Curated by RSF Research Staff
Black Hole Models Contradicted by Hands-on Tests
Black holes cannot be directly observed - only indirectly though their effects on the surrounding matter. Binary stars, where one ‘star’ is a black hole, are a perfect example of this. In these systems, the black hole can gravitationally pull - accrete - matter from its companion star.
Due to the conservation of angular momentum the transferring matter is forced into a circular orbit around the star where the plasma eventually spreads out to form a flat disc of gas around the compact object - known as an accretion disk. As the gas from the accretion disc shrinks deeper and deeper into the gravitational potential well of the black hole, the disc heats up through viscous dissipation as angular momentum is transported outwards and matter inwards.
The loss of gravitational potential energy is so great that the accreting material is heated up to tens of millions of Kelvin's emitting high energy radiation such as X-rays. The periodicity at which these X-rays are observed allows astrophysicists to determine the orbital dynamics and thus the mass of the ‘unseen’ black hole as well as its spin.
However, interpreting the emission spectra correctly can be extremely challenging - due to the number of variables involved - and requires complex models to make predictions. One of the resulting theoretical predictions is the amount of iron ionization that occurs in the accretion disks - which is higher than what is observed. Currently this disagreement between theory and observation can be explained by the Auger effect which is a form of ionization in which the filling of an inner-shell vacancy of an atom results in the emission of an electron rather than a photon. The subsequent effect is that although ionization occurs no associated photon would be observed.
This explanation has never been verified and has since been challenged when higher ionization levels were observed in the X-ray Binaries Vela X-1 and Cyg X-1.
Now, for the first time, a team of scientist at Sandia labs, have demonstrated that the Auger effect used to interpret black hole accretion spectra is inaccurate. To do this the team, led by Dr. Guillaume Loisel, utilized the most energetic laboratory X-ray source on Earth - the Z machine - to create a photoionized silicon plasma and duplicate the X-rays surrounding black holes. Their measurements showed that if the specific ions are present, then the associated photon would be observed too - which it isn’t.
These results have huge implications as not only do they constrain the phenomena responsible for the observations, but as well they offer a new technique in which to study accretion disc behavior and the associated X-ray emission.