13,557 research outputs found
Candidates for a possible third-generation gravitational wave detector: comparison of ring-Sagnac and sloshing-Sagnac speedmeter interferometers
Speedmeters are known to be quantum non-demolition devices and, by potentially providing sensitivity beyond the standard quantum limit, become interesting for third generation gravitational wave detectors. Here we introduce a new configuration, the sloshing-Sagnac interferometer, and compare it to the more established ring-Sagnac interferometer. The sloshing-Sagnac interferometer is designed to provide improved quantum noise limited sensitivity and lower coating thermal noise than standard position meter interferometers employed in current gravitational wave detectors. We compare the quantum noise limited sensitivity of the ring-Sagnac and the sloshing-Sagnac interferometers, in the frequency range, from 5 Hz to 100 Hz, where they provide the greatest potential benefit. We evaluate the improvement in terms of the unweighted noise reduction below the standard quantum limit, and by finding the range up to which binary black hole inspirals may be observed. The sloshing-Sagnac was found to give approximately similar or better sensitivity than the ring-Sagnac in all cases. We also show that by eliminating the requirement for maximally-reflecting cavity end mirrors with correspondingly-thick multi-layer coatings, coating noise can be reduced by a factor of approximately 2.2 compared to conventional interferometers
Dark matter within high surface brightness spiral galaxies
We present results from a detailed dynamical analysis of five high surface
brightness, late type spirals, studied with the aim to quantify the
luminous-to-dark matter ratio inside their optical radii. The galaxies' stellar
light distribution and gas kinematics have been observed and compared to
hydrodynamic gas simulations, which predict the 2D gas dynamics arising in
response to empirical gravitational potentials, which are combinations of
differing stellar disk and dark halo contributions. The gravitational potential
of the stellar disk was derived from near-infrared photometry, color-corrected
to constant (M/L); the dark halo was modelled by an isothermal sphere with a
core. Hydrodynamic gas simulations were performed for each galaxy for a
sequence of five different mass fractions of the stellar disk and for a wide
range of spiral pattern speeds. These two parameters mainly determine the
modelled gas distribution and kinematics. The agreement between the
non-axisymmetric part of the simulated and observed gas kinematics permitted us
to conclude that the galaxies with the highest rotation velocities tend to
possess near-maximal stellar disks. In less massive galaxies, with v_max<200
km/s, the mass of the dark halo at least equals the stellar mass within 2-3
R_disk. The simulated gas morphology provides a powerful tool to determine the
dominant spiral pattern speed. The corotation radius for all galaxies was found
to be constant at R_corotation ~ 3 R_disk and encloses the strong part of the
stellar spiral in all cases.Comment: 28 pages, 7 figures; to appear in the Astrophysical Journal, Vol.
586, March 200
Modelling the Galaxy in the era of Gaia
The body of photometric and astrometric data on stars in the Galaxy has been
growing very fast in recent years (Hipparcos/Tycho, OGLE-3, 2-Mass, DENIS,
UCAC2, SDSS, RAVE, Pan Starrs, Hermes, ...) and in two years ESA will launch
the Gaia satellite, which will measure astrometric data of unprecedented
precision for a billion stars. On account of our position within the Galaxy and
the complex observational biases that are built into most catalogues, dynamical
models of the Galaxy are a prerequisite full exploitation of these catalogues.
On account of the enormous detail in which we can observe the Galaxy, models of
great sophistication are required. Moreover, in addition to models we require
algorithms for observing them with the same errors and biases as occur in real
observational programs, and statistical algorithms for determining the extent
to which a model is compatible with a given body of data.
