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

Creators: Rodriguez, Antonio C.; Kulkarni, Shrinivas R.; Prince, Thomas A.; Szkody, Paula; Burdge, Kevin B.; Caiazzo, Ilaria; van Roestel, Jan; Vanderbosch, Zachary P.; El-Badry, Kareem; Bellm, Eric C.; Gänsicke, Boris T.; Graham, Matthew J.; Mahabal, Ashish A.; Masci, Frank J.; Mróz, Przemek; Riddle, Reed; Rusholme, Ben

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_(orb) = 1.72 hr, white dwarf mass M_(WD) = 0.81 ± 0.08 M_⊙ and accretion rate M˙∼10⁻¹¹ M_⊙/yr. We suggest that ZTFJ0850+0443 is a low magnetic field strength polar, with B_(WD) ≲ 10 MG. We also discovered a non-eclipsing polar, ZTFJ0926+0105, with orbital period P_(orb) = 1.48 hr, magnetic field strength B_(WD)≳ 26 MG, and accretion rate M˙ ∼10⁻¹² M_⊙/yr.

Additional Information

Attribution 4.0 International (CC BY 4.0). ACR thanks the ZTF Variable Star Group for useful comments and discussions. ACR also thanks Axel Schwope, Paul Groot, Frank Verbunt, and Jim Fuller for insightful conversations that led to an improved final manuscript. This work is based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. Major funding has been provided by the U.S National Science Foundation under Grant No. AST-1440341 and by the ZTF partner institutions: the California Institute of Technology, the Oskar Klein Centre, the Weizmann Institute of Science, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron, the University of Wisconsin-Milwaukee, and the TANGO Program of the University System of Taiwan. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham). Some observations were made with the Apache Point 3.5m telescope, which is owned and operated be the Astrophysical Research Corporation. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular, the institutions participating in the Gaia Multilateral Agreement. This work is based on data from eROSITA, the soft X-ray instrument aboard SRG, a joint Russian-German science mission supported by the Russian Space Agency (Roskosmos), in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL) and its subcontractors, and is operated by NPOL with support from the Max Planck Institute for Extraterrestrial Physics (MPE). The development and construction of the eROSITA X-ray instrument was led by MPE, with contributions from the Dr. Karl Remeis Observatory Bamberg & ECAP (FAU Erlangen-Nuernberg), the University of Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP), and the Institute for Astronomy and Astrophysics of the University of Tübingen, with the support of DLR and the Max Planck Society. The Argelander Institute for Astronomy of the University of Bonn and the Ludwig Maximilians Universität Munich also participated in the science preparation for eROSITA.

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