arXiv subjects

Sedighe Sajadian

Publications and source records attributed to Sedighe Sajadian.

53 records · Page 3Linked to original sources

arXiv2019

Detection of Exoplanet as a Binary Source of Microlensing Events in WFIRST Survey

We investigate the possibility of exoplanet detection orbiting source stars in microlensing events through WFIRST observations. We perform a Monto Carlo simulation on the detection rate of exoplanets via microlensing, assuming that each source star has at least one exoplanet. The exoplanet can reflect part of the light from the parent star or emit internal thermal radiation. In this new detection channel, we use microlensing as an amplifier to magnify the reflection light from the planet. In the literature, this mode of detecting exoplanets has been investigated much less than the usual mode in which the exoplanets are considered as one companion in binary lens events. Assuming $72$ days of observation per season with the cadence of $15$ minutes, we find the probability of rocky planet detection with this method to be virtually zero. However, there is non-zero probability, for the detection of Jovian planets. We estimate the detection rates of the exoplanets by this method, using WFIRST observation to be $0.012\%$ in single lens events and $0.9\%$ in the binary lens events.

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arXiv2019

OGLE-2017-BLG-1186: first application of asteroseismology and Gaussian processes to microlensing

We present the analysis of the event OGLE-2017-BLG-1186 from the 2017 Spitzer microlensing campaign. This is a remarkable microlensing event because its source is photometrically bright and variable, which makes it possible to perform an asteroseismic analysis using ground-based data. We find that the source star is an oscillating red giant with average timescale of $\sim 9$ d. The asteroseismic analysis also provides us source properties including the source angular size ($\sim 27~μ{\rm as}$) and distance ($\sim 11.5$ kpc), which are essential for inferring the properties of the lens. When fitting the light curve, we test the feasibility of Gaussian Processes (GPs) in handling the correlated noise caused by the variable source. We find that the parameters from the GP model are generally more loosely constrained than those from the traditional $χ^2$ minimization method. We note that this event is the first microlensing system for which asteroseismology and GPs have been used to account for the variable source. With both finite-source effect and microlens parallax measured, we find that the lens is likely a $\sim 0.045~M_{\odot}$ brown dwarf at distance $\sim 9.0$ kpc, or a $\sim 0.073~M_{\odot}$ ultracool dwarf at distance $\sim 9.8$ kpc. Combining the estimated lens properties with a Bayesian analysis using a Galactic model, we find a $\sim 35$ per cent probability for the lens to be a bulge object and $\sim 65$ per cent to be a background disc object.

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arXiv2019

Polarization in caustic-crossing binary microlensing events

Here, we revisit the polarization signal in caustic-crossing binary microlensing and introduce the maps of the polarization signals of the source star as a function of its position projected on the lens plane. The behavior of these maps depends on the source size in comparison with the caustic. If the source size is so smaller than the caustic curve, the maximum polarization signal occurs while the source edge is crossing the nearest folds to the position of the primary. When the source size is larger than the caustic, the polarization signal maximizes over a circular ring around the primary whose radius normalized to the source radius is ~0.96. In cusp caustic crossings, the polarization curves have three peaks, the largest and widest one happens when the source edge is on the corner of the cusp and the source center is inside the caustic curve. When the source is entirely inside the caustic, the polarization signal significantly decreases. Despite the low magnification factor between two parts of planetary caustics, its polarization signal is considerable and can reach to 0.2\% for early-type stars. While crossing the connection line between different parts of caustic, the polarimetry curve have three close peaks that the middle one appears when the source center is upon the connection line.

