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**Ke****n Ng (Astronomy) **- MIT

**Title: Constraints on ultralight scalar bosons within black hole spin measurements from LIGO-Virgo’s GWTC-2**

Abstract: Clouds of ultralight bosons - such as axions - can form around a rapidly spinning black hole, if the black hole radius is comparable to the bosons' wavelength. The cloud rapidly extracts angular momentum from the black hole, and reduces it to a characteristic value that depends on the boson's mass as well as on the black hole mass and spin. Therefore, a measurement of a black hole mass and spin can be used to reveal or exclude the existence of such bosons. Using hierarchical Bayesian inference, we can simultaneously measure the black hole spin distribution at formation and the mass of the scalar boson. Based on the black holes released by LIGO and Virgo in their GWTC-2, the data strongly disfavors the existence of scalar bosons in the mass range between $1.3\times10^{-13}\,\mathrm{eV}$ and $2.7\times10^{-13}\,\mathrm{eV}$. Our mass constraint is valid for bosons with negligible self-interaction, that is with a decay constant $10^{14}~\mathrm{GeV}$. The statistical evidence is mostly driven by the two {binary black holes} systems GW190412 and GW190517, which host rapidly spinning black holes. The region where bosons are excluded narrows down if these two systems merged shortly ($\sim 10^5$ years) after the black holes formed. We also show that hundreds of high signal-to-noise ratio gravitational-wave detections are enough to exclude (confirm) the existence of non-interacting bosons everywhere in the mass range $\left[10^{-13},3\times 10^{-12}\right]$~eV $\left([10^{-13},10^{-12}]~\rm{eV}\right)$. The precise number depends on the distribution of black hole spins at formation and the mass of the boson.

Bio: Ken Ng is a graduate student at MIT’s Kavali Institute for Astrophysics and Space Research. He primarily studies physics in relation to lensing rate prediction of gravitational-wave detection and underlying astrophysics.