arXiv subjects

Richard Lieu

Publications and source records attributed to Richard Lieu.

At least 19 recordsLinked to original sources

arXiv2026

Dipole-Field Magnetic Windows for Radio-Frequency Transmission Through Hypersonic Plasma Sheaths: A Reduced-Order Scaling Model

Hypersonic vehicles and atmospheric-entry bodies can experience radio-frequency communication blackout when shock-heated gas surrounding the vehicle becomes sufficiently ionized that plasma cutoff and collisional attenuation restrict electromagnetic transmission. Magnetic-window approaches attempt to reduce this loss by exploiting the anisotropic dispersion of a magnetized plasma, in which selected right-hand or whistler-like modes may propagate along preferred directions. This paper develops a reduced-order scaling model for magnetic-window transmission through a finite-thickness hypersonic plasma sheath when the magnetic source is represented as an onboard axial dipole. The model gives a closed-form estimate of the collisionless angular aperture, compares the underlying projected-cyclotron criterion with full cold-plasma dispersion roots, and extends the aperture estimate to a loss-limited cone using a collisional optical-depth approximation. A simplified neutral-density, speed, and ionization closure is used only to generate qualitative parametric maps and sensitivity trends. The results clarify how dipole-field decay, sheath thickness, radio frequency, vehicle scale, electron density, and collisions jointly constrain the candidate transmission window. The contribution is intended as a screening framework for selecting cases for full-wave electromagnetic simulation, nonequilibrium aerothermochemistry, antenna-coupling analysis, and laboratory validation, rather than as a demonstrated engineering solution to plasma blackout.

physics.plasm-ph↗

arXiv2025

Fast radio bursts by stellar wind microlensing of a faint background source

By assuming the inverse square law of solar wind plasma density as representative of other stars, it is shown that just outside a star the {\it outward} deflection of a passing radio signal at $\nu\approx 1$~GHz (which is capable of penetrating the plasma) is about 5 times larger than the gravitational inward deflection by the star, and the ensuing lens equation which takes both effects into account is a cubic polynomial with three roots and a new strong lensing caustic. The geometric optics approach is valid for a radio source size $\lesssim 1$~pc. Microlensing magnification of a steady background source occurs typically over a timescale of milliseconds, resulting in $\approx 80$ Fast Radio Bursts (FRBs) per day over the whole sky, which can only perturb the isotropy of FRB distribution at the several \% level. Moreover, repeating FRBs could be triggered by the periodic interception of the line-of-sight of the background source by members of a binary system. The temporal signatures of such FRBs are consistent with the power spectrum of solar wind density fluctuations on corresponding scales, except the mean density of the wind is a few times higher than the solar value.

astro-ph.HE↗

arXiv2025

X-ray emission in IllustrisTNG circum-cluster environments. II -- Possible origins of the soft X-ray excess emission

An excess of soft X-ray emission (0.2-1 keV) above the contribution from the hot intra-cluster medium (ICM) has been detected in a number of galaxy clusters, including the Coma cluster. The physical origin of this emitting medium above hot ICM has not yet been determined, especially whether it be thermal or non-thermal. We aim to investigate which gas phase and gas structure more accurately reproduce the soft excess radiation from the cluster core to the outskirts, using simulations. By using the simulation TNG300, we predict the radial profile of thermodynamic properties and the Soft-X-ray surface brightness of 138 clusters within 5 $R_{200}$. Their X-ray emission is simulated for the hot ICM gas phase, the entire Warm-Hot medium, the diffuse and low-density Warm-Hot Intergalactic Medium (WHIM). Inside clusters, the soft excess appears to be produced by substructures of the WARM gas phase which host dense warm clumps (i.e, the Warm Circum-Galactic Medium, WCGM), and in fact the inner soft excess is strongly correlated with substructure and WCGM mass fractions. Outside of the virial radius, the fraction of WHIM gas that is mostly inside filaments connected to clusters boosts the soft X-ray excess. The more diffuse the gas is, the higher the soft X-ray excess beyond the virial region. The thermal emission of WARM gas phase, in the form of WCGM clumps and WHIM diffuse filaments, reproduces well the soft excess emission that was observed up to the virial radius in Coma and in the inner regions of other massive clusters. Moreover, our analysis suggests that soft X-ray excess is a proxy of cluster dynamical state, with larger excess being observed in the most unrelaxed clusters.

astro-ph.CO↗

arXiv2025

Are dark matter and dark energy omnipresent?

