421 research outputs found
The POINT-AGAPE survey II: An Unrestricted Search for Microlensing Events towards M31
An automated search is carried out for microlensing events using a catalogue
of 44554 variable superpixel lightcurves derived from our three-year monitoring
program of M31. Each step of our candidate selection is objective and
reproducible by a computer. Our search is unrestricted, in the sense that it
has no explicit timescale cut. So, it must overcome the awkward problem of
distinguishing long-timescale microlensing events from long-period stellar
variables. The basis of the selection algorithm is the fitting of the
superpixel lightcurves to two different theoretical models, using variable star
and blended microlensing templates. Only if microlensing is preferred is an
event retained as a possible candidate. Further cuts are made with regard to
(i) sampling, (ii) goodness of fit of the peak to a Paczynski curve, (iii)
consistency of the microlensing hypothesis with the absence of a resolved
source, (iv) achromaticity, (v) position in the colour-magnitude diagram and
(vi) signal-to-noise ratio. Our results are reported in terms of first-level
candidates, which are the most trustworthy, and second-level candidates, which
are possible microlensing but have lower signal-to-noise and are more
questionable. The pipeline leaves just 3 first-level candidates, all of which
have very short full-width half-maximum timescale (<5 days) and 3 second-level
candidates, which have timescales of 31, 36 and 51 days respectively. We also
show 16 third-level lightcurves, as an illustration of the events that just
fail the threshold for designation as microlensing candidates. They are almost
certainly mainly variable stars. Two of the 3 first-level candidates correspond
to known events (PA 00-S3 and PA 00-S4) already reported by the POINT-AGAPE
project. The remaining first-level candidate is new.Comment: 22 pages, 18 figures, MNRAS, to appea
The POINT-AGAPE Survey: Comparing Automated Searches of Microlensing Events toward M31
Searching for microlensing in M31 using automated superpixel surveys raises a
number of difficulties which are not present in more conventional techniques.
Here we focus on the problem that the list of microlensing candidates is
sensitive to the selection criteria or "cuts" imposed and some subjectivity is
involved in this. Weakening the cuts will generate a longer list of
microlensing candidates but with a greater fraction of spurious ones;
strengthening the cuts will produce a shorter list but may exclude some genuine
events. We illustrate this by comparing three analyses of the same data-set
obtained from a 3-year observing run on the INT in La Palma. The results of two
of these analyses have been already reported: Belokurov et al. (2005) obtained
between 3 and 22 candidates, depending on the strength of their cuts, while
Calchi Novati et al. (2005) obtained 6 candidates. The third analysis is
presented here for the first time and reports 10 microlensing candidates, 7 of
which are new. Only two of the candidates are common to all three analyses. In
order to understand why these analyses produce different candidate lists, a
comparison is made of the cuts used by the three groups...Comment: 28 pages, 24 figures, 9 table
Confluence of geodesic paths and separating loops in large planar quadrangulations
We consider planar quadrangulations with three marked vertices and discuss
the geometry of triangles made of three geodesic paths joining them. We also
study the geometry of minimal separating loops, i.e. paths of minimal length
among all closed paths passing by one of the three vertices and separating the
two others in the quadrangulation. We concentrate on the universal scaling
limit of large quadrangulations, also known as the Brownian map, where pairs of
geodesic paths or minimal separating loops have common parts of non-zero
macroscopic length. This is the phenomenon of confluence, which distinguishes
the geometry of random quadrangulations from that of smooth surfaces. We
characterize the universal probability distribution for the lengths of these
common parts.Comment: 48 pages, 33 color figures. Final version, with one concluding
paragraph and one reference added, and several other small correction
Cosmic shear requirements on the wavelength-dependence of telescope point spread functions
Cosmic shear requires high precision measurement of galaxy shapes in the
presence of the observational Point Spread Function (PSF) that smears out the
image. The PSF must therefore be known for each galaxy to a high accuracy.
