Relativistic Beaming and Flux Variability in Active Galactic Nuclei

We discuss the impact of special relativistic effects on the observed light curves and variability duty cycles of AGNs. We model the properties of AGN light curves at radio wavelengths using a simulated shot noise process in which the occurrence of major flaring events in a relativistic jet is governed by Poisson statistics. We show that flaring sources whose radiation is highly beamed toward us are able to reach very high flux levels, but will in fact spend most of their time in relatively low flaring states due to relativistic contraction of flare time scales in the observer frame. The fact that highly beamed AGNs do not return to a steady-state quiescent level between flares implies that their weakly beamed counterparts should have highly stable flux densities that result from a superposition of many long-term, low-amplitude flares. The ``apparent'' quiescent flux levels of these weakly beamed AGNs (identified in many unified models as radio galaxies) will be significantly higher than their ''true'' quiescent (i.e., non-flaring) levels. We use Monte Carlo simulations to investigate flux variability bias in the selection statistics of flat-spectrum AGN samples. In the case of the Caltech-Jodrell Flat-spectrum survey, the predicted orientation bias towards jets seen end-on is weakened if the parent population is variable, since the highly beamed sources have a stronger tendency to be found in low flaring states. This effect is small, however, since highly beamed sources are relatively rare, and their fluxes tend to be boosted sufficiently above the survey limit such that they are selected regardless of their flaring level. We find that for larger flat-spectrum AGN surveys with fainter flux cutoffs, variability should not be an appreciable source of selection bias.Comment: Accepted for publication in the Astrophysical Journa

Detecting very-high-frequency relic gravitational waves by electromagnetic wave polarizations in a waveguide

The polarization vector (PV) of an electromagnetic wave (EW) will experience a rotation in a region of spacetime perturbed by gravitational waves (GWs). Based on this idea, Cruise's group has built an annular waveguide to detect GWs. We give detailed calculations of the rotations of the polarization vector of an EW caused by incident GWs from various directions and in various polarization states, and then analyze the accumulative effects on the polarization vector when the EW passes n cycles along the annular waveguide. We reexamine the feasibility and limitation of this method to detect GWs of high frequency around 100 MHz, in particular, the relic gravitational waves (RGWs). By comparing the spectrum of RGWs in the accelerating universe with the detector sensitivity of the current waveguide, it is found that the amplitude of the RGWs is too low to be detected by the waveguide detectors currently running. Possible ways of improvements on detection are discussed also.Comment: 18pages, 10 figures, accepted by ChJA

Upper limits on the amplitude of ultra-high-frequency gravitational waves from graviton to photon conversion

In this work, we present the first experimental upper limits on the presence of stochastic gravitational waves in a frequency band with frequencies above 1 THz. We exclude gravitational waves in the frequency bands from (2.7 - 14) × 10 14 Hz and (5 - 12) × 10 18 Hz down to a characteristic amplitude of hcmin≈6×10-26 and hcmin≈5×10-28 at 95% confidence level, respectively. To obtain these results, we used data from existing facilities that have been constructed and operated with the aim of detecting weakly interacting slim particles, pointing out that these facilities are also sensitive to gravitational waves by graviton to photon conversion in the presence of a magnetic field. The principle applies to all experiments of this kind, with prospects of constraining (or detecting), for example, gravitational waves from light primordial black-hole evaporation in the early universe

Upper limits on the amplitude of ultra-high-frequency gravitational waves from graviton to photon conversion

In this work, we present the first experimental upper limits on the presence of stochastic gravitational waves in a frequency band with frequencies above 1 THz. We exclude gravitational waves in the frequency bands from (2.7−14)× 10 14 (2.7−14)×1014 Hz and (5−12)× 10 18 (5−12)×1018 Hz down to a characteristic amplitude of h min c ≈6× 10 −26 hcmin≈6×10−26 and h min c ≈5× 10 −28 hcmin≈5×10−28 at 95% confidence level, respectively. To obtain these results, we used data from existing facilities that have been constructed and operated with the aim of detecting weakly interacting slim particles, pointing out that these facilities are also sensitive to gravitational waves by graviton to photon conversion in the presence of a magnetic field. The principle applies to all experiments of this kind, with prospects of constraining (or detecting), for example, gravitational waves from light primordial black-hole evaporation in the early universe

A gravitational wave window on extra dimensions

We report on the possibility of detecting a submillimetre-sized extra dimension by observing gravitational waves (GWs) emitted by pointlike objects orbiting a braneworld black hole. Matter in the `visible' universe can generate a discrete spectrum of high frequency GWs with amplitudes moderately weaker than the predictions of general relativity (GR), while GW signals generated by matter on a `shadow' brane hidden in the bulk are potentially strong enough to be detected using current technology. We know of no other astrophysical phenomena that produces GWs with a similar spectrum, which stresses the need to develop detectors capable of measuring this high-frequency signature of large extra dimensions.Comment: 9 pages, 5 figure