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arXiv2019

Spitzer Microlensing parallax reveals two isolated stars in the Galactic bulge

We report the mass and distance measurements of two single-lens events from the 2017 Spitzer microlensing campaign. The ground-based observations yield the detection of finite-source effects, and the microlens parallaxes are derived from the joint analysis of ground-based observations and Spitzer observations. We find that the lens of OGLE-2017-BLG-1254 is a $0.60 \pm 0.03 M_{\odot}$ star with $D_{\rm LS} = 0.53 \pm 0.11~\text{kpc}$, where $D_{\rm LS}$ is the distance between the lens and the source. The second event, OGLE-2017-BLG-1161, is subject to the known satellite parallax degeneracy, and thus is either a $0.51^{+0.12}_{-0.10} M_{\odot}$ star with $D_{\rm LS} = 0.40 \pm 0.12~\text{kpc}$ or a $0.38^{+0.13}_{-0.12} M_{\odot}$ star with $D_{\rm LS} = 0.53 \pm 0.19~\text{kpc}$. Both of the lenses are therefore isolated stars in the Galactic bulge. By comparing the mass and distance distributions of the eight published Spitzer finite-source events with the expectations from a Galactic model, we find that the Spitzer sample is in agreement with the probability of finite-source effects occurrence in single lens events.

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arXiv2019

$Spitzer$ Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-Rotating Low-Mass Brown Dwarf

The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens parallax and finite-source effects are needed in order to recover the full information. Here, we combine $Spitzer$ satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either $18\pm1M_J$ or $20\pm1M_J$. This is the lowest isolated-object mass measurement to date, only $\sim$45\% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either $3.9\pm0.1$ kpc or $4.1\pm0.1$ kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population.

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arXiv2018

Prediction on detection and characterization of Galactic disk microlensing events by LSST

Upcoming LSST survey gives an unprecedented opportunity for studying populations of intrinsically faint objects using microlensing technique. Large field of view and aperture allow effective time-series observations of many stars in Galactic disk and bulge. Here, we combine Galactic models (for |b|<10 deg) and simulations of LSST observations to study how different observing strategies affect the number and properties of microlensing events detected by LSST. We predict that LSST will mostly observe long duration microlensing events due to the source stars with the averaged magnitude around 22 in r-band, rather than high-magnification events due to fainter source stars. In Galactic bulge fields, LSST should detect on the order of 400 microlensing events per square degree as compared to 15 in disk fields. Improving the cadence increases the number of detectable microlensing events, e.g., improving the cadence from 6 to 2 days approximately doubles the number of microlensing events throughout the Galaxy. According to the current LSST strategy, it will observe some fields 900 times during a 10-year survey with the average cadence of ~4-days (I) and other fields (mostly toward the Galactic disk) around 180 times during a 1-year survey only with the average 1-day cadence (II). We anticipate that the number of events corresponding to these strategies are 7900 and 34000, respectively. Toward similar lines of sight, LSST with the first observing strategy (I) will detect more and on average longer microlensing events than those observable with the second strategy. If LSST spends enough time observing near Galactic plane, then the large number of microlensing events will allow studying Galactic distribution of planets and finding isolated black holes among wealth of other science cases.

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arXiv2017

Understanding EROS2 observations toward the spiral arms within a classical Galactic model framework

EROS has searched for microlensing toward four directions in the Galactic plane away from the Galactic center. The interpretation of the catalog optical depth is complicated by the spread of the source distance distribution. We compare the EROS microlensing observations with Galactic models, tuned to fit the EROS source catalogs, and take into account all observational data such as the microlensing optical depth, the Einstein crossing durations, and the color and magnitude distributions of the catalogued stars. We simulated EROS-like source catalogs using the Hipparcos database, the Galactic mass distribution, and an interstellar extinction table. Taking into account the EROS star detection efficiency, we were able to produce simulated color-magnitude diagrams that fit the observed diagrams. This allows us to estimate average microlensing optical depths and event durations that are directly comparable with the measured values. Both the Besancon model and our Galactic model allow us to fully understand the EROS color-magnitude data. The average optical depths and mean event durations calculated from these models are in reasonable agreement with the observations; consequently our simulation allows a better understanding of the lens and source spatial distributions in the microlensing events. Varying the Galactic structure parameters through simulation, we were also able to deduce contraints on the kinematics of the disk, the disk stellar mass function (IMF at a few kpc distance from the Sun), and the maximum contribution of a thick disk of compact objects in the Galactic plane (Mthick < 5 - 7 x 10**10 Msun at 95%, depending on the model). We also show that the microlensing data toward one of our monitored directions are significantly sensitive to the galactic bar parameters, although much larger statistics are needed to provide competitive constraints.

