A starquake 50,000 light years away can affect Earth, yet starquakes and the stars that cause them are still sources of mystery. [Illustration Credit: NASA's Goddard Space Flight Center/S. Wiessinger
On Dec. 27, 2004, for a tenth of second, a blast of energy knocked satellites offline, disrupted submarine and radio transmissions, and shifted the magnetic field of the Earth. Within minutes, everything was back to normal, but astrophysicists all over the world were left staring at their instruments asking, “what was that?”
Every researcher with an instrument pointed at the sky was bombarded with emails and phone calls, despite the holiday season. David Palmer, an astrophysicist at Los Alamos National Laboratory, got an email asking if the pulse detection software he had designed for the SWIFT satellite had gotten any weird readings that day. The satellite had only been in space for slightly over a month, but Palmer logged in anyway to check. Although the satellite was looking the wrong way, gamma rays, which are powerful bursts of energy, had gone straight through it — more gamma rays than are emitted from the Sun in the course of 150,000 years.
“I thought, it was probably a giant burst [from a star], or there was something going wrong in the instrument,” Palmer said. But he and researchers all over the world concluded that the satellite was fine — and that the mass of radiation that hit the earth came from something called a starquake.
A starquake is vaguely similar to an earthquake but occurs on a magnetar, a mysterious type of star that is extremely dense and magnetic. To date, scientists have only identified 23 magnetars, and recorded three starquakes: one each in 1979, 1998 and 2004. Researchers cannot predict starquakes, so while they wait for one they are working on tools to better understand these events and the stars that create them. Two of those new tools are almost ready: a new technique to look at the magnetic interior of stars, and NASA’s Neutron Star Interior Composition Explorer mission, or NICER, due to launch in late 2016.
Only about 15 miles across, magnetars are likely the cores that remain after the deaths of supermassive stars. They have the strongest magnetic fields of any object in the universe by several orders of magnitude, says Anna Watts, an astrophysicist at the University of Amsterdam who studies neutron stars and black holes. In fact, a magnetar’s magnetic field is about two quadrillion times more powerful than the magnetic field of the Earth, and a thousand times stronger than a neutron star — the bright cores that remain after the death of a supernova. Conditions inside a magnetar are at a scale that cannot be replicated anywhere else, even at the largest particle physics lab in the world, she says. “CERN is never going to get to the energy and density we [see in these stars].”
Deep inside the magnetar “everything just becomes a soup of neutrons and protons,” the basic building blocks of atoms, explains Tod Strohmayer, a NASA astrophysicist and co-investigator on the NICER mission. In fact, he says, a magnetar is so dense that it may be crushing these atomic components into a soup of their fundamental particles, called quarks. Structured like a cosmic M&M, the magnetar’s soupy core may be surrounded by a crust resembling superhot, dense, iron crystal. “A neutron star crust is the strongest material that we know of in the universe,” says Strohmayer — but it’s not quite strong enough to contain the phenomenal power of a bursting starquake.
“We have no idea what the trigger process is for these things,” says Watts. The current theory suggests that, similarly to earthquakes, the crust rips as the magnetar’s powerful magnetic field moves. The shift pulls the inside of the star like a ball of rubber bands that eventually snap under the pressure. When the crust heats up and finally tears, a fireball of electrons, photons and plasma emerges as a bubble on the side of the star, researchers believe. A bright beam of radiation attaches the fiery bubble to the magnetar, and emits a giant burst of energy. As the bubble rotates around the star, it slowly shrinks back down its beam like the ball dropping on New Years Eve, eventually merging back into the core.
Even though it originated 50,000 light years away, the giant pulse of energy from the 2004 starquake was enough to knock all research and commercial satellites offline, says Brian Gaensler, director of the Dunlap Institute of Astronomy and Astrophysics at the University of Toronto in Canada. Satellites are designed to withstand short bursts of radiation, like what happens during solar flares, and can typically reboot and go back online after the event is over. As the satellites rebooted, researchers were able to track the direction of the starquake by mapping when each satellite was knocked offline. Gaensler says that he suspects the blast also affected military satellites but that researchers were not supplied with that data.
Some U.S. Navy’s stealthy communication equipment was briefly knocked out too, because the starquake temporarily altered the shape of the ionosphere, the outer edge of Earth’s atmosphere. As the pulses of energy hit the Earth, the powerful waves caused the ionosphere to expand and contract as each wave passed. The weird alteration knocked out low-frequency radio communications that rely on bouncing signals off the ionosphere, a technique used by Navy submarines, Gaensler says. Once the flare was over, transmission resumed as normal, but the magnetic field of the Earth remains slightly shifted, a constant reminder of the power contained in these dim stars.
Magnetars are difficult for astronomers to see except during a starquake, but can be identified by their semi-regular pulsation of radiation. Researchers believe that what they see as pulses are actually hotspots of radiation on the magnetar’s surface. Like a lighthouse beam, the spinning of the star moves the hotspot in and out of view as the magnetar rotates. Scientists have been tracking these soft pulses of radiation since 1973, when U.S. satellites created to monitor nuclear testing by the Soviet Union picked up this background noise during the Cold War. Today, the pulses provide most of the data researchers have on magnetars.
Charting the radiation pulses allows researchers to estimate the strength of a magnetar’s magnetic field — the more slowly the magnetar pulses, the stronger the field — as well as to estimate its composition and size. As telescopes get better, researchers are able to get more information on subtle shifts of the magnetar pulses. But the technique is limited by the fact that magnetars are relatively small and dim, so it’s hard to get sensitive data unless they’re flaring, says NASA’s Strohmayer. “You always want to build a bigger telescope.”
Soon, Strohmayer may get the better data he’s pining for. In late 2016, NASA is set to send its NICER mission to the International Space Station. Its primary objective is to measure gamma radiation to better understand the size and mass of neutron stars, including magnetars. Strohmayer hopes that a starquake will occur while NICER is operational, but if it doesn’t, he is already scouting new technology to follow NICER. “Currently there’s nothing being built, although there are things on the drawing board,” he says.
One of those tools could potentially come from a different field of astrophysics called astroseismology that uses the tiny changes in starlight to study the interior of a star. In October, two astroseismologists published the results of a technique they developed to detect the interior magnetic field of a star. Using minuscule variations in star brightness, Jim Fuller, a postdoctoral fellow in astrophysics at the California Institute of Technology, and Matteo Cantiello, an astrophysicist at the University of California at Santa Barbara, created a model that exposed the magnetic core hidden in a red giant star. They hope their work could one day be turned to other types of stars and maybe even help scientists understand how magnetars form when giant stars explode and die.
“This will open up a whole new window into the interiors of stars,” says Fuller, though he adds that there is still a long way to go before the technique can be applied to the super giant stars that form magnetars or to magnetars themselves. But magnetar specialists like Amsterdam’s Watts are already excited to have recently learned that the core of a red giant is more magnetic than previously predicted. If this finding applies to other star types, it would explain how magnetars become so magnetic, she says.
If all stars are more magnetic than we thought, “it’s not so hard to make a magnetar,” says Watts. Other possible explanations of magnetars’ super-magnetism involve rapidly spinning the dying super giant star. But the truth about magnetars and starquakes, she says, is that “we don’t know why they’re so odd.”