X-ray characteristics and the spectral energy distribution of AE Aquarii

Abstract

Using data from contemporaneous observations with Chandra and Swift, it is shown that the X‐ray emission below 10 keV is predominantly thermal, characterized by flares and emission lines and dominated by the soft component. The Chandra and Swift X‐ray spectra (EX≤ 10 keV) can be reproduced by multicomponent thermal emission models with a time‐averaged X‐ray luminosity of LX∼ 1031 erg s−1. The pulsed 33‐s soft X‐ray emission below 10 keV is confirmed in both the Chandra and Swift data sets. The epoch of pulse maximum of the 33‐s white dwarf spin period is consistent with the recently derived ephemeris based upon Suzaku measurements. The recently detected Suzaku hard X‐ray component above 10 keV shows a non‐thermal power‐law nature, with a photon index of Γ∼ 1.2, possibly the result of synchrotron emission of high‐energy electrons in the white dwarf magnetosphere. The hard X‐ray luminosity of LX,hard≤ 5 × 1030 erg s−1 also constitutes κ∼ 0.1 per cent of the total spin‐down luminosity of the white dwarf. This places AE Aquarii in the same category as young spin‐powered pulsars between 2 and 20 keV. Additionally, it is shown that electrons can be accelerated to energies in excess of 10 TeV outside the light cylinder radius, providing interesting possibilities for VHE‐TeV follow‐up observations. The X‐ray emission below EX≤ 10 keV, on the other hand, is explained in terms of plasma heating at the magnetospheric radius, the result of the dissipation of gravitational potential energy. It is found that a conversion efficiency of α∼ 0.01 is sufficient to heat the plasma at the magnetospheric boundary to temperatures kT≤ 10 keV, sufficient to drive the X‐ray emission below 10 keV. Only a small fraction (β∼ 0.3 per cent) of the mass flow at the magnetospheric radius eventually accretes on to the surface of the white dwarf, emphasizing the very effective magnetospheric propeller process in the system.

A Model of White Dwarf Pulsar Ar Scorpii

“…These peculiar observational properties make AR Sco a unique system. Although AR Sco could be in the evolutionary stage of the so-called intermediate polar (Bookbinder & Lamb 1987;Patterson 1994;Oruru & Meintjes 2012), of which accretion is the main power source, the absence of accretion features in AR Sco demands another mechanism to explain the observations. It has been suggested that if the dipole magnetic field of a WD is strong enough, it would behave like a WD radio pulsar (Zhang & Gil 2005).…”

The Physics of Transient High-Energy Cosmic Sources: from Radio to VHE Gamma-rays

“…The nature of the non-thermal emission from AE Aquarii, from radio to VHE gamma-ray gamma-rays, have been investigated in detail by [38], [27] and [39]. It has been shown that episodes of particle acceleration may be associated with the propeller process in AE Aquarii.…”

“…in the white dwarf magnetosphere in AE Aquarii has been investigated by [35] and [39]. It has been shown that huge potentials of the order of V ∼ 3 × 10 12 P −2 33 µ 33 Volt can exist between the white dwarf surface and the light cylinder, which can accelerate charged particles to VHE-TeV energies.…”

“…Keeping in mind synchrotron losses, these authors showed that relativistic electrons with Lorentz factors of the order of γ ∼ 10 5 can be produced inside the light cylinder of the white dwarf, and will radiate synchrotron radiation in the magnetospheric field (B lc ∼ 100 G) in the proximity of the light cylinder at frequencies ν ∼ few × 10 18 B 100 γ 2 5 Hz, where B 100 and γ 5 represent the magnetic field (∼ 100 G) in the proximity of the light cylinder and Lorentz factor (∼ 10 5 ) respectively. It was further shown that the inverse-Compton scattering of γ ∼ 10 5 electrons on optical photons from the secondary star (ε ph < 5 eV), provide interesting possibilities for the production of gamma-rays with energies of the order ε γ ≤ 0.2γ 2 5 (ε ph /5eV) TeV, which would place it in the Fermi-LAT energy range [35,39].…”

White Dwarf Pulsars and Very Massive Compact Ultra Magnetized White Dwarfs

“…This rapidly rotating magnetized WD would mimic the neutron star pulsars as pointed out by Geng et al 24 Another specific example is AE Aquarii, the first white dwarf pulsar (an intermediate polar WD), very fast with a short period P = 33 s, and spinning down at a rateṖ = 5.64 × 10 −14 s/s. The luminosity of this source and the nature of pulse hard X-ray emission detected with SUZAKU space telescope 25,26 can be explained in terms of spin-powered pulsar mechanism 27, 28 and emit detectable high-energy radiation.…”