A technique for narrowband time series photometry: The X-ray star V2116 Ophiuchi

We have used innovative features of the Taurus Tunable Filter instrument on the 3.9 m AngloAustralian Telescope to obtain nearly continuous, high-throughput, linear photometry of V2116 Oph in a 7 Å bandpass at the center of the O I λ8446 emission line. This instrumental technique shows promise for applications requiring precise, rapid, narrowband photometry of faint objects. The spectrum of V2116 Oph, the counterpart of GX 1+4 ( = X1728-247), is exotic, even among the unusual spectra of other optical counterparts of compact Galactic X-ray sources. The second strongest emission line is an unusual one, namely extremely prominent O I λ8446, which is likely to result from pumping by an intense Lyβ radiation field. As the X-radiation from GX 1+4 is steadily pulsed, with typical pulsed fractions of 0.4. the O I λ8446 emission in V2116 Oph may also be strongly modulated with the current 127 s period of the X-ray source. If so, this may well allow us to obtain high signal-to-noise ratio radial velocity measurements and thus to determine the system parameters. However, no such pulsations are detected, and we set an upper limit of ∼1% (full amplitude) on periodic λ8446 oscillations at the X-ray frequency. This value is comparable to the amplitude of continuum oscillations observed on some nights by other workers. Thus we rule out an enhancement of the pulsation amplitude in O I emission, at least at the time of our observations.