Confirmation of SRGt 062340.2-265751 as a nova-like cataclysmic variable with a possible magnetic nature

TL;DR

This study tackles SRGt 062340.2-265751, a CV with large X-ray variability and conflicting periods, by combining XMM-Newton timing/spectroscopy with eROSITA all-sky data and ASAS-SN photometry, plus broadband SED modelling. It finds a likely orbital period of $P_orb ~ 3.6h$ and potential spin modulations near $P_spin ~ 36min$ or $43min$, with X-ray emission from a multi-temperature plasma and a disc-dominated SED yielding dotM ~ 6.8x10^-9 Msun/yr and T_WD ~ 4.6x10^4 K. The results favor a nova-like CV with possible magnetic nature, i.e., a magnetic nova-like or IP, but confirmation requires polarimetry or unambiguous spin signatures. This work contributes to growing evidence that magnetic fields may be present in a broader subset of nova-like CVs and informs accretion and CV evolution models.

Abstract

SRGt 062340.2-265751, a cataclysmic variable identified by SRG/eROSITA thanks to its significant X-ray variability, remains poorly characterised despite the multi-wavelength follow-up. We present spectral and timing analyses from the first dedicated X-ray and ultraviolet observations with XMM-Newton, complemented by SRG/eROSITA data from four all-sky surveys (eRASS1-4) and ASAS-SN optical photometry. Our timing analysis reveals a >8$σ$ significant modulation at 3.6 $\pm$ 0.5 hours, likely representing the orbital period. Long-term ASAS-SN monitoring confirms the source as a VY Sculptoris-type nova-like system, while short-timescale X-ray and ultraviolet variability, down to a few minutes, suggests a possible underlying magnetic white dwarf. Two additional significant X-ray modulations at 43 $\pm$ 1 min and 36.0 $\pm$ 0.7 min tentatively point to the spin period of an intermediate polar. The best-fit XMM-Newton energy spectra reveal a multi-temperature thermal plasma ($kT$ = 0.23, 0.94, and 5.2 keV), while the SRG/eROSITA spectra are consistent with a single-temperature thermal plasma of a few keV. We estimate unabsorbed X-ray luminosities of $\gtrsim$$10^{32}$ erg s$^{-1}$ (0.2-12 keV). Broadband spectral energy distribution modelling, from near-ultraviolet to infrared, indicates a disc-dominated system consistent with a nova-like classification. We discuss these results in the context of the source's confirmed nova-like classification and its possible magnetic nature, a scenario increasingly supported by discoveries of intermediate polars exhibiting VY Sculptoris-type nova-like features.

Figures (7)

• Figure 1: Left panel: Long-term ASAS-SN light curve of SRGt 062340, combining V- and g-band magnitudes. The purple vertical lines indicate the epochs of the eROSITA observations, while the lilac line marks the epoch of the XMM-Newton observation. Right panel: Detailed view of the light curve between MJD 60280 and MJD 60470.
• Figure 2: Top panel: eROSITA light curve of SRGt 062340 in the 0.2$-$8 keV energy band across the four all-sky surveys (eRASS1-4). Each data point represents the mean rate of a single scan over the source position, separated by an eroday (4 h; see text for more details). Second to fifth panels:XMM-Newton/EPIC-pn, EPIC-MOS1, and EPIC-MOS2 30 s binned light curves of SRGt 062340 in the 0.2$-$12 keV energy band, followed by the OM 30 s binned light curve obtained with the UVM2 filter.
• Figure 3: Lomb-Scargle power spectra of the EPIC-pn light curve in the 0.2$-$12 keV energy band (top) and the OM light curve obtained with the UVM2 filter (bottom). The dashed horizontal lines indicate the 3$\sigma$ and 5$\sigma$ power significance levels. The grey vertical lines (dashed for EPIC-pn and dotted for OM) denote the frequency of the peaks with a significance higher than 5$\sigma$. The frequencies and significance levels of the marked peaks are summarised in Table \ref{['peaks']}.
• Figure 4: Left panel:XMM-Newton energy spectra of SRGt 062340, shown with the best-fitting model (TBabs*(apec+apec+apec)). The bottom panel shows the normalised residuals. The best-fit parameters are listed in Table \ref{['spec_param']}. Top right panel: Zoomed-in image of the Fe K complex, fitted with a TBabs*(apec+gaussian) model. The dashed lines represent the model components for the EPIC-MOS1 spectrum, shown as an example. The vertical dotted lines mark the expected energies of the different Fe-line components. Bottom right panel: RGS spectrum and best-fitting model, identical to those shown in the left panel, but plotted as a function of wavelength. The vertical lines indicate several prominent and commonly observed transitions.
• Figure 5: eROSITA energy spectra of SRGt 062340 from the four observing epochs, shown with the best-fitting model (TBabs*apec). For clarity, we show the eRASS1 spectrum rebinned to 10 counts per bin, although the analysis was performed using a 1 count per bin spectrum (see text). For eRASS2-4, we show the 25 counts per bin spectra used in the analysis. Normalised residuals are shown in the bottom panels. The best-fitting parameters are listed in Table \ref{['spec_param']}.