arXiv subjects

Sedighe Sajadian

Publications and source records attributed to Sedighe Sajadian.

At least 37 records · Page 2Linked to original sources

arXiv2023

Detecting Multi-Planetary Systems with Gravitational Microlensing and the Roman Space Telescope

It is plausible that most of the Stars in the Milky Way (MW) Galaxy, like the Sun, consist of planetary systems, instead of a single planet. Out of the estimately discovered 3,950 planet-hosting stars, about 860 of them are known to be multiplanetary systems (as of March, 2023). Gravitational microlensing, which is the magnification in the light of a source star, due to a single or several lenses, has proven to be one of the most useful Astrophysical phenomena with many applications. Until now, many extrasolar planets (exoplanets) have been discovered through binary microlensing, where the lens system consists of a star with one planet. In this paper, we discuss and explore the detection of multi-planetary systems that host two exoplanets via microlensing. This is done through the analysis and modeling of possible triple lens configurations (one star and two planets) of a microlensing event. Furthermore, we examine different magnifications and caustic areas of the second planet, by comparing the magnification maps of triple and binary models in different settings. We also discuss the possibility of detecting the corresponding light curves of such planetary systems with the future implementation of the Nancy Grace Roman (Roman) Space Telescope and its Galactic Time Domain survey.

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arXiv2023

Numerically studying the degeneracy problem in extreme finite-source microlensing events

Most transit microlensing events due to very low-mass lens objects suffer from extreme finite-source effects. While modeling their light curves, there is a known continuous degeneracy between their relevant lensing parameters, i.e., the source angular radius normalized to the angular Einstein radius $ρ_{\star}$, the Einstein crossing time $t_{\rm E}$, the lens impact parameter $u_{0}$, the blending parameter, and the stellar apparent magnitude. In this work, I numerically study the origin of this degeneracy. I find that these light curves have 5 observational parameters (i.e., the baseline magnitude, the maximum deviation in the magnification factor, the Full Width at Half Maximum $\rm{FWHM}=2 t_{\rm{HM}}$, the deviation from top-hat model, the time of the maximum time-derivative of microlensing light curves $T_{\rm{max}}=t_{\rm E}\sqrt{ρ_{\star}^{2}-u_{0}^{2}}$). For extreme finite-source microlensing events due to uniform source stars we get $t_{\rm{HM}}\simeq T_{\rm{max}}$, and the deviation from the top-hat model tends to zero which both cause the known continuous degeneracy. When either $ρ_{\star}\lesssim10$ or the limb-darkening effect is considerable $t_{\rm{HM}}$, and $T_{\rm{max}}$ are two independent observational parameters. I use a numerical approach, i.e., Random Forests containing $100$-$120$ Decision Trees, to study how these observational parameters are efficient in yielding the lensing parameters. These machine learning models find the mentioned 5 lensing parameters for finite-source microlensing events from uniform, and limb-darkened source stars with the average $R^{2}$-scores of $0.87$, and $0.84$, respectively. $R^{2}$-score for evaluating the lens impact parameter gets worse on adding limb darkening, and for extracting the limb-darkening coefficient itself this score falls as low as $0.67$.

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arXiv2023

Orbital Period Change of Dimorphos Due to the DART Kinetic Impact

The Double Asteroid Redirection Test (DART) spacecraft successfully performed the first test of a kinetic impactor for asteroid deflection by impacting Dimorphos, the secondary of near-Earth binary asteroid (65803) Didymos, and changing the orbital period of Dimorphos. A change in orbital period of approximately 7 minutes was expected if the incident momentum from the DART spacecraft was directly transferred to the asteroid target in a perfectly inelastic collision, but studies of the probable impact conditions and asteroid properties indicated that a considerable momentum enhancement ($β$) was possible. In the years prior to impact, we used lightcurve observations to accurately determine the pre-impact orbit parameters of Dimorphos with respect to Didymos. Here we report the change in the orbital period of Dimorphos as a result of the DART kinetic impact to be -33.0 +/- 1.0 (3$σ$) minutes. Using new Earth-based lightcurve and radar observations, two independent approaches determined identical values for the change in the orbital period. This large orbit period change suggests that ejecta contributed a significant amount of momentum to the asteroid beyond what the DART spacecraft carried.

