Hubble
Space Telescope images of a microlens system. The image on the left was
taken 3.7 years after an observed microlensing event; the one on the
right was taken 8.9 years later after the moving foreground (lensing)
source had changed position. The lens and source components (A and B)
are clearly resolved in the later image. Credit: NASA/Hubble
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.
Reference(s):
"OGLE-2017-BLG-1130: The First Binary Gravitational Microlens Detected from Spitzer Only," Tianshu Wang et al. ApJ 860, 25 (2018).
