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

Abstract

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 42.4 ks, 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 ∼ 0.2 cycles in phase, in contrast to approximately sinusoidal profiles observed at energies below ∼ 4 keV. 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 2.9O+0.20-0.16 keV and 0.53 +0.14-0.13 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+0.63-0.62^ or a third thermal component with a temperature of 54 +26-47 keV. At a distance of 102 pc, the 4-30 keV luminosities of the thermal and the additional components become 1.7 +1.3-0.6 and 5.3 +15.3-0.3 × 1029 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. © 2008. Astronomical Society of Japan.