V Sge: Supersoft Source or Exotic Hot Binary? I. An X-Shooter campaign in the high state

Abstract

V Sge is a peculiar, highly luminous long-period (12.34h) binary star that can display a super-soft X-ray emitting component when in the faint phase of its V~ 10-13mag variability range. Apparently undergoing Eddington-limited accretion from its more massive secondary, it is in a very rare, short-lived evolutionary phase towards the double degenerate channel. Its complex and highly variable optical emission features, from Balmer and Heii to high-ionisation lines, including strong fluorescence features, have been challenging to interpret, especially given the absence of any absorption lines associated with photospheric features from either stellar component. With the detailed properties of V Sge, especially the donor, still controversial, we undertook a VLT/X-Shooter campaign over three months in 2023, obtaining high S/N, high resolution spectra that revealed multiple components in both high- and low-ionisation lines. This allows us to track V Sge's principal emitting regions via Doppler tomography, obtaining new insights into high accretion-rate dynamics. In particular, we identify a stationary, double-peaked emission core which we interpret as a circumbinary ring, analogous to SS433. This enables us to derive limits on the system masses. Furthermore, we find very broad emission-line wings whose mean velocity can vary over hundreds of kilometres per second on timescales of decades, yet ``flip'' between states in <1 week. We show that the super-soft X-ray source interpretation is able to account for these and other observational attributes significantly better than the hot binary model, concluding that V Sge could be one of the brightest known Galactic super-soft sources.