A Technique for Narrowband Time Series Photometry: The X‐Ray Star V2116 Ophiuchi

Abstract

ABSTRACTWe have used innovative features of the Taurus Tunable Filter instrument on the 3.9 m Anglo‐Australian 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.