A set of temporal singularities (transients) in the mass-energy density and pressure, bearing a specific mathematical structure which represents a new solution to the continuity equation (\ie~conservation of mass-energy) and satisfying the strong energy condition, is proposed to account for the expansion history of a homogeneous Universe, and the formation and binding of large scale structures as a continuum approximation of their cumulative effects. These singularities are unobservable because they occur rarely in time and are unresolvably fast, and that could be the reason why dark matter and dark energy have not been found. Implication on inflationary cosmology is discussed. The origin of these temporal singularities is unknown, safe to say that the same is true of the moment of the Big Bang itself. This work complements a recent paper, where a topological defect in the form of a spatial, spherical shell of density singularity giving rise to a 1/r attractive force (to test particles of positive mass) but zero integrated mass over a large volume of space, was proposed to solve the dark matter problem in bound structures but not cosmic expansion. The idea also involved a negative density, which is not present in the current model.

gr-qc↗

arXiv2024

The interface of gravity and dark energy

At sufficiently large radii dark energy modifies the behavior of (a) bound orbits around a galaxy and (b) virialized gas in a cluster of galaxies. Dark energy also provides a natural cutoff to a cluster's dark matter halo. In (a) there exists a maximum circular orbit beyond which periodic motion is no longer possible, and orbital evolution near critical binding is analytically calculable using an adiabatic invariant integral. The finding implicates the study of wide galaxy pairs. In (b), dark energy necessitates the use of a generalized Virial Theorem to describe gas at the outskirts of a cluster. When coupled to the baryonic escape condition, aided by dark energy, the results is a radius beyond which the continued establishment of a hydrostatic halo of thermalized baryons is untenable. This leads to a theoretically motivated virial radius. We use this theory to probe the structure of a cluster's baryonic halo and apply it to X-ray and weak-lensing data collected on cluster Abell 1835. We find that gas in its outskirts deviates significantly from hydrostatic equilibrium beginning at $\sim 1.3\ {\rm Mpc}$, the `inner' virial radius. We also define a model dependent dark matter halo cutoff radius to A1835. The dark matter cutoff gives an upper limit to the cluster's total mass of $\sim7\times 10^{15}M_{\odot}$. Moreover, it is possible to derive an `outer' hydrostatic equilibrium cutoff radius given a dark matter cutoff radius. A region of cluster gas transport and turbulence occurs between the inner and outer cutoff radii.

astro-ph.CO↗

arXiv2024

The binding of cosmological structures by massless topological defects

Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field equations sourced by a topological defect, \ie~a singularity of a very specific form, the result is a localised gravitational field capable of driving flat rotation (\ie~Keplerian circular orbits at a constant speed for all radii) of test masses on a thin spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect light in the same manner as an equipotential (isothermal) sphere. Thus the need for dark matter or modified gravity theory is mitigated, at least in part.

physics.gen-ph↗

arXiv2023

Scale invariant curvature perturbations from a spontaneously decaying scalar field

The evolution of superhorizon curvature perturbations in a two-component interacting universe is considered. It is found that adiabatic modes conserve the total curvature perturbation $ζ$, unless there are stages in which the rate of dissipation of one component into another is not constant. Moreover, our result shows that when the rate is varying it is possible for 'isocurvature' perturbations generated during reheating to alter the amplitude of an adiabatic curvature mode even when the mode is outside the horizon. Specifically, if an indefinitely large rate $Γ$ for massive particles decaying into photons develops rapidly amid vanishingly small initial values (before decay) of the total curvature $ζ_i$ and Newtonian potential $Φ_i$, such that the product $Γζ_i$ and $ΓΦ_i$ become a pair of finite and universal constants for all superhorizon scales afterwards, Harrison-Zel'dovich scale-invariant power spectrum could be synthesized from a homogeneous state without inflation at all.

astro-ph.CO↗

arXiv2023

A Model of EUV Emission from Clusters of Galaxies

With tantalizing evidence of the recent e-Rosita mission, re-discovering very soft X-rays and EUV radiation from a cluster of galaxies or its environment, the question of the origin of cluster EUV excess is revisited in this work. It will be shown that the gas temperature, density, and frozen-in magnetic field of the intracluster medium, collectively support the emission and propagation of coherent uCerenkov radiation, which is low frequency and large amplitude radiation capable of accelerating charged particles to relativistic speeds. Owing to the spectrum of uCerenkov radiation, most of the incipient relativistic electrons undergo inverse-Compton scattering with the cosmic microwave background. It turns out the scattered radiation has observable ramifications only in the EUV band, of photon energy $70 -- 100$~eV, having a luminosity $\approx 10^{44}$~ergs~s$^{-1}$. This luminosity is on par with the EUV excess level detected from Abell 1795 and the Coma cluster. It should be stressed, as {\it caveat emptor}, that although the main subject is the putative large amplitude coherent uCerenkov modes which are highly nonlinear, the results presented were derived using a quasi-linear approach to highlight the observable features of the phenomenon, namely the EUV emission.