However, for several reasons, the PSF is usually wavelength dependent,
therefore the differences between the spectral energy distribution of the
observed objects introduces further complexity. In this paper we investigate
the effect of the wavelength-dependence of the PSF, focusing on instruments in
which the PSF size is dominated by the diffraction-limit of the telescope and
which use broad-band filters for shape measurement.
We first calculate biases on cosmological parameter estimation from cosmic
shear when the stellar PSF is used uncorrected. Using realistic galaxy and star
spectral energy distributions and populations and a simple three-component
circular PSF we find that the colour-dependence must be taken into account for
the next generation of telescopes. We then consider two different methods for
removing the effect (i) the use of stars of the same colour as the galaxies and
(ii) estimation of the galaxy spectral energy distribution using multiple
colours and using a telescope model for the PSF. We find that both of these
methods correct the effect to levels below the tolerances required for per-cent
level measurements of dark energy parameters. Comparison of the two methods
favours the template-fitting method because its efficiency is less dependent on
galaxy redshift than the broad-band colour method and takes full advantage of
deeper photometry.Comment: 10 pages, 8 figures, version accepted for publication in MNRA
The Anomaly in the Candidate Microlensing Event PA-99-N2
The lightcurve of PA-99-N2, one of the recently announced microlensing
candidates towards M31, shows small deviations from the standard Paczynski
form. We explore a number of possible explanations, including correlations with
the seeing, the parallax effect and a binary lens. We find that the
observations are consistent with an unresolved RGB or AGB star in M31 being
microlensed by a binary lens. We find that the best fit binary lens mass ratio
is about one hundredth, which is one of most extreme values found for a binary
lens so far. If both the source and lens lie in the M31 disk, then the standard
M31 model predicts the probable mass range of the system to be 0.02-3.6 solar
masses (95 % confidence limit). In this scenario, the mass of the secondary
component is therefore likely to be below the hydrogen-burning limit. On the
other hand, if a compact halo object in M31 is lensing a disk or spheroid
source, then the total lens mass is likely to lie between 0.09-32 solar masses,
which is consistent with the primary being a stellar remnant and the secondary
a low mass star or brown dwarf. The optical depth (or alternatively the
differential rate) along the line of sight toward the event indicates that a
halo lens is more likely than a stellar lens provided that dark compact objects
comprise no less than 15 per cent (or 5 per cent) of haloes.Comment: Latex, 23 pages, 9 figures, in press at The Astrophysical Journa
Classical novae from the POINT-AGAPE microlensing survey of M31 -- I. The nova catalogue
The POINT-AGAPE survey is an optical search for gravitational microlensing
events towards the Andromeda Galaxy (M31). As well as microlensing, the survey
is sensitive to many different classes of variable stars and transients. Here
we describe the automated detection and selection pipeline used to identify M31
classical novae (CNe) and we present the resulting catalogue of 20 CN
candidates observed over three seasons. CNe are observed both in the bulge
region as well as over a wide area of the M31 disk. Nine of the CNe are caught
during the final rise phase and all are well sampled in at least two colours.
