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Register a new userFirst Space-based Microlens Parallax Measurement of an Isolated Star: Spitzer Observations of OGLE-2014-BLG-0939
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We present the first space-based microlens parallax measurement of an isolated star. From the striking differences in the lightcurve as seen from Earth and from Spitzer (~1 AU to the West), we infer a projected velocity v_helio,projected ~ 250 km/s, which strongly favors a lens in the Galactic Disk with mass M=0.23 +- 0.07 M_sun and distance D_L=3.1 +- 0.4 kpc. An ensemble of such measurements drawn from our ongoing program could be used to measure the single-lens mass function including dark objects, and also is necessary for measuring the Galactic distribution of planets since the ensemble reflects the underlying Galactic distribution of microlenses. We study the application of the many ideas to break the four-fold degeneracy first predicted by Refsdal 50 years ago. We find that this degeneracy is clearly broken, but by two unanticipated mechanisms.
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We combine Spitzer and ground-based observations to measure the microlens parallax of OGLE-2005-SMC-001, the first such space-based determination since S. Refsdal proposed the idea in 1966. The parallax measurement yields a projected velocity tilde v ~ 230 km/s, the typical value expected for halo lenses, but an order of magnitude smaller than would be expected for lenses lying in the Small Magellanic Cloud (SMC) itself. The lens is a weak (i.e., non-caustic-crossing) binary, which complicates the analysis considerably but ultimately contributes additional constraints. Using a test proposed by Assef et al. (2006), which makes use only of kinematic information about different populations but does not make any assumptions about their respective mass functions, we find that the likelihood ratio is L_halo/L_SMC = 20. Hence, halo lenses are strongly favored but SMC lenses are not definitively ruled out. Similar Spitzer observations of additional lenses toward the Magellanic Clouds would clarify the nature of the lens population. The Space Interferometry Mission could make even more constraining measurements.
We report the first mass and distance measurement of a caustic-crossing binary system OGLE-2014-BLG-1050L using the space-based microlens parallax method. emph{Spitzer} captured the second caustic-crossing of the event, which occurred $sim$10 days before that seen from Earth. Due to the coincidence that the source-lens relative motion was almost parallel to the direction of the binary-lens axis, the four-fold degeneracy, which was known before only to occur in single-lens events, persists in this case, leading to either a lower-mass (0.2 $M_odot$ and 0.07 $M_odot$) binary at $sim$1.1 kpc or a higher-mass (0.9 $M_odot$ and 0.35 $M_odot$) binary at $sim$3.5 kpc. However, the latter solution is strongly preferred for reasons including blending and lensing probability. OGLE-2014-BLG-1050L demonstrates the power of microlens parallax in probing stellar and substellar binaries.
We analyze the single microlensing event OGLE-2015-BLG-1482 simultaneously observed from two ground-based surveys and from textit{Spitzer}. The textit{Spitzer} data exhibit finite-source effects due to the passage of the lens close to or directly over the surface of the source star as seen from textit{Spitzer}. Such finite-source effects generally yield measurements of the angular Einstein radius, which when combined with the microlens parallax derived from a comparison between the ground-based and the textit{Spitzer} light curves, yields the lens mass and lens-source relative parallax. From this analysis, we find that the lens of OGLE-2015-BLG-1482 is a very low-mass star with the mass $0.10 pm 0.02 M_odot$ or a brown dwarf with the mass $55pm 9 M_{J}$, which are respectively located at $D_{rm LS} = 0.80 pm 0.19 textrm{kpc}$ and $ D_{rm LS} = 0.54 pm 0.08 textrm{kpc}$, and thus it is the first isolated low-mass microlens that has been decisively located in the Galactic bulge. The fundamental reason for the degeneracy is that the finite-source effect is seen only in a single data point from textit{Spitzer} and this single data point gives rise to two solutions for $rho$. Because the $rho$ degeneracy can be resolved only by relatively high cadence observations around the peak, while the textit{Spitzer} cadence is typically $sim 1,{rm day}^{-1}$, we expect that events for which the finite-source effect is seen only in the textit{Spitzer} data may frequently exhibit this $rho$ degeneracy. For OGLE-2015-BLG-1482, the relative proper motion of the lens and source for the low-mass star is $mu_{rm rel} = 9.0 pm 1.9 textrm{mas yr$^{-1}$}$, while for the brown dwarf it is $5.5 pm 0.5 textrm{mas yr$^{-1}$}$. Hence, the degeneracy can be resolved within $sim 10 rm yrs$ from direct lens imaging by using next-generation instruments with high spatial resolution.
We analyze the gravitational binary-lensing event OGLE-2016-BLG-0156, for which the lensing light curve displays pronounced deviations induced by microlens-parallax effects. The light curve exhibits 3 distinctive widely-separated peaks and we find that the multiple-peak feature provides a very tight constraint on the microlens-parallax effect, enabling us to precisely measure the microlens parallax $pi_{rm E}$. All the peaks are densely and continuously covered from high-cadence survey observations using globally located telescopes and the analysis of the peaks leads to the precise measurement of the angular Einstein radius $theta_{rm E}$. From the combination of the measured $pi_{rm E}$ and $theta_{rm E}$, we determine the physical parameters of the lens. It is found that the lens is a binary composed of two M dwarfs with masses $M_1=0.18pm 0.01 M_odot$ and $M_2=0.16pm 0.01 M_odot$ located at a distance $D_{rm L}= 1.35pm 0.09 {rm kpc}$. According to the estimated lens mass and distance, the flux from the lens comprises an important fraction, $sim 25%$, of the blended flux. The bright nature of the lens combined with the high relative lens-source motion, $mu=6.94pm 0.50 {rm mas} {rm yr}^{-1}$, suggests that the lens can be directly observed from future high-resolution follow-up observations.
In this paper, we present the analysis of the binary gravitational microlensing event OGLE-2015-BLG-0196. The event lasted for almost a year and the light curve exhibited significant deviations from the lensing model based on the rectilinear lens-source relative motion, enabling us to measure the microlens parallax. The ground-based microlens parallax is confirmed by the data obtained from space-based microlens observations using the {it Spitzer} telescope. By additionally measuring the angular Einstein radius from the analysis of the resolved caustic crossing, the physical parameters of the lens are determined up to the two-fold degeneracy: $u_0<0$ and $u_0>0$ solutions caused by the well-known ecliptic degeneracy. It is found that the binary lens is composed of two M dwarf stars with similar masses $M_1=0.38pm 0.04 M_odot$ ($0.50pm 0.05 M_odot)$ and $M_2=0.38pm 0.04 M_odot$ ($0.55pm 0.06 M_odot$) and the distance to the lens is $D_{rm L}=2.77pm 0.23$ kpc ($3.30pm 0.29$ kpc). Here the physical parameters out and in the parenthesis are for the $u_0<0$ and $u_0>0$ solutions, respectively.
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