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arXiv2017

Polarimetry microlensing of close-in planetary systems

A close-in giant planetary (CGP) system has a net polarization signal whose value varies depending on the orbital phase of the planet. This polarization signal is either caused by the stellar occultation or by reflected starlight from the surface of the orbiting planet. When the CGP system is located in the Galactic bulge, its polarization signal becomes too weak to be measured directly. One method for detecting and characterizing these weak polarization signatures due to distant CGP systems is gravitational microlensing. In this work, we focus on potential polarimetric observations of highly-magnified microlensing events of CGP systems. When the lens is passing directly in front of the source star with its planetary companion, the polarimetric signature caused by the transiting planet is magnified. As a result some distinct features in the polarimetry and light curves are produced. In the same way microlensing amplifies the reflection-induced polarization signal. While the planet-induced perturbations are magnified, whenever these polarimetric or photometric deviations vanish for a moment the corresponding magnification factor or the polarization component(s) is equal to the related one due to the planet itself. In order to evaluate the observability of such systems through polarimetric or photometric observations of high-magnification microlensing events, we simulate these events by considering confirmed CGP systems as their source stars and conclude that the efficiency for detecting the planet-induced signal with the state-of-the-art polarimetric instrument (FORS2/VLT) is less than 0.1 %. Consequently, these planet-induced polarimetry perturbations can likely be detected under favorable conditions by high-resolution and short-cadence polarimeters of the next generation.

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arXiv2016

Stellar rotation effects in polarimetric microlensing

It is well known that the polarization signal in microlensing events of hot stars is larger than that of main-sequence stars. Most hot stars rapidly rotate around their stellar axes. The stellar rotation makes ellipticity and gravity-darkening effects which break the spherical symmetry of the source shape and the circular symmetry of the source surface brightness respectively. Hence, it causes a net polarization signal for the source star. This polarization signal should be considered in polarimetry microlensing of fast rotating stars. For moderate rotating stars, lensing can magnify or even characterize small polarization signals due to the stellar rotation through polarimetry observations. The gravity-darkening effect due to a rotating source star makes asymmetric perturbations in polarimetry and photometry microlensing curves whose maximum happens when the lens trajectory crosses the projected position of the rotation pole on the sky plane. The stellar ellipticity makes a time shift (i) in the position of the second peak of the polarimetry curves in transit microlensing events and (ii) in the peak position of the polarimetry curves with respect to the photometry peak position in bypass microlensing events. By measuring this time shift via polarimetry observations of microlensing events, we can evaluate the ellipticity of the projected source surface on the sky plane. Given the characterizations of the FOcal Reducer and low dispersion Spectrograph (FORS2) polarimeter at Very Large Telescope (VLT) telescope, the probability of observing this time shift is so small. The more accurate polarimeters of the next generation may likely measure these time shifts and evaluate the ellipticity of microlensing source stars.

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arXiv2016

The advantages of using a Lucky Imaging camera for observations of microlensing events

In this work, we study the advantages of using a Lucky Imaging camera for the observations of potential planetary microlensing events. Our aim is to reduce the blending effect and enhance exoplanet signals in binary lensing systems composed of an exoplanet and the corresponding parent star. We simulate planetary microlensing light curves based on present microlensing surveys and follow-up telescopes where one of them is equipped with a Lucky imaging camera. This camera is used at the Danish $1.54$-m follow-up telescope. Using a specific observational strategy, For an Earth-mass planet in the resonance regime, where the detection probability in crowded-fields is smaller, lucky imaging observations improve the detection efficiency which reaches 2 per cent. Given the difficulty of detecting the signal of an Earth-mass planet in crowded-field imaging even in the resonance regime with conventional cameras, we show that Lucky Imaging can substantially improve the detection efficiency.

