Superorbital Variability

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Science  16 Aug 2013:
Vol. 341, Issue 6147, pp. 696
DOI: 10.1126/science.341.6147.696-a

Many stars live in pairs, and sometimes they form rather intriguing binary systems. LS I ÷61°303 is one such case, where a Be star 10 times as massive as the Sun and a compact object orbit around their common center of mass. Be stars are rapidly rotating B-type stars that show hydrogen Balmer emission lines in their spectrum and lose mass to an equatorial circumstellar disk. The nature of the compact object in LS I ÷61°303 is unknown, but it is suspected to be a neutron star. The system has been detected across the electromagnetic spectrum all the way from radio to gamma rays, and it has been shown to be highly variable across all frequencies. At most wavelengths, the flux of LS I ÷61°303 is known to be modulated by the orbital period of about 26.5 days; at radio, x-ray, and optical frequencies, the flux is also modulated on a longer time scale, or superorbital period, of 1667 days. Based on data from the Fermi Gamma-ray Space Telescope, Ackermann et al. show that the gamma-ray emission of LS I ÷61°303 also varies according to the superorbital period. This modulation is more prominently seen at orbital phases around apastron (the point at which the stars in the binary are farthest apart), which could be explained by a quasi-cyclical evolution of the equatorial outflow of the Be star. The authors suggest that gamma-ray observations such as these could be used to study the ouflows of massive stars in eccentric binary systems.

Astrophys. J. 773, L35 (2013).

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