astro-ph.HE↗

arXiv2022

Damping of long wavelength gravitational waves by the intergalactic medium

The problem of radiation by the charged particles of the intergalactic medium (IGM) when a passing gravitational wave (GW) accelerate them is investigated. The largest acceleration (taking a charge from rest to a maximum speed which remains non-relativistic in the rest frame of the unperturbed spacetime) is found to be limited by the curvature of a propagating spherical gravitational wavefront. Interesting physics arises from the ensuing emission of radiation into the warm hot IGM, which to lowest order is a fully ionized hydrogen plasma with a frozen-in magnetic field $B$. It is found that for a vast majority of propagation directions, the right-handed polarized radiation can penetrate the plasma at frequencies below the plasma frequency $\om_p$, provided $\om<\om_b,$ where $\om_b=eB/m_e$ satisfies $\om_b<\om_p$ for typical IGM conditions. Moreover, the refractive index under such a scenario is $n\gg 1,$ resulting in an enhanced radiative dissipation of GW energy (relative to the vacuum scenario), which is more severe for electrons if both charge species are in thermal equilibrium and accelerated in the same way. The emission by the electrons then prevails, and is further amplified by coherent addition of amplitudes within the size one wavelength. The conversion of GWs of $\lam\gtrsim 5\times 10^{13}$~cm to electromagnetic waves means such GWs can only propagate a distance $\lesssim 1$~Gpc before being significantly damped by an IGM B field of $\sim10^{-8}$ G. The low-frequency GWs \textcolor{black}{targeted by pulsar-timing-arrays} will not survive unless the IGM magnetic field is much lower than expected. The \textcolor{black}{mHz} frequency GW inspirals targeted by future \textcolor{black}{space based} detectors such as the Laser Interferometer Space Antenna remain intact and can be detected.

gr-qc↗

arXiv2021

Galactic Orbital Effects on Pulsar Timing

In the currently accepted paradigm, dark matter is hypothesized as an explanation of the flat rotation curves of galaxies under the assumption of virialized orbits. The use of millisecond pulsar timing as a probe of Galactic dark matter content is explored as a means of relaxing this assumption. A method of inference of the Galactic potential using the frequency derivative $\dotν$ is produced, and an estimate for a virialized Galactic rotation curve is given through direct observation of acceleration. The data set used includes 210 pulsars with known $\dotν$ and astrometric properties, a subset of which also have measured $\ddotν$. In principle, this enables the exploration of kinematic effects, but in practice, $\ddotν$ values are found to be too imprecise at present to adequately constrain radial velocities of pulsars. Additionally, surface magnetic field strengths are inferred from $\dotν$ and the magnetic spin-down contribution to $\ddotν$ is estimated. For several pulsars the radial velocity is known, and the kinematic contribution to $\ddotν$ is estimated accordingly. The binary orbital periods of PSR J1713+0747 and other binary pulsars are also used to constrain Galactic mass density models.

astro-ph.GA↗

arXiv2020

A method of enhancing the detection sensitivity of transient sources in time series with Gaussian stationary noise

The Gaussian phase noise of intensity time series is demonstrated to be drastically reduced when the raw voltage data are digitally filtered through an arbitrarily large number $n$ of orthornormal bandpass profiles (eigen-filters) sharing the same intensity bandwidth, and the resulting intensity series are co-added. Specifically, the relative noise variance of the summed series at the resolution of one coherence time or less, goes down with increasing $n$ as $1/n$, although (consistent with the radiometer equation) the advantage gradually disappears when the series is bin averaged to lower resolution. Thus the algorithm is designed to enhance the sensitivity of detecting transients that are smoothed out by time averaging and too faint to be visible in the noisy unaveraged time series, as demonstrated by the simulation of a weak embedded time varying signal of either a periodic nature or a fast and unrepeated pulse. The algorithm is then applied to a 10 minute observation of the pulsar PSR 1937+21 by the VLA, where the theoretical predictions were verified by the data. Moreover, it is shown that microstructures within the time profile are better defined as the number $n$ of filters used increases, and a periodic signal of period $1.86 \times 10^{-5}$~s ($53.9$~kHz) is discovered in the pulse profile. Lastly, we apply the algorithm to the first binary black hole merger detected by LIGO, GW150914. We find the SNR of the mean peak intensity increases as $\sqrt{n}$ and cross correlation of the event between the LIGO-Hanford-Livingston detector pair increases with filter order $n$.

