Discovery of fast, large-amplitude optical variability of V648 Car (=SS73-17)

TL;DR

This paper reports the discovery of large-amplitude optical flickering in V648 Car, a hard X-ray emitting symbiotic star, supporting the idea that such systems are powered by accretion near the Chandrasekhar limit.

Contribution

It presents the first detailed optical variability study of V648 Car, revealing unprecedented flickering amplitudes and suggesting accretion-driven X-ray emission in symbiotic stars.

Findings

V648 Car exhibits large flickering amplitude over minutes.

No significant periodicity detected in flickering.

Long-term light curve indicates a ~520-day binary period.

Abstract

We report on the discovery of large-amplitude flickering from V648 Car (= SS73-17), a poorly studied object listed amongst the very few hard X-ray emitting symbiotic stars. We performed milli-magnitude precision optical photometry with the Swope Telescope at the Las Campanas Observatory, Chile, and found that V648 Car shows large U-band variability over time scales of minutes. To our knowledge, it is amongst the largest flickering of a symbiotic star ever reported. Our finding supports the hypothesis that symbiotic WDs producing hard X-rays are predominantly powered by accretion, rather than quasi-steady nuclear burning, and have masses close to the Chandrasekhar limit. No significant periodicity is evident from the flickering light curve. The ASAS long-term V light curve suggests the presence of a tidally distorted giant accreting via Roche Lobe overflow, and a binary period of about 520 days. On the basis of the outstanding physical properties of V648 Car as hinted by its fast and long-term optical variability, as well as by its nature as hard X-ray emitter, we therefore call for simultaneous follow-up observations in different bands, ideally combined with time-resolved optical spectroscopy.