A tale of two mergers: constraints on kilonova detection in two short GRBs at z ∼ 0.5

We present a detailed multiwavelength analysis of two short gamma-ray bursts (sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at z = 0.483 and GRB 200522A at z = 0.554. These sGRBs demonstrate very different properties in their observed emission and environment. GRB 160624A is associated with a late-type galaxy with an old stellar population (approximate to 3Gyr) and moderate ongoing star formation (approximate to 1M(circle dot) yr(-1)). Hubble and Gemini limits on optical/near-infrared emission from GRB 160624A are among the most stringent for sGRBs, leading to tight constraints on the allowed kilonova properties. In particular, we rule out any kilonova brighter than AT2017gfo, disfavouring large masses of wind ejecta (less than or similar to 0.03 M-circle dot). In contrast, observations of GRB 200522A uncovered a luminous (L-F125W approximate to 10(42) erg s(-1) at 2.3 d) and red (r - H approximate to 1.3 mag) counterpart. The red colour can be explained either by bright kilonova emission powered by the radioactive decay of a large amount of wind ejecta (0.03 M-circle dot less than or similar to M less than or similar to 0.1 M-circle dot) or moderate extinction, E(B - V) approximate to 0.1-0.2 mag, along the line of sight. The location of this sGRB in the inner regions of a young (approximate to 0.1 Gyr) star-forming (approximate to 2-6 M-circle dot yr(-1)) galaxy and the limited sampling of its counterpart do not allow us to rule out dust effects as contributing, at least in part, to the red colour.

Automated summary

We conducted a detailed multiwavelength analysis of two short gamma-ray bursts (sGRBs) observed by the Neil Gehrels Swift Observatory. The sGRBs, GRB 160624A and GRB 200522A, exhibited different properties in their emission and environments. GRB 160624A was associated with an older galaxy with moderate star formation, while GRB 200522A showed a luminous and red counterpart, possibly due to either kilonova emission or extinction effects.