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arXiv2015

Polarimetric microlensing of circumstellar disks

We study the benefits of polarimetry observations of microlensing events to detect and characterize circumstellar disks around the microlensed stars located at the Galactic bulge. These disks which are unresolvable from their host stars make a net polarization effect due to their projected elliptical shapes. Gravitational microlensing can magnify these signals and make them be resolved. The main aim of this work is to determine what extra information about these disks can be extracted from polarimetry observations of microlensing events in addition to those given by photometry ones. Hot disks which are closer to their host stars are more likely to be detected by microlensing, owing to more contributions in the total flux. By considering this kind of disks, we show that although the polarimetric efficiency for detecting disks is similar to the photometric observation, but polarimetry observations can help to constraint the disk geometrical parameters e.g. the disk inner radius and the lens trajectory with respect to the disk semimajor axis. On the other hand, the time scale of polarimetric curves of these microlensing events generally increases while their photometric time scale does not change. By performing a Monte Carlo simulation, we show that almost 4 optically-thin disks around the Galactic bulge sources are detected (or even characterized) through photometry (or polarimetry) observations of high-magnification microlensing events during 10 years monitoring of 150 million objects.

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arXiv2015

Detecting stellar spots through polarimetry observations of microlensing events in caustic-crossing

In this work, we investigate if gravitational microlensing can magnify the polarization signal of a stellar spot and make it be observable. A stellar spot on a source star of microlensing makes polarization signal through two channels of Zeeman effect and breaking circular symmetry of the source surface brightness due to its temperature contrast. We first explore the characteristics of perturbations in polarimetric microlensing during caustic-crossing of a binary lensing as follows: (a) The cooler spots over the Galactic bulge sources have the smaller contributions in the total flux, although they have stronger magnetic fields. (b) The maximum deviation in the polarimetry curve due to the spot happens when the spot is located near the source edge and the source spot is first entering the caustic whereas the maximum photometric deviation occurs for the spots located at the source center. (c) There is a (partial) degeneracy for indicating spot's size, its temperature contrast and its magnetic induction from the deviations in light or polarimetric curves. (d) If the time when the photometric deviation due to spot becomes zero (between positive and negative deviations) is inferred from microlensing light curves, we can indicate the magnification factor of the spot, characterizing the spot properties except its temperature contrast. The stellar spots alter the polarization degree as well as strongly change its orientation which gives some information about the spot position. Although, the photometry observations are more efficient in detecting stellar spots than the polarimetry ones, but polarimetry observations can specify the magnetic field of the source spots.

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arXiv2015

Binary astrometric microlensing with Gaia

We investigate whether Gaia can specify the binary fractions of massive stellar populations in the Galactic disk through astrometric microlensing. Furthermore, we study if some information about their mass distributions can be inferred via this method. In this regard, we simulate the binary astrometric microlensing events due to massive stellar populations according to the Gaia observing strategy by considering (a) stellar-mass black holes, (b) neutron stars, (c) white dwarfs and (d) main-sequence stars as microlenses. The Gaia efficiency for detecting the binary signatures in binary astrometric microlensing events is $\sim 10-20$ per cent. By calculating the optical depth due to the mentioned stellar populations, the number of the binary astrometric microlensing events being observed with Gaia with detectable binary signatures, for the binary fraction about 0.1, is estimated as 6, 11, 77 and 1316 respectively. Consequently, Gaia can potentially specify the binary fractions of these massive stellar populations. However, the binary fraction of black holes measured with this method has the large uncertainty owing to a low number of the estimated events. Knowing the binary fractions in massive stellar populations helps for studying the gravitational waves. Moreover, we investigate the number of massive microlenses which Gaia specifies their masses through astrometric microlensing of single lenses toward the Galactic bulge. The resulted efficiencies of measuring the mass of mentioned populations are 9.8, 2.9, 1.2 and 0.8 per cent respectively. The number of their astrometric microlensing events being observed in the Gaia era in which the lens mass can be inferred with the relative error less than 0.5 toward the Galactic bulge is estimated as 45, 34, 76 and 786 respectively.

