Dynamical Evidence for a Black Hole in the Microquasar XTE J1550-564

ATel #70; Jerome A. Orosz (Utrecht), Michiel van der Klis (Amsterdam), Jeffrey E. McClintock (CfA), Raj K. Jain (Yale), Charles D. Bailyn (Yale), Ronald A. Remillard (MIT)
on 30 May 2001; 18:40 UT
Credential Certification: Jerome A. Orosz (orosz@astro.psu.edu)

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We obtained direct CCD images and time series spectroscopy of the microquasar XTE J1550-564 on May 24-27, 2001 using the FORS1 instrument on Antu, which is the first 8.2 meter telescope at the European Southern Observatory, Paranal (visitor program 67.D-0229(A)). We obtained a total of 18 spectra covering a wavelength range of 4571 to 6927 Angstroms with a resolution of 3.6 Angstroms FWHM. The spectra show a broad, double-peaked H alpha emission line with an equivalent width of ~23 Angstroms and a peak-to-peak separation of 800 km/s. Absorption lines indicative of K-stars (e.g. the Mg b lines near 5169 Angstroms) are also apparent. Radial velocities were relatively easily to measure from 17 of the spectra, and the best cross correlation peaks were found using a K3III template. A four-parameter sinusoid fit to the radial velocity data yields the following elements: P = 1.552 +/- 0.010 days, K = 349 +/- 12 km/s, gamma = 28 +/- 11 km/sec, T_0 = HJD 2,452,054.296 +/- 0.014 (the epoch of maximum velocity), and chi^2/DOF = 1.38. The spectroscopic period is free of aliases and is consistent with the photometric period of P_photo = 1.541 +/- 0.009 days found by Jain et al. (2001, ApJ, 546, 1086). The resulting mass function is f(M) = 6.86 +/- 0.71 solar masses, which is the largest mass function known among the X-ray transients to date. The lack of X-ray eclipses and deep optical eclipses during the outbursts sets an upper limit on the inclination of i < 81 degrees for all mass ratios. Furthermore, the secondary star must be more massive than 0.15 solar masses in order for it to have evolved off the main sequence (Webbink, Rappaport, & Savonije, 1983, ApJ, 270, 678). The upper limit of i < 81 degrees and the lower limit of M_2 > 0.15 solar masses implies a lower limit on the mass of the compact object of M_1 > 7.4 +/- 0.7 solar masses. We conclude XTE J1550-564 contains a black hole since the mass of the compact object is well above the maximum mass of a stable neutron star.