The Intermediate Polar EI UMa: A Pre-Polar Cataclysmic Variable

TL;DR

This paper identifies EI UMa as a pre-polar cataclysmic variable with high accretion rate and magnetic field, based on optical and X-ray observations revealing white dwarf spin modulations.

Contribution

It provides the first detailed optical and X-ray timing analysis confirming EI UMa as an intermediate polar and suggests it is a rare pre-polar system.

Findings

Detected white dwarf spin and sideband modulations at 746 and 770 s.

Estimated high mass accretion rate (~3.6 x 10^{17} g/s).

Inferred large magnetic moment indicating an accretion ring.

Abstract

We present optical and X-ray time-series photometry of EI UMa that reveal modulation at 746 and 770 s, which we interpret as the white dwarf spin and spin-orbit sidebands. These detections, combined with previous X-ray studies, establish EI UMa as an intermediate polar. We estimate the mass accretion rate to be ~ 3.6 x 10^{17} g s^{-1}, which is close to, and likely greater than, the critical rate above which dwarf nova instabilities are suppressed. We also estimate the white dwarf to have a large magnetic moment mu > (3.4 +/- 0.2) x 10^{33} G cm^3. The high mass accretion rate and magnetic moment imply the existence of an accretion ring rather than a disk, and along with the relatively long orbital period, these suggest that EI UMa is a rare example of a pre-polar cataclysmic variable.