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Session 110 - X-ray, Binaries & Accretion Disks.
Display session, Saturday, January 10
Exhibit Hall,
The spectrum of V2116 Oph, the counterpart of GX1+4 (=X1728--247), is exotic, even among the unusual spectra of other optical counterparts of compact Galactic X-ray sources. The optical spectrum of the object has shown higher excitation emission lines than any other known X-ray star, extremely strong H\alpha emission, and marked time variability on scales from minutes to years. The second strongest emission line is a rare and remarkable one, namely extremely prominent O\,I \lambda8446, which is likely to result from pumping by an intense Ly\beta radiation field. As the X-radiation from GX1+4 is steadily pulsed, with typical pulsed fractions of 0.4, it is possible that the O\,I \lambda8446 emission in V2116 Oph may also be strongly modulated with the current 120 s period of the X-ray source. If so, this may well allow us to obtain high signal-to-noise radial velocity measurements, and thus to determine the system parameters.
In order to study the pulsations of O\,I \lambda8446 emission from V2116 Oph, we have used innovative features of the Taurus Tunable Filter (TTF) instrument on the AAT. After the TTF was tuned to a bandpass of 7Å\ at the center of the line, we repeatedly imaged V2116 Oph and a reference star through a 6'' by 3' slit, shuffling the charge down 10 pixels after each 12 s exposure. After this is done 102 times, the chip is full and is read out completely. Differential photometry with nearby reference stars reveals that despite the constantly variable clouds and poor seeing conditions, we may set an upper limit of \sim3% (full-amplitude) on periodic \lambda8446 oscillations at the X-ray frequency. This value is comparable to the amplitude of continuum oscillations observed on some nights by other workers. Therefore we are able to rule out an enhancement of the pulsation amplitude in O\,I emission, at least at the time of our observations.