JD5 reviewed the status of our knowledge of the Galaxy, the different ways in
which we could model the Galaxy, and what will be required to extract our
science goals from the data that will be on hand when the Gaia Catalogue
becomes available.Comment: Proceedings of Joint Discussion 5 at IAU XXVII, Rio de Janeiro,
August 2009; 31 page
Design of a speed meter interferometer proof-of-principle experiment
The second generation of large scale interferometric gravitational wave
detectors will be limited by quantum noise over a wide frequency range in their
detection band. Further sensitivity improvements for future upgrades or new
detectors beyond the second generation motivate the development of measurement
schemes to mitigate the impact of quantum noise in these instruments. Two
strands of development are being pursued to reach this goal, focusing both on
modifications of the well-established Michelson detector configuration and
development of different detector topologies. In this paper, we present the
design of the world's first Sagnac speed meter interferometer which is
currently being constructed at the University of Glasgow. With this
proof-of-principle experiment we aim to demonstrate the theoretically predicted
lower quantum noise in a Sagnac interferometer compared to an equivalent
Michelson interferometer, to qualify Sagnac speed meters for further research
towards an implementation in a future generation large scale gravitational wave
detector, such as the planned Einstein Telescope observatory.Comment: Revised version: 16 pages, 6 figure
The next detectors for gravitational wave astronomy
This paper focuses on the next detectors for gravitational wave astronomy
which will be required after the current ground based detectors have completed
their initial observations, and probably achieved the first direct detection of
gravitational waves. The next detectors will need to have greater sensitivity,
while also enabling the world array of detectors to have improved angular
resolution to allow localisation of signal sources. Sect. 1 of this paper
begins by reviewing proposals for the next ground based detectors, and presents
an analysis of the sensitivity of an 8 km armlength detector, which is proposed
as a safe and cost-effective means to attain a 4-fold improvement in
sensitivity. The scientific benefits of creating a pair of such detectors in
China and Australia is emphasised. Sect. 2 of this paper discusses the high
performance suspension systems for test masses that will be an essential
component for future detectors, while sect. 3 discusses solutions to the
problem of Newtonian noise which arise from fluctuations in gravity gradient
forces acting on test masses. Such gravitational perturbations cannot be
shielded, and set limits to low frequency sensitivity unless measured and
suppressed. Sects. 4 and 5 address critical operational technologies that will
be ongoing issues in future detectors. Sect. 4 addresses the design of thermal
compensation systems needed in all high optical power interferometers operating
at room temperature. Parametric instability control is addressed in sect. 5.
Only recently proven to occur in Advanced LIGO, parametric instability
phenomenon brings both risks and opportunities for future detectors. The path
to future enhancements of detectors will come from quantum measurement
technologies. Sect. 6 focuses on the use of optomechanical devices for
obtaining enhanced sensitivity, while sect. 7 reviews a range of quantum
measurement options
Gravitational wave astronomy
The first decade of the new millenium should see the first direct detections
of gravitational waves. This will be a milestone for fundamental physics and it
will open the new observational science of gravitational wave astronomy. But
gravitational waves already play an important role in the modeling of
astrophysical systems. I review here the present state of gravitational
radiation theory in relativity and astrophysics, and I then look at the
development of detector sensitivity over the next decade, both on the ground
(such as LIGO) and in space (LISA). I review the sources of gravitational waves
that are likely to play an important role in observations by first- and
second-generation interferometers, including the astrophysical information that
will come from these observations. The review covers some 10 decades of
gravitational wave frequency, from the high-frequency normal modes of neutron
stars down to the lowest frequencies observable from space. The discussion of
sources includes recent developments regarding binary black holes, spinning
neutron stars, and the stochastic background.Comment: 29 pages, 2 figures, as submitted for special millenium issue of
Classical and Quantum Gravit
Scattered light images of spiral arms in marginally gravitationally unstable discs with an embedded planet
Scattered light images of transition discs in the near-infrared often show
non-axisymmetric structures in the form of wide-open spiral arms in addition to
their characteristic low-opacity inner gap region. We study self-gravitating
discs and investigate the influence of gravitational instability on the shape
and contrast of spiral arms induced by planet-disc interactions.
Two-dimensional non-isothermal hydrodynamical simulations including viscous
heating and a cooling prescription are combined with three-dimensional dust
continuum radiative transfer models for direct comparison to observations. We
find that the resulting contrast between the spirals and the surrounding disc
in scattered light is by far higher for pressure scale height variations, i.e.
thermal perturbations, than for pure surface density variations. Self-gravity
effects suppress any vortex modes and tend to reduce the opening angle of
planet-induced spirals, making them more tightly wound. If the disc is only
marginally gravitationally stable with a Toomre parameter around unity, an
embedded massive planet (planet-to-star mass ratio of ) can trigger
gravitational instability in the outer disc. The spirals created by this
instability and the density waves launched by the planet can overlap resulting
in large-scale, more open spiral arms in the outer disc. The contrast of these
spirals is well above the detection limit of current telescopes.Comment: Accepted for publication in MNRAS; 13 pages, 8 figure
Resolving the dynamical mass of a z~1.3 QSO host galaxy using SINFONI and Laser Guide Star assisted Adaptive Optics
Recent studies of the tight scaling relations between the masses of
supermassive black holes and their host galaxies have suggested that in the
past black holes constituted a larger fraction of their host galaxies' mass.
