New explanation for greatest cosmic explosions

An artist's rendering shows a star as it explodes outward, causing a supernova.
An artist's rendering shows a star as it explodes outward, causing a supernova. Aurore Simonnet / NASA
/ Source:

There is a certain type of cosmic explosion that becomes, in a flash, the brightest thing in the universe, emitting for a few seconds as much radiation as a million galaxies. Don't bother looking for one in the sky, though, since most of the light is in the gamma-ray part of the spectrum, a realm we can't see.

Astronomers observe these colossal gamma-ray bursts with space-based telescopes, however.

They generally agree that only the birth of a black hole could supply enough spark for one of these intense flashes, but there remains a great deal of uncertainty over what converts the newborn black hole's energy into the radiation that astronomers detect.

Recent observations suggest that this "converter" is a high-powered magnetic beam, and not — as many theorists believe — a high-speed jet of hot material.

This is just the latest debate over these exceptionally luminous objects called gamma ray bursts. Researchers previously argued whether GRBs come from inside or outside our galaxy, then whether they emerge from a dying star or two neutron stars merging.

The current consensus is that most GRBs are the death knell of a massive star in a faraway galaxy. After exhausting its fuel supply, the star's core collapses into a black hole (or a comparatively dense neutron star), which acts as a "central engine" for two jets spouting out of the poles.

These jets are where the energy of the collapse is transformed into gamma rays, but we only observe a GRB if we happen to be lined up with the barrel of one of the jets.

This overall picture is fairly well-established, but the big question, according to Tsvi Piran of the Hebrew University in Jerusalem, is what makes up the jets.

When you're a jet
The widely-accepted fireball model assumes that the outer shells of the dying star are heated to very high temperature. This hot material expands outward in all directions, but the expansion is easiest along the star's rotational axis. Hence, fast moving material emerges from the poles as twin jets.

But the Swift satellite, NASA's dedicated GRB observatory, has detected a number of GRBs that appear to defy the fireball model.

"Swift has seen many puzzling GRBs," Piran told "I would say about half the cases have something unexpected."

From a launch out of the weeds to a special delivery in orbit, see the best space offerings from January 2014.

Of the more than 200 GRBs that Swift has recorded, some have had very long X-ray afterglows, while others have faded away and then suddenly rebrightened.

"What we are finding is that the central engine is not dying immediately but continues to inject energy into the flow for thousands of seconds," said theorist Dimitrios Giannios of the Max Planck Institute for Astrophysics in Garching, Germany. "This long activity is more consistent with magnetic models."

A star's magnetic fields is compressed and amplified when the star collapses to a black hole or a highly magnetized neutron star, called a magnetar. Models predict that the fields are strongest — roughly a million billion times that of Earth's magnetic field — along the rotational axis, where they spiral out like an ever-widening corkscrew, according to Giannios.

Since magnetic fields have no mass, they are much easier to accelerate than matter. The fields would therefore be more efficient at carrying energy out of the central engine.

Outward-moving magnetic fields would eventually dissipate their energy into gamma rays — most likely in a process similar to what happens in solar flares, said Erin McMahon of the University of Texas at Austin.

Theories predict that this gamma ray production occurs 10 billion miles from the central source, roughly 100 times further out than the fireball model. McMahon and her colleagues recently studied a sample of 10 GRBs and found that the estimated location of gamma ray emission was more consistent with magnetic outflows.

Polarized opposite
It is not easy to confirm the presence of magnetic fields in far-off astronomical objects, but the light coming from a magnetized source should be polarized, which means the light's electric field should point in a specific direction.

"The polarization gives you a handle on the magnetic fields," said astronomer Carole Mundell of Liverpool John Moores University in the UK.

Mundell and her colleagues have built a polarization detector for GRBs — which is not an easy thing to do, since the light fades away so quickly. The team recently caught their first burst, but failed to detect any polarization — apparently ruling out a strong, well-ordered magnetic field.

However, Giannios thinks that magnetic fields could be oriented in a way that hides the polarization signal. The trouble is that no one can say exactly how the fields will behave in a magnetic outflow, since astronomers have yet to observe one in any setting.

Conversely, matter-filled jets are often seen coming out of quasars.

Open field
Magnetic fields lag behind their fireball counterparts not only observationally but theoretically too.

"We don't know how to calculate all the effects of the magnetic fields yet," Piran said. "The model does not make enough predictions."

Giannios agrees that magnetic outflow researchers have their work cut out for them, especially in explaining what makes the afterglows that shine for minutes up to days.

"But the status has improved," Giannios said. "There have been detailed calculations [of the initial burst] and they compare well with data."

In light of all the unexpected results coming from Swift, it may turn out that there are two classes of GRBs: some born from fire, others magnetically driven.

"It is a very open field right now," Giannios said. "Every day brings a new surprise."