On high energy tails in inelastic gases

We study the formation of high energy tails in a one-dimensional kinetic model for granular gases, the so-called Inelastic Maxwell Model. We introduce a time- discretized version of the stochastic process, and show that continuous time implies larger fluctuations of the particles energies. This is due to a statistical relation between the number of inelastic collisions undergone by a particle and its average energy. This feature is responsible for the high energy tails in the model, as shown by computer simulations and by analytical calculations on a linear Lorentz model.Comment: 8 pages, 2 figures, submitted to physica

Topographically Scattered Infrasound Waves Observed on Microbarometer Arrays in the Lower Stratosphere

When an acoustic wave strikes a topographic feature, some of its energy is scattered. Sensors on the ground cannot capture these scattered signals when they propagate at high angles. We report observations of upwardly-scattered acoustic waves prior to refraction back to the ground, intercepting them with a set of balloon-borne infrasound microbarometers in the lower stratosphere over northern Sweden. We show that these scattered waves generate a coda whose presence can be related to topography beneath balloons and low-altitude acoustic ducts. The inclination of the coda signals changes systematically with time, as expected from waves arriving from scatterers successively closer to receivers. The codas are present when a temperature inversion channels infrasound from a set of ground chemical explosions along the ground, but are absent following the inversion's dissipation. Since scattering partitions energy away from the main arrival, these observations imply a mechanism of amplitude loss that had previously been inaccessible to measurement. As such, these data and results allow for a better comprehension of interactions between atmospheric infrasound propagation and the solid earth

Application of X-band radars for deriving intertidal bathymetries and characterising coastal behaviours

Coastal monitoring techniques aim to capture the relationship between physical forcing factors and morphological change, at a range of timescales to understand ongoing coastal processes and identify areas prone to erosion and flooding hazards posed by storms. Standard marine radar provides temporally and spatially continuous monitoring data over a wide area in all conditions, and images can be processed to generate intertidal bathymetries to assess morphological change across event (days-years) timescales. This research applies a series of intertidal bathymetries derived from a standard marine radar deployed at Camber Sands, southeast England in XBeach, a process-based, storm response model, to assess wave runup hazard at the coast during a high energy storm event from the deployment period. Wave runup is dependent on offshore wave climate and beach slope and used here as a proxy to explore the influence of nearshore morphological variability, represented by different processing techniques to derive intertidal bathymetries from the marine radar images, on a coastal hazard. XBeach is used in combination with beach survey data from the site to first demonstrate reasonable skill in reproducing wave runup observations. Intertidal bathymetries are derived from the marine radar images using either a local or regional water level signal, and an average of 1, 5, or 10 days of images preceding the storm event. Modelled wave runup shows up to 0.32 m sensitivity to input intertidal bathymetries, which could be important for overwash predictions. The slope and resolution of the radar-derived intertidal bathymetries is sensitive to the water level time series used. This research is the first time that radar-derived intertidal bathymetries have been used to assess a coastal hazard in a process-based model, and results show that ideally users would have a locally measured water level to accurately generate intertidal bathymetries, and extended beach surveys for ground truthing

Quantum Theory of Noncommutative Fields

Generalizing the noncommutative harmonic oscillator construction, we propose a new extension of quantum field theory based on the concept of "noncommutative fields". Our description permits to break the usual particle-antiparticle degeneracy at the dispersion relation level and introduces naturally an ultraviolet and an infrared cutoff. Phenomenological bounds for these new energy scales are given.Comment: LaTeX file, JHEP3.cls, subequations.sty; 12 pages, no figures. Final version published in JHEP with some references adde

Sub-arcsecond radio and optical observations of the likely counterpart to the gamma-ray source 2FGL J2056.7+4939

We have searched and reviewed all multi- wavelength data available for the region towards the gamma-ray source 2FGL J2056.7+4939 in order to con- strain its possible counterpart at lower energies. As a result, only a point-like optical/infrared source with flat-spectrum radio emission is found to be consistent with all X-ray and gamma-ray error circles. Its struc- ture is marginally resolved at radio wavelengths at the sub-arcsecond level. An extragalactic scenario appears to be the most likely interpretation for this object.Comment: 5 pages, 3 figures, 1 tabl

The second and third Sonine coefficients of a freely cooling granular gas revisited

In its simplest statistical-mechanical description, a granular fluid can be modeled as composed of smooth inelastic hard spheres (with a constant coefficient of normal restitution $\alpha$) whose velocity distribution function obeys the Enskog-Boltzmann equation. The basic state of a granular fluid is the homogeneous cooling state, characterized by a homogeneous, isotropic, and stationary distribution of scaled velocities, $F(\mathbf{c})$. The behavior of $F(\mathbf{c})$ in the domain of thermal velocities ($c\sim 1$) can be characterized by the two first non-trivial coefficients ($a_2$ and $a_3$) of an expansion in Sonine polynomials. The main goals of this paper are to review some of the previous efforts made to estimate (and measure in computer simulations) the $\alpha$-dependence of $a_2$ and $a_3$, to report new computer simulations results of $a_2$ and $a_3$ for two-dimensional systems, and to investigate the possibility of proposing theoretical estimates of $a_2$ and $a_3$ with an optimal compromise between simplicity and accuracy.Comment: 12 pages, 5 figures; v2: minor change

