A radio pulsing white dwarf binary star (1607.08265v1)

Abstract: White dwarfs are compact stars, similar in size to Earth but ~200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions, and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf / cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a delta-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56 hr period close binary, pulsing in brightness on a period of 1.97 min. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 s, and they are detectable at radio frequencies, the first such detection for any white dwarf system. They reflect the spin of a magnetic white dwarf which we find to be slowing down on a 10^7 yr timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they originate in large part from the cool star. AR Sco's broad-band spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.

• The paper identifies AR Scorpii as a unique spin-powered binary where a magnetic white dwarf pulses optically and at radio frequencies.
• The system exhibits broad-band synchrotron radiation across the electromagnetic spectrum, likely originating from the interaction between the white dwarf’s magnetosphere and the M-type companion star.
• AR Scorpii challenges existing binary star classifications like cataclysmic variables, providing new insights into magnetic white dwarf interactions and relativistic processes in these systems.

Review of "A Radio Pulsing White Dwarf Binary Star"

The analyzed paper presents a detailed paper of a unique binary star system known as AR Scorpii (AR Sco), featuring a peculiar white dwarf component. This system represents a novel discovery in the field of astrophysics due to its unusual emission across the electromagnetic spectrum, from X-ray to radio wavelengths. The star system consists of a white dwarf and a cool M-type main-sequence star in a close binary configuration.

Key Findings

1. Pulsation and Spin Characteristics:
   • AR Sco exhibits intense optical pulsations with a period of 1.97 minutes, detectable also at radio frequencies. This phenomenon is driven by the spin of a magnetic white dwarf, inferred to be slowing over a timescale of $10^7$ years.
   • The absence of noticeable accretion signatures indicates that the system is predominantly spin-powered, unlike typical accreting binaries.
2. Emission Profile:
   • The system displays a broad-band spectrum typical of synchrotron radiation, implying the presence of relativistic electrons. The emission likely originates almost entirely from the interaction between the white dwarf's magnetosphere and the M star.
   • Despite emissions being present across the spectrum, X-ray data revealed no significant pulsations, unlike other wavelengths.
3. Complex Radial Velocities and Mass Constraints:
   • Radial velocity measurements indicate the period of the binary system is 3.56 hours, with the M star’s velocity amplitude suggesting a lower limit on the mass of the white dwarf as $M_1 \geq 0.395 \, \text{M}_{\odot}$. The spectral emissions further restrict the mass ratios between the stars, placing the white dwarf mass between 0.81 and 1.29 $\text{M}_{\odot}$.
4. Distance and Luminosity:
   • The paper estimates the distance to AR Sco to be approximately 116 parsecs. The system's mean luminosity exceeds that of its stellar components alone, further ruling out accretion as the sole energy source.
5. Synchrotron Emission:
   • The interaction between the white dwarf's magnetosphere and the M star induces synchrotron emission. The paper explores scenarios where energy transfer occurs more efficiently than via direct isotropic radiation, proposing collimated particle outflows or direct magnetic interaction as possible mechanisms.

Implications and Theoretical Considerations

The findings provide new insights into the behavior of magnetic white dwarfs and their interactions with low-mass stellar companions. This interaction induces relativistic conditions that were previously only speculated in theoretical models, offering a real-world scenario that confirms this aspect of magnetic stellar dynamics.

AR Sco challenges existing classifications of binary stars, as it does not fit neatly into known categories like cataclysmic variables or pulsar-like systems. Its unique properties suggest an evolutionary link to intermediate polars but with distinctive features due to the apparent lack of notable accretion. This could imply a transient developmental phase or a distinct subclass within white dwarf binaries.

Future Developments

Further research could explore understanding the synchrotron emission mechanics and efficiency within such systems. Given the rarity of such occurrences, identifying and studying similar binaries can enhance understanding of white dwarf energetics and magnetospheric physics. Advancements in observational technologies and techniques will be pivotal in addressing these objectives, potentially uncovering more such systems or even predicting their emergence under certain conditions.

This paper of AR Sco provides a testament to the complexity and richness of stellar dynamics, paving pathways for future explorations in magnetic and relativistic processes within similar celestial entities.