1,320 research outputs found
Comparison of LISA and Atom Interferometry for Gravitational Wave Astronomy in Space
One of the atom interferometer gravitational wave missions proposed by
Dimopoulos et al.1 in 2008 was called AGIS-Sat. 2. It had a suggested
gravitational wave sensitivity set by the atom state detection shot noise level
that started at 1 mHz, was comparable to LISA sensitivity from 1 to about 20
mHz, and had better sensitivity from 20 to 500 mHz. The separation between the
spacecraft was 1,000 km, with atom interferometers 200 m long and shades from
sunlight used at each end. A careful analysis of many error sources was
included, but requirements on the time-stability of both the laser wavefront
aberrations and the atom temperatures in the atom clouds were not investigated.
After including these considerations, the laser wavefront aberration stability
requirement to meet the quoted sensitivity level is about 1\times10-8
wavelengths, and is far tighter than for LISA. Also, the temperature
fluctuations between atom clouds have to be less than 1 pK. An alternate atom
interferometer GW mission in Earth orbit called AGIS-LEO with 30 km satellite
separation has been suggested recently. The reduction of wavefront aberration
noise by sending the laser beam through a high-finesse mode-scrubbing optical
cavity is discussed briefly, but the requirements on such a cavity are not
given. Unfortunately, such an Earth-orbiting mission seems to be considerably
more difficult to design than a non-geocentric mission and does not appear to
have comparably attractive scientific goals.Comment: Submitted to Proc. 46th Rencontres de Moriond: Gravitational Waves
and Experimental Gravity, March 20 - 27, 2011, La Thuile, Ital
Estimating the detectable rate of capture of stellar mass black holes by massive central black holes in normal galaxies
The capture and subsequent inspiral of stellar mass black holes on eccentric
orbits by central massive black holes, is one of the more interesting likely
sources of gravitational radiation detectable by LISA. We estimate the rate of
observable events and the associated uncertainties. A moderately favourable
mass function could provide many detectable bursts each year, and a detection
of at least one burst per year is very likely given our current understanding
of the populations in cores of normal spiral galaxies.Comment: 3 pages 2-column revtex Latex macro. No figures. Classical and
Quantum Gravity, accepte
The influence of short term variations in AM CVn systems on LISA measurements
We study the effect of short term variations of the evolution of AM CVn
systems on their gravitational wave emissions and in particular LISA
observations. We model the systems according to their equilibrium mass-transfer
evolution as driven by gravitational wave emission and tidal interaction, and
determine their reaction to a sudden perturbation of the system. This is
inspired by the suggestion to explain the orbital period evolution of the
ultra-compact binary systems V407 Vul and RX-J0806+1527 by non-equilibrium mass
transfer. The characteristics of the emitted gravitational wave signal are
deduced from a Taylor expansion of a Newtonian quadrupolar emission model, and
the changes in signal structure as visible to the LISA mission are determined.
