X‐Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo. Robinson, R. D., Smith, M. A., & Henry, G. W. The Astrophysical Journal, 575(1):435–448, August, 2002. arXiv: astro-ph/0205278 Publisher: IOP Publishing
X‐Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo [link]Paper  doi  abstract   bibtex   
gamma Cas (B0.5e) is known to be a unique X-ray source because ot its moderate L_x, hard X-ray spectrum, and light curve punctuated by ubiquitous flares and slow undulations. Its X-ray peculiarities have led to a controversy concerning their origin: either from wind infall onto a putative degenerate companion, as for typical Be/X-ray binaries, or from the Be star per se. Recent progress has been made to address this: (1) the discovery that gamma Cas is an eccentric binary system (P = 203.59 d) with unknown secondary type, (2) the accumulation of RXTE data at 9 epochs in 1996-2000, and (3) the collation of robotic telescope B, V-band photometric observations over 4 seasons. The latter show a 3%, cyclical flux variation with cycle lengths 55-93 days. We find that X-ray fluxes at all 9 epochs show random variations with orbital phase. This contradicts the binary accretion model, which predicts a substantial modulation. However,these fluxes correlate well with the cyclical optical variations. Also, the 6 flux measurements in 2000 closely track the interpolated optical variations between the 2000 and 2001 observing seasons. Since the optical variations represent a far greater energy than that emitted as X-rays, the optical variability cannot arise from X-ray reprocessing. However, the strong correlation between the two suggests that they are driven by a common mechanism. We propose that this mechanism is a cyclical magnetic dynamo excited by a Balbus-Hawley instability located within the inner part of the circumstellar disk. In our model, variations in the field strength directly produce the changes in the magnetically related X-ray activity. Turbulence associated with the dynamo results in changes to the density distribution within the disk and creates the observed optical variations.
@article{robinson02,
	title = {X‐{Ray} and {Optical} {Variations} in the {Classical} {Be} {Star} γ {Cassiopeia}: {The} {Discovery} of a {Possible} {Magnetic} {Dynamo}},
	volume = {575},
	issn = {0004-637X},
	url = {http://stacks.iop.org/0004-637X/575/i=1/a=435},
	doi = {10.1086/341141},
	abstract = {gamma Cas (B0.5e) is known to be a unique X-ray source because ot its moderate L\_x, hard X-ray spectrum, and light curve punctuated by ubiquitous flares and slow undulations. Its X-ray peculiarities have led to a controversy concerning their origin: either from wind infall onto a putative degenerate companion, as for typical Be/X-ray binaries, or from the Be star per se. Recent progress has been made to address this: (1) the discovery that gamma Cas is an eccentric binary system (P = 203.59 d) with unknown secondary type, (2) the accumulation of RXTE data at 9 epochs in 1996-2000, and (3) the collation of robotic telescope B, V-band photometric observations over 4 seasons. The latter show a 3\%, cyclical flux variation with cycle lengths 55-93 days. We find that X-ray fluxes at all 9 epochs show random variations with orbital phase. This contradicts the binary accretion model, which predicts a substantial modulation. However,these fluxes correlate well with the cyclical optical variations. Also, the 6 flux measurements in 2000 closely track the interpolated optical variations between the 2000 and 2001 observing seasons. Since the optical variations represent a far greater energy than that emitted as X-rays, the optical variability cannot arise from X-ray reprocessing. However, the strong correlation between the two suggests that they are driven by a common mechanism. We propose that this mechanism is a cyclical magnetic dynamo excited by a Balbus-Hawley instability located within the inner part of the circumstellar disk. In our model, variations in the field strength directly produce the changes in the magnetically related X-ray activity. Turbulence associated with the dynamo results in changes to the density distribution within the disk and creates the observed optical variations.},
	number = {1},
	urldate = {2018-02-09},
	journal = {The Astrophysical Journal},
	author = {Robinson, Richard D. and Smith, Myron A. and Henry, Gregory W.},
	month = aug,
	year = {2002},
	note = {arXiv: astro-ph/0205278
Publisher: IOP Publishing},
	pages = {435--448},
}

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