X-Rays from the Mira AB Binary System

We present the results of XMM-Newton X-ray observations of the Mira AB binary system, which consists of a pulsating, asymptotic giant branch primary and nearby (~0.6" separation) secondary of uncertain nature. The European Photon Imaging Camera (EPIC) CCD (MOS and pn) X-ray spectra of Mira AB are relatively soft, peaking at ~1 keV, with only very weak emission at energies >3 keV; lines of Ne IX, Ne X, and O VIII are apparent. Spectral modeling indicates a characteristic temperature T_X~10⁷ K and intrinsic luminosity L_X~5×10²⁹ ergs s^-1 and suggests enhanced abundances of O and, possibly, Ne and Si in the X-ray-emitting plasma. Overall, the X-ray spectrum and luminosity of the Mira AB system more closely resemble those of late-type, pre-main-sequence stars or late-type, magnetically active main-sequence stars than those of accreting white dwarfs. We conclude that Mira B is most likely a late-type, magnetically active, main-sequence dwarf, and X-rays from the Mira AB system arise either from magnetospheric accretion of wind material from Mira A onto Mira B or from coronal activity associated with Mira B itself as a consequence of accretion-driven spin-up. In addition, one (or both) of these mechanisms could be responsible for the recently discovered, pointlike X-ray sources within planetary nebulae.