OGLE-2019-BLG-0825 ; Identified Xallarap Effect of ~5.5 days Affects Binary-lens Parameters in Planetary Candidate Microlensing Event.

The gravitational lensing is a phenomenon in which the way light travels changes around the gravitational source due to the distortion of space and time due to gravity explained by the general theory of relativity. When the gravity source (hereinafter called lens) is a galaxy or a cluster of galaxies, the phenomenon that the background body (hereinafter called source) is divided or distorted has been confirmed. When the lens is a low mass such as a star or a planet, it is generally difficult to resolve the image of the source because the effect of space-time distortion is small. However, it can be confirmed as a magnification of the source star.

When the lens is a single star, the time variation of the brightness of the source star is symmetrical, but if there is a companion, the light of source is secondarily magnified by the gravity of the companion, and an anomaly appears. Planetary exploration by the gravitational microlensing method utilizes this magnification that occurs when the lens passes in front of the source star. Planetary exploration by the gravitational microlensing method uses only the light from the source star and does not use the light of the host star or planet that is the lens bodies. Therefore, there is an advantage that it is possible to discover planets far from the earth and free floating planets that revolve directly in the galaxy.

We report on the analysis of the microlensing event OGLE-2019-BLG-0825. This event has been labeled as a planetary event by preliminary modeling. From detailed analysis, we find non-negligible residuals from the best-fit static binary-lens model exists. We detect xallarap effect with ~5.5 days that can fit the residuals very well and significantly improves χ² values in all data sets. On the other hand, by introducing the xallarap effect, we find that binary-lens parameters like q and s cannot be constrained well, i.e. various binary-lens models are possible. However, we also find the parameters for the source system like the orbital period and xallarap amplitude are consistent between all the possible models. We conclude that the source system consists of a main-sequence star (M-dwarf or K-dwarf) orbited by a low-mass companion (M-dwarf or brown dwarf) with P ~ 5.5 days and a ~ 0.06 AU. This analysis first demonstrates that xallarap effect with short periods can affect binary-lens parameters in planetary events.