42 research outputs found
High Frame-rate Imaging Based Photometry, Photometric Reduction of Data from Electron-multiplying Charge Coupled Devices (EMCCDs)
The EMCCD is a type of CCD that delivers fast readout times and negligible
readout noise, making it an ideal detector for high frame rate applications
which improve resolution, like lucky imaging or shift-and-add. This improvement
in resolution can potentially improve the photometry of faint stars in
extremely crowded fields significantly by alleviating crowding. Alleviating
crowding is a prerequisite for observing gravitational microlensing in main
sequence stars towards the galactic bulge. However, the photometric stability
of this device has not been assessed. The EMCCD has sources of noise not found
in conventional CCDs, and new methods for handling these must be developed.
We aim to investigate how the normal photometric reduction steps from
conventional CCDs should be adjusted to be applicable to EMCCD data. One
complication is that a bias frame cannot be obtained conventionally, as the
output from an EMCCD is not normally distributed. Also, the readout process
generates spurious charges in any CCD, but in EMCCD data, these charges are
visible as opposed to the conventional CCD. Furthermore we aim to eliminate the
photon waste associated with lucky imaging by combining this method with
shift-and-add.
A simple probabilistic model for the dark output of an EMCCD is developed.
Fitting this model with the expectation-maximization algorithm allows us to
estimate the bias, readout noise, amplification, and spurious charge rate per
pixel and thus correct for these phenomena. To investigate the stability of the
photometry, corrected frames of a crowded field are reduced with a PSF fitting
photometry package, where a lucky image is used as a reference.
We find that it is possible to develop an algorithm that elegantly reduces
EMCCD data and produces stable photometry at the 1% level in an extremely
crowded field.Comment: Submitted to Astronomy and Astrophysic
Bayesian photon counting with electron-multiplying charge coupled devices (EMCCDs)
The EMCCD is a CCD type that delivers fast readout and negligible detector
noise, making it an ideal detector for high frame rate applications. Because of
the very low detector noise, this detector can potentially count single
photons. Considering that an EMCCD has a limited dynamical range and negligible
detector noise, one would typically apply an EMCCD in such a way that multiple
images of the same object are available, for instance, in so called lucky
imaging. The problem of counting photons can then conveniently be viewed as
statistical inference of flux or photon rates, based on a stack of images. A
simple probabilistic model for the output of an EMCCD is developed. Based on
this model and the prior knowledge that photons are Poisson distributed, we
derive two methods for estimating the most probable flux per pixel, one based
on thresholding, and another based on full Bayesian inference. We find that it
is indeed possible to derive such expressions, and tests of these methods show
that estimating fluxes with only shot noise is possible, up to fluxes of about
one photon per pixel per readout.Comment: Fixed a few typos compared to the published versio
The Transiting System GJ1214: High-Precision Defocused Transit Observations and a Search for Evidence of Transit Timing Variation
Aims: We present 11 high-precision photometric transit observations of the
transiting super-Earth planet GJ1214b. Combining these data with observations
from other authors, we investigate the ephemeris for possible signs of transit
timing variations (TTVs) using a Bayesian approach.
Methods: The observations were obtained using telescope-defocusing
techniques, and achieve a high precision with random errors in the photometry
as low as 1mmag per point. To investigate the possibility of TTVs in the light
curve, we calculate the overall probability of a TTV signal using Bayesian
methods.
Results: The observations are used to determine the photometric parameters
and the physical properties of the GJ1214 system. Our results are in good
agreement with published values. Individual times of mid-transit are measured
with uncertainties as low as 10s, allowing us to reduce the uncertainty in the
orbital period by a factor of two.
Conclusions: A Bayesian analysis reveals that it is highly improbable that
the observed transit times is explained by TTV, when compared with the simpler
alternative of a linear ephemeris.Comment: Submitted to A&
OGLE-2009-BLG-092/MOA-2009-BLG-137: A Dramatic Repeating Event With the Second Perturbation Predicted by Real-Time Analysis
We report the result of the analysis of a dramatic repeating gravitational
microlensing event OGLE-2009-BLG-092/MOA-2009-BLG-137, for which the light
curve is characterized by two distinct peaks with perturbations near both
peaks. We find that the event is produced by the passage of the source
trajectory over the central perturbation regions associated with the individual
components of a wide-separation binary. The event is special in the sense that
the second perturbation, occurring days after the first, was
predicted by the real-time analysis conducted after the first peak,
demonstrating that real-time modeling can be routinely done for binary and
planetary events. With the data obtained from follow-up observations covering
the second peak, we are able to uniquely determine the physical parameters of
the lens system. We find that the event occurred on a bulge clump giant and it
was produced by a binary lens composed of a K and M-type main-sequence stars.
