New γ Cassiopeiae-like objects: X-ray and optical observations of SAO 49725 and HD 161103

¹ Observatoire Astronomique, UMR 7550 CNRS, 11 rue de l'Université, 67000 Strasbourg, France e-mail: rlopes@newb6.u-strasbg.fr
² Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, R. do Matão 1226, 05508-090 São Paulo, Brazil
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁴ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politécnica Superior, Universidad de Alicante, Ap. 99, 03080 Alicante, Spain

Received: 25 November 2005
Accepted: 28 February 2006

Abstract

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, SAO 49725 and HD 161103, 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 ~ 10^32-33 erg s^-1) 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 HD 161103 in which the H-like, He-like, and fluorescent components are resolved, while strong evidences also exist for the presence of similar features in SAO 49725. X-ray spectra can be equally well-fitted by a thermal plasma (mekal) with T ~ 10⁸ K 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 ~ 10⁷ K. The intensity of the fluorescence 6.4 keV 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 HD 161103 with P ~ 3200 s is readily detected. The X-ray light curve of SAO 49725 exhibits clear variability by ~80% on time scales as short as ~1000 s. New optical observations provide updated spectral types (B0.5 III-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 γ 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 γ Cas analogs.