RR Lyrae stars with variable mean magnitudes

TL;DR

This study investigates the genuine astrophysical causes of mean magnitude variability in RR Lyrae stars, using multi-survey light curves and spectral energy distributions, revealing that about 0.9% exhibit true variability likely due to dust extinction.

Contribution

It introduces a modified Fourier-series fitting method and provides the first substantial evidence that some RR Lyrae mean magnitude variations are astrophysical, not photometric errors.

Findings

72 stars show genuine mean-magnitude variability

Variation ratios suggest dust extinction as a cause in most cases

Approximately 0.9% of RR Lyrae in OGLE bulge fields exhibit this phenomenon

Abstract

Context. A number of RR Lyrae stars show variable mean magnitudes in the OGLE survey light curves of the Galactic bulge. Hitherto this phenomenon was not studied, as it was generally assumed to be related to problems with the photometry. Aims. We investigate whether the mean magnitude variability of RR Lyrae variables is due to genuine astrophysical phenomena. Methods. We make use of the extended, and in many cases overlapping, light curves from multiple microlensing surveys, to study RR Lyrae stars with apparent mean-magnitude variations. A modified Fourier-series based fitting method is introduced to analyze the light curves showing mean-magnitude variations. Data from infrared surveys are also used to construct spectral energy distributions (SEDs). Results. 72 stars are presented where the mean-magnitude variations are most probably of genuine astrophysical origin, and not the result of problems with the photometry. The ratio of variation between the V and I bands is compatible with variable extinction by dust in most cases, but no infrared excess is detected in the SEDs. The occurrence rate of the phenomenon, after correcting for selection effects, is $\sim0.9\%$ among RR Lyrae variables in the OGLE bulge fields.