However, these arguments are limited by selection effects and difficulties in
determining robust host galaxy masses at high redshifts. Here we report the
first results of a new, complementary diagnostic route: we directly determine a
dynamical host galaxy mass for the z=1.3 luminous quasar J090543.56+043347.3
through high-spatial-resolution (0.47", 4kpc FWHM) observations of the host
galaxy gas kinematics over 30x40 kpc using ESO/VLT/SINFONI with LGS/AO.
Combining our result of M_dyn = 2.05+1.68_0.74 x 10^11 M_sun (within a radius
5.25 +- 1.05 kpc) with M_BH,MgII = 9.02 \pm 1.43 x 10^8 M_sun, M_BH,Halpha =
2.83 +1.93-1.13 x 10^8 M_sun, we find that the ratio of black hole mass to host
galaxy dynamical mass for J090543.56+043347.3 matches the present-day relation
for M_BH vs. M_Bulge,Dyn, well within the IR scatter, deviating at most a
factor of two from the mean. J090543.56+043347.3 displays clear signs of an
ongoing tidal interaction and of spatially extended star formation at a rate of
50-100 M_sun/yr, above the cosmic average for a galaxy of this mass and
redshift. We argue that its subsequent evolution may move J090543.56+043347.3
even closer to the z=0 relation for M_BH vs. M_Bulge,Dyn. Our results support
the picture where any substantive evolution in these relations must occur prior
to z~1.3. Having demonstrated the power of this modelling approach we are
currently analyzing similar data on seven further objects to better constrain
such evolution.Comment: Accepted for publication in ApJ, 14 pages, 10 Figure
Testing hydrodynamics schemes in galaxy disc simulations
We examine how three fundamentally different numerical hydrodynamics codes follow the evolution of an isothermal galactic disc with an external spiral potential. We compare an adaptive mesh refinement code (RAMSES), a smoothed particle hydrodynamics code (SPHNG), and a volume-discretised meshless code (GIZMO). Using standard refinement criteria, we find that RAMSES produces a disc that is less vertically concentrated and does not reach such high densities as the SPHNG or gizmo runs. The gas surface density in the spiral arms increases at a lower rate for the RAMSES simulations compared to the other codes. There is also a greater degree of substructure in the SPHNG and GIZMOruns and secondary spiral arms are more pronounced. By resolving the Jeansâ length with a greater number of grid cells we achieve more similar results to the Lagrangian codes used in this study. Other alterations to the refinement scheme (adding extra levels of refinement and refining based on local density gradients) are less successful in reducing the disparity between RAMSES and SPHNG/GIZMO. Although more similar, SPHNG displays different density distributions and vertical mass profiles to all modes of gizmo (including the smoothed particle hydrodynamics version). This suggests differences also arise which are not intrinsic to the particular method but rather due to its implementation. The discrepancies between codes (in particular, the densities reached in the spiral arms) could potentially result in differences in the locations and timescales for gravitational collapse, and therefore impact star formation activity in more complex galaxy disc simulations
Modelling of the Surface Emission of the Low-Magnetic Field Magnetar SGR 0418+5729
We perform a detailed modelling of the post-outburst surface emission of the
low magnetic field magnetar SGR 0418+5729. The dipolar magnetic field of this
source, B=6x10^12 G estimated from its spin-down rate, is in the observed range
of magnetic fields for normal pulsars. The source is further characterized by a
high pulse fraction and a single-peak profile. Using synthetic temperature
distribution profiles, and fully accounting for the general-relativistic
effects of light deflection and gravitational redshift, we generate synthetic
X-ray spectra and pulse profiles that we fit to the observations. We find that
asymmetric and symmetric surface temperature distributions can reproduce
equally well the observed pulse profiles and spectra of SGR 0418. Nonetheless,
the modelling allows us to place constraints on the system geometry (i.e. the
angles and that the rotation axis makes with the line of sight and
the dipolar axis, respectively), as well as on the spot size and temperature
contrast on the neutron star surface. After performing an analysis iterating
between the pulse profile and spectra, as done in similar previous works, we
further employed, for the first time in this context, a Markov-Chain
Monte-Carlo approach to extract constraints on the model parameters from the
pulse profiles and spectra, simultaneously. We find that, to reproduce the
observed spectrum and flux modulation: (a) the angles must be restricted to
or ; (b) the
temperature contrast between the poles and the equator must be at least a
factor of , and (c) the size of the hottest region ranges between
0.2-0.7 km (including uncertainties on the source distance). Last, we interpret
our findings within the context of internal and external heating models.Comment: 13 pages, 10 figures. Accepted for publication in MNRA
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