GEMINI Spectroscopy of the Ultracompact Binary Candidate V407 Vulpeculae

We present time-resolved spectroscopy of the optical counterpart to the proposed ultracompact binary system V407 Vul (=RX J1914.4+2456). Our Gemini spectra resolve the 9.48 minute periodicity that has previously been reported for this source. We find that the optical counterpart is dominated by a reddened late-type spectrum of type G9 V and contains solely unresolved absorption features. No radial velocity signatures exceeding 10 km s^-1 could be detected on periods from minutes to hours. Using interstellar extinction estimates, we derive a distance to the G9 star of 1.1-3.5 kpc. In addition to this stellar spectrum, we detect a blue component that modulates solely on the 9.48 minute period and peaks ~0.15 in phase ahead of the X-ray peak. This blue component, which contributes up to 40% of the light, shows no evidence for emission-line features that are the usual hallmarks of an interacting binary. Good-seeing images obtained with the Magellan Telescopes indicate that the variable and the G star are aligned to better than 0.1". Despite the low probability of a chance alignment of a field star along the line of sight, the G9 light cannot be directly associated with the 9.48 minute variable that powers the luminous (~10³⁵ ergs s^-1) and highly variable X-ray source. The outlook for the detection of conclusive radial velocity measurements will remain challenging due to the extinction along the line of sight in conjunction with the contaminating effect of the G9 V star.

This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.