arXiv subjects

N. J. Rattenbury

Publications and source records attributed to N. J. Rattenbury.

At least 37 records · Page 2Linked to original sources

arXiv2017

The First Eclipsing Binary Catalogue from the MOA-II database

We present the first catalogue of eclipsing binaries in two MOA fields towards the Galactic bulge, in which over 8,000 candidates, mostly contact and semi-detached binaries of periods < 1 d, were identified. In this paper, the light curves of a small number of interesting candidates including eccentric binaries, binaries with noteworthy phase modulations and eclipsing RS CVn type stars are shown as examples. In addition, we identified three triple object candidates by detecting the light-travel-time effect in their eclipse time variation curves.

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arXiv2017

OGLE-2016-BLG-1469L: Microlensing Binary Composed of Brown Dwarfs

We report the discovery of a binary composed of two brown dwarfs, based on the analysis of the microlensing event OGLE-2016-BLG-1469. Thanks to detection of both finite-source and microlens-parallax effects, we are able to measure both the masses $M_1\sim 0.05\ M_\odot$, $M_2\sim 0.01\ M_\odot$, and distance $D_{\rm L} \sim 4.5$ kpc, as well as the projected separation $a_\perp \sim 0.33$ au. This is the third brown-dwarf binary detected using the microlensing method, demonstrating the usefulness of microlensing in detecting field brown-dwarf binaries with separations less than 1 au.

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arXiv2017

MOA Data Reveal a New Mass, Distance, and Relative Proper Motion for Planetary System OGLE-2015-BLG-0954L

We present the MOA Collaboration light curve data for planetary microlensing event OGLE-2015-BLG-0954, which was previously announced in a paper by the KMTNet and OGLE Collaborations. The MOA data cover the caustic exit, which was not covered by the KMTNet or OGLE data, and they provide a more reliable measurement of the finite source effect. The MOA data also provide a new source color measurement that reveals a lens-source relative proper motion of $μ_{\rm rel} = 11.8\pm 0.8\,$mas/yr, which compares to the value of $μ_{\rm rel} = 18.4\pm 1.7\,$mas/yr reported in the KMTNet-OGLE paper. This new MOA value for $μ_{\rm rel}$ has an a priori probability that is a factor of $\sim 100$ times larger than the previous value, and it does not require a lens system distance of $D_L < 1\,$kpc. Based on the corrected source color, we find that the lens system consists of a planet of mass $3.4^{+3.7}_{-1.6} M_{\rm Jup}$ orbiting a $0.30^{+0.34}_{-0.14}M_\odot$ star at an orbital separation of $2.1^{+2.2}_{-1.0}\,$AU and a distance of $1.2^{+1.1}_{-0.5}\,$kpc.

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arXiv2017

The Lowest Mass Ratio Planetary Microlens: OGLE 2016-BLG-1195Lb

We report discovery of the lowest mass ratio exoplanet to be found by the microlensing method in the light curve of the event OGLE~2016--BLG--1195. This planet revealed itself as a small deviation from a microlensing single lens profile from an examination of the survey data soon after the planetary signal. The duration of the planetary signal is $\sim 2.5\,$hours. The measured ratio of the planet mass to its host star is $q = 4.2\pm 0.7 \times10^{-5}$. We further estimate that the lens system is likely to comprise a cold $\sim$3 Earth mass planet in a $\sim\,$2 AU wide orbit around a 0.2 Solar mass star at an overall distance of 7.1 kpc.

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arXiv2017

OGLE-2016-BLG-1003: First Resolved Caustic-crossing Binary-source Event Discovered by Second-generation Microlensing Surveys

We report the analysis of the first resolved caustic-crossing binary-source microlensing event OGLE-2016-BLG-1003. The event is densely covered by the round-the-clock observations of three surveys. The light curve is characterized by two nested caustic-crossing features, which is unusual for typical caustic-crossing perturbations. From the modeling of the light curve, we find that the anomaly is produced by a binary source passing over a caustic formed by a binary lens. The result proves the importance of high-cadence and continuous observations, and the capability of second-generation microlensing experiments to identify such complex perturbations that are previously unknown. However, the result also raises the issues of the limitations of current analysis techniques for understanding lens systems beyond two masses and of determining the appropriate multiband observing strategy of survey experiments.

