Compact white dwarf binaries in the combined SRG/eROSITA/SDSS eFEDS survey

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: aschwope@aip.de
² Potsdam University, Institute for Physics and Astronomy, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany
³ Astronomy Department, Box 351580, University of Washington, Seattle, WA 98195, USA
⁴ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago 7820436, Chile
⁵ Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago 7820436, Chile
⁶ Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Of 104, Providencia, Santiago, Chile
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
⁸ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
¹¹ Department of Physics and Astronomy, Western Washington University, 516 High Street, Bellingham, WA 98225, USA
¹² Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
¹³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁴ Institut für Astronomie & Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁵ Department of Astronomy, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
¹⁶ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
¹⁷ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

Received: 30 October 2023
Accepted: 10 March 2024

Abstract

Context. X-ray surveys combined with optical follow-up observations are used to generate complete flux-limited samples of the main X-ray emitting source classes. eROSITA on the Spectrum-Roentgen-Gamma mission provides sufficient sensitivity to build significantly enhanced samples of rare X-ray emitting sources.

Aims. We strive to identify and classify compact white dwarf binaries, cataclysmic variables (CVs), and related objects, which were detected in the sky area of eFEDS, the eROSITA Final Equatorial Depths Survey, and they were observed in the plate program of SDSS-V.

Methods. Compact white dwarf binaries were selected from spectra obtained in the early SDSS-V plate program. A dedicated set of SDSS plate observations were carried out in the eFEDS field, providing spectroscopic classifications for a significant fraction of the optically bright end (r < 22.5) of the X-ray sample. The identification and subclassification rests on visual inspections of the SDSS spectra, spectral variability, color-magnitude and color-color diagrams involving optical and X-ray fluxes, optical variability, and literature work.

Results. Upon visual inspection of SDSS spectra and various auxiliary data products, we have identified 26 accreting compact white dwarf binaries (aCWDBs) in eFEDS, of which 24 are proven X-ray emitters. Among those 26 objects, there are 12 dwarf novae, three WZ Sge-like disk-accreting nonmagnetic CVs with low accretion rates, five likely nonmagnetic high accretion rate nova-like CVs, two magnetic CVs of the polar subcategory, and three double degenerates (AM CVn objects). Period bouncing candidates and magnetic systems are rarer than expected in this sample, but it is too small for a thorough statistical analysis. Fourteen of the systems are new discoveries, of which five are fainter than the Gaia magnitude limit. Thirteen aCWDBs have measured or estimated orbital periods, of which five were presented here. Through a Zeeman analysis, we revise the magnetic field estimate of the polar system J0926+0105, which is likely a low-field polar at B = 16 MG. We quantified the success of X-ray versus optical/UV selection of compact white dwarf binaries which will be relevant for the full SDSS-V survey. We also identified six white dwarf main sequence (WDMS) systems, among them there is one confirmed pre-CV at an orbital period of 17.6 h and another pre-CV candidate.

Conclusions. This work presents successful initial work in building large samples of all kinds of accreting and X-ray emitting compact white dwarf binaries that will be continued over the full hemisphere in the years to come.