488 research outputs found
Latest results from the POINT-AGAPE pixel-lensing survey of the Andromeda Galaxy
I report on recent results from the POINT-AGAPE pixel-lensing experiment,
which is engaged in a search towards the Andromeda galaxy (M31) for
gravitational microlensing signatures from massive compact halo objects
(Machos). An analysis of two years of data reveals over 360 light-curves
compatible with microlensing. The third year of data, currently being analysed,
will be crucial in determining how many of these candidates are long-period
variables rather than microlensing. Within the dataset we have isolated a
subset of four high signal-to-noise ratio, short duration events which are
compelling microlensing candidates. The properties and possible origins of
these events are discussed.Comment: 6 pages, 4 figures. Proceedings contribution for the 4th
International Workshop on "The Identification of Dark Matter", York,
September 2nd - 6th 200
Difference image photometry with bright variable backgrounds
Over the last two decades the Andromeda Galaxy (M31) has been something of a
test-bed for methods aimed at obtaining accurate time-domain relative
photometry within highly crowded fields. Difference imaging methods, originally
pioneered towards M31, have evolved into sophisticated methods, such as the
Optimal Image Subtraction (OIS) method of Alard & Lupton (1998), that today are
most widely used to survey variable stars, transients and microlensing events
in our own Galaxy. We show that modern difference image (DIA) algorithms such
as OIS, whilst spectacularly successful towards the Milky Way bulge, may
perform badly towards high surface brightness targets such as the M31 bulge.
Poor results can occur in the presence of common systematics which add spurious
flux contributions to images, such as internal reflections, scattered light or
fringing. Using data from the Angstrom Project microlensing survey of the M31
bulge, we show that very good results are usually obtainable by first
performing careful photometric alignment prior to using OIS to perform
point-spread function (PSF) matching. This separation of background matching
and PSF matching, a common feature of earlier M31 photometry techniques, allows
us to take full advantage of the powerful PSF matching flexibility offered by
OIS towards high surface brightness targets. We find that difference images
produced this way have noise distributions close to Gaussian, showing
significant improvement upon results achieved using OIS alone. We show that
with this correction light-curves of variable stars and transients can be
recovered to within ~10 arcseconds of the M31 nucleus. Our method is simple to
implement and is quick enough to be incorporated within real-time DIA
pipelines. (Abridged)Comment: 12 pages. Accepted for publication in MNRAS. Includes an expanded
discussion of DIA testing and results, including additional lightcurve
example
Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red
dwarfs, stars just massive enough to burn Hydrogen. The authors of this search
(Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less
than 6\% of the galactic halo. First, we extrapolate this result to include
brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a
mass function. Then the total mass of red dwarfs and brown dwarfs
is 18\% of the halo. This result is consistent with microlensing results
assuming a popular halo model. However, using new stellar models and parallax
observations of low mass, low metallicity stars, we obtain much tighter bounds
on low mass stars. We find the halo red dwarf density to be of the halo,
while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the
halo mass density of red dwarfs drops to 16-40 times less than the reported
result of Bahcall et al (1994). For a mass function, this suggests
a total density of red dwarfs and brown dwarfs of 0.25-0.67\% of the
halo, \ie , (0.9-2.5)\times 10^9\msun out to 50 kpc. Such a low result would
conflict with microlensing estimates by the \macho\ group (Alcock \etal
1995a,b).Comment: 13 pages, 2 figures. Figure one only available via fax or snail-mail
To be published in ApJL. fig. 2 now available in postscript. Some minor
changes in dealing with disk forground. Some cosmetic changes. Updated
reference
Exoplanetary atmosphere target selection in the era of comparative planetology
The large number of new planets expected from wide-area transit surveys means
that follow-up transmission spectroscopy studies of their atmospheres will be
limited by the availability of telescope assets. We argue that telescopes
covering a broad range of apertures will be required, with even 1m-class
instruments providing a potentially important contribution. Survey strategies
that employ automated target selection will enable robust population studies.
As part of such a strategy, we propose a decision metric to pair the best
target to the most suitable telescope, and demonstrate its effectiveness even
when only primary transit observables are available. Transmission spectroscopy
target selection need not therefore be impeded by the bottle-neck of requiring
prior follow-up observations to determine the planet mass. The decision metric
can be easily deployed within a distributed heterogeneous network of telescopes
equipped to undertake either broadband photometry or spectroscopy. We show how
the metric can be used either to optimise the observing strategy for a given
telescope (e.g. choice of filter) or to enable the selection of the best
telescope to optimise the overall sample size. Our decision metric can also
provide the basis for a selection function to help evaluate the statistical
completeness of follow-up transmission spectroscopy datasets. Finally, we
validate our metric by comparing its ranked set of targets against lists of
planets that have had their atmospheres successfully probed, and against some
existing prioritised exoplanet lists.Comment: 20 pages, 16 figures, 3 tables. Revision 3, accepted by MNRAS.
Improvements include always using planetary masses where available and
reliable, treatment for sky backgrounds and out-of-transit noise and a use
case for defocused photometr
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
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