MMT observations of the black hole candidate XTE J1118+480 near and in quiescence

We report on the analysis of new and previously published MMT optical spectra of the black hole binary XTE J1118+480 during the decline from, the 2000 outburst to true quiescence. From cross-correlation with template stars, we measure the radial velocity of the secondary to derive a new spectroscopic ephemeris. The observations acquired during approach to quiescence confirm the earlier reported modulation in the centroid of the double-peaked Hα emission line. In addition, our data combined with the results presented by Zurita et al. (2002) provide support for a modulation with a periodicity in agreement with the expected precession period of the accretion disk of ∼52 days. Doppler images during the decline phase of the Hα emission line show evidence for a hot spot and emission from the gas stream: the hot spot is observed to vary its position, which may be due to the precession of the disk. The data available during quiescence show that the centroid of the Hα emission line is offset by about -100 km s^-1 from the systemic velocity, which suggests that the disk continues to precess. An Hα tomogram reveals emission from near the donor star after subtraction of the ringlike contribution from the accretion disk, which we attribute to chromospheric emission. No hot spot is present, suggesting that accretion from the secondary has stopped (or decreased significantly) during quiescence. Finally, a comparison is made with the black hole XRN GRO J0422+32: we show that the Hα profile of this system also exhibits a behavior consistent with a processing disk.