The optical light curve of the low-mass X-ray binary GX 9 + 9

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GX 9 + 9 is a low-mass X-ray binary for which Hertz and Wood have recently found a weak and intermittent periodic modulation of the X-ray light curve. In this paper, I report on the detection of a small modulation in the light curve of the optical counterpart at the same period as determined from the X-ray data. The modulation is roughly sinusoidal with a period of 4.198 ± 0.0094 hours and an average peak-to-peak amplitude of 0.19 mag in the B bandpass in 1987. The B, V, and R light curves in 1988 all had roughly the same amplitude (somewhat larger than the 1987 value). There is a flickering about the mean light curve with a rms deviation of 0.060 mag. The likely cause for the optical modulation is the varying aspect of the X-ray illuminated regions of the companion star and the bulge associated with the bright spot (where the accretion stream impacts the accretion disk). Calculations suggest that the heated bright spot is more important for causing the modulation than is the heated companion star, a result which is mildly surprising since several other systems are known where the importance is reversed. The traditional explanation for the low-amplitude X-ray modulation is that the outer edge of the accretion disk is variable in height so that an accretion disk corona will be occulted by differing amounts. However, this model has several difficulties for GX 9 + 9 and similar systems. I suggest that the X-ray modulation might be due to the asymmetries of X-rays reflected off the bright spot. One of the stars near to GX 9 + 9 (star 1 of Doxsey et al.) was found to be a variable star (17.32 < B < 17.62), and is suggested to be an eclipsing binary with a 1.61 day period.

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Astrophysical Journal

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