impulse response function. We show that this is indeed the case for giant pulses from PSR B1957+20: from baseband observations at 327 MHz, we demonstrate that the observed voltages of a bright pulse allow one to coherently descatter nearby pulses. We find that while the scattering timescale is , the power in the descattered pulses is concentrated within a span almost two orders of magnitude shorter, of . This sets an upper limit to the intrinsic duration of the giant pulses. We verify that the response inferred from the giant pulses is consistent with the scintillation pattern obtained by folding the regular pulsed emission, and that it decorrelates on the same timescale, of . In principle, with large sets of giant pulses, it should be possible to constrain the structure of the scattering screen much more directly than with other current techniques, such as holography on the dynamic spectrum and cyclic spectroscopy.

A Millimeter-Wave Digital Link for Wireless MRI.

Abstract: A millimeter (mm) wave radio is presented in this work to support wireless MRI data transmission High path loss and availability of wide bandwidth make mm-waves an ideal candidate for short range, high data rata communication required for wireless MRI The proposed system uses a custom designed integrated chip (IC) mm-wave radio with 60 GHz as radio frequency carrier In this work, we assess performance in a 15 T MRI field, with the addition of optical links between the console room and magnet The system uses ON-OFF keying (OOK) modulation for data transmission and supports data rates from 200 Mb/s to 25 Gb/s for distances up-to 65 cm The presence of highly directional, linearly polarized, on-chip dipole antennas on the mm-wave radio along with the time division multiplexing (TDM) circuitry allows multiple wireless links to be created simultaneously with minimal inter-channel interference This leads to a highly scalable solution for wireless MRI

The Accuracy of Radio Direction Finding in the Extremely Low Frequency Range

Abstract: In this work, we study the accuracy of direction finding in the Extremely Low Frequency (ELF) range using a newly installed broadband receiver equipped with two active magnetic antennas. The main natural source of ELF radio waves is lightning. In this work, we analyzed 1000 atmospheric discharges at distances of up to 5000 km from the receiver. We identified the most important factors influencing the accuracy of the angle of arrival: the deviation of the radio waves propagating through the day-night terminator zone and the signal-to-noise ratio resulting from local electromagnetic noise and Schumann Resonance background. The obtained results clearly show that the accuracy of estimating the direction of arrival is very high (an average error of 0.1° with the standard deviation of 2.3°) when the signal-to-noise ratio is large (the amplitude of the magnetic field component above 100 pT), except for short periods in the local morning and evening, when the day-night terminator is present on the propagation path of the direct wave. For the day-night propagation paths, the refraction angle was larger than the incidence angle, and for the night-day propagation paths, the refraction angle was smaller than the incidence angle, which is consistent with theory. Using our analytical ELF radio propagation model allowed us to explain the obtained results.

Hybrid Visible Light and Ultrasound-Based Sensor for Distance Estimation

Abstract: Distance estimation plays an important role in location-based services, which has become very popular in recent years. In this paper, a new short range cricket sensor-based approach is proposed for indoor location applications. This solution uses Time Difference of Arrival (TDoA) between an optical and an ultrasound signal which are transmitted simultaneously, to estimate the distance from the base station to the mobile receiver. The measurement of the TDoA at the mobile receiver endpoint is proportional to the distance. The use of optical and ultrasound signals instead of the conventional radio wave signal makes the proposed approach suitable for environments with high levels of electromagnetic interference or where the propagation of radio frequencies is entirely restricted. Furthermore, unlike classical cricket systems, a double-way measurement procedure is introduced, allowing both the base station and mobile node to perform distance estimation simultaneously.

Exploring the connection between radio and GeV-TeV γ-ray emission in the 1FHL and 2FHL AGN samples

Abstract: Context. With the advent of the Fermi Large Area Telescope (LAT) it was revealed that blazars, representing the most extreme radio-loud active galactic nuclei (AGN) population, dominate the census of the γ -ray sky, and a significant correlation was found between radio and γ -ray emission in the 0.1−100 GeV energy range. However, the possible connection between radio and very high energy (VHE, E > 0.1 TeV) emission still remains elusive, owing to the lack of a homogeneous coverage of the VHE sky. Aims. The main goal of this work is to quantify and assess the significance of a possible connection between the radio emission on parsec scale measured by the very long baseline interferometry (VLBI) and GeV-TeV γ -ray emission in blazars, which is a central issue for understanding blazar physics and the emission processes in these objects. Methods. We investigate the radio VLBI and high energy γ -ray emission by using two large and unbiased AGN samples extracted from the first and second Fermi -LAT catalogs of hard γ -ray sources detected above 10 GeV (1FHL) and 50 GeV (2FHL). For comparison, we perform the same correlation analysis by using the 0.1−300 GeV γ -ray energy flux provided by the third Fermi -LAT source catalog (3FGL). We assess the correlation’s statistical significance by means of a method based on permutations of the luminosities, by taking into account the various observational biases, which may apparently enhance or spoil any intrinsic correlation. Results. We find that the correlation strength and significance depend on the γ -ray energy range, with a different behavior among the blazar sub-classes. Overall, the radio and γ -ray emission above 10 GeV turns out to be uncorrelated for the full samples and for all of the blazar sub-classes with the exception of high synchrotron peaked (HSP) objects, which show a strong and significant correlation. On the contrary, when 0.1−300 GeV γ -ray energies are considered, a strong and significant correlation is found for the full blazar sample as well as for all of the blazar sub-classes. Conclusions. We interpret and explain this correlation behavior within the framework of the blazar spectral energy distribution properties. In the most powerful blazars, which are in general of low synchrotron peaked type, the high energy emission component peaks at energies lower than those sampled by the LAT. On the contrary, in HSP blazars the part of the high energy spectrum affected by cooling effects is well beyond the energy range sampled by the LAT, showing a rising spectrum both in the 3FGL and 1FHL/2FHL energy ranges.