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arXiv2014

Photometric, Astrometric and Polarimetric observations of gravitational microlensing events

The gravitational microlensing as a unique astrophysical tool can be used for studying the atmosphere of stars thousands of parsec far from us. This capability results from the bending of light rays in the gravitational field of a lens which can magnify the light of a background source star during the lensing. Moreover, one of properties of this light bending is that the circular symmetry of the source is broken by producing distorted images at either side of the lens position. This property makes the possibility of the observation of the polarization and the light centroid shift of images. Assigning vectors for these two parameters, they are perpendicular to each other in the simple and binary microlensing events, except in the fold singularities. In this work, we investigate the advantages of polarimetric and astrometric observations during microlensing events for (i) studying the surface of the source star and spots on it and (ii) determining the trajectory of source stars with respect to the lens. Finally we analyze the largest sample of microlensing events from the OGLE catalog and show that for almost ~ 4.3 per cent of events in the direction of the Galactic bulge, the polarization signals with large telescopes would be observable.

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arXiv2014

Orbital motion effects in astrometric microlensing

We investigate lens orbital motion in astrometric microlensing and its detectability. In microlensing events, the light centroid shift in the source trajectory (the astrometric trajectory) falls off much more slowly than the light amplification as the source distance from the lens position increases. As a result, perturbations developed with time such as lens orbital motion can make considerable deviations in astrometric trajectories. The rotation of the source trajectory due to lens orbital motion produces a more detectable astrometric deviation because the astrometric cross-section is much larger than the photometric one. Among binary microlensing events with detectable astrometric trajectories, those with stellar-mass black holes have most likely detectable astrometric signatures of orbital motion. Detecting lens orbital motion in their astrometric trajectories helps to discover further secondary components around the primary even without any photometric binarity signature as well as resolve close/wide degeneracy. For these binary microlensing events, we evaluate the efficiency of detecting orbital motion in astrometric trajectories and photometric light curves by performing Monte Carlo simulation. We conclude that astrometric efficiency is 87.3 per cent whereas the photometric efficiency is 48.2 per cent.

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arXiv2011

Simulation of a strategy for the pixel lensing of M87 using the Hubble Space Telescope

In this work, we propose a new strategy for the pixel-lensing observation of M87 in the Virgo cluster using the {\it Hubble Space Telescope} (HST). In contrast to the previous observational strategy by Baltz et al., we show that a few days intensive observation with the duration of $\sim 90 $min in each HST orbit will increase the observational efficiency of the high-magnification events by more than one order of magnitude. We perform a Monte-Carlo simulation for this strategy and we show that the number of high magnification microlensing events will increase at the rate of 4.2 event per day with a typical transit time scale of 16 h. We also examine the possibility of observing mini-halo dark matter structures using pixel lensing.

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arXiv2010

Illuminating Hot Jupiters in caustic crossing

In recent years a large number of Hot Jupiters orbiting in a very close orbit around the parent stars have been explored with the transit and doppler effect methods. Here in this work we study the gravitational microlensing effect of a binary lens on a parent star with a Hot Jupiter revolving around it. Caustic crossing of the planet makes enhancements on the light curve of the parent star in which the signature of the planet can be detected by high precision photometric observations. We use the inverse ray shooting method with tree code algorithm to generate the combined light curve of the parent star and the planet. In order to investigate the probability of observing the planet signal, we do a Monte-Carlo simulation and obtain the observational optical depth of $τ\sim 10^{-8}$. We show that about ten years observations of Galactic Bulge with a network of telescopes will enable us detecting about ten Hot Jupiter with this method. Finally we show that the observation of the microlensing event in infra-red band will increase the probability for detection of the exo-planets.

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