# Extensive photometry of the intermediate polar V2069 Cyg (RX J2123.7+4217)

## TL;DR

This extensive photometric study of V2069 Cyg precisely measures the white dwarf's spin period, detects a new sideband oscillation, and observes significant changes in pulse profiles and amplitudes over years, providing valuable data for future spin evolution studies.

## Contribution

The paper provides the first detection of the sideband oscillation in V2069 Cyg and offers a highly precise spin period with a long-term ephemeris, along with detailed analysis of pulse profile variability.

## Key findings

- Precise spin period of 743.40650±0.00048 s determined.
- First detection of sideband oscillation at 764.5125±0.0049 s.
- Significant changes in pulse profiles and oscillation amplitudes over years.

## Abstract

To obtain the spin period of the white dwarf in the intermediate polar V2069 Cyg with high precision, we fulfilled extensive photometry. Observations were performed within 32 nights, which have a total duration of 119 hours and cover 15 months. We found the spin period of the white dwarf, which is equal to 743.40650+/-0.00048 s. Using our precise spin period, we derived the oscillation ephemeris with a long validity of 36 years. This ephemeris and the precise spin period can be used for future investigations of spin period changes of the white dwarf in V2069 Cyg. In addition, for the first time we detected the sideband oscillation with a period of 764.5125+/-0.0049 s. The spin and sideband oscillations revealed unstable amplitudes both in a time-scale of days and in a time-scale of years. On average, the semi-amplitude of the spin oscillation varied from 17 mmag in 2014 to 25 mmag in 2015. The semi-amplitude of the sideband oscillation varied from 12 mmag in 2014 to an undetectable level of less than 7 mmag in 2015. In a time-scale of years, the optical spin pulse profile revealed essential changes from an asymmetric double-peaked shape in 2014 to a quasi-sinusoidal shape in 2015. Such drastic changes of the optical spin pulse profile seem untypical of most intermediate polars and, therefore, are of great interest. The pulse profile of the sideband oscillation was quasi-sinusoidal. Moreover, we note that V2069 Cyg possesses strong flickering with a peak-to-peak amplitude of 0.4-0.6 mag.