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arXiv2017

OGLE-2013-BLG-1761Lb: A Massive Planet Around an M/K Dwarf

We report the discovery and the analysis of the planetary microlensing event, OGLE-2013-BLG-1761. There are some degenerate solutions in this event because the planetary anomaly is only sparsely sampled. But the detailed light curve analysis ruled out all stellar binary models and shows that the lens to be a planetary system. There is the so-called close/wide degeneracy in the solutions with the planet/host mass ratio of $q \sim (7.5 \pm 1.5) \times 10^{-3}$ and $q \sim (9.3 \pm 2.9) \times 10^{-3}$ with the projected separation in Einstein radius units of $s = 0.95$ (close) and $s = 1.19$ (wide), respectively. The microlens parallax effect is not detected but the finite source effect is detected. Our Bayesian analysis indicates that the lens system is located at $D_{\rm L}=6.9_{-1.2}^{+1.0} \ {\rm kpc}$ away from us and the host star is an M/K-dwarf with the mass of $M_{\rm L}=0.33_{-0.18}^{+0.32} \ M_{\odot}$ orbited by a super-Jupiter mass planet with the mass of $m_{\rm P}=2.8_{-1.5}^{+2.5} \ M_{\rm Jup}$ at the projected separation of $a_{\perp}=1.8_{-0.5}^{+0.5} \ {\rm AU}$. The preference of the large lens distance in the Bayesian analysis is due to the relatively large observed source star radius. The distance and other physical parameters can be constrained by the future high resolution imaging by ground large telescopes or HST. If the estimated lens distance is correct, this planet provides another sample for testing the claimed deficit of planets in the Galactic bulge.

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arXiv2016

Faint source star planetary microlensing: the discovery of the cold gas giant planet OGLE-2014-BLG-0676Lb

We report the discovery of a planet --- OGLE-2014-BLG-0676Lb --- via gravitational microlensing. Observations for the lensing event were made by the MOA, OGLE, Wise, RoboNET/LCOGT, MiNDSTEp and $μ$FUN groups. All analyses of the light curve data favour a lens system comprising a planetary mass orbiting a host star. The most favoured binary lens model has a mass ratio between the two lens masses of $(4.78 \pm 0.13)\times 10^{-3}$. Subject to some important assumptions, a Bayesian probability density analysis suggests the lens system comprises a $3.09_{-1.12}^{+1.02}$ M_jup planet orbiting a $0.62_{-0.22}^{+0.20}$ M_sun host star at a deprojected orbital separation of $4.40_{-1.46}^{+2.16}$ AU. The distance to the lens system is $2.22_{-0.83}^{+0.96}$ kpc. Planet OGLE-2014-BLG-0676Lb provides additional data to the growing number of cool planets discovered using gravitational microlensing against which planetary formation theories may be tested. Most of the light in the baseline of this event is expected to come from the lens and thus high-resolution imaging observations could confirm our planetary model interpretation.

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arXiv2016

The First Circumbinary Planet Found by Microlensing: OGLE-2007-BLG-349L(AB)c

We present the analysis of the first circumbinary planet microlensing event, OGLE-2007-BLG-349. This event has a strong planetary signal that is best fit with a mass ratio of $q \approx 3.4\times10^{-4}$, but there is an additional signal due to an additional lens mass, either another planet or another star. We find acceptable light curve fits with two classes of models: 2-planet models (with a single host star) and circumbinary planet models. The light curve also reveals a significant microlensing parallax effect, which constrains the mass of the lens system to be $M_L \approx 0.7 M_\odot$. Hubble Space Telescope images resolve the lens and source stars from their neighbors and indicate excess flux due to the star(s) in the lens system. This is consistent with the predicted flux from the circumbinary models, where the lens mass is shared between two stars, but there is not enough flux to be consistent with the 2-planet, 1-star models. So, only the circumbinary models are consistent with the HST data. They indicate a planet of mass $m_c = 80\pm 13\,M_\oplus$, orbiting a pair of M-dwarfs with masses of $M_A = 0.41\pm 0.07 M_\odot$ and $M_B = 0.30\pm 0.07 M_\oplus$, which makes this the lowest mass circumbinary planet system known. The ratio of the separation between the planet and the center-of-mass to the separations of the two stars is $\sim 40$, so unlike most of the circumbinary planets found by Kepler, the planet does not orbit near the stability limit.

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arXiv2016

First simultaneous microlensing observations by two space telescopes: $Spitzer$ & $Swift$ reveal a brown dwarf in event OGLE-2015-BLG-1319

Simultaneous observations of microlensing events from multiple locations allow for the breaking of degeneracies between the physical properties of the lensing system, specifically by exploring different regions of the lens plane and by directly measuring the "microlens parallax". We report the discovery of a 30-55$M_J$ brown dwarf orbiting a K dwarf in microlensing event OGLE-2015-BLG-1319. The system is located at a distance of $\sim$5 kpc toward the Galactic bulge. The event was observed by several ground-based groups as well as by $Spitzer$ and $Swift$, allowing the measurement of the physical properties. However, the event is still subject to an 8-fold degeneracy, in particular the well-known close-wide degeneracy, and thus the projected separation between the two lens components is either $\sim$0.25 AU or $\sim$45 AU. This is the first microlensing event observed by $Swift$, with the UVOT camera. We study the region of microlensing parameter space to which $Swift$ is sensitive, finding that while for this event $Swift$ could not measure the microlens parallax with respect to ground-based observations, it can be important for other events. Specifically, for detecting nearby brown dwarfs and free-floating planets in high magnification events.

