597 research outputs found
Magnitude bias of microlensed sources towards the Large Magellanic Cloud
There are lines of evidence suggesting that some of the observed microlensing
events in the direction of the Large Magellanic Cloud (LMC) are caused by
ordinary star lenses as opposed to dark Machos in the Galactic halo. Efficient
lensing by ordinary stars generally requires the presence of one or more
additional concentrations of stars along the line of sight to the LMC disk. If
such a population behind the LMC disk exists, then the source stars (for
lensing by LMC disk objects) will be drawn preferentially from the background
population and will show systematic differences from LMC field stars. One such
difference is that the (lensed) source stars will be farther away than the
average LMC field stars, and this should be reflected in their apparent
baseline magnitudes. We focus on red clump stars: these should appear in the
color-magnitude diagram at a few tenths of a magnitude fainter than the field
red clump. Suggestively, one of the two near-clump confirmed events,
MACHO-LMC-1, is a few tenths of magnitude fainter than the clump.Comment: To appear in ApJ Letters. Shortened to match the accepted version, 8
pages plus 1 ps figur
Microlens Parallax Asymmetries Toward the LMC
If the microlensing events now being detected toward the Large Magellanic
Cloud (LMC) are due to lenses in the Milky Way halo, then the events should
typically have asymmetries of order 1% due to parallax from the reflex motion
of the Earth. By contrast, if the lenses are in the LMC, the parallax effects
should be negligible. A ground-based search for such parallax asymmetries would
therefore clarify the location of the lenses. A modest effort (2 hours per
night on a 1 m telescope) could measure 15 parallax asymmetries over 5 years
and so marginally discriminate between the halo and the LMC as the source of
the lenses. A dedicated 1 m telescope would approximately double the number of
measurements and would therefore clearly distinguish between the alternatives.
However, compared to satellite parallaxes, the information extracted from
ground-based parallaxes is substantially less useful for understanding the
nature of the halo lenses (if that is what they are). The backgrounds of
asymmetries due to binary-source and binary-lens events are estimated to be
approximately 7% and 12% respectively. These complicate the interpretation of
detected parallax asymmetries, but not critically.Comment: Submitted to ApJ, 17 pages, including 2 embedded figure
Microlens Parallaxes with SIRTF
The Space Infrared Telescope Facility (SIRTF) will drift away from the Earth
at about 0.1 AU/yr. Microlensing events will therefore have different
characteristics as seen from the satellite and the Earth. From the difference,
it is possible in principle to measure v-tilde, the transverse velocity of the
lens projected onto the observer plane. Since v-tilde has very different values
for different populations (disk, halo, Large Magellanic Cloud), such
measurements could help identify the location, and hence the nature, of the
lenses. I show that the method previously developed by Gould for measuring such
satellite parallaxes fails completely in the case of SIRTF: it is overwhelmed
by degeneracies which arise from fact that the Earth and satellite observations
are in different band passes. I develop a new method which allows for
observations in different band passes and yet removes all degeneracies. The
method combines a purely ground-based measurement of the "parallax asymmetry"
with a measurement of the delay between the time the event peaks at the Earth
and satellite. In effect, the parallax asymmetry determines the component of
v-tilde in the Earth-Sun direction, while the delay time measures the component
of v-tilde in the direction of the Earth's orbit.Comment: 21 pages plus 3 figure
Complete Parallax and Proper Motion Solutions For Halo Binary-Lens Microlensing Events
A major problem in the interpretation of microlensing events is that the only
measured quantity, the Einstein time scale t_E, is a degenerate combination of
the three quantities one would like to know, the mass, distance, and speed of
the lens. This degeneracy can be partly broken by measuring either a "parallax"
or a "proper motion" and completely broken by measuring both. Proper motions
can easily be measured for caustic-crossing binary-lens events. Here we examine
the possibility (first discussed by Hardy & Walker) that one could also measure
a parallax for some of these events by comparing the light curves of the
caustic crossing as seen from two observatories on Earth. We derive analytic
expressions for the signal-to-noise ratio of the parallax measurement in terms
of the characteristics of the source and the geometry of the event. For
Galactic halo binary lenses seen toward the LMC, the light curve is delayed
from one continent to another by a seemingly minuscule 15 seconds (compared to
t_E ~ 40 days). However, this is sufficient to cause a difference in
magnification of order 10%. To actually extract complete parallax information
(as opposed to merely detecting the effect) requires observations from three
non-collinear observatories. Parallaxes cannot be measured for binary lenses in
the LMC but they can be measured for Galactic halo binary lenses seen toward
M31. Robust measurements are possible for disk binary lenses seen toward the
