The polar IL Leo in a low accretion state

TL;DR

IL Leo is a period-bouncer polar studied in a low accretion state using 20 years of multiwavelength photometry and phase-resolved spectroscopy from the BTA and VLT. Phase-resolved data reveal cyclotron harmonics, Zeeman splitting, and H$\alpha$ emission dominated by the accretion stream, enabling a coherent picture of the magnetic geometry and accretion flow. Spectral-energy-distribution modelling yields a white dwarf with $T_{\mathrm{eff}} = 12700 \pm 360$ K, $\log g = 8.2 \pm 0.4$, and $M_{\mathrm{wd}} = 0.74 \pm 0.05\,M_\odot$, while cyclotron-emission modelling in the bombardment regime constrains $i$, $\beta$, $\psi$, $B_m \approx 41$ MG, and $\dot{M} \approx (2.4-4.1) \times 10^{-13} M_\odot$ yr$^{-1}$. Doppler tomography and H$\alpha$-line analysis place the emission in the accretion stream and magnetic funnel rather than the donor surface, with $B_{\mathrm{wd}} \approx 40$ MG. X-ray data provide context for higher accretion states, suggesting $\dot{M}$ increases by roughly an order of magnitude in intermediate states, underscoring IL Leo as a key system for studying magnetic accretion at the tail end of cataclysmic-variable evolution.

Abstract

We performed an optical study of the magnetic period-bouncer candidate IL Leo. Long-term photometric analysis over $\approx 20$ years reveals multiple state transitions. Modelling the ultraviolet and optical spectral energy distribution refined the white dwarf parameters, yielding a mass of $M_\textrm{wd} = 0.74 \pm 0.05 M_{\odot}$ and an effective temperature of $T_\mathrm{eff} = 12700 \pm 360$ K. We analyzed phase-resolved spectroscopy obtained with the 6-m BTA telescope and the VLT during the low state. Orbital variability of the H$α$ emission, inferred from dynamical spectra and Doppler tomograms, suggests that it originates in the accretion stream. Zeeman splitting gives a mean magnetic field of $B = 40.7 \pm 0.5$ MG. Modelling two sets of cyclotron spectra determined a low-state accretion rate of $\dot{M} = (2.5 - 4.1) \times 10^{-13}~M_{\odot}$ yr$^{-1}$ and a magnetic field of $B_\mathrm{m} \approx 41$ MG near magnetic pole.

Figures (8)

• Figure 1: Long-term optical light curve of IL Leo. Photometric data are presented from ZTF (bands $g$, $r$, $i$), PTF (bands $g$, $R$), CSS (band $V$), Gaia (bands $G$, $BP$, $RP$), and Pan-STARRS (bands $g$, $r$, $i$) catalogs. Vertical lines indicate the times of spectroscopic observations with the BTA and VLT telescopes.
• Figure 2: Left panel: Lomb--Scargle periodogram derived from the low-state photometry of IL Leo in the ZTF $zr$ and PTF $R$ bands. The highest power peak at a frequency of $17.4699 \pm 0.0004$ d$^{-1}$ corresponds to a period of $82.428 \pm 0.001$ min. Right panel: phase-folded light curves of IL Leo in the low state for the optical ZTF $zr$ and PTF $R$ bands and the ultraviolet GALEX $FUV$ and $NUV$ bands.
• Figure 3: Bottom panel: the spectral energy distribution of IL Leo in the low state. The plot presents the observed photometric fluxes (with horizontal bars indicating the effective bandwidths), the averaged VLT spectrum, and the white dwarf model spectrum. Top panel: the $(O - C)/\sigma$ diagram showing the residuals between the observed fluxes and the synthetic fluxes used in the spectral fitting.
• Figure 4: Optical dynamic spectrum of IL Leo constructed from BTA observations. The 4th and 5th cyclotron harmonics appear at 6600 Å and 5400 Å, respectively. In the blue part of the spectrum ($\lambda < 5000$ Å), the Zeeman components of the photospheric H$\beta$ line can be seen.
• Figure 5: Left panel: the observed dynamic spectrum of the H$\alpha$ line with superimposed sinusoids tracing the centroids of the bright narrow (red) and faint broad (blue) components. Middle panel: the composite emission source model fitted with two Gaussians. Right panel: the residuals between the observed and model dynamic spectra.