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arXiv2016

The frequency of snowline-region planets from four-years of OGLE-MOA-Wise second-generation microlensing

We present a statistical analysis of the first four seasons from a "second-generation" microlensing survey for extrasolar planets, consisting of near-continuous time coverage of 8 deg$^2$ of the Galactic bulge by the OGLE, MOA, and Wise microlensing surveys. During this period, 224 microlensing events were observed by all three groups. Over 12% of the events showed a deviation from single-lens microlensing, and for $\sim$1/3 of those the anomaly is likely caused by a planetary companion. For each of the 224 events we have performed numerical ray-tracing simulations to calculate the detection efficiency of possible companions as a function of companion-to-host mass ratio and separation. Accounting for the detection efficiency, we find that $55^{+34}_{-22}\%$ of microlensed stars host a snowline planet. Moreover, we find that Neptunes-mass planets are $\sim10$ times more common than Jupiter-mass planets. The companion-to-host mass ratio distribution shows a deficit at $q\sim10^{-2}$, separating the distribution into two companion populations, analogous to the stellar-companion and planet populations, seen in radial-velocity surveys around solar-like stars. Our survey, however, which probes mainly lower-mass stars, suggests a minimum in the distribution in the super-Jupiter mass range, and a relatively high occurrence of brown-dwarf companions.

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arXiv2016

Black Hole, Neutron Star and White Dwarf Candidates from Microlensing with OGLE-III

Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III database of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 MSun and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission.

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arXiv2015

MOA-2010-BLG-353Lb A Possible Saturn Revealed

We report the discovery of a possible planet in microlensing event MOA-2010-BLG-353. This event was only recognised as having a planetary signal after the microlensing event had finished, and following a systematic analysis of all archival data for binary lens microlensing events collected to date. Data for event MOA-2010-BLG-353 were only recorded by the high cadence observations of the OGLE and MOA survey groups. If we make the assumptions that the probability of the lens star hosting a planet of the measured mass ratio is independent of the lens star mass or distance, and that the source star is in the Galactic bulge, a probability density analysis indicates the planetary system comprises a 0.9^{+1.6}_{-0.53} M_{Saturn} mass planet orbiting a 0.18^{+0.32}_{-0.11} M_{sun} red dwarf star, 6.43^{+1.09}_{-1.15} kpc away. The projected separation of the planet from the host star is 1.72^{+0.56}_{-0.48} AU. Under the additional assumption that the source is on the far side of the Galactic bulge, the probability density analysis favours a lens system comprising a slightly lighter planet.

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arXiv2015

Reanalyses of Anomalous Gravitational Microlensing Events in the OGLE-III Early Warning System Database with Combined Data

We reanalyze microlensing events in the published list of anomalous events that were observed from the OGLE lensing survey conducted during 2004-2008 period. In order to check the existence of possible degenerate solutions and extract extra information, we conduct analyses based on combined data from other survey and follow-up observation and consider higher-order effects. Among the analyzed events, we present analyses of 8 events for which either new solutions are identified or additional information is obtained. We find that the previous binary-source interpretations of 5 events are better interpreted by binary-lens models. These events include OGLE-2006-BLG-238, OGLE-2007-BLG-159, OGLE-2007-BLG-491, OGLE-2008-BLG-143, and OGLE-2008-BLG-210. With additional data covering caustic crossings, we detect finite-source effects for 6 events including OGLE-2006-BLG-215, OGLE-2006-BLG-238, OGLE-2006-BLG-450, OGLE-2008-BLG-143, OGLE-2008-BLG-210, and OGLE-2008-BLG-513. Among them, we are able to measure the Einstein radii of 3 events for which multi-band data are available. These events are OGLE-2006-BLG-238, OGLE-2008-BLG-210, and OGLE-2008-BLG-513. For OGLE-2008-BLG-143, we detect higher-order effect induced by the changes of the observer's position caused by the orbital motion of the Earth around the Sun. In addition, we present degenerate solutions resulting from the known close/wide or ecliptic degeneracy. Finally, we note that the masses of the binary companions of the lenses of OGLE-2006-BLG-450 and OGLE-2008-BLG-210 are in the brown-dwarf regime.

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arXiv2014

Can the masses of isolated planetary-mass gravitational lenses be measured by terrestrial parallax?

