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TESS light curves of two new magnetic cataclysmic variables: an asynchronous polar at the period minimum, and an eclipsing system with a large spin-to-orbit ratio

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Key takeaway

Astronomers discovered two new types of rare binary stars with extremely fast-spinning white dwarfs, which could help reveal how these unusual systems form and evolve.

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Quick Explainer

The two newly discovered magnetic cataclysmic variable stars, Gaia21akb and ZTF18aazmehw, exhibit unique properties that challenge the established classification of these systems. These variables contain an accreting, magnetized white dwarf with a spin rate that is not perfectly synchronized with the orbital period, a phenomenon known as asynchronous rotation. Gaia21akb is a candidate for an "asynchronous polar" at the period minimum, while ZTF18aazmehw is an eclipsing system with a particularly large spin-to-orbit ratio. These systems expand the recently identified population of mCVs that occupy this transitional parameter space, providing important insights into the evolution of magnetic cataclysmic variables and the mechanisms responsible for their asynchronous rotation.

Deep Dive

Technical Deep Dive

Overview

Two new magnetic cataclysmic variable stars (mCVs) were identified in TESS light curve data:

  1. Gaia21akb: A candidate asynchronous polar (AP) at the period minimum, with a suspected orbital period of 77.55 min and a spin-to-orbit ratio of 0.9879.
  2. ZTF18aazmehw: An eclipsing mCV with a 89.5 min orbital period and a large spin-to-orbit ratio of 0.867.

These systems expand the recently identified population of asynchronous mCVs with short orbital periods (< 2 hours) and high spin-to-orbit ratios (0.5 < Pspin/Porb < 0.9). This provides important insights into the evolution of magnetic cataclysmic variables.

Problem & Context

Magnetic cataclysmic variables (mCVs) are short-period binary systems containing an accreting, magnetized white dwarf primary. In synchronous mCVs (polars), the white dwarf's magnetic field synchronizes its rotation to the binary orbit. Asynchronous rotation is also commonly observed in several classes of mCVs, including:

  • Asynchronous polars (APs): Share many characteristics with polars, but their spin and orbital periods differ by up to several percent.
  • Intermediate polars (IPs): Highly asynchronous, with typical spin-to-orbit ratios Pspin/Porb ≲ 0.1.

Until recently, there was a relatively clear distinction between polars, APs, and IPs based on their spin-to-orbit ratios. However, a new population of systems with 0.5 ≲ Pspin/Porb ≲ 0.9 has been identified, challenging this established classification scheme.

Methodology

The authors analyzed TESS light curve data of the two newly identified mCVs, Gaia21akb and ZTF18aazmehw, to determine their orbital and spin properties:

  • For Gaia21akb, the TESS data showed large-amplitude variability consistent with polar behavior, and a power spectrum indicative of an asynchronous polar. The authors provisionally identified the orbital period as 77.55 min and the spin period as 76.61 min, resulting in a spin-to-orbit ratio of 0.9879.
  • For ZTF18aazmehw, the TESS data revealed an eclipsing system with a 89.5 min orbital period and an 77.6 min spin period, giving a spin-to-orbit ratio of 0.867.

The authors also searched for evidence of nova shells around these systems, which could explain their asynchronous rotation, but found no such evidence.

Results

The key findings are:

  • Gaia21akb and ZTF18aazmehw are two new mCVs with short orbital periods (< 2 hours) and high spin-to-orbit ratios (0.9879 and 0.867, respectively).
  • These systems expand the recently identified population of asynchronous mCVs in this parameter space, which was previously thought to be sparsely occupied.
  • The authors did not find evidence of nova shells around these systems, despite theories suggesting nova eruptions could cause asynchronous rotation.

Interpretation

The authors suggest several interpretations:

  • These systems may be evolving towards synchronization, as proposed by the Chanmugam & Ray (1984) model. The long synchronization timescales could allow observing them in this transitional stage.
  • The lack of nova shells challenges the nova eruption hypothesis for the origin of asynchronous rotation in these systems. Other mechanisms, such as the stream-overflow accretion model, may better explain their properties.
  • The simple power spectrum and stable spin profile of ZTF18aazmehw suggest its accretion geometry may differ from other asynchronous mCVs, perhaps due to better alignment between the spin and magnetic axes.

Limitations & Uncertainties

  • The authors could not definitively identify the orbital period of Gaia21akb due to ambiguities in the power spectrum.
  • The nature of the newly discovered system Gaia19bxc, with a proposed orbital period below the period minimum, remains uncertain and requires further observations to confirm.

What Comes Next

The authors suggest several avenues for future research:

  • Measuring the white dwarf masses and spin period derivatives in these systems to test theoretical models of their evolution.
  • Investigating the accretion geometries and the reasons for the differences observed between ZTF18aazmehw and other asynchronous mCVs.
  • Continuing the search for nova shells around asynchronous mCVs to better understand the origins of their asynchronous rotation.

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