astro-ph.IM↗

arXiv2018

Exclusion of standard $\hbarω$ gravitons by LIGO observation

Dyson (2013) argued that the extraordinarily large number of gravitons in a gravitational wave makes them impossible to be resolved as individual particles. While true, it is shown in this paper that a LIGO interferometric detector also undergoes frequent and {\it discrete} quantum interactions with an incident gravitational wave, in such a way as to allow the exchange of energy and momentum between the wave and the detector. This opens the door to another way of finding gravitons. The most basic form of an interaction is the first order Fermi acceleration (deceleration) of a laser photon as it is reflected by a test mass mirror oscillating in the gravitational wave, resulting in a frequency blueshift (redshift) of the photon depending on whether the mirror is advancing towards (receding from) the photon before the reflection. If e.g. a blueshift occurred, wave energy is absorbed and the oscillation will be damped. It is suggested that such energy exchanging interactions are responsible for the observed radiation reaction noise of LIGO (although the more common way of calculating the same amplitude for this noise is based on momentum considerations). Most importantly, in each interaction the detector absorbs or emits wave energy in amounts far smaller than the standard graviton energy $\hbarω$ where $ω$ is the angular frequency of the gravitational wave. This sets a very tight upper limit on the quantization of the wave energy, viz. it must be at least $\approx 10^{11}$ times below $\hbarω$, independently of the value of $ω$ itself.

astro-ph.HE↗

arXiv2018

Photon flux and bunching noise from measurement of the shot noise variance

We report the experimental observation of photon bunching noise through shot noise measurements made on a pseudo-thermal state of light using balanced detection. A full theory describing the measurement is developed, and in agreement with theory it is found that the shot noise variance in the balanced signal reproduces the time series of the flux of the primary incoherent beam. Moreover, when the average power of the pseudo-thermal light is varied, the balanced detection is seen to track this change. A comparison of direct detection and balanced detection of the thermal field, shows that the balanced detection performs at least as well the direct detection and under some conditions appears to outperform the direct detection. There is not necessarily a contradiction with quantum field theory which predicts that at best the performance of the balanced detection should be equal to the direct detection, because the direct detection process is subject to nonlinearity that has not been excluded by measurements (even though any tests we performed suggest such effects are small). This is the first time that the bunching noise effect of high occupation number chaotic light via the shot noise of the field has successfully been measured, to the point of using it to infer the flux of the field. The findings may be relevant to radio receiver design, specifically from the viewpoint of sensitivity improvement.

physics.ins-det↗

arXiv2017

Does light from steady sources bear any observable imprint of the dispersive intergalactic medium?

There has recently been some interest in the prospect of detecting ionized intergalactic baryons by examining the properties of incoherent light from background cosmological sources, namely quasars. Although the paper by \cite{lieu13} proposed a way forward, it was refuted by the later theoretical work of \cite{hir14} and observational study of \cite{hal16}. In this paper we investigated in detail the manner in which incoherent radiation passes through a dispersive medium both from the frameworks of classical and quantum electrodynamics, which led us to conclude that the premise of \cite{lieu13} would only work if the pulses involved are genuinely classical ones involving many photons per pulse, but unfortunately each photon must not be treated as a pulse that is susceptible to dispersive broadening. We are nevertheless able to change the tone of the paper at this juncture, by pointing out that because current technology allows one to measure the phase of individual modes of radio waves from a distant source, the most reliable way of obtaining irrefutable evidence of dispersion, namely via the detection of its unique signature of a quadratic spectral phase, may well be already accessible. We demonstrate how this technique is only applied to measure the column density of the ionized intergalactic medium.

