Cygnus X-1
Compact object

From various techniques, the mass of the compact object appears to be greater than the maximum mass for a neutron star. Stellar evolutionary models suggest a mass of 20 ± 5 solar masses, while other techniques resulted in 10 solar masses. Measuring periodicities in the X-ray emission near the object yielded a more precise value of 14.8 ± 1 solar masses. In all cases, the object is most likely a black hole —a region of space with a gravitational field that is strong enough to prevent the escape of electromagnetic radiation from the interior. The boundary of this region is called the event horizon and has an effective radius called the Schwarzschild radius, which is about 44 km for Cygnus X-1. Anything (including matter and photons) that passes through this boundary is unable to escape. New measurements published in 2021 yielded an estimated mass of 21.2 ± 2.2 solar masses.

Evidence of just such an event horizon may have been detected in 1992 using ultraviolet (UV) observations with the High Speed Photometer on the Hubble Space Telescope. As self-luminous clumps of matter spiral into a black hole, their radiation is emitted in a series of pulses that are subject to gravitational redshift as the material approaches the horizon. That is, the wavelengths of the radiation steadily increase, as predicted by general relativity. Matter hitting a solid, compact object would emit a final burst of energy, whereas material passing through an event horizon would not. Two such "dying pulse trains" were observed, which is consistent with the existence of a black hole.

The spin of the compact object is not yet well determined. Past analysis of data from the space-based Chandra X-ray Observatory suggested that Cygnus X-1 was not rotating to any significant degree. However, evidence announced in 2011 suggests that it is rotating extremely rapidly, approximately 790 times per second.
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