The estimated masses of the binary components are
and , respectively, and they are separated in
projection by . The measured distance to the
lens is . We also detect the orbital motion
of the lens system.Comment: 18 pages, 5 figures, 1 tabl
MOA-2009-BLG-387Lb: A massive planet orbiting an M dwarf
We report the discovery of a planet with a high planet-to-star mass ratio in
the microlensing event MOA-2009-BLG-387, which exhibited pronounced deviations
over a 12-day interval, one of the longest for any planetary event. The host is
an M dwarf, with a mass in the range 0.07 M_sun < M_host < 0.49M_sun at 90%
confidence. The planet-star mass ratio q = 0.0132 +- 0.003 has been measured
extremely well, so at the best-estimated host mass, the planet mass is m_p =
2.6 Jupiter masses for the median host mass, M = 0.19 M_sun. The host mass is
determined from two "higher order" microlensing parameters. One of these, the
angular Einstein radius \theta_E = 0.31 +- 0.03 mas, is very well measured, but
the other (the microlens parallax \pi_E, which is due to the Earth's orbital
motion) is highly degenate with the orbital motion of the planet. We
statistically resolve the degeneracy between Earth and planet orbital effects
by imposing priors from a Galactic model that specifies the positions and
velocities of lenses and sources and a Kepler model of orbits. The 90%
confidence intervals for the distance, semi-major axis, and period of the
planet are 3.5 kpc < D_L < 7.9 kpc, 1.1 AU < a < 2.7AU, and 3.8 yr < P < 7.6
yr, respectively.Comment: 20 pages including 8 figures. A&A 529 102 (2011
Characterizing Low-Mass Binaries From Observation of Long Time-scale Caustic-crossing Gravitational Microlensing Events
Despite astrophysical importance of binary star systems, detections are
limited to those located in small ranges of separations, distances, and masses
and thus it is necessary to use a variety of observational techniques for a
complete view of stellar multiplicity across a broad range of physical
parameters. In this paper, we report the detections and measurements of 2
binaries discovered from observations of microlensing events MOA-2011-BLG-090
and OGLE-2011-BLG-0417. Determinations of the binary masses are possible by
simultaneously measuring the Einstein radius and the lens parallax. The
measured masses of the binary components are 0.43 and 0.39
for MOA-2011-BLG-090 and 0.57 and 0.17 for
OGLE-2011-BLG-0417 and thus both lens components of MOA-2011-BLG-090 and one
component of OGLE-2011-BLG-0417 are M dwarfs, demonstrating the usefulness of
microlensing in detecting binaries composed of low-mass components. From
modeling of the light curves considering full Keplerian motion of the lens, we
also measure the orbital parameters of the binaries. The blended light of
OGLE-2011-BLG-0417 comes very likely from the lens itself, making it possible
to check the microlensing orbital solution by follow-up radial-velocity
observation. For both events, the caustic-crossing parts of the light curves,
which are critical for determining the physical lens parameters, were resolved
by high-cadence survey observations and thus it is expected that the number of
microlensing binaries with measured physical parameters will increase in the
future.Comment: 8 pages, 5 figures, 4 table
MOA-2010-BLG-477Lb: constraining the mass of a microlensing planet from microlensing parallax, orbital motion and detection of blended light
Microlensing detections of cool planets are important for the construction of
an unbiased sample to estimate the frequency of planets beyond the snow line,
which is where giant planets are thought to form according to the core
accretion theory of planet formation. In this paper, we report the discovery of
a giant planet detected from the analysis of the light curve of a
high-magnification microlensing event MOA-2010-BLG-477. The measured
planet-star mass ratio is and the projected
separation is in units of the Einstein radius. The angular
Einstein radius is unusually large mas. Combining
this measurement with constraints on the "microlens parallax" and the lens
flux, we can only limit the host mass to the range . In
this particular case, the strong degeneracy between microlensing parallax and
planet orbital motion prevents us from measuring more accurate host and planet
masses. However, we find that adding Bayesian priors from two effects (Galactic
model and Keplerian orbit) each independently favors the upper end of this mass
range, yielding star and planet masses of
and at a distance of kpc,
