EDP Sciences
Free Access
Issue
A&A
Volume 454, Number 1, July IV 2006
Page(s) 265 - 276
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361:20054589


A&A 454, 265-276 (2006)
DOI: 10.1051/0004-6361:20054589

New $\mathsf{\gamma}$ Cassiopeiae-like objects: X-ray and optical observations of SAO 49725 and HD 161103

R. Lopes de Oliveira1, 2, C. Motch1, F. Haberl3, I. Negueruela4 and E. Janot-Pacheco2

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 ($L_{\rm X}$ ~ 1032-33 erg s-1) and the hardness of their X-ray spectra, which are both unusual for normal early type stars. An iron K$\alpha$ 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  $\gamma$ 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.


Key words: stars: emission-line, Be -- stars: individual: SAO 49725 -- stars: individual: HD 161103



© ESO 2006

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.