An improved spin-down rate for the proposed white dwarf pulsar AR scorpii

ABSTRACT We analyse rapid-cadence, multiwavelength photometry of AR Scorpii from three observatories, covering five observing seasons. We measure the arrival times of the system’s beat pulses and use them to compute an updated ephemeris. The white dwarf spin-down rate is estimated with an uncertainty of only 4 per cent. These results confirm, beyond any doubt, that the white dwarf’s spin period is increasing at the rate consistent with by that of Stiller et al. (2018). We study the evolution of the beat pulse’s colour index across the orbit. The colour of the primary pulse maxima varies significantly across the orbit, with the peaks being bluer after superior conjunction than in the first half of the orbit. Specifically, at orbital phase 0.5, the colour index of the primary pulse shows a very sharp discontinuity towards bluer indices. This supports the Potter & Buckley (2018b) synchrotron emission model where the two emitting poles differ significantly in colour. However, no corresponding jump in the colour of the secondary pulses is seen. Furthermore, our analysis reveals that the arrival times of the pulses can differ by as much as 6 s in simultaneous u and r photometry, depending on the binary orbital phase. If left uncorrected, this wavelength-dependent timing offset could lead to erroneous measurements of the spin-period derivative, particularly with heterogeneous data sets.

One-dimensional analyses of striker impact on bar with different general impedance

The particle velocity and stress in the striker and bar generated by the striker impact on a bar with different general impedance are formulated based on one-dimensional assumptions. Departure of the impact-generated stress wave towards the rear end of the striker and arrival of the release wave from the rear to the front of the striker constitute one impact cycle. In cases where Zs ≤ Zb ( Z is the general impedance, and subscripts ‘s’ and ‘b’ denote striker and bar, respectively), only one impact cycle takes place because the striker is stationary or separated from the impact surface after the first impact cycle. As a result, only a single (primary) pulse is observed in the bar and striker. In the case where Zs > Zb, however, multiple impact cycles take place because the striker is compressing the bar continually after the first cycle. As a result, a series of step-wise residual pulses follow the primary pulse in the bar and striker. The magnitudes of the stress and particle velocity in the bar and striker calculated using the formulated equations are quantitatively consistent with the results of the numerical simulations, verifying the formulated one-dimensional equations. The equations formulated in this study may be useful for better understanding the various wave interaction phenomena that take place in a pseudo-one-dimensional impact system and for modifying/designing an impact system.

NOISE AND TARGET SIGNALS SIMULATION IN PASSIVE SEISMIC LOCATION SYSTEMS

The paper proposes the model of a person seismic signal with noise for the investigation of passive seismic location system characteristics. The known models based on Gabor and Berlage pulses have been analyzed. These models are not able wholly to consider statistical properties of seismic signals. The proposed model is based on almost cyclic character of seismic signals, Gauss character of fluctuations inside a pulse, random amplitude change from pulse to pulse and relatively small fluctuation of separate pulses positions. The simulation procedure consists of passing the white noise through a linear generating filter with characteristics formed by real steps of a person, and the primary pulse sequence modulation by Gauss functions. The model permits to control the signal-to-noise ratio after its reduction to unity and to vary pulse shifts with respect to person steps irregularity. It has been shown that the model of a person seismic signal with noise agrees with experimental data.

Wave Characteristics of Falling Film on Inclination Plate at Moderate Reynolds Number

Falling water film on an inclined plane is studied by shadowgraphy. The ranges of inclination angle and the film Reynolds number are, respectively, up to 21° and 60. Water is used as working fluid. The scenario of wave regime evolution is identified as three distinctive regimes, namely, initial quiescent smooth film flow, two-dimensional regular solitary wave pattern riding on film flow, and three-dimensional irregular wave pattern. Three characteristic parameters of two-dimensional solitary wave pattern, namely, inception length, primary pulse spacing, and propagation velocity, are examined, which are significant in engineering applications for estimation of heat and mass transfer on film flow. The present experimental data are well in agreement with the Koizumi correlations, the deviation from which is limited to 20% and 15%, respectively, for primary pulse spacing and propagation velocity. Through the scrutiny of the present experimental observation, it is concluded that wave evolution on film flow at the moderate Reynolds number is controlled by gravity and drag and the Rayleigh-Taylor instability that occurred on the steep front of primary pulse triggers the disintegration of continuous two-dimensional regular solitary wave pattern into three-dimensional irregular wave pattern.

