Black holes are fundamentally simple objects: only their mass and spin are enough to properly describe them. However, direct measurements of these two properties are not simple. Often, we look for observables which, upon being inserted in a physical model, may give us information about one of these fundamental properties of black holes. One such observable is luminosity. Our paper “Jet efficiencies and black hole spins in jetted quasars”, recently accepted by MNRAS (see preprint here), aims to relate jet properties with black hole spins.
With a sample of 154 flat-spectrum radio quasars (FSRQs), a subclass of blazars, whose masses had been previously estimated, we set out to find their gamma-ray luminosities in the Fermi 4FGL catalog, which comprises 8 years of observations performed with the Fermi Large Area Telescope (Fermi-LAT). Our first result is a correlation between jet luminosity and black hole mass, suggesting that more luminous jets are powered by more massive black holes.
We also estimated the jet power of these blazars. For this, we used a relation found in Nemmen et al. 2012. This, along with the assumption that these black holes are accreting at around 10 per cent of the Eddington rate — such high accretion rates are necessary for the thin discs believed to feed FSRQs — allowed us to estimate the jet efficiencies in these objects.
Having estimated the jet efficiencies, we used a simulation-based model to estimate the black hole spins. We found that, overall, these black holes are rotating very fast: the average spin was 0.84. This result is consistent with scenarios for the cosmological evolution of SMBHs which support rapidly rotating black holes as their host galaxies — and the black holes themselves — merge.
The preprint of the accepted version, with a full discussion, can be found here.