4,457 research outputs found
M31 Pixel Lensing PLAN Campaign: MACHO Lensing and Self Lensing Signals
We present the final analysis of the observational campaign carried out by
the PLAN (Pixel Lensing Andromeda) collaboration to detect a dark matter signal
in form of MACHOs through the microlensing effect. The campaign consists of
about 1 month/year observations carried out during 4 years (2007-2010) at the
1.5m Cassini telescope in Loiano ("Astronomical Observatory of BOLOGNA", OAB)
plus 10 days of data taken in 2010 at the 2m Himalayan Chandra Telescope (HCT)
monitoring the central part of M31 (two fields of about 13'x12.6'). We
establish a fully automated pipeline for the search and the characterization of
microlensing flux variations: as a result we detect 3 microlensing candidates.
We evaluate the expected signal through a full Monte Carlo simulation of the
experiment completed by an analysis of the detection efficiency of our
pipeline. We consider both "self lensing" and "MACHO lensing" lens populations,
given by M31 stars and dark matter halo MACHOs, in the M31 and the Milky Way
(MW), respectively. The total number of events is compatible with the expected
self-lensing rate. Specifically, we evaluate an expected signal of about 2
self-lensing events. As for MACHO lensing, for full 0.5 (0.01) solar mass MACHO
halos, our prediction is for about 4 (7) events. The comparatively small number
of expected MACHO versus self lensing events, together with the small number
statistics at disposal, do not enable us to put strong constraints on that
population. Rather, the hypothesis, suggested by a previous analysis, on the
MACHO nature of OAB-07-N2, one of the microlensing candidates, translates into
a sizeable lower limit for the halo mass fraction in form of the would be MACHO
population, f, of about 15% for 0.5 solar mass MACHOs.Comment: ApJ accepted, 13 pages, 5 figures, 2 table
Microlensing events from the 11-year observations of the Wendelstein Calar Alto Pixellensing Project
We present the results of the decade-long M31 observation from the
Wendelstein Calar Alto Pixellensing Project (WeCAPP). WeCAPP has monitored M31
from 1997 till 2008 in both R- and I-filters, thus provides the longest
baseline of all M31 microlensing surveys. The data are analyzed with the
difference imaging analysis, which is most suitable to study variability in
crowded stellar fields. We extracted light curves based on each pixel, and
devised selection criteria that are optimized to identify microlensing events.
This leads to 10 new events, and sums up to a total of 12 microlensing events
from WeCAPP, for which we derive their timescales, flux excesses, and colors
from their light curves. The color of the lensed stars fall between (R-I) =
0.56 to 1.36, with a median of 1.0 mag, in agreement with our expectation that
the sources are most likely bright, red stars at post main-sequence stage. The
event FWHM timescales range from 0.5 to 14 days, with a median of 3 days, in
good agreement with predictions based on the model of Riffeser et al. (2006).Comment: 44 pages, 16 figures, 5 tables. ApJ accepte
LMC self lensing for OGLE-II microlensing observations
In the framework of microlensing searches towards the Large Magellanic Cloud
(LMC), we discuss the results presented by the OGLE collaboration for their
OGLE-II campaign \citep{lukas09}. We evaluate the optical depth, the duration
and the expected rate of events for the different possible lens populations:
both luminous, dominated by the LMC self lensing, and "dark", the would be
compact halo objects (MACHOs) belonging to either the Galactic or to the LMC
halo. The OGLE-II observational results, 2 microlensing candidate events
located in the LMC bar region with duration of 24.2 and 57.2 days, compare well
with the expected signal from the luminous lens populations:
, with typical duration, for LMC self lensing, of about 50
days. Because of the small statistics at disposal, however, the conclusions
that can be drawn as for the halo mass fraction, , in form of compact halo
objects are not too severe. By means of a likelihood analysis we find an
\emph{upper} limit for , at 95% confidence level, of about 15% in the mass
range and 26% for .Comment: accepted for publication in MNRA
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
Microlensing towards M31 with MDM data
We report the final analysis of a search for microlensing events in the
direction of the Andromeda galaxy, which aimed to probe the MACHO composition
of the M31 halo using data collected during the 1998-99 observational campaign
at the MDM observatory. In a previous paper, we discussed the results from a
first set of observations. Here, we deal with the complete data set, and we
take advantage of some INT observations in the 1999-2000 seasons. This merging
of data sets taken by different instruments turns out to be very useful, the
study of the longer baseline available allowing us to test the uniqueness
characteristic of microlensing events. As a result, all the candidate
microlensing events previously reported turn out to be variable stars. We
further discuss a selection based on different criteria, aimed at the detection
of short--duration events. We find three candidates whose positions are
consistent with self--lensing events, although the available data do not allow
us to conclude unambiguously that they are due to microlensing.Comment: Accepted for publication in Astronomy and Astrophysic
OGLE-2017-BLG-1130: The First Binary Gravitational Microlens Detected From Spitzer Only
We analyze the binary gravitational microlensing event OGLE-2017-BLG-1130
(mass ratio q~0.45), the first published case in which the binary anomaly was
only detected by the Spitzer Space Telescope. This event provides strong
evidence that some binary signals can be missed by observations from the ground
alone but detected by Spitzer. We therefore invert the normal procedure, first
finding the lens parameters by fitting the space-based data and then measuring
the microlensing parallax using ground-based observations. We also show that
the normal four-fold space-based degeneracy in the single-lens case can become
a weak eight-fold degeneracy in binary-lens events. Although this degeneracy is
resolved in event OGLE-2017-BLG-1130, it might persist in other events.Comment: 14 pages, 5 figure
Pixel lensing: Microlensing towards M31
Pixel lensing is gravitational microlensing of unresolved stars. The main
target explored up to now has been the nearby galaxy of Andromeda, M31. The
scientific issues of interest are the search for dark matter in form of compact
halo objects, the study of the characteristics of the luminous lens and source
populations and the possibility of detecting extra-solar (and extra-galactic)
planets. In the present work we intend to give an updated overview of the
observational status in this field.Comment: Invited article for the GRG special issue on gravitational lensing
(Ph. Jetzer, Y. Mellier and V. Perlick Eds.
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