Recently Sumi et al. (2011) reported evidence for a large population of planetary-mass objects (PMOs) that are either unbound or orbit host stars in orbits > 10 AU. Their result was deduced from the statistical distribution of durations of gravitational microlensing events observed by the MOA collaboration during 2006 and 2007. Here we study the feasibility of measuring the mass of an individual PMO through microlensing by examining a particular event, MOA-2011-BLG-274. This event was unusual as the duration was short, the magnification high, the source-size effect large and the angular Einstein radius small. Also, it was intensively monitored from widely separated locations under clear skies at low air masses. Choi et al. (2012) concluded that the lens of the event may have been a PMO but they did not attempt a measurement of its mass. We report here a re-analysis of the event using re-reduced data. We confirm the results of Choi et al. and attempt a measurement of the mass and distance of the lens using the terrestrial parallax effect. Evidence for terrestrial parallax is found at a 3 sigma level of confidence. The best fit to the data yields the mass and distance of the lens as 0.80 +/- 0.30 M_J and 0.80 +/- 0.25 kpc respectively. We exclude a host star to the lens out to a separation ~ 40 AU. Drawing on our analysis of MOA-2011-BLG-274 we propose observational strategies for future microlensing surveys to yield sharper results on PMOs including those down to super-Earth mass.

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arXiv2014

OGLE-2013-BLG-0102LA,B: Microlensing binary with components at star/brown-dwarf and brown-dwarf/planet boundaries

We present the analysis of the gravitational microlensing event OGLE-2013-BLG-0102. The light curve of the event is characterized by a strong short-term anomaly superposed on a smoothly varying lensing curve with a moderate magnification $A_{\rm max}\sim 1.5$. It is found that the event was produced by a binary lens with a mass ratio between the components of $q = 0.13$ and the anomaly was caused by the passage of the source trajectory over a caustic located away from the barycenter of the binary. From the analysis of the effects on the light curve due to the finite size of the source and the parallactic motion of the Earth, the physical parameters of the lens system are determined. The measured masses of the lens components are $M_{1} = 0.096 \pm 0.013~M_{\odot}$ and $M_{2} = 0.012 \pm 0.002~M_{\odot}$, which correspond to near the hydrogen-burning and deuterium-burning mass limits, respectively. The distance to the lens is $3.04 \pm 0.31~{\rm kpc}$ and the projected separation between the lens components is $0.80 \pm 0.08~{\rm AU}$.

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arXiv2014

OGLE-BLG182.1.162852: An Eclipsing Binary with a Circumstellar Disk

We present the discovery of a plausible disk-eclipse system OGLE-BLG182.1.162852. The OGLE light curve for OGLE-BLG182.1.162852 shows three episodes of dimming by $I \simeq 2 - 3$ magnitudes, separated by 1277 days. The shape of the light curve during dimming events is very similar to that of known disk eclipse system OGLE-LMC-ECL-11893 (Dong et al. 2014). The event is presently undergoing a dimming event, predicted to end on December 30th, 2014. We encourage spectroscopic and multi-band photometric observations now. The next dimming episode for OGLE-BLG182.1.162852 is expected to occur in March 2018.

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arXiv2014

A Terrestrial Planet in a ~1 AU Orbit Around One Member of a ~15 AU Binary

We detect a cold, terrestrial planet in a binary-star system using gravitational microlensing. The planet has low mass (2 Earth masses) and lies projected at $a_{\perp,ph}$ ~ 0.8 astronomical units (AU) from its host star, similar to the Earth-Sun distance. However, the planet temperature is much lower, T<60 Kelvin, because the host star is only 0.10--0.15 solar masses and therefore more than 400 times less luminous than the Sun. The host is itself orbiting a slightly more massive companion with projected separation $a_{\perp,ch}=$10--15 AU. Straightforward modification of current microlensing search strategies could increase their sensitivity to planets in binary systems. With more detections, such binary-star/planetary systems could place constraints on models of planet formation and evolution. This detection is consistent with such systems being very common.

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arXiv2014

MOA-2013-BLG-220Lb: Massive Planetary Companion to Galactic-Disk Host

We report the discovery of MOA-2013-BLG-220Lb, which has a super-Jupiter mass ratio $q=3.01\pm 0.02\times 10^{-3}$ relative to its host. The proper motion, $μ=12.5\pm 1\, {\rm mas}\,{\rm yr}^{-1}$, is one of the highest for microlensing planets yet discovered, implying that it will be possible to separately resolve the host within $\sim 7$ years. Two separate lines of evidence imply that the planet and host are in the Galactic disk. The planet could have been detected and characterized purely with follow-up data, which has important implications for microlensing surveys, both current and into the LSST era.

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