and with a semi-major axis of AU. Finally, we show that the
lens mass can be determined from future high-resolution near-IR adaptive optics
observations independently from two effects, photometric and astrometric.Comment: 3 Tables, 12 Figures, accepted in Ap
Frequency of Solar-Like Systems and of Ice and Gas Giants Beyond the Snow Line from High-Magnification Microlensing Events in 2005-2008
We present the first measurement of planet frequency beyond the "snow line"
for planet/star mass-ratios[-4.5<log q<-2]: d^2 N/dlog q/dlog
s=(0.36+-0.15)/dex^2 at mean mass ratio q=5e-4, and consistent with being flat
in log projected separation, s. Our result is based on a sample of 6 planets
detected from intensive follow-up of high-mag (A>200) microlensing events
during 2005-8. The sample host stars have typical mass M_host 0.5 Msun, and
detection is sensitive to planets over a range of projected separations
(R_E/s_max,R_E*s_max), where R_E 3.5 AU sqrt(M_host/Msun) is the Einstein
radius and s_max (q/5e-5)^{2/3}, corresponding to deprojected separations ~3
times the "snow line". Though frenetic, the observations constitute a
"controlled experiment", which permits measurement of absolute planet
frequency. High-mag events are rare, but the high-mag channel is efficient:
half of high-mag events were successfully monitored and half of these yielded
planet detections. The planet frequency derived from microlensing is a factor 7
larger than from RV studies at factor ~25 smaller separations [2<P<2000 days].
However, this difference is basically consistent with the gradient derived from
RV studies (when extrapolated well beyond the separations from which it is
measured). This suggests a universal separation distribution across 2 dex in
semi-major axis, 2 dex in mass ratio, and 0.3 dex in host mass. Finally, if all
planetary systems were "analogs" of the Solar System, our sample would have
yielded 18.2 planets (11.4 "Jupiters", 6.4 "Saturns", 0.3 "Uranuses", 0.2
"Neptunes") including 6.1 systems with 2 or more planet detections. This
compares to 6 planets including one 2-planet system in the actual sample,
implying a first estimate of 1/6 for the frequency of solar-like systems.Comment: 42 pages, 10 figure
Physical properties of the planetary systems WASP-45 and WASP-46 from simultaneous multiband photometry
Accurate measurements of the physical characteristics of a large number of exoplanets are useful to strongly constrain theoretical models of planet formation and evolution, which lead to the large variety of exoplanets and planetary-system configurations that have been observed. We present a study of the planetary systems WASP-45 and WASP-46, both composed of a main-sequence star and a close-in hot Jupiter, based on 29 new high-quality light curves of transits events. In particular, one transit of WASP-45 b and four of WASP-46 b were simultaneously observed in four optical filters, while one transit of WASP-46 b was observed with the NTT obtaining a precision of 0.30 mmag with a cadence of roughly 3 min. We also obtained five new spectra of WASP-45 with the FEROS spectrograph. We improved by a factor of 4 the measurement of the radius of the planet WASP-45 b, and found that WASP-46 b is slightly less massive and smaller than previously reported. Both planets now have a more accurate measurement of the density (0.959 ± 0.077 ρJup instead of 0.64 ± 0.30 ρJup for WASP-45 b, and 1.103 ± 0.052 ρJup instead of 0.94 ± 0.11 ρJup for WASP-46 b). We tentatively detected radius variations with wavelength for both planets, in particular in the case of WASP-45 b we found a slightly larger absorption in the redder bands than in the bluer ones. No hints for the presence of an additional planetary companion in the two systems were found either from the photometric or radial velocity measurements
Microlensing discovery of a population of very tight, very low mass binary brown dwarfs
Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs (BDs) are poorly understood. The multiplicity properties and minimum mass of the BD mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 M [SUB]⊙[/SUB] and 0.034 M [SUB]⊙[/SUB], and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field BD binaries known. The discovery of a population of such binaries indicates that BD binaries can robustly form at least down to masses of ~0.02 M [SUB]⊙[/SUB]. Future microlensing surveys will measure a mass-selected sample of BD binary systems, which can then be directly compared to similar samples of stellar binaries