How did the Christmas gamma-ray burst explode? No, it's not a geeky Christmas cracker joke, it's a real question scientists have been trying to answer since Christmas day last year, when a gamma-ray burst called GRB 101225A first lit up the sky. The Christmas burst, as its come to be known, exhibted some rather unusual characteristics.

Gamma-ray bursts are short-lived flashes of gamma rays, made up of light that is more energetic than x-rays. Most are thought to be the result of massive stellar explosions in distant galaxies. Bursts can be over in milliseconds or last for several minutes, but no longer than that. After they finish they leave behind a longer-lived afterglow that can survive into weeks and months. While they last, they are the brightest objects in the known universe.

The Christmas burst was unusual. It went on for at least half an hour, but its afterglow faded much faster than was expected and displayed some features that are incompatible with current models. When it was first observed, astronomers were unable to work out how far away it was from Earth.

Now, two groups of scientists have come up with explanations of why the Christmas burst exploded the way it did. Both put forward their explanations in the same issue of the journal Nature earlier this month. Each group's hypothesis attempts to explain why the Christmas burst lasted for longer than most others and why its afterglow faded so fast. In doing so, they also provide an estimate of the burst's distance — and each come up with different answers.

Sergio Campana, an astronomer at the Istituto Nazionale di Astrofisica in Italy, and his group think that the Christmas burst was created in an atypical way — by a comet falling onto an extremely dense star. On the other hand, Christina Thöne, from the Instituto de Astrofísica de Andalucía in Spain, and her group propose a more conventional creation, involving a supernovae born out of two stars.

In Campana's hypothesis a "small" object — a comet or asteroid — is whizzing past a dense neutron star just a little too close. It gets pulled in by the gravity of the star, but before it reaches the star's surface, the smaller object is overwhelmed by the force of the gravity and breaks up into fragments. This debris is thrown into orbit around the star and then falls back to form a disk. Eventually, the debris falls on to the star itself and explodes, releasing vast amounts of energy. If this version of events is true, the explosion must have happened in our own galaxy, no more than 10,000 light years away in the Perseus spiral arm.

Thöne's explanation involves a burst that starts as as binary system — two stars, one an extremely dense neutron star and the other a supergiant helium star, orbiting each other. The neutron star sucks the mass off the helium star until both are surrounded by an envelope of gas. The two stars then merge into a black hole or magnetar, a dense star with a powerful magnetic field, creating a jet of intense energy that bursts through the gas envelope. In this scenario, the explosion happened over four and a half billion light years away.

Enrico Costa, independent of both groups, pointed out that though both groups' suggestions are "plausible", at least one must be wrong. Once the burst's host has been found, one (or perhaps even both) of these models will be ruled out. For now, though, the jury is still out on how this unusual Christmas burst came to be, so perhaps we should keep an open mind about the origins of some of the brightest explosions in the universe.



Astronomy Journal Club (#astrojc on twitter) dicussed this paper in their final meeting of the year. This is the review (and you can find the full meeting here).

For some animations of the two possible scenarios, go here.


Thöne, C., de Ugarte Postigo, A., Fryer, C., Page, K., Gorosabel, J., Aloy, M., Perley, D., Kouveliotou, C., Janka, H., Mimica, P., Racusin, J., Krimm, H., Cummings, J., Oates, S., Holland, S., Siegel, M., De Pasquale, M., Sonbas, E., Im, M., Park, W., Kann, D., Guziy, S., García, L., Llorente, A., Bundy, K., Choi, C., Jeong, H., Korhonen, H., Kubànek, P., Lim, J., Moskvitin, A., Muñoz-Darias, T., Pak, S., & Parrish, I. (2011). The unusual γ-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33 Nature, 480 (7375), 72-74 DOI: 10.1038/nature10611

Campana, S., Lodato, G., D’Avanzo, P., Panagia, N., Rossi, E., Valle, M., Tagliaferri, G., Antonelli, L., Covino, S., Ghirlanda, G., Ghisellini, G., Melandri, A., Pian, E., Salvaterra, R., Cusumano, G., D’Elia, V., Fugazza, D., Palazzi, E., Sbarufatti, B., & D.Vergani, S. (2011). The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star Nature, 480 (7375), 69-71 DOI: 10.1038/nature10592