V838 Mon: A slow waking up of Sleeping Beauty?

TL;DR

Over 13 years of optical monitoring reveals V838 Mon's gradual brightening, spectral changes, and potential reappearance of its B3V companion, indicating a slow recovery process of the post-outburst stellar system.

Contribution

This study provides the first long-term optical spectroscopic and photometric analysis of V838 Mon, showing its slow brightening and spectral evolution post-outburst.

Findings

Steady brightening observed in all bands, especially in blue.

Gradual increase in continuum flux and Hα emission line.

Decreasing strength of molecular absorption bands indicating rising temperature.

Abstract

Context. V838 Monocerotis is a peculiar binary that underwent an immense stellar explosion in 2002, leaving behind an expanding cool supergiant and a hot B3V companion. Five years after the outburst, the B3V companion disappeared from view, and so far did not recover. Aims. We investigate the changes in the light curve and spectral features Methods. A monitoring campaign has been performed during the past 13 years with the Nordic Optical Telescope to obtain optical photometric and spectroscopic data. The data sets are used to analyse the temporal evolution of the spectral features and the spectral energy distribution, and to characterize the object. Results. Our photometric data show a steady brightening in all bands during the past 13 years, which is particularly prominent in the blue. This rise is also reflected in the spectra, showing a gradual relative increase in the continuum flux at shorter wavelengths. In addition, a slow brightening of the Ha emission line starting in 2015 was detected. These changes might imply that the B3V companion is slowly reappearing. During the same time interval, our analysis reveals a considerable change in the observed colours of the object along with a steady decrease in the strength and width of molecular absorption bands in our low-resolution spectra. These changes suggest a rising temperature of the cool supergiant along with a weakening of its wind, most likely combined with a slow recovery of the secondary due to the evaporation of the dust and accretion of the material from the shell in which the hot companion is embedded. From our medium-resolution spectra, we find that the heliocentric radial velocity of the atomic absorption line of TiI 6556.06 A has been stable for more than a decade. We propose that TiI lines are tracing the velocity of the red supergiant in V838 Mon, and not representing the infalling matter as previously stated.