Search for Non-Thermal Emissions from an Isolated Magnetic White Dwarf, EUVE J0317 855, with Suzaku

Abstract

Abstract To search for possible non-thermal emissions from magnetic white dwarfs (WDs), we selected the most suitable candidates from among magnetic cataclysmic variables and isolated WDs. After the magnetic cataclysmic variable AE Aquarii, which emits hard X-ray pulses, the isolated WD EUVE J0317$-$855 is expected to emit the second most powerful magnetic dipole radiation with energies reaching $\sim$1.1 $\times$10$^{29}$ erg s$^{-1}$. EUVE J0317$-$855 was observed with Suzaku from 2009 July 16 to 2009 July 17 for about 60 ks. No significant emissions were detected from the object in the 0.5–10 keV band, and the upper limit of the X-ray flux was estimated to be 1.7 $\times$10$^{-13}$ erg cm$^{-2}$ s$^{-1}$ in the 2–10 keV band. This value corresponds to less than 0.05% of the calculated spin-down energy. It excludes the possibility of explaining soft gamma-ray repeaters and anomalous X-ray pulsars as WDs, but may indicate that the activity in the magnetosphere of WDs is not so different from that occurring in radio pulsars. Curvature radiation, which is one of the suggested mechanism for emissions from the magnetosphere of compact objects, may explain the observed X-ray flux of AE Aquarii, and is consistent with the non-detection of EUVE J0317$-$855.