Discovery of Two Polars from a Crossmatch of ZTF and the SRG/eFEDS X-ray Catalog

TL;DR

This paper reports the discovery of two new magnetic cataclysmic variables, or polars, by crossmatching the eROSITA X-ray catalog with ZTF optical data, providing insights into their properties and magnetic field strengths.

Contribution

The study introduces a novel method of identifying magnetic CVs by combining X-ray and optical surveys, leading to the discovery of two previously unknown polars with detailed characterization.

Findings

Discovered two new polars: ZTFJ0850+0443 and ZTFJ0926+0105.

ZTFJ0850+0443 is an eclipsing polar with a 1.72 hr orbital period.

ZTFJ0926+0105 is a non-eclipsing polar with a 1.48 hr orbital period.

Abstract

Magnetic CVs are luminous Galactic X-ray sources but have been difficult to find in purely optical surveys due to their lack of outburst behavior. The eROSITA telescope aboard the Spektr-RG (SRG) mission is conducting an all-sky X-ray survey and recently released the public eROSITA Final Equatorial Depth Survey (eFEDS) catalog. We crossmatched the eFEDS catalog with photometry from the Zwicky Transient Facility (ZTF) and discovered two new magnetic cataclysmic variables (CVs). We obtained high-cadence optical photometry and phase-resolved spectroscopy for each magnetic CV candidate and found them both to be polars. Among the newly discovered magnetic CVs is ZTFJ0850+0443, an eclipsing polar with orbital period $P_\textrm{orb} = 1.72$ hr, white dwarf mass $M_\textrm{WD} = 0.81 \pm 0.08 M_\odot$ and accretion rate $\dot{M} \sim 10^{-11} M_\odot$/yr. We suggest that ZTFJ0850+0443 is a low magnetic field strength polar, with $B_\textrm{WD} \lesssim 10$ MG. We also discovered a non-eclipsing polar, ZTFJ0926+0105, with orbital period $P_\textrm{orb} = 1.48$ hr, magnetic field strength $B_\textrm{WD} \gtrsim 26$ MG, and accretion rate $\dot{M} \sim 10^{-12} M_\odot$/yr.