Optical identification and follow-up observations of SRGA J213151.5+491400 -- a new magnetic cataclysmic variable discovered with SRG Observatory

We report results of optical identification and multi-wavelength study of a new polar-type magnetic cataclysmic variable (MCV), SRGA J213151.5+491400, discovered by Spectrum Roentgen-Gamma ($SRG$) observatory in the course of the all-sky survey. We present optical data from telescopes in Turkey (RTT-150 and T100 at the T\"UBITAK National Observatory), and in Russia (6-m and 1-m at SAO RAS), together with the X-ray data obtained with $ART-XC$ and $eROSITA$ telescopes aboard $SRG$ and the $NICER$ observatory. We detect SRGA J213151.5+491400 in a high state in 2020 (17.9 mag) that decreases about 3 mag into a low state (21 mag) in 2021. We find only one significant period using optical photometric time series analysis which reveals the white dwarf spin/orbital period to be 0.059710(1) days (85.982 min). The long slit spectroscopy in the high state yields a power law continuum increasing towards the blue with a prominent He II line along with the Balmer line emissions with no cyclotron humps; consistent with MCV nature. Doppler Tomography confirms the polar nature revealing ballistic stream accretion along with magnetic stream during the high state. These characteristics show that the new source is a polar-type MCV. $SRG$ $ART-XC$ detections yield an X-ray flux of (4.0-7.0)$\times$10$^{-12}$ erg cm$^2$ s$^{-1}$ in the high state. $eROSITA$ detects a dominating hot plasma component (kT$_{\rm{max}}$ $>$ 21 keV in the high state) declining to (4.0-6.0)$\times$10$^{-13}$ erg cm$^2$ s$^{-1}$ in 2021 (low state). The $NICER$ data obtained in the low state reveal a two-pole accretor showing a soft X-ray component at (6-7)$\sigma$ significance with a blackbody temperature of 15-18 eV. A soft X-ray component has never been detected for a polar in the low state before.