The First Short GRB Millimeter Afterglow: The Wide-angled Jet of the Extremely Energetic SGRB 211106A

We present the discovery of the first millimeter afterglow of a short-duration 7-ray burst (SGRB) and the first confirmed afterglow of an SGRB localized by the GUANO system on Swift. Our Atacama Large Millimeter/Submillimeter Array (ALMA) detection of SGRB 211106A establishes an origin in a faint host galaxy detected in Hubble Space Telescope imaging at 0.7 less than or similar to z less than or similar to 1.4. From the lack of a detectable optical afterglow, coupled with the bright millimeter counterpart, we infer a high extinction, A(v) greater than or similar to 2.6 mag along the line of sight, making this one of the most highly dust-extincted SGRBs known to date. The millimeter-band light curve captures the passage of the synchrotron peak from the afterglow forward shock and reveals a jet break at t(jet) = 29.2(-4.0)(+4.5) days. For a presumed redshift of z = 1, we infer an opening angle, theta(jet)= (15 degrees.5 +/- 1 degrees.4), and beaming-corrected kinetic energy of log(E-K/erg) = 51.8 +/- 0.3, making this one of the widest and most energetic SGRB jets known to date. Combining all published millimeter-band upper limits in conjunction with the energetics for a large sample of SGRBs, we find that energetic outflows in high-density environments are more likely to have detectable millimeter counterparts. Concerted afterglow searches with ALMA should yield detection fractions of 24%-40% on timescales of greater than or similar to 2 days at rates of per year, outpacing the historical discovery rate of SGRB centimeterband afterglows.

Automated summary

We report the discovery of the first millimeter afterglow of a short-duration gamma-ray burst (SGRB) and confirm its localization using the GUANO system on Swift. The ALMA detection of SGRB 211106A reveals its origin in a faint host galaxy with high extinction along the line of sight. The millimeter light curve captures the synchrotron peak and reveals a jet break at 29.2 days. We find that energetic outflows in high-density environments are more likely to have detectable millimeter counterparts.