I found this interesting article recently:
Famous black hole confirmed after 40 years
It gives a really good step-by-step breakdown on how they’ve concluded that Cygnus X-1 contains a black hole. I’ll try and summarise the steps in this post.
As background info, the dark star in the Cygnus X-1 binary system has been difficult to classify as being a black hole as opposed to a neutron star, because historical distance measurements for the accompanying blue star, lacked the precision to determine its intrinsic luminosity. (The closer the system is, the less intrinsically luminous the blue star must be).
My attempt at describing the steps is as follows:
- the less intrinsically luminous the blue star is, the less massive it must be;
- the less massive it is, the less massive its orbiting dark star must be;
- if the dark star is less than three times our Sun, from theoretical models, it would fall into the neutron star category.
- using the parallax angular measurement method in the X-Ray spectrum (via the VBLA array), an accurate distance measurement of 6050 lyrs, uncertain to 400 years, was obtained;
- from this new distance measurement (and known mass/luminosity relationships), the blue star is calculated to be 19 solar masses;
- from the known dark-blue star orbital period of 5.6 days, the dark star mass is calculated to be 14.8 solar masses to an uncertainty of 1 solar mass. (This is done using its distance from the centre of mass of the blue star and the star’s orbital period);
- the dark star object thus falls into the black hole category of mass scales;
- the X-Ray emitting gas is calculated to be revolving at 0.5c, or 670 orbits per second (by measuring cyclical variations in the ‘hardness’ of the X-Ray spectrum);
- the black hole also spins at 97% of its theoretical speed. This is deduced from accretion disk x-ray emissions and a relativistic model which assumes that the X-Ray emissions are coming from the hottest part, and thus the closest part, to the black hole event horizon itself.
There are three papers published recently on these findings. They cover: the distance
, the mass
and the spin.
I find the rationale and logic leading to this conclusion to be about as straight-forward as it gets when it comes to astronomical measurements/deductions, which is why I found this topic irresistable matter for a thread.
Next time someone doubts a black hole conclusion, we should point them to the Cygnus X1 example, as I don’t think it gets much more straight-forward than this.
It is also another example of the use of Relativistic modelling, which would not be possible if one can’t accept the theories of Relativity in the first place. So, the next time someone asks what tangible value comes from an SR and GR view of the universe, here is one classic example.