2006A&A...454..265L

A growing number of early Be stars exhibit X-ray luminosities that are intermediate between those typical of early type stars and those emitted by most Be/X-ray binaries in quiescence. We report on XMM-Newton observations of two such Be stars, SAO49725 and HD161103, which were originally discovered in a systematic cross-correlation between the ROSAT all-sky survey and SIMBAD. The new observations confirm the X-ray luminosity excess (L_X∼10^32–33erg/s) and the hardness of their X-ray spectra, which are both unusual for normal early type stars. An iron Kα complex is clearly detected in HD161103 in which the H-like, He-like, and fluorescent components are resolved, while strong evidences also exist for the presence of similar features in SAO49725. X-ray spectra can be equally well-fitted by a thermal plasma (mekal) with T∼10⁸K and solar abundances or by a power law + iron line model with photon index ∼1.5-1.8, both with a soft thermal component with T∼10⁷K. The intensity of the fluorescence 6.4keV line reflects the presence of large amounts of cold material close to the X-ray sources and strongly argues against accretion onto a companion neutron star in a large orbit. On the other hand, the probable thermal origin of the X-ray emission, as supported by the ionised iron lines, disagrees with those observed in all known Be/X-ray binaries, in which a non-thermal component is always required. Remarkably, the X-ray features are similar to those of white dwarves in several cataclysmic variables. There is no evidence of high frequency pulsations in neither of the two systems. However, a large oscillation in the light curve of HD161103 with P∼3200s is readily detected. The X-ray light curve of SAO49725 exhibits clear variability by ∼80% on time scales as short as ∼1000s. New optical observations provide updated spectral types (B0.5III-Ve) and disclose a dense, large, and apparently stable circumstellar disc for both stars. The nature of the excess X-ray emission is discussed in light of the models proposed for {gamma} Cas, magnetic disc-star interaction, or accretion onto a compact companion object - whether neutron star or white dwarf. These two new objects, added to similar cases discovered in XMM-Newton surveys, point to the emergence of a new class of {gamma} Cas analogs.