The optical photometric and spectroscopic periodicities of the cataclysmic variable SRGt 062340.2-265751

TL;DR

The study targets SRGt 062340.2-265751, a CV discovered by eROSITA, to constrain its sub-type using time-resolved optical spectroscopy and photometry across ground-based facilities and TESS, supplemented by archival data. Spectroscopy reveals very low radial-velocity amplitudes in Hβ/Hγ (K ≈ 14 km s$^{-1}$) and an orbital period of $P_{ ext{orb}} \,\approx\, 3.645$ h, while photometry uncovers high-frequency variability with a possible WD spin period near $P_{ ext{spin}} \,\approx\, 24.905$ min and multiple nocturnal periodicities. Ground-based and TESS data show multiple periodicities without a stable spin signal, though TESS Sector 6 hints at a positive superhump with $P_{+}$ nearby $P_{ ext{orb}}$, and He II 4686 along with Bowen blend features support a nova-like CV, likely of the VY Scl subclass, with a low orbital inclination. Collectively, the results favor a nova-like, possibly VY Scl, CV with a truncated disc or weak magnetism, though an IP interpretation remains plausible but unconfirmed without definitive spin detection and X-ray timing. The work highlights the complexity of period timing in magnetic NLs and underscores the need for complementary X-ray timing and broader wavelength coverage to fully characterize the system’s accretion geometry and magnetic nature.

Abstract

We report on optical spectroscopic and photometric follow-up observations of the eROSITA discovered transient SRGt 062340.2-265751 and show that it displays the characteristics of a nova-like cataclysmic variable (CV), with possible indications of being a magnetic system. We try to put better constraints on the classification of SRGt 062340.2-265751 using optical time-resolved spectroscopic and photometric observations to find any periodicities in the system. From these periodicities we can classify the CV sub-type that it belongs to. Spectroscopic observations revealed a very low amplitude, K $\sim$ 14 km s$^{-1}$, in the radial velocity of the H$β$ and H$γ$ emission lines, suggesting that the system is likely observed at a low inclination angle. High-speed photometric observations revealed highly stochastic variability, characteristic of many magnetic cataclysmic variable systems. A probable 3.645 $\pm$ 0.006 hour orbital period was found by applying Lomb-Scargle period analysis to the H$β$ and H$γ$ emission line radial velocities. A 24.905 $\pm$ 0.065 min period was found from photometric observations, which we associate with the white dwarf spin. However, it was also found that the photometry revealed multiple periodicities from night to night. TESS observations in three sectors did not reveal any of the periodicities found from ground-based observations, but did show a prominent period in only one sector, which might be attributed to a positive superhump period. These multiple periodicities as well as the HeII $λ$4686 and Bowen blend emission lines seen in the spectra indicate that SRGt 062340.2-265751 is likely a nova-like CV, and might belong to the VY Scl sub-type.

Figures (20)

• Figure 1: CRTS $V$-band (Catalina ID 3027043037227) and ASAS-SN light curve of SRGt 062340 taken using g and V filters. Superimposed are the photometric and spectroscopic observations reported on in this study. Refer to the appendix and text for more detail on observation epochs. The inset plot has the same axes as the main plot, and focuses on the epochs of photometric TESS and SAAO observations. The magnitude ranges from 12.14 - 12.93 during the SAAO photometry campaign
• Figure 2: Median combined flux calibrated spectrum ranging from 4100Å - 5000Å of the nine observations made on 2021 February 2. 'BB' indicates the Bowen fluorescence blend.
• Figure 3: lmfit H$\gamma$ (top) and H$\beta$ (bottom) modelling for an observation obtained on 2021 February 26, where the bottom panels of each line shows the respective residuals of the fit.
• Figure 4: Power spectra obtained using the H$\beta$ (top) and H$\gamma$ (bottom) emission line radial velocity measurements using all the datasets. The inset plot shows the frequency range of the suspected orbital period of the system. It is found that all indicated periods are one-day aliases.
• Figure 5: Power spectra obtained using the H$\beta$ (top) and H$\gamma$ (bottom) absorption line radial velocity measurements using all the datasets. The inset plot shows the frequency range of the suspected orbital period of the system. It is found that all indicated periods are one-day aliases.