A TiO study of the black hole binary GRO J0422+32 in a very low state

Webb, N. A.; Naylor, T.; Ioannou, Z.; Charles, P. A.; Shahbaz, T.
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Monthly Notices of the Royal Astronomical Society, Volume 317, Issue 3, pp. 528-534.

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We present 53 simultaneous photometric (I band) and spectroscopic (6900-9500Å) observations of GRO J0422+32, taken during 1997 December. From these we determine that J0422+32 was in its lowest state yet observed, at I=20.44+/-0.08. Using relative spectrophotometry, we show that it is possible to correct very accurately for telluric absorption. Following this, we use the TiO bands at 7055 and 7589Å for a radial velocity study and thereby obtain a semi-amplitude of 378+/-16kms-1, which yields f(M)=1.191+/-0.021Msolar and q9.0+2.2-2.7, consistent with previous observations. We further demonstrate that this little-explored method is very powerful for such systems. We also determine a new orbital ephemeris of HJD=2450274.4156+/-0.0009+0.2121600+/-0.0000002E. We see some evidence for an ellipsoidal modulation, from which we determine the orbital inclination of J0422+32 to be less than 45°. We therefore calculate a minimum mass for the primary of 2.22Msolar, consistent with a black hole, but not necessarily the supermassive one proposed recently (1997) by Beekman et al. We obtain an M4-5 spectral type for the secondary star, and determine that the secondary contributes 38+/-2per cent of the flux that we observe from J0422+32 over the range 6950-8400Å. From this we calculate the distance to the system to be 1.39+/-0.15kpc.