Galactic bulge, but are difficult for bulge binary lenses.Comment: Revised to take account of important work by Hardy & Walker (1995
Inferring the IGM thermal history during reionisation with the Lyman-α forest power spectrum at redshift z â 5
We use cosmological hydrodynamical simulations to assess the feasibility of constraining the thermal history of the intergalactic medium during reionisation with the Lyα forest at z â 5. The integrated thermal history has a measureable impact on the transmitted flux power spectrum that can be isolated from Doppler broadening at this redshift. We parameterise this using the cumulative energy per proton, uâ, deposited into a gas parcel at the mean background density, a quantity that is tightly linked with the gas density power spectrum in the simulations. We construct mock observations of the line of sight Lyα forest power spectrum and use a Markov Chain Monte Carlo approach to recover uâ at redshifts 5âČzâČ12. A statistical uncertainty of ⌠20 per cent is expected (at 68 per cent confidence) at z â 5 using high resolution spectra with a total redshift path length of Îz = 4 and a typical signal-to-noise ratio of S/N = 15 per pixel. Estimates for the expected systematic uncertainties are comparable, such that existing data should enable a measurement of uâ to within ⌠30 per cent. This translates to distinguishing between reionisation scenarios with similar instantaneous temperatures at z â 5, but with an energy deposited per proton that differs by 2â3eV over the redshift interval 5âČzâČ12. For an initial temperature of T ⌠10âŽK following reionisation, this corresponds to the difference between early (zre = 12) and late (zre = 7) reionisation in our models
Detection of Ly\beta auto-correlations and Ly\alpha-Ly\beta cross-correlations in BOSS Data Release 9
The Lyman- forest refers to a region in the spectra of distant quasars
that lies between the rest-frame Lyman- and Lyman- emissions.
The forest in this region is dominated by a combination of absorption due to
resonant Ly and Ly scattering. When considering the 1D Ly
forest in addition to the 1D Ly forest, the full statistical
description of the data requires four 1D power spectra: Ly and
Ly auto-power spectra and the Ly-Ly real and imaginary
cross-power spectra. We describe how these can be measured using an optimal
quadratic estimator that naturally disentangles Ly and Ly
contributions. Using a sample of approximately 60,000 quasar sight-lines from
the BOSS Data Release 9, we make the measurement of the one-dimensional power
spectrum of fluctuations due to the Ly resonant scattering. While we
have not corrected our measurements for resolution damping of the power and
other systematic effects carefully enough to use them for cosmological
constraints, we can robustly conclude the following: i) Ly power
spectrum and Ly-Ly cross spectra are detected with high
statistical significance; ii) the cross-correlation coefficient is
on large scales; iii) the Ly measurements are contaminated by the
associated OVI absorption, which is analogous to the SiIII contamination of the
Ly forest. Measurements of the Ly forest will allow extension of
the usable path-length for the Ly measurements while allowing a better
understanding of the physics of intergalactic medium and thus more robust
cosmological constraints.Comment: 26 pages, 10 figures; matches version accepted by JCA
Photometric selection of Type Ia supernovae in the Supernova Legacy Survey
We present a sample of 485 photometrically identified Type Ia supernova
candidates mined from the first three years of data of the CFHT SuperNova
Legacy Survey (SNLS). The images were submitted to a deferred processing
independent of the SNLS real-time detection pipeline. Light curves of all
transient events were reconstructed in the g_M, r_M, i_M and z_M filters and
submitted to automated sequential cuts in order to identify possible
supernovae. Pure noise and long-term variable events were rejected by light
curve shape criteria. Type Ia supernova identification relied on event
characteristics fitted to their light curves assuming the events to be normal
SNe Ia. The light curve fitter SALT2 was used for this purpose, assigning host
galaxy photometric redshifts to the tested events. The selected sample of 485
candidates is one magnitude deeper than that allowed by the SNLS spectroscopic
identification. The contamination by supernovae of other types is estimated to
be 4%. Testing Hubble diagram residuals with this enlarged sample allows us to
measure the Malmquist bias due to spectroscopic selections directly. The result
is fully consistent with the precise Monte Carlo based estimate used to correct
SN Ia distance moduli in the SNLS 3-year cosmological analyses. This paper
demonstrates the feasibility of a photometric selection of high redshift
supernovae with known host galaxy redshifts, opening interesting prospects for
cosmological analyses from future large photometric SN Ia surveys.Comment: (The SNLS collaboration) 23 pages, 28 figures, Accepted in A&
Artificial Intelligence Approach to the Determination of Physical Properties of Eclipsing Binaries. I. The EBAI Project
Achieving maximum scientific results from the overwhelming volume of
astronomical data to be acquired over the next few decades will demand novel,
fully automatic methods of data analysis. Artificial intelligence approaches
hold great promise in contributing to this goal. Here we apply neural network
