Quasi-periodic oscillations in the TESS light curve of TX Col, a diskless intermediate polar on the precipice of forming an accretion disk

TL;DR

This study investigates quasi-periodic oscillations in TX Col's light curve, suggesting a transitional accretion regime involving diamagnetic blobs that could lead to disk formation in a previously diskless system.

Contribution

It introduces the concept of a diamagnetic-blob accretion regime to explain observed QPOs and transitional accretion states in TX Col, expanding understanding of accretion processes in intermediate polars.

Findings

QPOs appear during enhanced accretion episodes.

QPOs are within 10-25 cycles per day frequency range.

Evidence suggests formation of a diamagnetic-blob torus near the circularization radius.

Abstract

One of the fundamental properties of an intermediate polar is the dynamical nature of the accretion flow as it encounters the white dwarf's magnetosphere. Many works have presumed a dichotomy between disk-fed accretion, in which the WD accretes from a Keplerian disk, and stream-fed accretion, in which the matter stream from the donor star directly impacts the WD's magnetosphere without forming a disk. However, there is also a third, poorly understood regime in which the accretion flow consists of a torus of diamagnetic blobs that encircles the WD. This mode of accretion is expected to exist at mass-transfer rates below those observed during disk-fed accretion, but above those observed during pure stream-fed accretion. We invoke the diamagnetic-blob regime to explain the exceptional TESS light curve of the intermediate polar TX Col, which transitioned into and out of states of enhanced accretion during Cycles 1 and 3. Power-spectral analysis reveals that the accretion was principally stream-fed. However, when the mass-transfer rate spiked, large-amplitude quasi-periodic oscillations (QPOs) abruptly appeared and dominated the light curve for weeks. The QPOs have two striking properties: they appear in a stream-fed geometry at elevated accretion rates, and they occur preferentially within a well-defined range of frequencies (~10-25 cycles per day). We propose that during episodes of enhanced accretion, a torus of diamagnetic blobs forms near the binary's circularization radius and that the QPOs are beats between the white dwarf's spin frequency and unstable blob orbits within the WD's magnetosphere. We discuss how such a torus could be a critical step in producing an accretion disk in a formerly diskless system.