Stochastic Optical Variability and an rms–Flux Relation in the Intermediate Polar EP240309a

Abstract

Magnetic cataclysmic variables provide a natural laboratory for studying how accretion interacts with compact-object magnetospheres and generates stochastic variability. We present an optical variability study of the intermediate-polar candidate EP240309a, an Einstein Probe X-ray transient, using BOOTES photometry, high-cadence TESS light curves, and a Southern Astrophysical Research (SOAR)/Goodman optical spectrum. Previous studies found a white-dwarf spin period of 3.97 minutes (Pspin ≃ 238 s) and an orbital period of Porb = 3.7614(4) hr. Power spectral densities from the BOOTES data are consistent with single power laws with slopes α ≃ 1.2–1.8, with no statistically significant evidence for a bend across the sampled frequency range. Using red-noise simulations and injection–recovery tests, we place one-sided constraints on any putative break frequency, which translate, under standard dynamical identifications, into an upper limit on the magnetospheric radius of Rm ≲ few × 1010 cm for MWD = 0.8 M⊙. In the TESS data, we detect a linear rms–flux relation on hour timescales in three high-cadence sectors, while two other sectors do not show a robust detection, indicating epoch-dependent rms–flux behavior. The SOAR spectrum shows Balmer and He ii emission lines with FWHM ≈ 1000–1600 km s−1; under a Keplerian interpretation, these imply characteristic radii of r ≈ (0.9–3.4) × 1010 cm, broadly comparable to the timing-based constraints. Overall, the data provide conservative, order-of-magnitude radius constraints consistent with accretion onto a magnetic white dwarf, but they do not establish the detailed accretion geometry or exclude stream-fed or mixed accretion scenarios.