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A 106-d period in the nuclear source X-8 in M33

G. Dubus·P. A. Charles·K. S. Long·P. J. Hakala
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Abstract

With an X-ray luminosity of about 10^39 erg/s, the source X-8 coincident with the optical center of M33 is the most luminous X-ray source in the Local Group. However, its nature remains a mystery. We present here new and archival ROSAT observations of X-8 spread over 6 years which show variability and a ~ 106-d periodicity. This implies that (most of) the emission from M33 X-8 arises from a single object, perhaps a binary system with a ~ 10 Msol black hole primary. We suggest that the companion is a giant orbiting with a ~ 10 day period, and that the observed modulation is ``super-orbital'', analogous to that seen in Cyg X-2 and X1820-30.

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G. Dubus, P. A. Charles, K. S. Long, P. J. Hakala. 1997-10-01. A 106-d period in the nuclear source X-8 in M33. https://doi.org/10.1086/311013

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