Discovery of eRASSt J192932.9–560346: A bright, two-pole accreting, eclipsing polar

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: aschwope@aip.de
² South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa
³ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁴ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
⁵ Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
⁶ Dr. Karl Remeis-Sternwarte & Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁷ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland

Received: 28 June 2021
Accepted: 4 August 2021

Abstract

We report the discovery of a bright (V ~ 15), eclipsing, two-pole accreting magnetic cataclysmic variable (CV), a polar, as counterpart of the SRG/eROSITA and Gaia transients eRASSt J192932.9–560346 and Gaia21bxo. Frequent large-amplitude changes of its brightness at X-ray and optical wavelengths by more than four magnitudes are indicative of a CV nature of the source. Identification spectra obtained with the 10m SALT telescope revealed the typical features of a magnetic CV: strong, broad HeI, HeII, and hydrogen Balmer emission lines superposed on a blue continuum. Time-resolved photoelectric polarimetry revealed that the circular polarization varies from −20 to +20% and the linear polarization varies from 0 to 10%, confirming the system to be a magnetic CV of the polar subclass. High-cadence photometry revealed deep, structured eclipses, indicating that the system is a two-pole accretor. The orbital period determined from the eclipse times is 92.5094 ± 0.0002 min. The X-ray spectrum is thermal only, and the implied luminosity is L_X = 2.2 × 10³¹ erg s⁻¹ at the Gaia-determined distance of 376 pc.