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Orbital Parameters for the Black Hole Binary XTE J1650-500
We present R-band photometry of the X-ray transient and candidate blackhole binary XTE J1650-500 obtained between 2003 May and August with the6.5 m Clay telescope. A timing analysis of these data reveals aphotometric period of 0.3205+/-0.0007 days (i.e., 7.63 hr) with apossible alias at 0.3785 days (9.12 hr). Our photometry completely rulesout the previously published spectroscopic period of 0.212 days (5.09hr). Consequently, we reanalyzed the 15 archival ESO VLT spectra(obtained 2002 June by Sanchez-Fernandez et al.) that were the basis ofthe previously published spectroscopic period. We used a ``rest-framesearch'' technique that is well suited for cases in which thesignal-to-noise ratio of individual spectra is low. For each of roughly1.1 million binary ephemerides, we summed all of the spectra in a trialrest frame of the secondary star, and each rest-frame spectrum wascross-correlated against a template spectrum. We then searched for theset of orbital parameters that produced the strongest cross-correlationvalue. The results confirmed the photometric period of 0.3205 days andruled out the alias period near 0.38 days. The best value for thevelocity semiamplitude of the companion star is K2=435+/-30km s-1, and the corresponding optical mass function isf(M)=2.73+/-0.56 Msolar. The spectral type of the companionstar is not well constrained because we only have six template spectraavailable to us. The K4 V template provides the best match; next-bestmatches are provided by the G5 V and K2 III templates. We also find thatthe accretion disk dominates the light in the R band where the diskfraction is 80% or higher, although this value should be treated withcaution owing to the poor signal-to-noise ratio and the limited numberof templates. The amplitude of the phased R-band light curve is 0.2 mag,which gives a lower limit to the inclination of50deg+/-3deg in the limiting case of nocontribution to the R-band light curve from the accretion disk. If themass ratio of XTE J1650-500 is similar to the mass ratios of other blackhole binaries, such as A0620-00 or GRS 1124-683 (e.g., Q>~10), thenour lower limit to the inclination gives an upper limit to the mass ofthe black hole in XTE J1650-500 of M1<~7.3Msolar. However, the mass can be considerably lower if theR-band flux is dominated by the accretion disk. For example, if theaccretion disk does contribute 80% of the flux, as our preliminaryresults suggest, then the black hole mass would be only about 4Msolar.Based on observations made with the Magellan 6.5 m Clay telescope at LasCampanas Observatory of the Carnegie Institution and with EuropeanSouthern Observatory (ESO) telescopes at the Paranal Observatory underprogram ID 69.D-0644(A).
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