Distance of a Microlensing Event Measured for the First Time

For the first time, astronomers have combined the observational power of a ground-based survey with a space telescope to measure the distance to a stellar-mass object that was detected through a chance microlensing event.

Microlenses occur when a massive object, such as a dim star, brown dwarf (a failed star), planet or even a black hole - known collectively as "dark objects" - drifts in front of distant starlight. As predicted by Einstein's theory of relativity, when a free-floating (and otherwise hidden) planet drifts through our galaxy, its gravitational field will slightly warp spacetime.

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From Earth's vantage point, should the planet drift directly in front of a distant star, the starlight may bend around the planet, creating a lensing effect. Like a magnifying glass passing in front of a light bulb, the gravitational lens will brighten the starlight for a short time.

With all the billions of stars in our galaxy and the unknown millions or even billions of free-floating planets, brown dwarfs and dim stars, it is impossible to predict when or where these random events will occur - unless, of course, we have prior knowledge of a celestial object that may position itself between us and a star.

Because the detection of microlensing events is down to luck, astronomers have devised several ground-based survey networks that use wide-angle telescopes to continuously monitor large swathes of sky. When a microlensing event is detected, an automated notification system alerts the astronomical community so as much data as possible can be collected.

A wealth of information is contained withing a microlens "light curve" (the evolution of the starlight intensity throughout the event); these events contribute to the statistical estimations about the population of dark objects floating between the stars and after analysis of the curve, signatures of exoplanets or even exomoons may be teased out.

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This is awesome, but there's one thing that's lacking in microlens event analysis - there is little information about the distance from Earth to the lens, so huge uncertainties as to the object's location in the galaxy often remain.

But now, by combining the fast response of a ground-based microlens survey and NASA's Spitzer Space Telescope, astronomers have come up with a novel technique for adding some precision to measuring the distance to dark lenses.

In a new paper published in The Astrophysical Journal, astronomer Jennifer Yee of the Harvard-Smithsonian Center for Astrophysics (CfA), Mass., led a study into the detection and first ever distance measurement of a microlensing event called "OGLE-2014-BLG-0939."

Detected by the 1.3 meter Warsaw Telescope at the Las Campanas Observatory in Chile and alerted through the Optical Gravitational Lens Experiment (OGLE) community on May 28, 2014, Yee's team seized the opportunity to use Spitzer to focus on the transient brightening. Both telescopes recorded a light curve of the event.

Spitzer orbits the sun at the same distance as the Earth, but lags behind the Earth about one-sixth of its orbital path around the sun. This unique opportunity allowed the astronomers to set up a base-line for some astronomical trigonometry.

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Usually, when measuring the distance to objects many light years away, astronomers will take measurements of a celestial body 6 months apart. As the Earth orbits the sun at 1 AU (astronomical unit), this 6 month delay between observations provides a 2 AU baseline. If you know the baseline distance and you know the angular displacement of the celestial object, you can estimate the distance to that object. This technique is known as a parallax measurement.

With 2 near-simultaneous measurements of the same microlensing event, the astronomers were able to estimate the distance to the "dark" object that caused OGLE-2014-BLG-0939. Calculations estimated it to be 10,200 (+/- 1,300) light-years away. Interestingly, when comparing the light curve between the ground-based and Spitzer observations, the brightenings appeared at slightly different times and had different brightening profiles, potentially revealing some interesting characteristics of the lens.

These observations also allowed the mass of the object to be measured - around 0.23 solar masses, hinting that it is of stellar mass.

More work is needed to characterize the nature of this particular lens, but the method to measure its distance - through employing the help of Spitzer to provide a parallax measurement - is obviously extremely powerful.

Source: CfA

The Milky Way hangs above the ESO's Very Large Telescope at the Paranal Observatory in Chile. Billions of stars are visible in our galaxy, but through microlensing, the unseen "dark objects" become dramatically visible for a short time.