A white dwarf magnifier

Diagram of lensing of a star by a white dwarf in the same binary system. [Credit: Eric Agol]
Diagram of lensing of a star by a white dwarf in the same binary system. [Credit: Eric Agol]
Gravitational lenses provide some of the most incredible images in astronomy. As the gravity from massive objects — generally galaxies or galaxy clusters — bends the path of light from more distant sources, it produces multiple distorted images, rings, crosses, and other wonderful things. Even stars can serve as lenses, but they are too small and too low-mass for the effect to be seen except under special circumstances, when they serve as gravitational “microlenses”. When one star passes between us and a more distant star, the background star will experience a brief magnification, seeming to grow brighter as the foreground star lenses its light.

One situation where this rarely happens is in a binary system: two stars locked in mutual orbit. The reason is that microlensing is a small effect, usually not detectable compared with other variations (starspots, star flares, and other stellar weather). But what if one of the objects is a white dwarf rather than a star? White dwarfs are the remnants of the cores of stars like the Sun; they are as massive as stars, but only the size of Earth. That intensifies their gravitational field, making it possible for them to serve as a microlens in a binary system. I wrote about one such system for Ars Technica:

In the binary system known as KOI-3278, microlensing from the white dwarf boosts the light of its companion by 0.1 percent. (KOI stands for “Kepler object of interest”, meaning it was discovered using the Kepler telescope and identified as a possible exoplanet system.) Each eclipse lasts about five hours, and each orbit takes about 88 days, coincidentally the same as Mercury’s orbit around the Sun. [Read more….]

This is also an exciting discovery because white dwarfs in binaries could eventually become type Ia supernovas: the kinds of explosions astronomers use to measure the expansion of the Universe.

As a final note, I wanted to point out how hard this kind of observation is. Gravitational lensing depends on the distance between the lens — in this case, the white dwarf — and the source object. When both source and lens are in the same binary system, that number is very small compared with the distance between the binary and Earth. Maybe that’s another post in its own right!

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