Optical identification and follow-up observations of SRGA J213151.5+491400

A new magnetic cataclysmic variable discovered with the SRG observatory

¹ Department of Astronomy and Space Sciences, Istanbul University, Faculty of Science, Beyazit, Istanbul 34119, Türkiye
e-mail: solen.balman@istanbul.edu.tr, solen.balman@gmail.com
² Faculty of Engineering and Natural Sciences, Kadir Has University, Cibali, Istanbul 34083, Türkiye
³ Department of Astronomy and Satellite Geodesy, Kazan Federal University, Kremlevskaya Str. 18, Kazan 420008, Russia
⁴ TÜBİTAK National Observatory, Akdeniz University Campus, Antalya 07058, Türkiye
⁵ Special Astrophysical Observatory of Russian Academy of Sciences, Nizhnij Arkhyz, 369167 Karachai-Cherkessian Rep., Russia
⁶ Department of Physics, Süleyman Demirel University, Isparta 32000, Türkiye
⁷ Department of Astronomy and Space Sciences, Ataturk University, Faculty of Science, Yakutiye, Erzurum 25240, Türkiye
⁸ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow 117997, Russia
⁹ National Research University Higher School of Economics, Moscow 101000, Russia
¹⁰ Department of Space Sciences and Technologies, Akdeniz University, Faculty of Sciences, Antalya 07058, Türkiye
¹¹ Department of Astronomy and Space Sciences, Ege University, Science Faculty, Bornova, Izmir 35100, Türkiye
¹² Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, Postfach 1317, 85741 Garching, Germany
¹³ Department of Physics, Adıyaman University, Adıyaman 02040, Türkiye
¹⁴ Astrophysics Application and Research Center, Adıyaman University, Adiyaman 02040, Türkiye

Received: 1 November 2023
Accepted: 6 January 2024

Abstract

Context. The paper is comprised of optical identification and multiwavelength studies of a new X-ray source discovered by the Spectrum Roentgen-Gamma (SRG) observatory during the ART-XC survey and its follow-up optical and X-ray observations.

Aims. We aim to identify SRGA J213151.5+491400 in the optical wavelengths. We determine spectra and light curves in the optical high and low states to find periodicities in the light curves and resolve emission lines in the system using optical ground-based data. We intend to study the spectral and temporal X-ray characteristics of the new source using the SRG surveys in the high and low states and NICER data in the low state.

Methods. We present optical data from telescopes in Türkiye (RTT-150 and T100 at the TÜBİTAK 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. Using the optical data, we performed astrometry, photometry, spectroscopy, and power spectral analysis of the optical time series. We present optical Doppler tomography along with X-ray data analysis producing light curves and spectra.

Results. We detected SRGA J213151.5+491400 in a high state in 2020 (17.9 mag) that decreased by 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, which is consistent with a magnetic cataclysmic variable (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 source. ART-XC detections yield an X-ray flux of (4.0−7.0) × 10⁻¹² erg s⁻¹ cm⁻² in the high state. eROSITA detects a dominating hot plasma component (kT_max > 21 keV in the high state) declining to (4.0−6.0) × 10⁻¹³ erg s⁻¹ cm⁻² 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)σ 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.