Is Gamma-ray Burst 221009A Really a Once-in-10,000 Year Event?

TL;DR

This paper challenges the idea that GRB 221009A is a once-in-10,000-year event, proposing a separate, more frequent narrow-jet GRB population with implications for cosmic rays and life on Earth.

Contribution

It introduces a potential new population of narrow-jet GRBs with higher occurrence rates and discusses their properties, detection prospects, and implications for cosmic rays and life.

Findings

Narrow-jet GRBs could occur as frequently as once every 200 years.

This population must have a maximum redshift of about 0.38.

The rate of nearby narrow-jet GRBs capable of affecting Earth is about 1 per 500 million years.

Abstract

Gamma-ray bursts (GRBs) brighter than the GRB 221009A, the brightest yet observed, have previously been estimated to occur at a rate of 1 per 10,000 years, based on the extrapolation of the distribution of fluences of the Long GRB population. We show that bursts this bright could instead have a rate as high as approximately one per 200 years if they are from a separate population of narrow-jet GRBs. This population must have a maximum redshift of about $z\approx 0.38$ in order to avoid over-producing the observed rate of fainter GRBs. We show that it will take $> 100$ years to confirm this new population based on observing another GRB from it with a $\gamma$-ray detector; observing an orphan optical afterglow from this population with Vera Rubin Observatory or an orphan radio afterglow with the Square Kilometer Array will also take similarly long times to observe, and it is unclear if they could be distinguished from the standard GRB population. We show that the nearby narrow-jet population has more favorable energetics for producing ultra-high energy cosmic rays than standard GRBs. The rate of bursts in the Milky Way bright enough to cause mass extinctions of life on Earth from the narrow jet population is estimated to be approximately 1 per 500 Myr. This GRB population could make life in the Milky Way less likely, with implications for future searches for life on exoplanets.