The excellent light-curve coverage has allowed us to detect and classify CNe
over a wide range of speed class, from very fast to very slow. Among the
light-curves is a moderately fast CN exhibiting entry into a deep transition
minimum, followed by its final decline. We have also observed in detail a very
slow CN which faded by only 0.01 mag day over a 150 day period. We
detect other interesting variable objects, including one of the longest period
and most luminous Mira variables. The CN catalogue constitutes a uniquely
well-sampled and objectively-selected data set with which to study the
statistical properties of classical novae in M31, such as the global nova rate,
the reliability of novae as standard-candle distance indicators and the
dependence of the nova population on stellar environment. The findings of this
statistical study will be reported in a follow-up paper.Comment: 21 pages, 13 figures, re-submitted for publication in MNRAS, typos
corrected, references updated, figures 5-9 made cleare
The Angstrom Project: a microlensing survey of the structure and composition of the bulge of the Andromeda galaxy
The Andromeda Galaxy Stellar Robotic Microlensing Project (The Angstrom
Project) aims to use stellar microlensing events to trace the structure and
composition of the inner regions of the Andromeda Galaxy (M31). We present
microlensing rate and timescale predictions and spatial distributions for
stellar and sub-stellar lens populations in combined disk and barred bulge
models of M31. We show that at least half of the stellar microlenses in and
around the bulge are expected to have characteristic durations between 1 and 10
days, rising to as much as 80% for brown-dwarf dominated mass functions. These
short-duration events are mostly missed by current microlensing surveys that
are looking for Macho candidates in the M31 dark matter halo. Our models
predict that an intensive monitoring survey programme such as Angstrom, which
will be able to detect events of durations upwards of a day, could detect
around 30 events per season within ~5 arcminutes of the M31 centre, due to
ordinary low-mass stars and remnants. This yield increases to more than 60
events for brown-dwarf dominated mass functions. The overall number of events
and their average duration are sensitive diagnostics of the bulge mass, in
particular the contribution of low-mass stars and brown dwarfs. The combination
of an inclined disk, an offset bar-like bulge, and differences in the bulge and
disk luminosity functions results in a four-way asymmetry in the number of
events expected in each quadrant defined by the M31 disk axes. The asymmetry is
sensitive to the bar prolongation, orientation and mass.Comment: 9 pages, submitted to MNRA
The Point-Agape survey: 4 high signal-to-noise ratio microlensing candidates detected towards M31
We have carried out a survey of the Andromeda galaxy for unresolved
microlensing (pixel lensing). We present a subset of four short timescale, high
signal-to-noise microlensing candidates found by imposing severe selection
criteria: the source flux variation exceeds the flux of an R=21 magnitude star
and the full width at half maximum timescale is less than 25 days. Remarkably,
in three out of four cases, we have been able to measure or strongly constrain
the Einstein crossing time of the event. One event, which lies projected on the
M31 bulge, is almost certainly due to a stellar lens in the bulge of M31. The
other three candidates can be explained either by stars in M31 and M32 or by
MACHOs
A Short Timescale Candidate Microlensing Event in the POINT-AGAPE Pixel Lensing Survey of M31
We report the discovery of a short-duration microlensing candidate in the
northern field of the POINT-AGAPE pixel lensing survey towards M31. The
full-width half-maximum timescale is very short, just 1.8 days. Almost
certainly, the source star has been identified on Hubble Space Telescope
archival images, allowing us to infer an Einstein crossing time of 10.4 days, a
maximum magnification of about 18, and a lens-source proper motion greater than
0.3 microarcsec/day. The event lies projected at 8' from the center of M31,
which is beyond the bulk of the stellar lens population. The lens is likely to
reside in one of three locations. It may be a star in the M31 disk, or a
massive compact halo object (Macho) in either M31 or the Milky Way. The most
probable mass is 0.06 solar masses for an M31 Macho, 0.02 solar masses for a
Milky Way Macho and 0.2 solar masses for an M31 stellar lens. Whilst the
stellar interpretation is plausible, the Macho interpretation is the most
probable for halo fractions above 20%.Comment: 20 pages, 4 figures, submitted to The Astrophysical Journal (Letters
Non-holomorphic Corrections from Threebranes in F Theory
We construct solutions of type IIB supergravity dual to N=2 super Yang-Mills
theories. By considering a probe moving in a background with constant coupling
and an AdS_{5} component in its geometry, we are able to reproduce the exact
low energy effective action for the theory with gauge group SU(2) and N_{f}=4
massless flavors. After turning on a mass for the flavors we find corrections
to the AdS_{5} geometry. In addition, the coupling shows a power law dependence
on the energy scale of the theory. The origin of the power law behaviour of the
coupling is traced back to instanton corrections. Instanton corrections to the
four derivative terms in the low energy effective action are correctly obtained
from a probe analysis. By considering a Wilson loop in this geometry we are
also able to compute the instanton effects on the quark-antiquark potential.
Finally we consider a solution corresponding to an asymptotically free field
theory. Again, the leading form of the four derivative terms in the low energy
effective action are in complete agreement with field theory expectations.Comment: 23 pages, uses harvmac, References added, typos corrected and minor
improvements to discussion of N dependence, to appear in Phys. Rev.
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