Spectroscopy of AT 2016blu's recurring supernova impostor outbursts

TL;DR

This study presents decade-long spectral observations of the recurrent supernova impostor AT 2016blu, revealing quasiperiodic outbursts likely caused by binary interactions, with spectral features showing complex, phase-independent behavior.

Contribution

It provides the first detailed spectral analysis of AT 2016blu's recurring outbursts, linking their periodicity to binary interactions and characterizing spectral line variations over time.

Findings

Quasiperiodic outbursts with a 113-day cycle linked to binary periastron passages.

Spectral features remain largely consistent, with subtle line-profile changes.

Recent outburst observed around May/June 2023, confirming the periodicity.

Abstract

We present spectra of the supernova (SN) impostor AT 2016blu spanning over a decade. This transient exhibits quasiperiodic outbursts with a $\sim$113 d period, likely triggered by periastron encounters in an eccentric binary system where the primary star is a luminous blue variable (LBV). The overall spectrum remains fairly consistent during quiescence and eruptions, with subtle changes in line-profile shapes and other details. Some narrow emission features indicate contamination from a nearby H~{\sc ii} region in the host galaxy, NGC 4559. Broader H$\alpha$ profiles exhibit Lorentzian shapes with full width at half-maximum intensity (FWHM) values that vary significantly, showing no correlation with photometric outbursts or the 113 d phase. At some epochs, H$\alpha$ exhibits asymmetric profiles with a stronger redshifted wing, while broad and sometimes multicomponent P Cygni absorption features occasionally appear, but are again uncorrelated with brightness or phase. These P Cygni absorptions have high velocities compared to the FWHM of the H$\alpha$ emission line, perhaps suggesting that the absorption component is not in the LBV's wind, but is instead associated with a companion. The lack of phase dependence in line-profile changes may point to interaction between a companion and a variable or inhomogeneous primary wind, in an orbit with only mild eccentricity. Recent photometric data indicate that AT 2016blu experienced its \nth{21} outburst around 2023 May/June, as predicted based on its period. This type of quasiperiodic LBV remains poorly understood, but its spectra and erratic light curve resemble some pre-SN outbursts like those of SN 2009ip.