Changing Planet

Mysterious Christmas Day “Starburst” Explained?

On December 25, 2010, a NASA telescope spotted a bright “star” that suddenly appeared in the sky.

The brilliant object was a gamma-ray burst—a distant and mysterious flash of some of the most intense light in the cosmos. (Related: “Ultrabright Gamma-ray Burst ‘Blinded’ NASA Telescope.”)

In general, gamma-ray bursts are divided into two categories: long and short. Short GRBs last less than two seconds, while long GRBs can last from seconds to minutes.

Once the gamma rays fade, there’s usually an afterglow in other wavelengths of light that can last for weeks or months.

The Christmas Day burst, called GRB 101225A, was one of the longest events yet observed by NASA’s Swift satellite. Its gamma rays were detected for almost half an hour, but then its x-ray afterglow was unusually short, lasting just two days.

Optical emission from GRB 101225A over time.
—Image set courtesy Christina Thöne et al.

These observations are at odds with the current theory that long GRBs occur due to spectacular Type Ic supernovas —explosions triggered by the collapse of stars massive enough to leave behind black holes.

In such a supernova, the theory goes, the star explodes with such force that it spews high-speed jets of particles from its poles. If one of those jets is aimed at Earth, we see it as a long GRB. Light from the supernova itself will later be visible.

But in addition to not quite fitting the timing for this model, the Christmas Day burst could be linked to only a very faint possible supernova, and groups collecting data on the burst couldn’t definitively pinpoint a host galaxy for the event.

These and other inconsistencies in the nature of the afterglow got astronomers thinking that maybe the Christmas Day burst was an unusual event that somehow produced a GRB-like flare.

In this week’s issue of the journal Nature, two groups propose their theories for what sparked the Christmas Day explosion.

In one model, the flash happened due to the merger of a massive star and a stellar corpse.

Illustration of a star merger that could have spawned a GRB.
Image courtesy Aurore Simonnet, NASA E/PO, Sonoma State University

In a binary system of two massive stars, a huge helium star survived the death of its companion, which left behind a dense core called a neutron star, according a team led by Christina Thöne of the Instituto de Astrofísica de Andalucía in Granada, Spain.

The two objects were so close that, as the helium star aged and expanded, it engulfed the neutron star, which started spiraling into the helium star’s center.

As the two objects merged, angular momentum from the neutron star created a disk of material that in turn spewed GRB-like jets.

(Related: “Black Hole Caught Eating a Star, Gamma-Ray Flash Hints.”)

In the other model, the burst was created by the breakup of a comet as it neared a lone neutron star.

Illustration of a comet-like object being torn apart by a neutron star.
Image courtesy A. Simonnet, NASA, E/PO, Sonoma State University

Scientists led by Sergio Campana of the Osservatorio Astronomico di Brera in Merate, Italy, think the data fits if an object about half the mass of the dwarf planet Ceres got within about a million kilometers of a neutron star that’s 1.4 times the mass of the sun.

As the comet approached the star, it got ripped apart by gravitational forces—just as the comet Shoemaker-Levy 9 (picture) broke up when it got too close to Jupiter.

In the GRB model, though, the remains of the comet fell back toward the neutron star, with some pieces hitting its surface, spewing gamma rays, and other pieces forming a disk around the star that glowed in other wavelengths as matter was gathered and compressed.

(Related: “Space-Time ‘Wrinkles’ Igniting Odd Gamma-ray Bursts?”)

“Both hypotheses are plausible and explain numerous and complex data,” Enrico Costa, of the Istituto di Astrofisica Spaziale e Fisica Cosmica in Rome, Italy, writes in a commentary accompanying the Nature studies.

But, Costa says, both theories also raise a number of questions and can’t account for all the data on the Christmas Day burst.

“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,” he writes.

“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.”

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