ar X iv : a st ro - p h / 04 04 34 3 v 1 1 7 A pr 2 00 4 Orbital Parameters for the Black Hole Binary XTE J 1650 - 500 1

We present R-band photometry of the X-ray transient and candidate black hole binary XTE J1650-500 obtained between 2003 May and August with the 6.5m Clay Telescope. A timing analysis of these data reveals a photometric period of 0.3205 ± 0.0007 days (i.e. 7.63 hr) with a possible alias at 0.3785 days (9.12 hr). Our photometry completely rules out the previously published spectroscopic period of 0.212 days (5.09 hr). Consequently, we reanalyzed the 15 archival ESO/VLT spectra (obtained 2002 June by others) that were the basis of the previously published spectroscopic period. We used a “restframe search” technique that is well suited for cases when the signal-to-noise ratio of individual spectra is low. For each of roughly 1.1 million binary ephemerides, we summed all of the spectra in a trial restframe of the secondary star, and each restframe spectrum was cross-correlated against a template spectrum. We then searched for the set of orbital parameters that produced the strongest cross-correlation value. The results confirmed the photometric period of 0.3205 days, and rule out the alias period near 0.38 days. The best value for the velocity semiamplitude of the companion star is K2 = 435 ± 30 km s , and the corresponding optical mass function is f(M) = 2.73± 0.56M⊙. The limited number of template spectra available to us suggest a spectral type of K3V to K5V for the companion star. We also find that the accretion disk dominates the light in the R-band where the disk fraction is 80% or higher, although this value should be treated with caution owing to the poor signalnoise-ratio and the limited number of templates. The amplitude of the phased R-band light Based on observations made with the Magellan 6.5m Clay telescope at Las Campanas Observatory of the Carnegie Institution and on observations made with European Southern Observatory (ESO) Telescopes at the Paranal Observatory under programme ID 69.D-0644(A)