695 research outputs found
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
and the inflation index for a given scalar spectral index . As a
comparison, the diagram in the slow-roll inflation model is also
given. Thus, the observational limits of from CMB lead to the constraints
on the value of . Then, we illustrate the energy density spectrum of
RGWs with the quantum normalization for different values of and the
corresponding . For comparison, the energy density spectra of RGWs with
parameters based on slow-roll inflation are also discussed. We find that the
values of 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
and gravitational wave frequencies of the order of 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
- …