Gamma-ray bursts strike Earth's atmosphere from random directions in space about twice a day. In 1997, the discovery1 of the emission, or afterglow, that follows the prompt release of γ-rays placed their progenitors in remote galaxies. The mechanisms put forward to explain the bursts and their afterglows are complex and differ from one to the next. Any theory for the origin of γ-ray bursts must account for both the commonalities and the differences between them. Writing in this issue, Campana et al.2 (page 69) and Thöne et al.3 (page 72) use two completely different models to explain the especially unusual 'Christmas' γ-ray burst, so dubbed because it occurred on Christmas Day 2010.
Although each one is a unique event, γ-ray bursts (GRBs) can be roughly divided into two classes — long and short — according to the duration of the prompt emission. Long GRBs, which last from 2 seconds to a few minutes, are thought to originate from hypernovae, an unusual class of supernova. They involve the collapse of a massive star (a hundred times more massive than the Sun) that not only explodes as a supernova, but also generates two opposing high-speed jets of particles, leaving behind a black hole. If one of the jets happens to point towards an observer, a bright emission — the prompt γ-ray release — is detected.
The GRB afterglow, which follows minutes after the initial burst and fades away over the course of weeks or months, is emitted at various wavelengths and is caused by the impact of the jet on the surrounding interstellar medium. Once the GRB fades away, the faint galaxy that hosts the collapsing star can be identified, and the distance of the GRB from Earth measured from the galaxy's spectrum. Short GRBs, which typically last less than 2 seconds, are better explained by two neutron stars bound in a tight binary system. The stars produce gravitational waves and eventually merge to form a black hole, producing the GRB's jets and afterglow in the process of merging.
As GRB research has progressed, established models have been challenged by observations, even of a single GRB. It is conceivable that the GRB population has been 'polluted' — or, alternatively, enriched — by a few puzzling events that mimic GRBs but have a completely different origin. One such event is the Christmas GRB, which was detected by the Swift satellite and is known technically as GRB 101225A. The Christmas GRB's γ-ray emission was exceptionally long — it lasted for at least half an hour — and the X-ray afterglow faded much faster than usual, an observation that is incompatible with prevailing GRB models. The energy spectrum of the afterglow displayed a shape (the Planckian form) that is characteristic of a plasma whose electrons arrive at such an equilibrium that we can say that it has a certain temperature. Conversely, typical energy spectra of GRBs and their afterglows follow power laws that well describe the collision of materials at high velocities, such as that resulting from the impact of an explosion on its environment.
If the Christmas GRB originates from a remote hypernova, as do long-lasting GRBs, why is it so unusual? Could it be a completely different phenomenon, only apparently similar to a GRB? So far, observations have not unambiguously disclosed a host galaxy. The location of the GRB in the sky is such that it could belong to the Perseus arm of the Milky Way, but also to a satellite of the Andromeda galaxy. In their studies, Campana et al.2 and Thöne et al.3 and set out to address these questions and to explain the origin of the GRB.
Thöne et al.3 interpret a change in slope and colour of the Christmas GRB afterglow's optical emission as being due to the addition of a supernova that emerges after 10 days, when the afterglow has almost vanished. By comparing the system with similar composite supernova–GRB objects, they derive a distance for the system of 1.6 gigaparsecs and an energy of about 1.4 × 1051 erg, compatible with a typical GRB.
Thöne and colleagues posit that the GRB originates from a tightly bound binary system composed of a neutron star and a supergiant helium star. The helium star would transfer mass to the neutron star, and the system would evolve to a phase in which the stars would be surrounded by a common gas envelope produced by the expansion of the external layers of the helium star. The neutron star and the core of the helium star would merge to form a black hole or a highly magnetized neutron star, known as a magnetar4, producing a GRB-like jet that would eventually emerge through the common envelope. The authors find that this model agrees with the observed thermal spectrum of the GRB. However, the case for the additional supernova, which would be unusually faint, is not compelling. Therefore, the deduced GRB distance from Earth is uncertain. Moreover, it isn't clear whether a host galaxy exists. The only optical object seen near the position at which the GRB took place is point-like and extremely faint: were it a galaxy it would be the faintest ever to host a GRB.
Meanwhile, Campana et al.2 revive a model proposed5 in 1973, soon after the discovery of GRBs. In this model, a minor body, such as a comet or an asteroid, flying by a neutron star at a distance of less than 5,000 kilometres, gets tidally disrupted, breaks into fragments and produces a multi-peak GRB. The authors2 used recent models for tidal-disruption events6 and further assumed that the debris of the minor body forms a transient disk around the star. They compared these models with the available data for the Christmas GRB and found that a model in which a minor body of 5 × 1020 grams falls onto a neutron star in our Galaxy is in excellent agreement, in both temporal and spectral behaviour, with the data.
Both hypotheses2,3 are plausible and explain numerous and complex data. But at least one is wrong and the definitive proof — namely, unambiguous determination of the GRB's distance from Earth — is missing. Thöne and colleagues' study raises several questions. How many binaries pass through a phase of a common envelope and produce GRB-like explosions? Do all jets emerge from the envelope, or is that the case for only a few of them? How highly collimated is the jet? But Campana and colleagues' study also opens up queries. How many minor bodies pass in close proximity to neutron stars? And is the deduced mass for the minor body conceivable? The proposed mass is relatively large (and consequently rare) for an object in the Solar System, but larger bodies have actually been found to orbit neutron stars. We could compute the likelihood of each hypothesis, and perhaps discard one on the basis of statistical considerations. Unfortunately, however, such a computation would inevitably involve considerable conjecture.
In short, not much more can be said about the nature of the Christmas GRB — except that the odds are that the event is a rare phenomenon that looks like a GRB but falls outside this category. Whatever the case, it's hard to escape the fascination of a possible comet death on Christmas Day.