Christmas Gamma Ray Burst:

Naughty or Nice?

Last Christmas, NASA’s SWIFT satellite observed an unusual and spectacular cosmic explosion from the direction of the constellation Andromeda. Everyone agrees this was a gamma ray burst with peculiar characteristics. Two competing explanations have emerged, both involving a neutron star**. In one, a very distant neutron star smashed into another star creating a black hole and a horrific explosion — we’ll call that “naughty.” In the other, a nearby, innocent neutron star was hit by a comet-sized body, releasing a modest flash — we’ll call that “nice.”

SWIFT has dramatically increased our understanding of Gamma Ray Bursts (GRBs.) Launched seven years ago, SWIFT rapidly scans the sky for energetic bursts and then hones in on the source with its X-ray, ultraviolet, and visible light telescopes. SWIFT can determine the source location to arc-second precision and follow the decaying burst across a wide spectrum of light frequencies.

GRBs are our universe’s most spectacular explosions and we believe they’re related to collapsed stars, either neutron stars or black holes. GRBs are conventionally divided into two types, long and short, depending on their duration. Long GRBs last 20 to 40 seconds and are attributed to the collapse of massive stars. By contrast, most short GRBs come and go in under 1/5th of a second. A phenomenon that fast can only originate from something less than 1/5th of a light-second across, comparable to the planet Uranus. Merging neutron stars or black holes are the leading candidates.

The Christmas Burst, technically named GRB 101225A, was remarkable because it lasted 28 minutes, about 100 times longer than some long GRBs.

The leading proponent of the “naughty” Xmas Burst story is Christina Thoene of the Institute of Astrophysics of Andalusia in beautiful Granada, Spain. Christina proposes a two-star system—a neutron star and a normal star orbiting one another, held together by their mutual gravity. When a normal star comes to the end of its life, it expands enormously into a red giant. If its neutron star partner is close enough, it may become enveloped by the extended, diffuse outer layers of the red giant. This would cause a drag that would force the stars to spiral together, creating a black hole with an immensely energetic explosion. Based on the amount of energy reaching Earth, Christina’s team estimates the Xmas Burst occurred 5.5 billion light-years away (33 billion, trillion miles). Indeed, her team has identified what seems like a faint galaxy at the expected location; perhaps this galaxy hosted this very naughty outburst.

In a field where so much remains undiscovered, there’s often another viewpoint. Sergio Campana leads a group at Brera Observatory in Merate, Italy that proposes a “nice” alternative. (Since I’ve been to Granada and seen the Alhambra, perhaps I should go to Merate, just to be fair.) Sergio proposes a much smaller explosion that is much closer to Earth. If a large comet or small dwarf planet crashed into a neutron star only 10,000 light-years from Earth, Sergio says the blast seen here would look the same as Christina’s immense explosion 550,000 times farther away. Sergio suggests searching for an X-ray point source with NASA’s Chandra satellite and searching for a pulsar with a radio telescope. Either would support the nice neutron star version and contradict the naughty version with the new black hole.

We said GRB’s are spectacularly energetic. Just how energetic is an interesting question. Originally, some observers took the amount of GRB energy they saw and assumed it was spread equally in all directions. That made the total energy phenomenal—about 100 million, million, million times brighter than our Sun, and equivalent to converting the entire mass of an average star into radiation energy in just a few seconds, with this equivalence based on Einstein’s equation E=mc². Instead, astrophysicists now believe the total energy in a GRB is equivalent to “only” a thousandth of a star’s mass but that its energy is concentrated in two narrow jets that shoot out in opposite directions. We only see the GRBs that happen to shoot our way, and miss the vast majority that are aimed elsewhere. When we do see a GRB, we observe the energy in the highly concentrated jet.

The brightest known GRB was GRB 080319B, seen on March 19, 2008, brighter than 10 million galaxies and exploded 7.5 billion light-years away (shown at left). It occurred when our universe was less than half its current age; its light took that long to reach us. For about one minute, this object was visible to the naked eye in the constellation Bootes.

Fortunately GRBs haven’t happened close to Earth, at least not recently. A GRB that originated in our galaxy and was aimed our way could kill every form of life on Earth by its combination of gamma ray, X-ray, and UV radiation, and by depleting Earth’s ozone layer that shields us from the Sun’s deadly rays. More distant GRBs would be much less deadly. Some scientists estimate that GRBs harmful to life have hit Earth about once every 5 million years, or about 1000 times since Earth formed. They even postulate that a major extinction event 450 million years ago could have been due to a GRB that was too close for comfort. Might want to consider lead-lined long-johns.

**Lastly, here’s some background on neutron stars. Stars are powered by nuclear fusion—fusing small nuclei into larger ones. When a star exhausts its nuclear fuel, gravity crushes its core, forming a white dwarf, a neutron star, or a black hole. The least massive stars become white dwarfs; the most massive become black holes; and middling stars become neutron stars. The self-gravity of a neutron star is strong enough to convert electrons and protons into neutrons and neutrinos. The long-wavelength electrons disappear, leaving only short-wavelength neutrons, which pack together at an immense density. Neutron stars can pack more than the mass of our Sun into an object only six miles across—a teaspoon full of neutron would contain 3 billion tons of mass.

Best Regards,
Robert

Correction: in my last newsletter, I erroneously said "Caltech Professor Kip Throne", his name is Kip Thorne. Sorry Kip.

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