Evidence for Particle Acceleration in a Magnetized White Dwarf from Radio and Gamma-Ray Observations

Abstract

The DQ Her-type magnetic cataclysmic variable AE Aqr contains the most rapidly spinning white dwarf (P = 33 s) in a close binary and can be considered as the "millisecond pulsar" equivalent of white dwarfs. It shows flare-like UV emission on timescales of an hour during which strong QPO activity is seen, as well as flare-like radio synchrotron emission on similar timescales up to frequencies of at least 250 GHz. It is one of the most efficient converters of accretion power to MeV electrons of all X-ray binaries, LMXB, and cataclysmic variables. TeV emission was also reported by two independent groups with a periodic and burst-like behavior similar to that seen in optical. The detection of a period derivative near 10~1 3 s/s in optical implies that the white dwarf is spinning down at a rate of ~6 X 10 33 ergs s"' which is at least an order of magnitude larger than the quiescent accretion luminosity and a few times larger than the typical UV flare luminosities. This spindown energy is not seen as disk luminosity, and the conditions appear to be favorable for the release of relativistic particles in a pulsar-type mechanism to explain the radio synchrotron emission in magnetic reconnection events, and the acceleration of particles to TeV energies in double layers if conditions are favorable.

Exploring the nature of AE Aquarii at higher energies using Sazuku and Fermi-LAT

“…The spin-down luminosity −IΩΩ was determined from the white dwarf moment of inertia I ≈ 0.2 M WD R 2 WD ≈ 2 × 10 50 g cm 2 with M WD the WD mass and R WD the WD radius, with the 0.2 factor extrapolated from [6] for a WD mass of ∼ 0.48 M [7], in addition with Ω = 2π/P 33 = 0.19 s −1 andΩ = −2πṖ/P 2 = −3.24 × 10 −16 s −2 . This large spin down power is ∼ 120 times greater than the inferred accretion luminosities derived from UV [8] and X-ray emission [9], and could act as a reservoir that drives non-thermal particle acceleration [10], explaining the radio synchrotron and possible high energy (γ-ray) emission.…”