Subjects:High Energy Astrophysical Phenomena (astro-ph.HE)
Here, we report the discovery of a kilonova associated with the nearby (350
Mpc) minute-duration GRB 211211A. In tandem with deep optical limits that rule
out the presence of an accompanying supernova to MI>−13 mag at 17.7 days
post-burst, the identification of a kilonova confirms that this burst's
progenitor was a compact object merger. While the spectrally softer tail in GRB
211211A's gamma-ray light curve is reminiscent of previous extended emission
short GRBs (EE-SGRBs), its prompt, bright spikes last ≳12 s,
separating it from past EE-SGRBs. GRB 211211A's kilonova has a similar
luminosity, duration and color to AT2017gfo, the kilonova found in association
with the gravitational wave (GW)-detected binary neutron star (BNS) merger
GW170817. We find that the merger ejected ≈0.04M⊙ of
r-process-rich material, and is consistent with the merger of two neutron stars
(NSs) with masses close to the canonical 1.4M⊙. This discovery
implies that GRBs with long, complex light curves can be spawned from compact
object merger events and that a population of kilonovae following GRBs with
durations ≫2 s should be accounted for in calculations of the NS merger
r-process contribution and rate. At 350 Mpc, the current network of GW
interferometers at design sensitivity would have detected the merger
precipitating GRB 211211A, had it been operating at the time of the event.
Further searches for GW signals coincident with long GRBs are therefore a
promising route for future multi-messenger astronomy.