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arXiv2023

Microlensing due to free-floating moon-planet systems

Gravitational microlensing is a powerful method for detecting and characterizing free-floating planetary-mass objects (FFPs). FFPs could have exomoons rotating them. In this work, we study the probability of realizing these systems (i.e., free-floating moon-planet ones) through microlensing observations. These systems make mostly close caustic configurations with a considerable finite-source effect. We investigate finite-source microlensing light curves owing to free-floating moon-planet systems. We conclude that crossing planetary caustics causes an extensive extra peak at light curves' wing that only changes its width if the source star does not cross the central caustic. If the source trajectory is normal to the moon-planet axis, the moon-induced perturbation has a symmetric shape with respect to the magnification peak, and its light curve is similar to a single-lens one with a higher finite-source effect. We evaluate the \wfirst~efficiency for realizing moon-induced perturbations, which is $\left[0.002-0.094\right]\%$ by assuming a log-uniform distribution for moon-planet mass ratio in the range $\in\left[-9,~-2\right]$. The highest detection efficiency (i.e., $\simeq 0.094\%$) happens for Saturn-mass planets when moon-planet distance is $\sim 43 R_{\rm p}$, where $R_{\rm p}$ is the Saturn radius. Enhancing planetary mass extends the event's time scale and decreases the finite-source effect, but it reduces the projected moon-planet distance normalized to the Einstein radius $s(R_{\rm E})$ which in turn decreases the size of planetary caustics and takes them away from the host planet's position in close caustic configurations.

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arXiv2023

Detecting isolated stellar-mass black holes by the Roman telescope

Isolated Stellar-Mass BlackHoles (ISMBHs) are potentially discernible through microlensing observations. In this work, we study detecting and characterizing ISMBHs with the Roman observations. We simulate a big ensemble of these events as seen by Roman and estimate the errors in the physical parameters of the lens objects, including their masses, distances, and proper motions through calculating Fisher and Covariance matrices. Since the ~2.3-year time gap between Roman's first three observing seasons and the others may lower the efficiency of realizing microlensing events and characterizing ISMBHs, we additionally consider a scenario where we add a small amount of additional observations -- one hour of observations every 10 days when the Bulge is observable during the large time gap -- which is equivalent to a total of about one additional day of observations with the Roman telescope. These extra observations increase Roman's efficiency for characterizing ISMBHs by ~$1-2\%$ and, more importantly, improve the robustness of the results by avoiding possible degenerate solutions. By considering uniform, and power-law mass functions ($dN/dM ~ M^{-α}$, $α=2,~1,~0.5$) for ISMBHs in the range of $[2,~50] M_{\odot}$, we conclude that the Roman telescope will determine the physical parameters of the lenses within $<5\%$ uncertainty, with efficiencies of $21\%$, and $16$-$18\%$, respectively. By considering these mass functions, we expect that the Roman telescope during its mission will detect and characterize $3$-$4$, $15$-$17$ and $22$-$24$ ISMBHs through astrometric microlensing, with the relative errors for all physical parameters less than $1,~5,~10\%$, respectively. Microlensing events owing to ISMBHs with a mass $\simeq 10$-$25 M_{\odot}$ and located close to the observer with $D_l \lesssim 0.5 D_s$ while the source is inside the Galactic disk can be characterized with least errors.

