# Variations on a theme -- the puzzling behaviour of Schulte 12

## TL;DR

This study reveals a 108-day X-ray flux modulation in the massive star Schulte 12, linked to non-radial pulsations, with multi-wavelength data supporting its evolutionary status and complex variability.

## Contribution

It reports the first detection of a regular 108-day X-ray modulation in Schulte 12 and links it to stellar pulsations, revising its distance and evolutionary understanding.

## Key findings

- Discovery of a 108-day X-ray flux modulation.
- Correlation of X-ray variability with stellar pulsations.
- Revised distance placing Schulte 12 in an instability region.

## Abstract

One of the first massive stars detected in X-rays, Schulte 12 has remained a puzzle in several aspects. In particular, its extreme brightness both in the visible and X-ray ranges is intriguing. Thanks to Swift and XMM-Newton observations covering ~5000d, we report the discovery of a regular 108d modulation in X-ray flux of unknown origin. The minimum in the high-energy flux appears due to a combination of increased absorption and decreased intrinsic emission. We examined in parallel the data from a dedicated spectroscopic and photometric monitoring in the visible and near-IR domains, complemented by archives. While a similar variation timescale is found in those data, they do not exhibit the strict regular clock found at high energies. Changes in line profiles cannot be related to binarity but rather correspond to non-radial pulsations. Considering the substantial revision of the distance of Schulte 12 from the second GAIA data release, the presence of such oscillations agrees well with the evolutionary status of Schulte 12, as it lies in an instability region of the HR diagram.