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Gravitational Microlens Detection from Spitzer

Spitzer images of OB171130. The five images show, in descending order, a time sequence image of the star field in which a microlensing event suddenly appears and experiences two peaks, as highlighted by the arrows.

NASA/Spitzer/J. Yee

The path of light from a star as it passes by a massive body, like an exoplanet, will be bent and an observer looking towards the star will see its image distorted. Like an object seen through the stem of a wineglass, the stellar image could even be deformed into two bright peaks. That mass could influence light in this way was first confirmed in 1919, but some of the more subtle effects have only been detected in the past twenty-five years. In one such process, microlensing, a flash of light is produced when the path of a moving cosmic body (perhaps otherwise unknown) passes fortuitously in front of a star and briefly increases the intensity of its light.

The Spitzer Space Telescope circles the Sun in an Earth-trailing orbit, and it is currently 1.66 astronomical units away from Earth (one AU is the average distance of the Earth form the Sun). Scientists had predicted that if it ever became possible to observe a microlensing flash from two well-separated vantage points, a parallax measurement (the apparent angular difference between the positions of the star as seen from the two separated sites) would determine the distance of the dark object. In fact, since 2014 Spitzer has been used successfully to measure the parallax for hundreds of microlensing events. In all these cases, Spitzer was used after ground-based observations had first identified a microlensing event underway.

CfA astronomers Jennifer Yee, Y. Jung, and In-Gu Shin were members of a collaboration that used Spitzer to record the first microlensing event in which only Spitzer (but not the ground-based sites) saw a double flash. Although this was in principle always possible, the effect had never before been observed, and proves that some double-peaked signals can be missed by ground-only observations. The result increases the importance of Spitzer observations beyond simply measuring parallax to include observing multiple-peak images and thus enabling a more precise characterization of the lens, which in the current instance is a binary system with a low-mass star and an orbiting companion.

"OGLE-2017-BLG-1130: The First Binary Gravitational Microlens Detected from Spitzer Only," Tianshu Wang et al. ApJ 860, 25 (2018).