A Long Story

NASA/Swift/Cruz deWilde

Gamma-ray bursts (GRBs) are the most powerful phenomena in the Universe. First detected during the Cold War, these events beam a tremendous amount of high-energy light our way in a short period of time. They come in two types: short GRBs that last for less than two seconds and long GRBs that last for minutes. Both types have mysterious origins. Short GRBs could be caused by the collisions of neutron stars or perhaps the powerful flares of a magnetar. Observations of long GRBs suggest they are caused by a powerful supernova called a hypernova, where a massive star collapses to become a black hole. But a new study suggests that the origins of long GRBs are more diverse.¹

One of the challenges in explaining long gamma-ray bursts is that they have a wide range of behaviors. Some are less bright, while others are immensely powerful. Some have long afterglows of radio light, while others have almost no afterglows. The general idea to explain this is that the most powerful GRBs occur when a jet of the hypernova is pointed in our direction. When the jet isn’t quite aimed our way, the GRB can be fainter. The afterglows could be explained by high-energy light ionizing the surrounding interstellar medium. This new study suggests this isn’t enough to explain the observed range of long GRBs.

The authors base their argument on observations gathered by the Neil Gehrels Swift Observatory, which has observed more than 1,600 GRBs and has measured the redshifts of about 500 of them. From this data, the authors focus on 280 GRBs for which Swift gathered good light curve profiles. Since the light curves of supernovae are well understood, hypernove light curves can be modeled in a similar way. We know how hypernovae should brighten and fade over time. By comparing this to the light curves of the GRBs, the authors find that only about half of them are a good fit to hypernovae. This suggests that long GRBs don’t have a singular origin. The authors suggest that other phenomena, such as the mergers of black holes or a black hole merging with a neutron star or white dwarf, could explain the diversity of observed GRBs.

The team also notes that matching light curves is challenging. There are assumptions you have to make when accounting for the effects of redshift and observational bias. The Swift data isn’t detailed enough to completely rule out one origin model over the other in numerous instances. But the data clearly suggests that the astrophysical processes creating GRBs are much more diverse than we have thought.