The ngEHT's Role in Measuring Supermassive Black Hole Spins

Angelo Ricarte, Paul Tiede, Razieh Emami, Aditya Tamar, Priyamvada Natarajan

Submitted on 7 November 2022


While supermassive black hole masses have been cataloged across cosmic time, only a few dozen of them have robust spin measurements. By extending and improving the existing Event Horizon Telescope (EHT) array, the next-generation Event Horizon Telescope (ngEHT) will enable multifrequency, polarimetric movies on event horizon scales, which will place new constraints on the space-time and accretion flow. By combining this information, it is anticipated that the ngEHT may be able to measure tens of supermassive black hole masses and spins. In this white paper, we discuss existing spin measurements and many proposed techniques with which the ngEHT could potentially measure spins of target supermassive black holes. Spins measured by the ngEHT would represent a completely new sample of sources that, unlike pre-existing samples, would not be biased towards objects with high accretion rates. Such a sample would provide new insights into the accretion, feedback, and cosmic assembly of supermassive black holes.


Comment: Submitted for Galaxies Special Issue "From Vision to Instrument: Creating a Next-Generation Event Horizon Telescope for a New Era of Black Hole Science"

Subject: Astrophysics - High Energy Astrophysical Phenomena


Shape of the $n=\infty$ photon ring or ``critical curve'' as a function of spin and inclination, using the analytic formulae provided in \citet{Chael+2021}.  For face-on viewing angles (left), the critical curve remains circular and shrinks only by about 7\% between 0 and maximal spin.  For edge-on viewing angles (right), the critical curve becomes horizontally displaced and asymmetric as spin increases.