An alternative method for modeling close-range interactions between air guns

We evaluated the problem of modeling the decay of the primary pulse amplitudes of air-gun clusters caused by the traditional assumption of sphericality. This was done by generalizing the Rayleigh equation to work with arbitrary bubble shapes, while retaining the assumption of incompressibility. To approximate the coalescence of the bubbles, we let the shapes be isosurfaces of the velocity potential. With this method, it is possible to model the firing of clustered air guns at any separation distance, including small distances that would cause two spherical bubbles to overlap. In this way, we obtained results matching the relative decay shown to be present for air-gun clusters. In addition, this method also allowed a way of calibrating the model such that effects created by the presence of the gun, compared to just a single spherical air bubble, may be estimated and included.

Nonlinear Pulse Equipartition in Weakly Coupled Ordered Granular Chains With No Precompression

We report on the strongly nonlinear dynamics of an array of weakly coupled, noncompressed, parallel granular chains subject to a local initial impulse. The motion of the granules in each chain is constrained to be in one direction that coincides with the orientation of the chain. We show that in spite of the fact that the applied impulse is applied to one of the granular chains, the resulting pulse that initially propagates only in the excited chain gets gradually equipartitioned between its neighboring chains and eventually in all chains of the array. In particular, the initially strongly localized state of energy distribution evolves towards a final stationary state of formation of identical solitary waves that propagate in each one of the chains. These solitary waves are synchronized and have identical speeds. We show that the phenomenon of primary pulse equipartition between the weakly coupled granular chains can be fully reproduced in coupled binary models that constitute a significantly simpler model that captures the main qualitative features of the dynamics of the granular array. The results reported herein are of major practical significance since it indicates that the weakly coupled array of granular chains is a medium in which an initially localized excitation gets gradually defocused, resulting in drastic reduction of propagating pulses as they are equipartitioned among all chains.

Strongly Nonlinear Spatially Periodic Traveling Waves in Granular Dimer Chains

In the present work we study the dynamics of spatially periodic traveling waves in granular 1:1 (each bead is followed and preceded by a bead of different mass and/or stiffness) dimer chain with no pre-compression. The dynamics of a 1:1 dimer chain is governed by a single parameter, the mass ratio of the two beads forming each dimer pair of the chain. In particular, we demonstrate numerically the formation of special families of traveling waves with spatially periodic waveforms that are realized in semi-infinite dimer chains with the application of an arbitrary impulse. These traveling waves were first observed in the form of oscillatory tails in the trail of the propagating primary pulse. The energy radiated by the propagating primary pulse manifests in the form of traveling waves of varying spatial periodicity depending on the mass ratio. These traveling waves depend only on the mass ratio and are rescalable with respect to any arbitrary applied energy. The dynamics of these families of traveling waves is systematically studied by considering finite dimer chains (termed the ‘reduced systems’) subject to periodic boundary conditions. We demonstrate that these waves may exhibit interesting bifurcations or loss of stability as the system parameter varies. In turn, these bifurcations and stability exchanges in infinite dimer chains are correlated to previous studies of pulse attenuation in finite dimer chains through efficient energy radiation from the propagating pulse to the far field, mainly in the form of traveling waves. Based on these results a new formulation of attenuation and propagation zones (stop and pass bands) in semi-infinite granular dimer chains is proposed.

Solitary Waves in 1:N Dimer Chains

In the present work we report the discovery of new families of solitary waves in a 1:N (N>1) granular dimers (a heavy bead followed and preceded by N light beads) wherein the Hertzian interaction law governs the interaction between spherical beads. We consider the dimer chain with zero precompression. The dynamics of such a dimer chain is governed by two system parameters, the stiffness ratio and the mass ratio between the light and the heavy beads. In particular we study in detail the solitary waves in 1:2 dimer chains [11]. The solitary waves in a 1:2 dimer are contrastingly different from that in a homogeneous chain and 1:1 dimer chain. Solitary waves realized in homogeneous and 1:1 dimer chains possess symmetric velocity waveforms. In contrast, in a 1:2 dimer chain we realize solitary waves that have symmetric velocity waveforms on the heavy beads, whereas that on the light beads is non-symmetric. The existence of families of solitary waves in these systems is attributed to the dynamical phenomenon to ‘anti-resonance’. This leads to the complete elimination of radiating waves in the trail of the propagating pulse. Anti-resonances are associated with certain symmetries of the velocity waveforms of the beads of the dimer. We conjecture that a countable infinity of family of solitary waves can be realized in 1:2 dimer chains. Interestingly, solitary waves in a general 1:N (N>2) dimer chain are far more difficult to realize. For the case of 1:2 dimers, we can vary the two parameters to satisfy conditions such that the oscillatory tails in the trail of the primary pulse of the two light beads decay to zero. In contrast, for a 1:N (N>2) dimer chain, we have the same two parameters but need to satisfy the decaying conditions on N light beads simultaneously. This leads to a mathematically ill-posed problem and as such rigorously no solitary waves can be realized in general.