White Dwarf Pulsars as Possible Gamma-Ray Sources

Abstract

The newly discovered white dwarf binary system AR Scorpii (AR Sco) with its fascinating nonthermal dominated multi-frequency emission has sparked renewed interest in potential high energy gamma-ray emission from white dwarf pulsars. Recent studies revealed that the optical and X-ray components of the emission in AR Sco can be powered entirely by the spin-down luminosity of the white dwarf in the absence of any mass accretion in this system. The Spectral Energy Distribution (SED) below and above optical shows a clear indication of a non-thermal synchrotron component, with pulsed emission in optical and X-ray bands at the white dwarf spin period as well as a beat with the binary period. From an energy perspective the highly magnetic rotating white dwarf can accelerate particles to TeV energies. In this study, a search for high energy gamma-ray emission was conducted between 100 MeV - 500 GeV by analysing the newly available Fermi-LAT Pass 8 data with the new Fermi 1.0.1 Science Tools. We find low significance (4.89 s) integrated gamma-ray activity between 100 MeV - 500 GeV at a photon flux level of 1.076±0.318×10−8photons cm−2s−1 from the selected Region of Interest (ROI) centred on AR Sco’s position. However, the close-by unidentified source (∼ 0.576◦) 4FGLJ1623.7−2315 may also contribute to this emission. AR Sco’s location inside the Rho Ophiuchi (Rho Oph) molecular cloud complex combined with the poor spatial resolution of Fermi-LAT complicates any unambiguous identification of the source of low-level gamma-ray activity at the location that coincides with the position of AR Sco.