Curated by RSF Research Staff
New discoveries challenging existing models of black holes
Black holes are very intriguing objects and while having been observed for more than 50 years are still a source of mysteries for astrophysicists. Every new study shows we understand even less about them than we thought we did. One of the difficulties is that they are very difficult to observe and only indirect observations can help understand their dynamic. One way is the observations of accreting black hole systems revealing the existence of an accretion disk coronae. However, we don’t see it, we just measure the relativistic jet outflows supposedly induced by an energetic electron cloud rotating around the compact object. A second way to study black holes consists of looking at their emissions, in particular in the case of quasars. They constitute the most luminous types of active galactic nuclei, where a supermassive black hole is powered by significant mass accretion through a thin accretion disk. They are visible thank to their radio emission (quasar=“quasi-stellar radio source”). However, only about 10% of all quasars are radio loud. The majority have much weaker core radio emission meaning they don’t manifest any relativistic jets. Interestingly, new discoveries using these two technic are challenging the current models of black holes.
We need to understand black holes in general. If we go back to the very earliest point in our universe, just after the big bang, there seems to have always been a strong correlation between black holes and galaxies. It seems that the birth and evolution of black holes and galaxies, our cosmic island, are intimately linked. Our results are surprising and one that we're still trying to puzzle out.
Chris Packham, Department of Physics and Astronomy, University of Texas
An international team studying the x-ray emission coming from binary stars containing an accreting black hole. Using simultaneous infrared, optical, x-ray, and radio observations of the Galactic black hole system V404 Cygni, they measured precisely the magnetic field in the corona. Surprisingly, this new measurement was substantially lower than previous estimates for such systems questioning the current physical models of accretion physics in black hole. A difficulty is there is no direct information regarding the process that accelerates the electron generating jet launching corona. More data will be necessary to propose a proper and viable mechanism.
A second team from Japan is trying to understand why radio-quiet and radio-loud quasars. The most promising explanation is provided by the spin paradigm, which suggests that radio-loud quasars have a higher black hole spin. However, the measurement of black hole spin remains extremely challenging.
In their last study, they compared the mean radiative efficiencies of carefully matched samples of radio-loud and radio-quiet Sloan Digital Sky Survey quasars. They found that the radio-loud sample shows an enhancement probably caused by differences in the spectral energy distribution, suggesting higher average bolometric luminosities at fixed accretion rate in the radio-loud population. This suggests that the radio-loud quasar population has on average systematically higher radiative efficiencies and therefore higher black hole spin than the radio-quiet population, providing observational support for the black hole spin paradigm.
Our approach, like others, relies on a number of key assumptions. Our results certainly don't mean that spin must be the only factor for differentiation between radio-loud and radio-quiet quasars. The results do suggest, however, that we shouldn't count spin out of the game. It might be determining the loudness of these distant accreting monsters.
Andreas Schulze, National Astronomical Observatory of Japan
Continue reading at: https://phys.org/news/2018-01-black-holes.html