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arXiv2022

A Statistical Relation between Mass, Age and Velocity Dispersion in the Solar Neighborhood

The stellar kinematics of the Galactic disk are main factors for constraining disk formation and evolution processes in the Milky Way (MW) Galaxy. In this paper we investigate a statistical relation between stellar Mass, Age and Velocity Dispersion for stars in the Solar neighborhood. Age-Velocity dispersion relations (AVR), with their applications, have been studied in details before. But their correlation with mass was mostly neglected. To investigate this relation, we use the proper motion data of more than 113035 stars in the Galactic disk (with Solar distances less than 150 parsecs) provided by the third data release of the \textit{Gaia} mission and for stellar Mass and Age, \textit{Gaia}'s Final Luminosity Age Mass Estimator (\textit{FLAME}) is implemented. We analyze this data and the correlations between the parameters with Random Forrest (RF) Regression, which is an Ensemble statistical learning technique. Finally, we show that by considering the stellar mass alongside age, we can determine Velocity Dispersions with the average relative error, and mean absolute error of about $9\%$, and $2.68~\rm{km/s}$, respectively. We also find that the correlation of stellar age with Velocity Dispersion is $3$ to $8$ times more than mass, which varies due to the different stellar types and masses.

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arXiv2022

An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing

We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t_E~270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462 (hereafter designated as MOA-11-191/OGLE-11-462), in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Ground-based photometry of MOA-11-191/OGLE-11-462 shows a parallactic signature of the effect of the Earth's motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 +/- 1.3 Msun and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a 'natal kick' from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial-velocity measurements of Galactic X-ray binaries, and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first for an isolated stellar-mass BH using any technique.

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arXiv2022

Mass-velocity dispersion relation by using the \textit{Gaia} data and its effect on interpreting short-duration and degenerate microlensing events

Gravitational microlensing, the lensing of stars in the Milky Way galaxy with other stars, has been used for exploring compact dark matter objects, exoplanets, and black holes. The duration of microlensing events, the so-called Einstein crossing time, is a function of distance, mass, and velocities of lens objects. Lenses with different ages and masses might have various characteristic velocities inside the galaxy and this might lead to our misinterpretation of microlensing events. In this work, we use the \gaia~archived data to find a relation between the velocity dispersion and mass, and the age of stars. This mass-velocity dispersion relation confirms the known age-velocity relation for early-type and massive stars, and additionally reveals a dependence of stellar velocity dispersion on the mass for low-mass and late-type stars at $2$-$3$ sigma level. By considering this correlation, we simulate short-duration microlensing events due to brown dwarfs. From this simulation, we conclude that lens masses are underestimated by $\sim 2.5$-$5.5\%$ while modeling short-duration and degenerate microlensing events with the Bayesian analysis.

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arXiv2022

Physical properties of near-Earth asteroid (2102) Tantalus from multi-wavelength observations

Between 2010 and 2017 we have collected new optical and radar observations of the potentially hazardous asteroid (2102)~Tantalus from the ESO NTT and Danish telescopes at the La Silla Observatory and from the Arecibo planetary radar. The object appears to be nearly spherical, showing a low amplitude light-curve variation and limited large-scale features in the radar images. The spin-state is difficult to constrain with the available data; including a certain light-curve subset significantly changes the spin-state estimates, and the uncertainties on period determination are significant. Constraining any change in rotation rate was not possible, despite decades of observations. The convex lightcurve-inversion model, with rotational pole at $λ=210{\pm}41$° and $β=-30{\pm}35$°, is more flattened than the two models reconstructed by including radar observations: with prograde ($λ=36{\pm}23$°, $β=30{\pm}15$°), and with retrograde rotation mode ($λ=180{\pm}24$°, $β=-30{\pm}16$°). Using data from WISE we were able to determine that the prograde model produces the best agreement in size determination between radar and thermophysical modelling. Radar measurements indicate possible variation in surface properties, suggesting one side might have lower radar albedo and be rougher at centimetre-to-decimetre scale than the other. However, further observations are needed to confirm this. Thermophysical analysis indicates a surface covered in fine-grained regolith, consistent with radar albedo and polarisation ratio measurements. Finally, geophysical investigation of the spin-stability of Tantalus shows that it could be exceeding its critical spin-rate via cohesive forces.

