Ellipsoidal Variations

The companion star is tidally distorted by the gravitational force of the black hole, into a teardrop shape. As it goes around in its orbit, one sees the star side on twice per orbit, and edge on twice per orbit. Since one sees less of the star when it is edge on, the flux from the star changes during the orbit, with two maxima and two minima. Below is a set of lightcurves from GRO J1655-40, from the undergraduate senior thesis of Jenny Greene (to be published in the Astrophysical Journal and available at astro-ph/0101337). The data were obtained at YALO. The different curves are different filters; B and V are blue and green light, and I, J, and K are different bandpasses of infrared light. The line is a model of the ellipsoidal variations from a code written by former graduate student Jerry Orosz. The excellent fit of the model to the data allows us to determine that the mass of the black hole in this system is between 5.8 and 6.8 times the mass of the Sun.

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