INTEGRAL observations of TeV plerions

Amongst the sources seen in very high gamma-rays several are associated with Pulsar Wind Nebulae (``TeV plerions''). The study of hard X-ray/soft gamma-ray emission is providing an important insight into the energetic particle population present in these objects. The unpulsed emission from pulsar/pulsar wind nebula systems in the energy range accessible to the INTEGRAL satellite is mainly synchrotron emission from energetic and fast cooling electrons close to their acceleration site. Our analyses of public INTEGRAL data of known TeV plerions detected by ground based Cherenkov telescopes indicate a deeper link between these TeV plerions and INTEGRAL detected pulsar wind nebulae. The newly discovered TeV plerion in the northern wing of the Kookaburra region (G313.3+0.6 powered by the middle aged PSR J1420-6048) is found to have a previously unknown INTEGRAL counterpart which is besides the Vela pulsar the only middle aged pulsar detected with INTEGRAL. We do not find an INTEGRAL counterpart of the TeV plerion associated with the X-ray PWN ``Rabbit'' G313.3+0.1 which is possibly powered by a young pulsar.Comment: 4 pages, 6 figures, proceedings of conference "The Multi-Messenger Approach to High-Energy Gamma-ray Sources" Barcelona/Spain (2006

Navier-Stokes transport coefficients of dd-dimensional granular binary mixtures at low density

The Navier-Stokes transport coefficients for binary mixtures of smooth inelastic hard disks or spheres under gravity are determined from the Boltzmann kinetic theory by application of the Chapman-Enskog method for states near the local homogeneous cooling state. It is shown that the Navier-Stokes transport coefficients are not affected by the presence of gravity. As in the elastic case, the transport coefficients of the mixture verify a set of coupled linear integral equations that are approximately solved by using the leading terms in a Sonine polynomial expansion. The results reported here extend previous calculations [V. Garz\'o and J. W. Dufty, Phys. Fluids {\bf 14}, 1476 (2002)] to an arbitrary number of dimensions. To check the accuracy of the Chapman-Enskog results, the inelastic Boltzmann equation is also numerically solved by means of the direct simulation Monte Carlo method to evaluate the diffusion and shear viscosity coefficients for hard disks. The comparison shows a good agreement over a wide range of values of the coefficients of restitution and the parameters of the mixture (masses and sizes).Comment: 6 figures, to be published in J. Stat. Phy

Towards a population of HMXB/NS microquasars as counterparts of low-latitude unidentified EGRET sources

The discovery of the microquasar LS 5039 well within the 95% conficence contour of the Unidentified EGRET Source (UES) 3EG J1824-1514 was a major step towards the possible association between microquasars (MQs) and UESs. The recent discovery of precessing relativistic radio jets in LS I +61 303, a source associated for long time with 2CG 135+01 and with the UES 3EG J0241+6103, has given further support to this idea. Finally, the very recently proposed association between the microquasar candidate AX J1639.0-4642 and the UES 3EG J1639-4702 points towards a population of High Mass X-ray Binary (HMXB)/Neutron Star (NS) microquasars as counterparts of low-latitude unidentified EGRET sources.Comment: 12 pages, 7 figures. Proceedings of the Conference "The Multiwavelength Approach to Unidentified Gamma-ray Sources", to appear in the journal Astrophysics and Space Scienc

Monitoring changes in human activity during the COVID-19 shutdown in Las Vegas using infrasound microbarometers

While studies of urban acoustics are typically restricted to the audio range, anthropogenic activity also generates infrasound (<20 Hz, roughly at the lower end of the range of human hearing). Shutdowns related to the COVID-19 pandemic unintentionally created ideal conditions for the study of urban infrasound and low frequency audio (20-500 Hz), as closures reduced human-generated ambient noise, while natural signals remained relatively unaffected. An array of infrasound sensors deployed in Las Vegas, NV, provides data for a case study in monitoring human activity during the pandemic through urban acoustics. The array records a sharp decline in acoustic power following the temporary shutdown of businesses deemed nonessential by the state of Nevada. This decline varies spatially across the array, with stations close to McCarran International Airport generally recording the greatest declines in acoustic power. Further, declines in acoustic power fluctuate with the time of day. As only signals associated with anthropogenic activity are expected to decline, this gives a rough indication of periodicities in urban acoustics throughout Las Vegas. The results of this study reflect the city's response to the pandemic and suggest spatiotemporal trends in acoustics outside of shutdowns