Detection of LIGO-Virgo binary black holes in the pair-instability mass gap

By probing the population of binary black hole (BBH) mergers detected by LIGO-Virgo, we can infer properties about the underlying black hole formation channels. A mechanism known as pair-instability (PI) supernova is expected to prevent the formation of black holes from stellar collapse with mass greater than $sim 40-65,M_odot$ and less than $sim 120,M_odot$. Any BBH merger detected by LIGO-Virgo with a component black hole in this gap, known as the PI mass gap, likely originated from an alternative formation channel. Here, we firmly establish GW190521 as an outlier to the stellar-mass BBH population if the PI mass gap begins at or below $65, M_{odot}$. In addition, for a PI lower boundary of $40-50, M_{odot}$, we find it unlikely that the remaining distribution of detected BBH events, excluding GW190521, is consistent with the stellar-mass population.