# $12\;660$ spotted stars toward the OGLE Galactic bulge fields

## TL;DR

This study analyzes over 12,000 spotted variable stars in the Galactic bulge, revealing correlations between luminosity, rotation, and variability, and introduces a new dereddening method for highly extincted regions.

## Contribution

The paper presents a novel dereddening technique for stars in the Galactic bulge and provides a comprehensive statistical analysis of spotted stars' properties and variability behaviors.

## Key findings

- Identified correlations between luminosity, amplitude, and rotation period in giants.
- Discovered three groups of stars based on light and color variation correlations.
- Detected 79 flaring stars with brightness changes linked to amplitude.

## Abstract

We present the discovery and statistical analysis of $12\;660$ spotted variable stars toward and inside the Galactic bulge from over two-decade-long Optical Gravitational Lensing Experiment (OGLE) data. We devise a new method of dereddening of individual stars toward the Galactic bulge where strong and highly nonuniform extinction is present. In effect, $11\;812$ stars were classified as giants and $848$ as dwarfs. Well defined correlations between the luminosity, variability amplitude and rotation period were found for the giants. Rapidly rotating dwarfs with periods $P \leq 2$ d show I-band amplitudes lower than 0.2 mag which is substantially less than the amplitudes of up to 0.8 mag observed in giants and slowly rotating dwarfs. We also notice that amplitudes of stars brighter than $I_0 \approx 16$ mag do not exceed 0.3-0.4 mag. We divide the stars into three groups characterized by correlation between light and color variations. The positive correlation is characteristic for stars that are cooler when fainter, which results from the variable coverage of the stellar surface with spots similar to the sunspots. The variability of stars that are cooler when brighter (negative correlation) can be characterized by chemical spots with overabundance of heavy elements inside and variable line-blanketing effect, which is observed in chemically peculiar stars. The null correlation may results from very high level of the magnetic activity with rapidly variable magnetic fields. This division is readily visible on the color-magnitude diagram (CMD), which suggests that it may depend on the radius of the stars. We detect 79 flaring objects and discuss briefly their properties. Among others, we find that relative brightening during flares is correlated with brightness amplitude.