Suzaku Discovery of Hard X-Ray Pulsations from a Rotating Magnetized White Dwarf, AEAquarii

Abstract

A fast rotating magnetized white dwarf, AEAquarii, was observed with Suzaku, in 2005 October-November and 2006 October with exposures of 53.1 and 42.4ks, respectively. In addition to clear spin modulation in the 0.5-10keV band of the XIS data at the barycentric period of 33.0769±0.0001s, the 10-30keV HXD data in the second half of the 2005 observation also showed statistically significant periodic signals at a consistent period. On that occasion, the spin-folded HXD light curve exhibited two sharp spikes separated by ∼0.2cycles in phase, in contrast to approximately sinusoidal profiles observed at energies below ∼4keV. The folded 4-10keV XIS light curves are understood to be a superposition of those two types of pulse profiles. The phase-averaged 1.5-10keV spectra can be reproduced by two thermal components with temperatures of 2.90_-0.16^+0.20keV and 0.53_-0.13^+0.14keV, but the 12-25keV HXD data show a significant excess above the extrapolated model. This excess can be explained by either a power-law model with a photon index of 1.12_-0.62^+0.63 or a third thermal component with a temperature of 54_-47⁺²⁶keV. At a distance of 102pc, the 4-30keV luminosities of the thermal and the additional components become 1.7_-0.6^+1.3 and 5.3_-0.3^+15.3×10²⁹ergs^-1, respectively. The latter corresponds to 0.09% of the spin-down energy of the object. Possible emission mechanisms of the hard pulsations are discussed, including non-thermal ones, in particular.