S.S. Doğru

A massive binary black-hole system in OJ 287 and a test of general relativity

Tests of Einstein’s general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton’s theory. Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened 20 days later.

The chromospherically active binary CF Tuc revisited

This paper presents results derived from analysis of new spectroscopic and photometric observations of the chromospherically active binary system CFTuc. New high-resolution spectra, taken at the Mt. John University Observatory in 2007, were analysed using two methods: cross-correlation and Fourier-based disentangling. As a result, new radial velocity curves of both components were obtained. The resulting orbital elements of CF Tuc are a 1 sin i = 0.0254 ± 0.0001 au, a 2 sin i = 0.0228 ± 0.0001 au, M 1 sin i = 0.902 ± 0.005 M ⊙ and M 2 sin i = 1.008 ± 0.006 M ⊙ . The cooler component of the system shows Hα and Ca II H&K emissions. Using simultaneous spectroscopic and photometric observations, an anticorrelation between the Hα emission and the BV light curve maculation effects was found. This behaviour indicates a close spatial association between photospheric and chromospheric active regions. Our spectroscopic data and recent BV light curves were solved simultaneously using the Wilson-Devinney code. A dark spot on the surface of the cooler component was assumed to explain large asymmetries observed in the light curves. The following absolute parameters of the components were determined: M 1 = 1.11 ± 0.01 M ⊙ , M 2 = 1.23 ± 0.01 M ⊙ , R 1 = 1.63 ± 0.02 R ⊙ , R 2 = 3.60 ± 0.02 R⊙, L 1 = 3.32 ± 0.51 L ⊙ and L 2 = 3.91 ± 0.84 L ⊙ . The primary component has an age of about 5 Gyr and is approaching its main-sequence terminal age. The distance to CF Tuc was calculated to be 89 ± 6 pc from the dynamic parallax, neglecting interstellar absorption, in agreement with the Hipparcos value. The orbital period of the system was studied using the O-C analysis. The O-C diagram could be interpreted in terms of either two abrupt changes or a quasi-sinusoidal form superimposed on a downward parabola. These variations are discussed by reference to the combined effect of mass transfer and mass loss, the Applegate mechanism and also a light-time effect due to the existence of a third body in the system.