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arXiv2022

Improved Aberth-Ehrlich root-finding algorithm and its further application for Binary Microlensing

In gravitational microlensing formalism and for modeling binary light curves, the key step is solving the binary lens equation. Currently, a combination of the Newton's and Laguerre's methods which was first introduced by Skowron \& Gould (SG) is used while modeling binary light curves. In this paper, we first introduce a fast root-finding algorithm for univariate polynomials based on the Aberth-Ehrlich (AE) method which was first developed in 1967 as an improvement over the Newton's method. AE algorithm has proven to be much faster than Newton's, Laguerre's and Durand-Kerner methods and unlike other root-finding algorithms, it is able to produce all the roots simultaneously. After improving the basic AE algorithm and discussing its properties, we will optimize it for solving binary lens equations, which are fifth degree polynomials with complex coefficients. Our method is about $1.8$ to $2.0$ times faster than the SG algorithm. Since, for calculating magnification factors for point-like or finite source stars, it is necessary to solve the binary lens equation and find the positions of the produced images in the image plane first, this new method will improve the speed and accuracy of binary microlensing modeling.

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arXiv2021

Sensitivity to habitable planets in the \textit{Roman} microlensing survey

We study the \textit{Roman} sensitivity to exoplanets in the Habitable Zone (HZ). The \textit{Roman}~efficiency for detecting habitable planets is maximized for three classes of planetary microlensing events with close caustic topologies. (a) The events with the lens distances of $D_{\rm l} \gtrsim 7$ kpc, the host lens masses of $M_{\rm h}\gtrsim 0.6M_{\odot}$. By assuming Jupiter-mass planets in the HZs, these events have $q\lesssim 0.001$ and $d\gtrsim 0.17$ ($q$ is their mass ratio and $d$ is the projected planet-host distance on the sky plane normalized to the Einstein radius). The events with primary lenses, $M_{\rm h} \lesssim 0.1 M_{\odot}$, while their lens systems are either (b) close to the observer with $D_{\rm l}\lesssim 1$ kpc or (c) close to the Galactic bulge, $D_{\rm l}\gtrsim 7$ kpc. For Jupiter-mass planets in the HZs of the primary lenses, the events in these two classes have $q\gtrsim 0.01$, $d\lesssim 0.04$. The events in the class (a) make larger caustics. By simulating planetary microlensing events detectable by \textit{Roman},~we conclude that the \textit{Roman}~efficiencies for detecting Earth- and Jupiter-mass planets in the Optimistic HZs (OHZs, which is the region between $[0.5,~2]$ AU around a Sun-like star) are $0.01\%$ and $5\%$, respectively. If we assume that one exoplanet orbits each microlens in microlensing events detectable by \textit{Roman}~( i.e., $\sim 27000$ ),~this telescope has the potential to detects $35$ exoplanets with the projected planet-host distances in the OHZs with only one having a mass $\lesssim 10M_{\oplus}$. According to the simulation, $27$ of these exoplanets are actually in the OHZs.

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arXiv2021

Variation of the stellar color in high-magnification and caustic-crossing microlensing events

To a first approximation, the microlensing phenomenon is achromatic, and great advancement has been achieved in the interpretation of the achromatic signals, which among other achievements has led to the discovery and characterization of well above $100$ new exoplanets. At higher order accuracy in the observations, microlensing has a chromatic component (a color term) which has so far been much less explored. Here, we analyze the chromatic microlensing effect of $4$ different physical phenomena, which have the potential to add important new knowledge about the stellar properties not easily reachable with other methods of observations. Our simulation is limited to the case of main-sequence source stars. Microlensing is particularly sensitive to giant and sub-giant stars near the Galactic center. While this population can be studied in short snapshots by use of the largest telescopes in the world, a general monitoring and characterization of the population can be achieved by use of more accessible medium-sized telescopes with specialized equipments through dual-color monitoring from observatories at sites with excellent seeing. We quantify our results to what will be achievable from the Danish $1.54$m telescope at La Silla observatory by use of the existing dual-color lucky imaging camera. Such potential monitoring programs of the bulge population from medium-sized telescopes include the characterization of starspots, limb-darkening, the frequency of close-in giant planet companions, and gravity darkening for blended source stars. We conclude our simulations with quantifying the likelihood of detecting these different phenomena per object where they are present to be $\sim 60\%$ and $\sim 30\%$ for the mentioned phenomena, when monitored during high magnification and caustic crossings, respectively.