Revisit relic gravitational waves based on the latest CMB observations

According to the CMB observations, Mielczarek (\cite{Mielczarek}) evaluated the reheating temperature, which could help to determine the history of the Universe. In this paper, we recalculate the reheating temperature using the new data from WMAP 7 observations. Based on that, we list the approximate solutions of relic gravitational waves (RGWs) for various frequency bands. With the combination of the quantum normalization of RGWs when they are produced and the CMB observations, we obtain the relation between the tensor-to-scalar ratio $r$ and the inflation index $\beta$ for a given scalar spectral index $n_s$. As a comparison, the diagram $r-\beta$ in the slow-roll inflation model is also given. Thus, the observational limits of $r$ from CMB lead to the constraints on the value of $\beta$. Then, we illustrate the energy density spectrum of RGWs with the quantum normalization for different values of $r$ and the corresponding $\beta$. For comparison, the energy density spectra of RGWs with parameters based on slow-roll inflation are also discussed. We find that the values of $n_s$ affect the spectra of RGWs sensitively in the very high frequencies. Based on the current and planed gravitational wave detectors, we discuss the detectabilities of RGWs.Comment: 16 pages, 6 figures, accepted for publication in Class. Quantum Gra

Gravitational waves from galaxy encounters

We discuss the emission of gravitational radiation produced in encounters of dark matter galactic halos. To this aim we perform a number of numerical simulations of typical galaxy mergers, computing the associated gravitational radiation waveforms as well as the energy released in the processes. Our simulations yield dimensionless gravitational wave amplitudes of the order of $10^{-13}$ and gravitational wave frequencies of the order of $10^{-16}$ Hz, when the galaxies are located at a distance of 10 Mpc. These values are of the same order as those arising in the gravitational radiation originated by strong variations of the gravitational field in the early Universe, and therefore, such gravitational waves cannot be directly observed by ground-based detectors. We discuss the feasibility of an indirect detection by means of the B-mode polarization of the Cosmic Microwave Background (CMB) induced by such waves. Our results show that the gravitational waves from encounters of dark matter galactic halos leave much too small an imprint on the CMB polarization to be actually observed with ongoing and future missions.Comment: 9 pages with revtex style, 3 ps figures; to be published in Physical Review

A systematic review of the antecedents and prevalence of suicide, self-harm and suicide ideation in Australian Aboriginal and Torres Strait Islander youth

Suicide and self-harm represent serious global health problems and appear to be especially elevated amongst indigenous minority groups, and particularly amongst young people (aged 24 years or younger). This systematic review investigates for the first time the antecedents and prevalence of suicide, self-harm and suicide ideation among Australian Aboriginal and Torres Strait Islander youth. Web of Science, PubMed, PsychINFO, CINAHL databases and grey literature were searched from earliest records to April 2019 for eligible articles. Twenty-two empirical articles met the inclusion criteria. The data confirmed that indigenous youth in Australia have elevated rates of suicide, self-harm and suicidal ideation relative to the nonindigenous population. Risk factors included being incarcerated, substance use and greater social and emotional distress. Notably, though, information on predictors of suicide and self-harm remains scarce. The findings support and justify the increasing implementation of public health programs specifically aimed at tackling this crisis. Based on the review findings, we argued that Aboriginal communities are best positioned to identify and understand the antecedents of youth self-harm, suicide ideation and suicide, and to take the lead in the development of more effective mental health preventive strategies and public policies within their communities

Gravitational eigenstates in weak gravity I: dipole decay rates of charged particles

The experimental demonstration that neutrons can reside in gravitational quantum stationary states formed in the gravitational field of the Earth indicates a need to examine in more detail the general theoretical properties of gravitational eigenstates. Despite the almost universal study of quantum theory applied to atomic and molecular states very little work has been done to investigate the properties of the hypothetical stationary states that should exist in similar types of gravitational central potential wells, particularly those with large quantum numbers. In this first of a series of papers, we attempt to address this shortfall by developing analytic, non-integral expressions for the electromagnetic dipole state-to-state transition rates of charged particles for any given initial and final gravitational quantum states. The expressions are non-relativistic and hence valid provided the eigenstate wavefunctions do not extend significantly into regions of strong gravity. The formulae may be used to obtain tractable approximations to the transition rates that can be used to give general trends associated with certain types of transitions. Surprisingly, we find that some of the high angular momentum eigenstates have extremely long lifetimes and a resulting stability that belies the multitude of channels available for state decay.Comment: 25 pages, 2 tables, 2 figure