Stochastic Variability of Luminous Blue Variables

TL;DR

This study analyzes the long-term stochastic variability of Luminous Blue Variables using observational data, revealing power-law behavior in their power density spectra and suggesting multiple noise components influence their brightness fluctuations.

Contribution

It provides the first comprehensive analysis of LBV variability spectra, identifying power-law slopes and proposing a multi-component stochastic model for their brightness variations.

Findings

Power density spectra follow a power-law with slopes near 2 for highly variable LBVs.

Less variable LBVs show shallower slopes around 1.3.

Flux variations are correlated over timescales much longer than pseudo-photosphere models suggest.

Abstract

Using the archives of the American Association of Variable Stars Observers and our own data, we analyse the long-term variability of several well-studied Luminous Blue Variables (LBVs) aiming on a general picture of stochastic variability of these objects. The power density spectra of all the selected objects may be generally described by a single power law contaminated by observational noise at higher frequencies. The slopes of the power-law component are close to p=2 (where PDS ~ f^{-p}, and f is frequency) for strongly variable flaring objects like AG Car and significantly smaller (p~1.3) for P Cyg where brightness variation amplitude is <~ 1 mag and dominated by slow low-amplitude variability. The slope holds for about two orders of magnitude in the frequency domain, though peaks and curvatures are present at f ~ 10^{-2}.. 10^{-3} d^{-1}. We show that pseudo-photosphere approach to variability may explain the power-law shape of the variability spectrum at higher frequencies. However, the observed spectra are actually rather "red" than "brown": flux variations are correlated up to tens of years that is much longer than the characteristic refreshment time scales of the pseudo-photosphere. We propose that several stochastic noise components produce the power spectra of LBV stars.