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arXiv2021

Identifying low-amplitude pulsating stars through microlensing observations

One possibility for detecting low-amplitude pulsational variations is through gravitational microlensing. During a microlensing event, the temporary brightness increase leads to improvement in the signal-to-noise ratio, and thereby better detectability of pulsational signatures in light curves. We explore this possibility under two primary considerations. The first is when the standard point-source and point-lens approximation applies. In this scenario, dividing the observed light curve by the best-fitted microlensing model leads to residuals that result in pulsational features with improved uncertainties. The second is for transit events (single lens) or caustic crossing (binary lens). The point-source approximation breaks down, and residuals relative to a simple best-fitted microlensing model display more complex behavior. We employ a Monte-Carlo simulation of microlensing of pulsating variables toward the Galactic bulge for the surveys of OGLE and of KMTNet. We demonstrate that the efficiency for detecting pulsational signatures with intrinsic amplitudes of $<0.25$ mag during single and binary microlensing events, at differences in $χ^{2}$ of $Δχ^{2} >350$, is $\sim 50-60\%$. The maximum efficiency occurs for pulsational periods $P \simeq 0.1-0.3$ days. We also study the possibility that high-magnification microlensing events of non-radially pulsating stars (NRPs) could be misinterpreted as planetary or binary microlensing events. We conclude that small asymmetric features around lightcurve peaks due to stellar pulsations could be misdiagnosed with crossing (or passing close to) small caustic curves.

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arXiv2021

On the detection of free-floating planets through microlensing towards the Magellanic Clouds

In this work, we study detecting free-floating planets (FFPs) by microlensing observations towards the Magellanic Clouds (MCs). In comparison to similar events toward the Galactic bulge, an FFP in the Galactic halo produces on average longer microlensing events with smaller projected source radii toward these clouds. For these microlensing events, the relative lens-source velocities are on average smaller. The MC self-lensing events due to FFPs suffer from severe finite-source effects. We first simulate microlensing events due to FFPs towards MCs and assume a log-uniform step function for their mass. The efficiencies for capturing their lensing signatures (with signal-to-noise greater than $50$) are found to be $0.1$-$0.6\%$ and $3$-$6\%$ through ground-based optical surveys and space-based near-infrared surveys, respectively. We then promote these simulations and assume the \wfirst~telescope continuously observes each MC during one $72$-day season with the $15$min observing cadence. From simulated microlensing events with the resolvable source stars at the baseline due to FFPs with the masses $\sim 0.01$-$10^{4}M_{\oplus}$, \wfirst~discovers their lensing effects with the efficiencies $\sim 10$-$80\%$, respectively. By adopting $5\%$ as halos fraction from FFPs we estimate the expected number of events. The highest number of detectable FFPs which is $\sim1700$-$2200$ per season per square degree happens for ones with masses $\sim 0.01 M_{\oplus}$. Our simulations show that \wfirst~potentially extends the mass range of detectable FFPs in halos down to $5.9\times 10^{-7} M_{\oplus}$ with continuous observations during one observing season from the Large Magellanic Cloud.

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arXiv2020

Measuring the stellar atmosphere parameters using follow-up polarimetry microlensing observations

We present an analysis of the potential follow-up polarimetry microlensing observation to study the stellar atmospheres of the distant stars. First, we produce synthetic microlensing events using the Galactic model, stellar population, and interstellar dust toward the Galactic Bulge. We simulate the polarization microlensing light curves and pass them through the instrument specifications of FOcal Reducer and low dispersion Spectrograph (FORS2) polarimeter at Very Large Telescope (VLT), and then analyze them. We find that the accuracy of the VLT telescope lets us constrain the atmosphere of cool RGB stars. Assuming detection of about $3000$ microlensing events per year by the OGLE telescope, we expect to detect almost $20,~10,~8, $ and $5$ of polarization microlensig events for the four different criteria of being three consecutive polarimetry data points above the baseline with $1σ$, $2σ$, $3σ$, and $4σ$, respectively in the polarimetry light curves. We generalize the covariance matrix formulation and present the combination of polarimetry and photometry information that leads us to measure the scattering optical depth of the atmosphere and the inner radius of the stellar envelope of red giant stars. These two parameters could determine the dust opacity of the atmosphere of cool RGB source stars and the radius where dust can be formed.

