Tuesday, November 18, 2008

A Brief Mystery: What are Short Gamma-ray Bursts?

October 20, 2008: For decades it was baffling. Out of the still night sky, astronomers peering through their telescopes would occasionally glimpse quick bursts of high-energy light popping off like flashbulbs at the far side of the universe.

These bursts seemed impossibly powerful: to appear so bright from so very far away, they must vastly outshine entire galaxies containing hundreds of billions of stars. These explosions, called gamma ray bursts (GRBs), are by far the brightest and most energetic phenomena in the known universe, second only to the Big Bang itself. Scientists were at a loss to imagine what could possibly cause them.

Right: An artist's concept of a gamma-ray burst.

Astronomers now know what the longer-lasting GRBs are: the collapse and explosion of an ultra-massive star to form a black hole at its core, an explanation first proposed by Stan Woosley of the University of California in San Diego. But there’s a second category of GRBs that still remains a mystery.

"The short-lived ones are very poorly understood. It's where the frontier [of research] is now," says Neil Gehrels, principal investigator for the GRB-detecting Swift satellite at NASA's Goddard Space Flight Center.

Gehrels and other researchers have gathered this week at the Sixth Huntsville Gamma Ray Burst Symposium in Huntsville, Ala., to discuss progress on this and other mysteries surrounding GRBs. Short gamma-ray bursts are a hot topic at today's sessions: agenda.

"We have had good evidence since the 1990s that the short bursts and long bursts were different classes," Gehrels explains. "It had to do with their gamma ray properties." Not only do the short bursts last less than about 2 seconds, the spectrum of light they emit is distinct. Gamma rays from short bursts lean toward the high-energy end of the spectrum, while long GRBs emit lower-energy gamma rays.

The differences were highlighted in 2005 when, for the first time, telescopes caught sight of short GRB afterglows. The fading debris contained no supernova, arguing against the collapse of a massive star. George Ricker of MIT, principal investigator of NASA's HETE (High Energy and Transient Explorer) satellite, famously likened a short burst on July 9, 2005, to "the dog that didn't bark."

Ultimately, the cause of short bursts is unknown. But scientists do have some good guesses.

Above: An artist's concept of a neutron star-neutron star collision.

The leading theory is that these bursts are extremely violent collisions between pairs of neutron stars. These stars aren't gassy, wispy giants like other stars — a neutron star is more like an atomic nucleus that's 12 kilometers across. Since the atoms that make up normal, "solid" matter are mostly empty space, a star made almost entirely of tightly packed neutrons is extraordinarily dense: a fingernail's worth of a neutron star would have a mass of more than a trillion kilograms. A neutron star's density and gravity is second only to a black hole. "When you have these two hard stars that run into each other, it's a very rapid fiery explosion. It's kind of like a crash."

So how could scientists know whether this explanation is true?

One way could be to detect gravitational waves. Before the two neutron stars collide, they would orbit each other as a binary system. Because their fields of gravity are so intense, the stars ought to send waves rippling outward in the fabric of space-time: gravitational waves. As the neutron stars spiral in toward each other, the frequency of those waves would ramp up in a characteristic pattern called a chirp signal.

"Scientists are trying to [detect] that now," Gehrels says. "It's the ultimate way of verifying the model."

Scientists at the Huntsville symposium are discussing the progress of gravitational wave detectors such as the Laser Interferometer Gravitational-wave Observatory (LIGO) located in Hanford, Washington, and Livingston, Louisiana. By using lasers to carefully measure the distances between pairs of mirrors at these observatories, LIGO scientists can notice tiny changes in these distances that would occur if subtle gravitational waves were passing through the Earth.

Other possible explanations for short GRBs exist as well, but only hard data from experiments such as LIGO can settle what is the real cause of these mysterious celestial bursts.

Magnetic Portals Connect Sun and Earth

Oct. 30, 2008: During the time it takes you to read this article, something will happen high overhead that until recently many scientists didn't believe in. A magnetic portal will open, linking Earth to the sun 93 million miles away. Tons of high-energy particles may flow through the opening before it closes again, around the time you reach the end of the page.

"It's called a flux transfer event or 'FTE,'" says space physicist David Sibeck of the Goddard Space Flight Center. "Ten years ago I was pretty sure they didn't exist, but now the evidence is incontrovertible."

Indeed, today Sibeck is telling an international assembly of space physicists at the 2008 Plasma Workshop in Huntsville, Alabama, that FTEs are not just common, but possibly twice as common as anyone had ever imagined.

Right: An artist's concept of Earth's magnetic field connecting to the sun's--a.k.a. a "flux transfer event"--with a spacecraft on hand to measure particles and fields. [Larger image]

Researchers have long known that the Earth and sun must be connected. Earth's magnetosphere (the magnetic bubble that surrounds our planet) is filled with particles from the sun that arrive via the solar wind and penetrate the planet's magnetic defenses. They enter by following magnetic field lines that can be traced from terra firma all the way back to the sun's atmosphere.