learning technology to the specific domain of eclipsing binary (EB) stars, of
which only some hundreds have been rigorously analyzed, but whose numbers will
reach millions in a decade. Well-analyzed EBs are a prime source of
astrophysical information whose growth rate is at present limited by the need
for human interaction with each EB data-set, principally in determining a
starting solution for subsequent rigorous analysis. We describe the artificial
neural network (ANN) approach which is able to surmount this human bottleneck
and permit EB-based astrophysical information to keep pace with future data
rates. The ANN, following training on a sample of 33,235 model light curves,
outputs a set of approximate model parameters (T2/T1, (R1+R2)/a, e sin(omega),
e cos(omega), and sin i) for each input light curve data-set. The whole sample
is processed in just a few seconds on a single 2GHz CPU. The obtained
parameters can then be readily passed to sophisticated modeling engines. We
also describe a novel method polyfit for pre-processing observational light
curves before inputting their data to the ANN and present the results and
analysis of testing the approach on synthetic data and on real data including
fifty binaries from the Catalog and Atlas of Eclipsing Binaries (CALEB)
database and 2580 light curves from OGLE survey data. [abridged]Comment: 52 pages, accepted to Ap
New approach for precise computation of Lyman-alpha forest power spectrum with hydrodynamical simulations
Current experiments are providing measurements of the flux power spectrum from the Lyman-α forests observed in quasar spectra with unprecedented accuracy. Their interpretation in terms of cosmological constraints requires specific simulations of at least equivalent precision. In this paper, we present a suite of cosmological N-body simulations with cold dark matter and baryons, specifically aiming at modeling the low-density regions of the inter-galactic medium as probed by the Lyman-α forests at high redshift. The simulations were run using the GADGET-3 code and were designed to match the requirements imposed by the quality of the current SDSS-III/BOSS or forthcoming SDSS-IV/eBOSS data. They are made using either 2 Ă 7683 1 billion or 2 Ă 1923 14 million particles, spanning volumes ranging from (25 Mpc hâ1)3 for high-resolution simulations to (100 Mpc hâ1)3 for large-volume ones. Using a splicing technique, the resolution is further enhanced to reach the equivalent of simulations with 2 Ă 30723 58 billion particles in a (100 Mpc hâ1)3 box size, i.e. a mean mass per gas particle of 1.2 Ă 105Mâ hâ1. We show that the resulting power spectrum is accurate at the 2% level over the full range from a few Mpc to several tens of Mpc. We explore the effect on the one-dimensional transmitted-flux power spectrum of four cosmological parameters (ns, Ï8, Ωm and H0) and two astrophysical parameters (T0 and Îł) that are related to the heating rate of the intergalactic medium. By varying the input parameters around a central model chosen to be in agreement with the latest Planck results, we built a grid of simulations that allows the study of the impact on the flux power spectrum of these six relevant parameters. We improve upon previous studies by not only measuring the effect of each parameter individually, but also probing the impact of the simultaneous variation of each pair of parameters. We thus provide a full second-order expansion, including cross-terms, around our central model. We check the validity of the second-order expansion with independent simulations obtained either with different cosmological parameters or different seeds. Finally, a comparison to the one-dimensional Lyman-α forest power spectrum obtained with BOSS by [1] shows an excellent agreement
Photometric selection of high-redshift type Ia supernovae
We present a method for selecting high-redshift type Ia supernovae (SNe Ia)
located via rolling SN searches. The technique, using both color and magnitude
information of events from only 2-3 epochs of multi-band real-time photometry,
is able to discriminate between SNe Ia and core collapse SNe. Furthermore, for
the SNe Ia, the method accurately predicts the redshift, phase and light-curve
parameterization of these events based only on pre-maximum-light data. We
demonstrate the effectiveness of the technique on a simulated survey of SNe Ia
and core-collapse SNe, where the selection method effectively rejects most
core-collapse SNe while retaining SNe Ia. We also apply the selection code to
real-time data acquired as part of the Canada-France-Hawaii Telescope Supernova
Legacy Survey (SNLS). During the period May 2004 to January 2005 in the SNLS,
440 SN candidates were discovered of which 70 were confirmed spectroscopically
as SNe Ia and 15 as core-collapse events. For this test dataset, the selection
technique correctly identifies 100% of the identified SNe II as non-SNe Ia with
only a 1-2% false rejection rate. The predicted parameterization of the SNe Ia
has a precision of |delta_z|/(1+z_spec)<0.09 in redshift, and +/- 2-3
rest-frame days in phase, providing invaluable information for planning
spectroscopic follow-up observations. We also investigate any bias introduced
by this selection method on the ability of surveys such as SNLS to measure
cosmological parameters (e.g., w and omega matter), and find any effect to be
negligible.Comment: Accepted for publication in A
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