gamma Cassiopeiae: an X-ray Be star with personality

Autor(es)

de Oliveira, R. Lopes ^[1] ; Smith, M. A. ^[2] ; Motch, C. ^[3]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Inst Astron Geofis & Ciencias Atmosfer, BR-05508090 Sao Paulo - Brazil

^[2] Catholic Univ Amer, Baltimore, MD 21218 - USA

^[3] Univ Strasbourg, CNRS, UMR 7550, Astron Observ, F-67000 Strasbourg - France

• Tipo de documento: Artigo Científico
• Fonte: Astronomy & Astrophysics; v. 512, MAR-APR 2010.
• Citações Web of Science: 19

Resumo

An exciting unsolved problem in the study of high energy processes of early type stars concerns the physical mechanism for producing X-rays near the Be star gamma Cassiopeiae. By now we know that this source and several ``gamma Cas analogs{''} exhibit an unusual hard thermal X-ray spectrum, compared both to normal massive stars and the non-thermal emission of known Be/X-ray binaries. Also, its light curve is variable on almost all conceivable timescales. In this study we reanalyze a high dispersion spectrum obtained by Chandra in 2001 and combine it with the analysis of a new (2004) spectrum and light curve obtained by XMM-Newton. We find that both spectra can be fit well with 3-4 optically thin, thermal components consisting of a hot component having a temperature kT(Q) similar to 12-14 keV, perhaps one with a value of similar to 2.4 keV, and two with well defined values near 0.6 keV and 0.11 keV. We argue that these components arise in discrete (almost monothermal) plasmas. Moreover, they cannot be produced within an integral gas structure or by the cooling of a dominant hot process. Consistent with earlier findings, we also find that the Fe abundance arising from K-shell ions is significantly subsolar and less than the Fe abundance from L-shell ions. We also find novel properties not present in the earlier Chandra spectrum, including a dramatic decrease in the local photoelectric absorption of soft X-rays, a decrease in the strength of the Fe and possibly of the Si K fluorescence features, underpredicted lines in two ions each of Ne and N (suggesting abundances that are similar to 1.5-3x and similar to 4x solar, respectively), and broadening of the strong NeXLy alpha and OVIII Ly alpha lines. In addition, we note certain traits in the gamma Cas spectrum that are different from those of the fairly well studied analog HD110432 - in this sense the stars have different ``personalities.{''} In particular, for gamma Cas the hot X-ray component remains nearly constant in temperature, and the photoelectric absorption of the X-ray plasmas can change dramatically. As found by previous investigators of gamma Cas, changes in flux, whether occurring slowly or in rapidly evolving flares, are only seldomly accompanied by variations in hardness. Moreover, the light curve can show a ``periodicity{''} that is due to the presence of flux minima that recur semiregularly over a few hours, and which can appear again at different epochs. (AU)

• Processo FAPESP: 06/57824-1 - Aplicação de modelos teórico-computacionais em Astrofísica
• Beneficiário: Gustavo Amaral Lanfranchi
• Modalidade de apoio: Auxílio à Pesquisa - Jovens Pesquisadores
• Processo FAPESP: 07/04710-1 - Natureza e propriedades de grupos fósseis
• Beneficiário: Raimundo Lopes de Oliveira Filho
• Modalidade de apoio: Bolsas no Brasil - Pós-Doutorado