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Estimating the inclination of Cygnus X-1 System
Evidence for both the binary nature of HDE226868 and the link with
Cygnus X-1 came when the Copernicus satellite took a closer look at the
X-ray source. It was discovered that the overall intensity of the X-rays
drops off slightly every 5.6 days--the same time it take for HDE226868 to
make one revolution around its partner. However, since the X-rays do not
drop off completely, the x-ray source obviously does not move completely
behind the star. In other words, from our vantage point on Earth we can
only view the pair at an particular angle, and this angle is apparently not
edge on. Since the X-rays do diminish, it is clear that the system is not
face-on either, since in that case both stars in the system would always
be visible and the X-ray intensity would remain the same. The
inclination, or angle of the Cyg X-1 system relative to the Earth,
is somewhere in-between, like the third image below. From more detailed
information, the best estimate for the inclination angle of the Cyg X-1
system is ~30 degrees, though we may have to wait for future missions to get
a more definitive answer.
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