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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.

inclination angle 90 degrees inclination angle 0 degrees inclination angle 45 degrees
Return Click here to return to solve for the mass of Cyg X-1.

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