The relationship between γ Cassiopeiae’s X-ray emission and its circumstellar environment^⋆

¹ Catholic University of America, 3700 San Martin Dr., Baltimore, MD 21218, USA
e-mail: msmith@stsci.edu
² Universidade Federal de Sergipe, Departamento de Física, Av. Marechal Rondon s/n, 49100-000 São Cristóvão, SE, Brazil
³ Universidade de São Paulo, Instituto de Física de São Carlos, Caixa Postal 369, 13560-970, São Carlos, SP, Brazil
⁴ Université de Strasbourg, CNRS, Observatoire Astronomique, 11 rue de l’Université, 67000 Strasbourg, France
⁵ Center of Excellence in Information Systems, Tennessee State University, 3500 John Merritt Blvd., Nashville, TN, USA
⁶ Center for High Angular Resolution Astronomy, Department of Physics and Astronomy, Georgia State University, PO Box 4106, Atlanta, GA 30202-4106, USA
⁷ Ritter Astrophysical Research Center, Department of Physics & Astronomy, University of Toledo, 2801 W. Bancroft, Toledo, OH 43606, USA
⁸ Laboratoire Lagrange, UMR 7293 UNS-CNRS-OCA, Bd de l’Observatoire, BP 4229, 06304 Nice Cedex 4, France
⁹ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
¹⁰ Anna-von-Gierke-Ring 147, 21035 Hamburg, Germany
¹¹ Emil-Nolde-Str. 12, 51375 Leverkusen, Germany
¹² National Optical Astronomical Observatory, 950 North Cherry Ave., Tucson, AZ 85719, USA

Received: 27 October 2011
Accepted: 22 January 2012

Abstract

γ Cas is the prototypical classical Be star and is recently best known for its variable hard X-ray emission. To elucidate the reasons for this emission, we mounted a multiwavelength campaign in 2010 centered around four XMM-Newton observations. The observational techniques included long baseline optical interferometry (LBOI) from two instruments at CHARA, photometry carried out by an automated photometric telescope and Hα observations. Because γ Cas is also known to be in a binary, we measured radial velocities from the Hα line and redetermined its period as 203.55 ± 0.20 days and its eccentricity as near zero. The LBOI observations suggest that the star’s decretion disk was axisymmetric in 2010, has an system inclination angle near 45°, and a larger radius than previously reported. In addition, the Be star began an “outburst” at the beginning of our campaign, made visible by a brightening and reddening of the disk during our campaign and beyond. Our analyses of the new high resolution spectra disclosed many attributes also found from spectra obtained in 2001 (Chandra) and 2004 (XMM-Newton). As well as a dominant hot ( ≈ 14 keV) thermal component, the familiar attributes included: (i) a fluorescent feature of Fe K even stronger than observed at previous times; (ii) strong lines of N VII and Ne XI lines indicative of overabundances; and (iii) a subsolar Fe abundance from K-shell lines but a solar abundance from L-shell ions. We also found that two absorption columns are required to fit the continuum. While the first one maintained its historical average of 1 × 10²¹ cm^-2, the second was very large and doubled to 7.4 × 10²³ cm^-2 during our X-ray observations. Although we found no clear relation between this column density and orbital phase, it correlates well with the disk brightening and reddening both in the 2010 and earlier observations. Thus, the inference from this study is that much (perhaps all?) of the X-ray emission from this source originates behind matter ejected by γ Cas into our line of sight.