CSS1603+19: a low-mass polar near the cataclysmic variable period minimum

TL;DR

This study characterizes CSS1603+19, a low-mass polar near the cataclysmic variable period minimum, revealing its magnetic white dwarf, accretion dynamics, and binary component masses through multi-wavelength observations.

Contribution

It provides the first detailed physical model of CSS1603+19, confirming its classification as a polar and constraining the masses of its binary components.

Findings

Magnetic field of white dwarf estimated at ~5MG.

Detection of cyclotron features and double-peaked emission lines.

System likely completed the period gap but not yet undergone period bounce.

Abstract

CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of cataclysmic variables. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here we present time-resolved multi-wavelength observations of this system. WISE photometry indicates that the mid-infrared excess displays a one-magnitude eclipsing-like variability during the orbit. We obtained near-infrared and optical spectroscopy using Gemini, MDM and APO telescopes. Near-infrared spectra show possible cyclotron features indicating that the white dwarf has a magnetic field of about 5MG. Optical and near-infrared spectra display double-peaked emission lines, with both components showing strong radial velocity variations during the orbital period and with the broad component leading the narrow component stably by about 0.2 of the orbital phase. We construct a physical model informed by existing observations of the system and determine that one component likely originates from the accretion column onto the magnetized white dwarf in synchronous rotation with the orbital motion and the other from the Roche overflow point. This allows us to constrain the masses of the binary components to be $M_1>0.24 M_{\odot}$ for the white dwarf accretor and $M_2=0.0644\pm0.0074 M_\odot$ for the donor. We classify the system as an AM Herculis star, or a polar. It has likely completed its stint on the period gap, but has not yet gone through the period bounce.