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arXiv2020

Non-radially pulsating stars as microlensing sources

We study the microlensing of Non-Radially Pulsating (NRP) stars. Pulsations are formulated for stellar radius and temperature using spherical harmonic functions with different values of l,m. The characteristics of the microlensing light curves from NRP stars are investigated in relation to different pulsation modes. For the microlensing of NRP stars, the light curve is not a simple multiplication of the magnification curve and the intrinsic luminosity curve of the source star, unless the effect of finite source size can be ignored. Three main conclusions can be drawn from the simulated light curves. First, for modes with $m\neq0$ and when the viewing inclination is more nearly pole-on, the stellar luminosity towards the observer changes little with pulsation phase. In this case, high-magnification microlensing events are chromatic and can reveal the variability of these source stars. Second, some combinations of pulsation modes produce nearly degenerate luminosity curves (e.g., (l,m)=(3,0), (5,0)). The resulting microlensing light curves are also degenerate, unless the lens crosses the projected source. Finally, for modes involving m=1, the stellar brightness centre does not coincide with the coordinate centre, and the projected source brightness centre moves in the sky with pulsation phase. As a result of this time-dependent displacement in the brightness centroid, the time of the magnification peak coincides with the closest approach of the lens to the brightness centre as opposed to the source coordinate centre. Binary microlensing of NRP stars and in caustic-crossing features are chromatic.

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arXiv2020

Detecting the inner regions of discs around sources of microlensing with Roman Space Telescope

The inner region of circumstellar discs makes an extra near-infrared emission (NIR bump). Detecting and studying these NIR bumps from nearby stars have been done mostly through infrared interferometry. In this work, we study the feasibility of detecting NIR bumps for Galactic bulge stars through microlensing from observations by The Nancy Grace Roman Space Telescope (RST) survey. We first simulate microlensing light curves from source stars with discs in near-infrared. Four main conclusions can be extracted from the simulations. (i) If the lens is crossing the disc inner radius, two extra and wide peaks appear and the main peak of microlensing light curve is flattened. (ii) In microlensing events with the lens impact parameters larger than the disc inner radius, the disc can break the symmetry of light curves with respect to the time of closest approach. (iii) In caustic-crossing binary microlensing, the discs produce wide peaks right before entering and immediately after exiting from the caustic curves. (iv) The disc-induced perturbations are larger in the W149 filter than in the Z087 filter, unless the lens crosses the disc condensation radius. By performing a Monte Carlo simulation, the probabilities of detecting the disc perturbations by \wfirst~are estimated ~3 and 20 per cent in single and binary microlensing, respectively. We anticipate that RST detects around 109 disc-induced perturbations during its microlensing survey if 5 per cent of its source stars have discs.

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arXiv2020

Microlensing of radially pulsating stars

Here, we study the microlensing of radially pulsating stars. Discerning and characterizing the properties of distant, faint pulsating stars is achievable through high-cadence microlensing observations. Combining stellar variability period with microlensing gives the source distance, type, and radius and helps better determine the lens parameters. Considering periodically variations in their radius and surface temperature, their microlensing light curves are resulted from multiplication of the magnification factor with variable finite size effect by the intrinsic brightness curves of pulsing source. The variable finite source size due to pulsation can be significant for transit and single microlensing and while caustic-crossing features. This kind of deviation in the magnification factor is considerable when the ratio of the source radius to the projected lens-source distance is in the range of $ρ_{\star}/u \in[0.4,10]$ and its duration is short and in the same order of the time of crossing the source radius. Other deviations due to variable source intensity and its area make colored and periodic deviations which are asymmetric with respect to the signs of pulsation phase. The positive phases makes deviations with larger amplitude that negative phase. These deviations dominate in filters with short wave lengths (e.g., $B-$band). The position of magnification peaks in microlensing of variable stars varies and this displacement differs in different filters.

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