Cyclotron emitting magnetic white dwarfs in post common-envelope binaries discovered with the Zwicky Transient Facility

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
² Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³ Department of Astronomy, University of Washington, Seattle, WA 98195, USA
⁴ Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
⁵ Department of Physics & Astronomy, Univ. of California Los Angeles, PAB 430 Portola Plaza, Los Angeles, CA 90095-1547, USA
⁶ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
⁷ DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
⁸ Aix Marseille University, CNRS/IN2P3, CPPM, Marseille, France
⁹ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹⁰ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden

^⋆ Corresponding author; jcjvanroestel@gmail.com

Received: 21 August 2024
Accepted: 12 December 2024

Abstract

We present the discovery of 14 new (and the recovery of four known) low accretion rate magnetic white dwarfs in post-common envelope binaries that emit strong cyclotron emission using the Zwicky Transient Facility (ZTF) light curves, doubling the known sample size. In addition, we discovered a candidate magnetic period bouncer and recovered three known ones. We confirmed the presence of cyclotron emission using low-resolution spectra in 19 objects. Using the ZTF light curves, follow-up spectra, and the spectral energy distribution, we measured the orbital period, magnetic field strength, and white dwarf temperature of each system. Although the phase-folded light curves have diverse shapes and show a much larger variability amplitude, we show that their intrinsic properties (e.g. period distribution, magnetic field strength) are similar to those of previously known systems. The diversity in light curve shapes can be explained by differences in the optical depth of the accretion spot and geometric differences, the inclination angle, and the magnetic spot latitude. The evolutionary states of the longer period binaries are somewhat uncertain but vary; we found systems consistent with being pre-polars, detached polars, or low-state polars. In addition, we discovered two new low-state polars that likely have brown dwarf companions and could be magnetic period bouncers.