X-ray and ultraviolet observations of the eclipsing cataclysmic variables OV Bootis and SDSS J103533.02+055158.3 with degenerate donors (2101.03059v1)

Abstract: The majority of cataclysmic variables are predicted to be post-period minimum systems with degenerate donor stars, the period bouncers. Owing to their intrinsic faintness, however, only a handful of these systems have so far been securely identified. We want to study the X-ray properties of two eclipsing period bouncers, OV Bootis and SDSS J103533.02+055158.3, that were selected for this study due to their proximity to Earth. We have obtained XMM-Newton phase-resolved X-ray and ultraviolet observations of the two objects for spectral and timing analysis. Owing to a recent dwarf nova outburst OV Boo was much brighter than SDSS J103533.02+055158.3 at X-ray and ultraviolet wavelengths and the eclipse could be studied in some detail. An updated eclipse ephemeris was derived. The X-rays were shown to originate close to the white dwarf, the boundary layer, with significant absorption affecting its spectrum. There was no absorption in SDSS J103533.02+055158.3, despite being observed at the same inclination indicating different shapes of the disk and the disk rim. The white-dwarf temperature was re-determined for both objects: the white dwarf in OV Boo was still hot (23,000 K) five months after a dwarf nova outburst, and the white dwarf in SDSS J103533.02+055158.3 hotter than assumed previously (Teff = 11,500 K). All three cataclysmic variables with degenerate donors studied so far in X-rays, including SDSS J121209.31+013627.7, were clearly discovered in X-rays and revealed mass accretion rates dot(M) >= 8 x 10^-15 Msun/ yr. If their X-ray behavior is representative of the subpopulation of period bouncers, the all-sky X-ray surveys with eROSITA together with comprehensive follow-up will uncover new objects in sufficient number to address the remaining questions concerning late-stage cataclysmic variable evolution.

• The paper presents phase-resolved X-ray and UV observations that refine orbital periods and accretion diagnostics in period bouncer cataclysmic variables.
• Methodologies combining Newton, Swift, and Chandra data enabled detailed spectral and timing analyses, highlighting differences in boundary layer emissions and absorption features.
• Findings include a post-outburst hot white dwarf in OV Bootis and revised temperature estimates in SDSSJ1035, underscoring diverse accretion geometries and evolutionary states.

Observations and Analysis of Eclipsing Cataclysmic Variables with Degenerate Donors

This paper investigates the X-ray and ultraviolet (UV) properties of two eclipsing cataclysmic variables (CVs) with degenerate donors: OV Bootis (OV Boo) and SDSS J103533.02+055158.3 (SDSSJ1035). The systems are postulated to be period bouncers, which are CVs having passed their orbital period minimum with secondary stars that are no longer capable of hydrogen fusion. These objects are predicted to be prevalent among CVs yet remain underrepresented in observational datasets due to their intrinsic faintness.

Methodology

The authors employed phase-resolved X-ray and UV observations from the Newton X-ray satellite alongside archival Swift and Chandra data. UV and optical data from the American Association of Variable Star Observers (AAVSO) helped refine the orbital periods for both systems. The combination of these observational platforms allowed for comprehensive spectral and timing analyses of the two targets.

Results

1. OV Bootis:
   • OV Boo displayed a marked X-ray brightness increase following a recent dwarf nova outburst, aiding the detailed paper of its eclipse.
   • X-ray emissions predominantly originated from a region near the white dwarf, specifically the boundary layer.
   • Significant intrinsic absorption was observed in its X-ray spectrum, contrasting with the absence of such absorption in SDSSJ1035, suggesting differences in accretion disk or disk rim structure.
   • The white dwarf in OV Boo was notably hot (23,000 K) several months post-outburst, indicating elevated accretion heating from the outburst.
2. SDSSJ1035:
   • This object was considerably fainter in X-rays than OV Boo. It was correctly associated with ongoing accretion due to its X-ray detection.
   • SDSSJ1035's weak X-ray signal limited a detailed eclipse investigation and constrained the evidence for absorption at comparable inclinations seen in OV Boo.
   • Revised observations suggested a higher white dwarf temperature than reported in prior studies (11,500 K), correlating with updated Gaia distance measurements.

Implications

The paper provides refined estimates of mass accretion rates and white dwarf temperatures for both binary systems, offering insights into their evolutionary paths. The differing spectral and timing characteristics between OV Boo and SDSSJ1035 illustrate the potential for diverse accretion geometries and disk properties within the period bouncer subpopulation.

The observational constraints and data derived here suggest that forthcoming surveys, particularly those with eROSITA, promise to extend the sample size of known CV period bouncers. By identifying and characterizing more systems, further investigation into the late-stage evolution of CVs and the role of degenerate secondaries in these configurations can be undertaken more robustly.

Future Directions

The findings underscore the necessity of enhanced all-sky X-ray surveys complemented by thorough optical follow-ups to identify a statistically significant number of period-bouncing CVs. Better constraints on accretion dynamics and evolutionary models will check the prevalence of such systems, predicted by population synthesis models.

This investigation lays a groundwork for leveraging future X-ray missions and extensive time-resolved optical studies to ascertain the population density and intrinsic properties of period bouncers, enriching our understanding of binary evolution and the lifecycle of low-mass accretors in the galaxy.