Suzaku discovery of hard X-ray pulsations from a rotating magnetized white dwarf, AE Aquarii

A fast rotating magnetized white dwarf, AE Aquarii, 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 0.5-10 keV band of the XIS data at the barycentric period of 33.0769 +/- 0.0001 s, the 10-30 keV 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 similar to 0.2 cycles in phase, in contrast to approximately sinusoidal profiles observed at energies below similar to 4 keV. The folded 4-10 keV XIS light curves are understood to be a superposition of those two types of pulse profiles. The phase-averaged 1.5-10 keV spectra can be reproduced by two thermal components with temperatures of 2.0(-0.16)(+0.20) keV and 0.53(-0.13)(+0.14) keV but the 12-25 keV 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)(+26) keV. 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) X 10(29) erg s(-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.