Short Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are highly energetic and bright explosions that come in two classes: short and long. Short GRBs emit gamma-rays for less than 2 seconds, whereas long GRBs emit gamma-rays for over 2 seconds (generally). Short and long GRBs differ in their burst and afterglow energetics and timescales as well as their host galaxies, so we can be sure they generally derive from different progenitor systems.

Short GRBs are associated with older, evolved progenitors due to their presence in a range of galaxies (including early-type, quiescent galaxies). On the other hand, long GRBs are exclusively found in star-forming, late-type galaxies, have associated supernovae, and are associated with a younger, massive star progenitor. Short GRBs are also found to be consistently offset from their hosts, implying that their progenitors undergo a significant migration prior to explosion. In combination with their large energies, this suggested that short GRBs formed from compact object mergers, specifically binary neutron star (BNS) or neutron star-black hole (NSBH) mergers.

This progenitor model, however, was only based on indirect evidence until August 17, 2017. On that day, the first BNS merger was detected through its gravitational wave (GW) radiation and was followed shortly by a short GRB. This multi-messenger event, GW170817/GRB170817, was revolutionary for astronomy, as it confirmed short GRBs have a common origin with BNS mergers and that BNS mergers are responsible for at least some heavy elements in the universe. To date, GW170817 remains the only GW event with a known EM counterpart and is generally regarded as a once-in-a-decade event. In contrast, there are over 100 short GRBs detected out to z=2.5.

The Importance of Host Galaxy Environments in Short GRB Studies

The robust associations of short GRBs to host galaxies are key to unlocking the scientific potential of a given event as they provide precise redshifts and properties of the environment that highlight the necessary conditions in which binaries can form and merge including their timescales. Short GRBs have now been found in both star-forming and quiescent galaxies, which suggests a wide range of merger timescales for their BNS progenitors. Locations of the transient can provide another key constraint. For instance, short GRBs are often found highly offset from their host galaxies, suggesting that they must migrate from their birth place and/or experience a large merger "kicks" to overcome the gravitational potential of their hosts.

Host galaxies provide a wealth of information on stellar population properties such as stellar population age, stellar mass, metallicity, and star formation rate. These in turn give insight into properties of the progenitor, including how delay times, or time between formation and merger, evolve with redshift and how compact object systems track the star formation and stellar mass in the universe, which are difficult to constrain through other methods. Thanks in large part to BRIGHT, about 84 of the 149 observed Swift short GRBs have robust host associations. Thus, we have a large sample of galaxies from which we can deduce binary properties.

The BRIGHT Papers

For more information on short GRBs and their host galaxies, we recommend you read the two-series BRIGHT Short GRB Hosts Papers. Paper I focuses on the sample, redshifts, and offsets of the short GRBs. Paper II focuses on the stellar population properties of the host galaxies and the implications for their progenitor systems.