Saturday, May 3, 2014

A Self-Lensing Binary Star System

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.

Figure 1: Schematic of the KOI-3278 binary star system. Credit: Eric Agol.

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.

- 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)