We show that short term variations can significantly change the higher order
terms in the expansion, and thus lead to spurious (non) detection of frequency
derivatives. This may hamper the estimation of the parameters of the system, in
particular their masses and distances. However, we find that overall detection
is still secured as signals still can be described by general templates. We
conclude that a better modelling of the effects of short term variations is
needed to prepare the community for astrophysical evaluations of real
gravitational wave data of AM CVn systems.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
The gravitational wave signal from the Galactic disk population of binaries containing two compact objects
We review the properties of Galactic binaries containing two compact objects,
as derived by means of population synthesis. Using this information we
calculate the gravitational wave signal of these binaries. At frequencies below
f ~< 2 mHz the double white dwarf population forms an unresolved background for
the low-frequency gravitational wave detector LISA. Above this limit some few
thousand double white dwarfs and few tens of binaries containing neutron stars
will be resolved. Of the resolved double white dwarfs ~500 have a total mass
above the Chandrasekhar limit. About ~95 of these have a measurable frequency
change allowing a determination of their chirp mass. We discuss the properties
of the resolved systems.Comment: Accepted for publication in A&
Delay time and tunneling transient phenomena
Analytic solutions to the time-dependent Schr\"odinger equation for cutoff
wave initial conditions are used to investigate the time evolution of the
transmitted probability density for tunneling. For a broad range of values of
the potential barrier opacity , we find that the probability density
exhibits two evolving structures. One refers to the propagation of a {\it
forerunner} related to a {\it time domain resonance} [Phys. Rev. A {\bf 64},
0121907 (2001)], while the other consists of a semiclassical propagating
wavefront. We find a regime where the {\it forerunners} are absent,
corresponding to positive {\it time delays}, and show that this regime is
characterized by opacities . The critical opacity
is derived from the analytical expression for the {\it delay time}, that
reflects a link between transient effects in tunneling and the {\it delay time}Comment: To be published in Physical Review
Transient tunneling effects of resonance doublets in triple barrier systems
Transient tunneling effects in triple barrier systems are investigated by
considering a time-dependent solution to the Schr\"{o}dinger equation with a
cutoff wave initial condition. We derive a two-level formula for incidence
energies near the first resonance doublet of the system. Based on that
expression we find that the probability density along the internal region of
the potential, is governed by three oscillation frequencies: one of them refers
to the well known Bohr frequency, given in terms of the first and second
resonance energies of the doublet, and the two others, represent a coupling
with the incidence energy . This allows to manipulate the above frequencies
to control the tunneling transient behavior of the probability density in the
short-time regim
Noise characterization for LISA
We consider the general problem of estimating the inflight LISA noise power
spectra and cross-spectra, which are needed for detecting and estimating the
gravitational wave signals present in the LISA data. For the LISA baseline
design and in the long wavelength limit, we bound the error on all spectrum
estimators that rely on the use of the fully symmetric Sagnac combination
(). This procedure avoids biases in the estimation that would otherwise
be introduced by the presence of a strong galactic background in the LISA data.
We specialize our discussion to the detection and study of the galactic white
dwarf-white dwarf binary stochastic signal.Comment: 9 figure
Annual modulation of the Galactic binary confusion noise bakground and LISA data analysis
We study the anisotropies of the Galactic confusion noise background and its
effects on LISA data analysis. LISA has two data streams of the gravitational
waves signals relevant for low frequency regime. Due to the anisotropies of the
background, the matrix for their confusion noises has off-diagonal components
and depends strongly on the orientation of the detector plane. We find that the
sky-averaged confusion noise level could change by a factor of 2
in three months, and would be minimum when the orbital position of LISA is
either around the spring or autumn equinox.Comment: 13 pages, 6 figure
An exploration of important factors in the decision-making process undertaken by foundation degree students with respect to level 6 progression
Deciding what degree to study and where are very important decisions to make for a range of practical and economic reasons, and much research has been undertaken in this area. What has not been considered previously is the decision-making processes of students who have completed an associate degree, such as a Higher National Diploma or Foundation Degree, and are now considering where to complete their Bachelor education. Associate degrees are often provided by further education colleges (FECs) in partnership with universities, and the choices available to college-based higher education students are whether to continue at the college or move to university. A mixed-methods approach was adopted comprising of an online questionnaire and subsequent focus groups of students studying at HE at a southwest FEC. The findings showed that equal importance is placed on the practical issues of proximity and familiarity as on course suitability, with the only differences between those remaining at college and those moving to university being financially based
Gravitational Helioseismology?
The magnitudes of the external gravitational perturbations associated with
the normal modes of the Sun are evaluated to determine whether these solar
oscillations could be observed with the proposed Laser Interferometer Space
Antenna (LISA), a network of satellites designed to detect gravitational
radiation. The modes of relevance to LISA---the , low-order , and
-modes---have not been conclusively observed to date. We find that the
energy in these modes must be greater than about in order
to be observable above the LISA detector noise. These mode energies are larger
than generally expected, but are much smaller than the current observational
upper limits. LISA may be confusion-limited at the relevant frequencies due to
the galactic background from short-period white dwarf binaries. Present
estimates of the number of these binaries would require the solar modes to have
energies above about to be observable by LISA.Comment: 8 pages; prepared with REVTEX 3.0 LaTeX macro
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