Dynamical Modeling of CXOGBS J175553.2-281633: A 10 Hour Long Orbital Period Cataclysmic Variable

Abstract

We present modeling of the long-term optical light curve and radial velocity curve of the binary stellar system CXOGBS J175553.2-281633, first detected in X-rays in the \textit{Chandra} Galactic Bulge Survey. We analyzed 7 years of optical I-band photometry from OGLE and found long-term variations from year to year. These long-term variations can most likely be explained with by either variations in the luminosity of the accretion disk or a spotted secondary star. The phased light curve has a sinusoidal shape, which we interpret as being due to ellipsoidal modulations. We improve the orbital period to be $P=10.34488\pm 0.00006$ h with a time of inferior conjunction of the secondary star $T_0=\mathrm{HJD}2455260.8204\pm 0.0008$. Moreover, we collected 37 spectra over 6 non-consecutive nights. The spectra show evidence for an evolved K7 secondary donor star, from which we obtain a semi-amplitude for the radial velocity curve of $K_2=161\pm 6$ km s${}^{-1}$. Using the light curve synthesis code {\tt XRbinary}, we derive the most likely orbital inclination for the binary of $i=63.0\pm 0.7$ deg, a primary mass of $M_1=0.83\pm 0.06$ M${}_{\odot }$, consistent with a white dwarf accretor, and a secondary donor mass of $M_2=0.65\pm 0.07$ M${}_{\odot }$, consistent with the spectral classification. Therefore, we identify the source as a long orbital period cataclysmic variable star.

16 pages, 6 figures, accepted to MNRAS