Gravitational lensing is a prediction from Einstein’s general
theory of relativity. It states that gravity can bend light and, consequently,
massive foreground objects can distort and magnify the light from background
sources. A. Maeder (1973) predicted that for edge-on binary star systems in
which one star is a compact object (i.e. a white dwarf, neutron star or black
hole), the gravity of the compact object can repeatedly magnify the light of its
companion star each time the compact object is observed to pass in front of its
companion star. Using data of high photometric precision from NASA’s Kepler
space telescope, E. Krusel & E. Agol (2014) report on the detection of such
a “self-lensing” system in a paper published in the April 18 issue of the
journal Science.
This is the first detection of a “self-lensing” system.
Dubbed KOI-3278, the system consists of a Sun-like star and a white dwarf - the
compact object. The white dwarf crosses in front of its companion Sun-like star
once every 88.18 days. Each time it does so, the white dwarf’s gravity acts as
a magnifying glass and slightly boosts the brightness of its companion star. The
brightness boost created by such a “self-lensing” system is small, typically
with amplitudes of a part in one thousand or less. In the case for KOI-3278,
each passage of the white dwarf in front of its companion star creates a 5 hour
pulse with 0.11 percent amplitude.
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Figure 1: Schematic of the KOI-3278 binary star system. Credit:
Eric Agol.
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Figure 2: Light curve of the KOI-3278 binary star system showing
the pulse when the white dwarf passes in front of its companion Sun-like star.
Source: E. Krusel & E. Agol (2014).
In the Kepler dataset, 16 gravitational lensing pulses were
found for KOI-3278, in addition to 16 occultations. The occultations happen
when the white dwarf passes behind its companion star. By modelling the
gravitational lensing along with orbital and stellar models, the white dwarf’s
mass is estimated to be 63 percent the Sun’s mass, and its size, 1.1 percent
the Sun’s radius - nearly the same size as Earth. In addition, the white
dwarf’s companion star has nearly the same mass and size as the Sun. The
companion star would eventually burn out and leave behind a second white dwarf
in the system, although not for another several billion years. Further analysis
of the Kepler dataset could turn up more “self-lensing” white dwarf/Sun-like
star binaries that are similar to KOI-3278.
References:
- A. Maeder, “Light Curves of the Gravitational Lens-like
Action for Binaries with Degenerate Stars”, Astronomy and Astrophysics, Vol.
26, p. 215-223 (1973)
- E. Krusel & E. Agol, “KOI-3278: A Self-Lensing Binary
Star System”, Science, Vol. 344 no. 6181 pp. 275-277 (2014)