"We used to think the connection was permanent and that solar wind could trickle into the near-Earth environment anytime the wind was active," says Sibeck. "We were wrong. The connections are not steady at all. They are often brief, bursty and very dynamic."

Several speakers at the Workshop have outlined how FTEs form: On the dayside of Earth (the side closest to the sun), Earth's magnetic field presses against the sun's magnetic field. Approximately every eight minutes, the two fields briefly merge or "reconnect," forming a portal through which particles can flow. The portal takes the form of a magnetic cylinder about as wide as Earth. The European Space Agency's fleet of four Cluster spacecraft and NASA's five THEMIS probes have flown through and surrounded these cylinders, measuring their dimensions and sensing the particles that shoot through. "They're real," says Sibeck.

Now that Cluster and THEMIS have directly sampled FTEs, theorists can use those measurements to simulate FTEs in their computers and predict how they might behave. Space physicist Jimmy Raeder of the University of New Hampshire presented one such simulation at the Workshop. He told his colleagues that the cylindrical portals tend to form above Earth's equator and then roll over Earth's winter pole. In December, FTEs roll over the north pole; in July they roll over the south pole.

Right: A "magnetic portal" or FTE mapped in cross-section by NASA's fleet of THEMIS spacecraft. [Larger image]

Sibeck believes this is happening twice as often as previously thought. "I think there are two varieties of FTEs: active and passive." Active FTEs are magnetic cylinders that allow particles to flow through rather easily; they are important conduits of energy for Earth's magnetosphere. Passive FTEs are magnetic cylinders that offer more resistance; their internal structure does not admit such an easy flow of particles and fields. (For experts: Active FTEs form at equatorial latitudes when the IMF tips south; passive FTEs form at higher latitudes when the IMF tips north.) Sibeck has calculated the properties of passive FTEs and he is encouraging his colleagues to hunt for signs of them in data from THEMIS and Cluster. "Passive FTEs may not be very important, but until we know more about them we can't be sure."

The Sun Shows Signs of Life

Nov. 7, 2008: After two-plus years of few sunspots, even fewer solar flares, and a generally eerie calm, the sun is finally showing signs of life.

"I think solar minimum is behind us," says sunspot forecaster David Hathaway of the NASA Marshall Space Flight Center.

His statement is prompted by an October flurry of sunspots. "Last month we counted five sunspot groups," he says. That may not sound like much, but in a year with record-low numbers of sunspots and long stretches of utter spotlessness, five is significant. "This represents a real increase in solar activity."

Above: New-cycle sunspot group 1007 emerges on Halloween and marches across the face of the sun over a four-day period in early November 2008. Credit: the Solar and Heliospheric Observatory (SOHO).

Even more significant is the fact that four of the five sunspot groups belonged to Solar Cycle 24, the long-awaited next installment of the sun's 11-year solar cycle. "October was the first time we've seen sunspots from new Solar Cycle 24 outnumbering spots from old Solar Cycle 23. It's a good sign that the new cycle is taking off."

Old Solar Cycle 23 peaked in 2000 and has since decayed to low levels. Meanwhile, new Solar Cycle 24 has struggled to get started. 2008 is a year of overlap with both cycles weakly active at the same time. From January to September, the sun produced a total of 22 sunspot groups; 82% of them belonged to old Cycle 23. October added five more; but this time 80% belonged to Cycle 24. The tables have turned.

At first glance, old- and new-cycle sunspots look the same, but they are not. To tell the difference, solar physicists check two things: a sunspot's heliographic latitude and its magnetic polarity. (1) New-cycle sunspots always appear at high latitude, while old-cycle spots cluster around the sun's equator. (2) The magnetic polarity of new-cycle spots is reversed compared to old-cycle spots. Four of October's five sunspot groups satisfied these two criteria for membership in Solar Cycle 24.

The biggest of the new-cycle spots emerged at the end of the month on Halloween. Numbered 1007, or "double-oh seven" for short, the sunspot had two dark cores each wider than Earth connected by active magnetic filaments thousands of kilometers long. Amateur astronomer Alan Friedman took this picture from his backyard observatory in Buffalo, New York:

On Nov. 3rd and again on Nov. 4th, double-oh seven unleashed a series of B-class solar flares. Although B-flares are considered minor, the explosions made themselves felt on Earth. X-rays bathed the dayside of our planet and sent waves of ionization rippling through the atmosphere over Europe. Hams monitoring VLF radio beacons noticed strange "fades" and "surges" caused by the sudden ionospheric disturbances.

Hathaway tamps down the excitement: "We're still years away from solar maximum and, in the meantime, the sun is going to have some more quiet stretches." Even with its flurry of sunspots, the October sun was mostly blank, with zero sunspots on 20 of the month's 31 days.