astro-ph.IM↗

arXiv2017

Large scale density perturbations from a uniform distribution by wave transport

It has long been known that a uniform distribution of matter cannot produce a Poisson distribution of density fluctuations on very large scales $1/k > ct$ by the motion of discrete particles over timescale $t$. The constraint is part of what is sometimes referred to as the Zel'dovich bound. We investigate in this paper the transport of energy by the propagation of waves emanating {\it incoherently} from a regular and infinite lattice of oscillators, each having the same finite amount of energy reserve initially. The model we employ does not involve the expansion of the Universe -- the scales of interest are all deeply sub-horizon -- but the size of regions over which perturbations are evaluated far exceed $ct$, where $t$ is the time elapsed since the start of emission (it is assumed that $t$ greatly exceeds the duration of emission). We find that to lowest order, when only wave fields $\propto 1/r$ are included, there is exact compensation between the energy loss of the oscillators and the energy emitted into space, which means $P(0)=0$ for the power spectrum of density fluctuations on the largest scales. This is consistent with the Zel'dovich bound. To the next order when near fields $\propto r^{-2}$ are included, however, $P(0)$ settles at late times to a positive value that depends only on time, as $t^{-2}$ (the same applies to an energy non-conserving term). Even though this effect looks like superluminal energy transport, there is no violation of causality because the two-point function vanishes completely for $r>t$ if the emission of each oscillator is truncated beyond some duration. The result calls to question any need of enlisting cosmic inflation to seed large scale density perturbations. When applied to fast radio bursts -- uniformly distributed transients (to lowest order) that repeat at other locations -- the result supports Hoyle's hypothesis of constant energy injection.

astro-ph.CO↗

arXiv2017

The speed of transmission of phase modulated signals through a plasma medium

The impossibility of sending pulses of radio waves (Morse codes) through an ionized medium, despite the superluminal phase velocity of the constituent modes, has been demonstrated in many and various ways; essentially the reason is because each pulse, or wave packet, propagate through the plasma at the group velocity, which is subluminal. Nevertheless, messages can also be encoded as {\it phase} modulations of a monochromatic carrier wave, with more than one constituent modes (which may mathematically be extracted by Fourier transform). These modes propagate at their respective phase velocities and, upon reassembling them on the receiver's side, can become the original signal with the original message it bore having propagated at the phase velocity of the carrier wave, \ie~superluminally. We provide a concrete working scenario of transmitting a message for arrival with a time lead (compared to vacuum propagation) which is an order magnitude more than the duration of the message itself. It is also shown that the distortion of the signal due to the multiplicity of modes induced by the phase modulation is minimal if the bandwidth of the signal, including the duration of its onset and offset, is a small fraction of the carrier wave frequency. Thus phase and pulse modulations are fundamentally different phenomena in terms of their propagation speeds.

physics.gen-ph↗

arXiv2017

Improvement in the accuracy of flux measurement of radio sources by exploiting an arithmetic pattern in photon bunching noise

A hierarchy of statistics of increasing sophistication and accuracy is proposed, to exploit an interesting and fundamental arithmetic structure in the photon bunching noise of incoherent light of large photon occupation number, with the purpose of suppressing the noise and rendering a more reliable and unbiased measurement of the light intensity. The method does not require any new hardware, rather it operates at the software level, with the help of high precision computers, to reprocess the intensity time series of the incident light to create a new series with smaller bunching noise coherence length. The ultimate accuracy improvement of this method of flux measurement is limited by the timing resolution of the detector and the photon occupation number of the beam (the higher the photon number the better the performance). The principal application is accuracy improvement in the bolometric flux measurement of a radio source.

astro-ph.IM↗

arXiv2017

The role of Cerenkov radiation in the pressure balance of cool core clusters of galaxies

Despite the substantial progress made recently in understanding the role of AGN feedback and associated non-thermal effects, the precise mechanism that prevents the core of some clusters of galaxies from collapsing catastrophically by radiative cooling remains unidentified. In this paper we demonstrate that the evolution of a cluster's cooling core, in terms of its density, temperature, and magnetic field strength, inevitably enables the plasma electrons there to quickly become Cerenkov loss dominated, with emission at the radio frequency of $\lesssim$ 350 Hz, and with a rate considerably exceeding free-free continuum and line emission. However, the same does not apply to the plasmas at the cluster's outskirts, which lacks such radiation. Owing to its low frequency, the radiation cannot escape, but because over the relevant scale size of a Cerenkov wavelength the energy of an electron in the gas cannot follow the Boltzmann distribution to the requisite precision to ensure reabsorption always occurs slower than stimulated emission, the emitting gas cools before it reheats. This leaves behind the radiation itself, trapped by the overlying reflective plasma, yet providing enough pressure to maintain quasi-hydrostatic equilibrium. The mass condensation then happens by Rayleigh-Taylor instability, at a rate determined by the outermost radius where Cerenkov radiation can occur. In this way, it is possible to estimate the rate at $\ap 2 M_\odot$~year$^{-1}$, consistent with observational inference. Thus the process appears to provide a natural solution to the long standing problem of `cooling flow' in clusters; at least it offers another line of defense against cooling and collapse, should gas heating by AGN feedback be inadequate in some clusters

astro-ph.HE↗