SRG/eROSITA X-ray selected cataclysmic variable candidates observed in SDSS-V DR20

Abstract

We report on the spectroscopic observations obtained during SDSS-V DR20 of accreting compact binaries (ACB), specifically the cataclysmic variables (CVs), that were identified as likely compact binary candidates from the SRG (Spectrum Roentgen Gamma) eROSITA eRASS1 and eRASS:3 observations. Our primary aim is to obtain a complete inventory of all CVs that were detected in eRASS1 and eRASS:3, with the goal to help better understand close-binary evolution, the population density, and demographics of these systems in the Galaxy. Previous population studies had their respective limitations, with volume-limited samples suffering from low-number statistics, while magnitude-limited observations were biased to only the brighter systems. All CVs are X-ray emitters, eROSITA, therefore, presents a unique opportunity to identify CVs based on their X-ray emission. Given eROSITA's sensitivity, we expect to find most X-ray active CVs within 500 pc, and magnetic CVs to several kpc. Using X-ray data from the eROSITA together with optical data from Gaia, three different approaches were explored to identify the Gaia optical counterpart to the eROSITA X-ray source of the likely ACB. From this, unique candidates were identified and were submitted in three different cartons to SDSS-V for optical spectroscopic observations as part of the Milky Way Mapper survey. From our submitted candidates, we found 538 likely CVs. We also identified CVs that were observed in other eROSITA based cartons, in which we identified an additional 49 systems that are likely CVs and which were not in our selection. We therefore identified 587 systems as CVs from the eROSITA based SDSS-V observations. We also attempted to sub classify the CVs as dwarf-novae, novalikes, or magnetic systems based on the eROSITA X-ray data, the emission line properties in the optical SDSS-V spectra, and other data in the public domain, if available.