NASA's Fermi Satellite Finds Hints of Starquakes in Magnetar 'Storm'

A rupture in the crust of a highly magnetized neutron star, shown here in an artist's rendering, can trigger high-energy eruptions. 

Fermi observations of these blasts include information on how the star's surface twists and vibrates, providing new insights into what lies beneath.

Image Credit: NASA's Goddard Space Flight Center/S. Wiessinger

NASA's Fermi Gamma-ray Space Telescope detected a rapid-fire "storm" of high-energy blasts from a highly magnetized neutron star, also called a magnetar, on Jan. 22, 2009.

Now astronomers analyzing this data have discovered underlying signals related to seismic waves rippling throughout the magnetar.

Such signals were first identified during the fadeout of rare giant flares produced by magnetars.

This image of NASA's Fermi Gamma-ray Space Telescope, shown here in May 2008 being readied for launch, highlights the spacecraft's instruments. 

The Gamma-ray Burst Monitor (GBM) is an array of 14 crystal detectors sensitive to short-lived gamma-ray blasts.

Image Credit: NASA/Jim Grossmann

Over the past 40 years, giant flares have been observed just three times, in 1979, 1998 and 2004, and signals related to starquakes, which set the neutron stars ringing like a bell, were identified only in the two most recent events.

"Fermi's Gamma-ray Burst Monitor (GBM) has captured the same evidence from smaller and much more frequent eruptions called bursts, opening up the potential for a wealth of new data to help us understand how neutron stars are put together," said Anna Watts, an astrophysicist at the University of Amsterdam in the Netherlands and co-author of a new study about the burst storm.

"It turns out that Fermi's GBM is the perfect tool for this work."


In the midst of SGR J1550-5418's 2009 burst storm, Swift's X-Ray Telescope captured an expanding halo produced by the magnetar's brightest bursts. 

The rings formed as X-rays from the brightest bursts scattered off of intervening dust clouds. Clouds closer to Earth produced larger rings.

Image Credit: NASA/Swift/Jules Halpern, Columbia University

Neutron stars are the densest, most magnetic and fastest-spinning objects in the universe that scientists can observe directly.

Each one is the crushed core of a massive star that ran out of fuel, collapsed under its own weight, and exploded as a supernova.

A neutron star packs the equivalent mass of half-a-million Earths into a sphere about 12 miles across, roughly the length of Manhattan Island in New York City.

While typical neutron stars possess magnetic fields trillions of times stronger than Earth's, the eruptive activity observed from magnetars requires fields 1,000 times stronger still. To date, astronomers have confirmed only 23 magnetars.

Because a neutron star's solid crust is locked to its intense magnetic field, a disruption of one immediately affects the other.

A fracture in the crust will lead to a reshuffling of the magnetic field, or a sudden reorganization of the magnetic field may instead crack the surface.

Either way, the changes trigger a sudden release of stored energy via powerful bursts that vibrate the crust, a motion that becomes imprinted on the burst’s gamma-ray and X-ray signals.

It takes an incredible amount of energy to convulse a neutron star. The closest comparison on Earth is the 9.5-magnitude Chilean earthquake of 1960, which ranks as the most powerful ever recorded on the standard scale used by seismologists.

On that scale, said Watts, a starquake associated with a magnetar giant flare would reach magnitude 23.

Fermi finds a mysterious 'transformer' pulsar

These artist's renderings show one model of pulsar J1023 before (top) and after (bottom) its radio beacon (green) vanished. 

Normally, the pulsar's wind staves off the companion's gas stream. 

When the stream surges, an accretion disk forms and gamma-ray particle jets (magenta) obscure the radio beam. 

Credit: NASA's Goddard Space Flight Center

In late June 2013, an exceptional binary containing a rapidly spinning neutron star underwent a dramatic change in behavior never before observed.

The pulsar's radio beacon vanished, while at the same time the system brightened fivefold in gamma rays, the most powerful form of light, according to measurements by NASA's Fermi Gamma-ray Space Telescope.

"It's almost as if someone flipped a switch, morphing the system from a lower-energy state to a higher-energy one," said Benjamin Stappers, an astrophysicist at the University of Manchester, England, who led an international effort to understand this striking transformation.

"The change appears to reflect an erratic interaction between the pulsar and its companion, one that allows us an opportunity to explore a rare transitional phase in the life of this binary."

A binary consists of two stars orbiting around their common center of mass. This system, known as AY Sextantis, is located about 4,400 light-years away in the constellation Sextans.

It pairs a 1.7-millisecond pulsar named PSR J1023+0038 (J1023) with a star containing about one-fifth the mass of the sun.

The stars complete an orbit in only 4.8 hours, which places them so close together that the pulsar will gradually evaporate its companion.

When a massive star collapses and explodes as a supernova, its crushed core may survive as a compact remnant called a neutron star or pulsar, an object squeezing more mass than the sun's into a sphere no larger than Washington, D.C.

Young isolated neutron stars rotate tens of times each second and generate beams of radio, visible light, X-rays and gamma rays that astronomers observe as pulses whenever the beams sweep past Earth.

Pulsars also generate powerful outflows, or "winds," of high-energy particles moving near the speed of light.

The power for all this comes from the pulsar's rapidly spinning magnetic field, and over time, as the pulsars wind down, these emissions fade.

More than 30 years ago, astronomers discovered another type of pulsar revolving in 10 milliseconds or less, reaching rotational speeds up to 43,000 rpm.

While young pulsars usually appear in isolation, more than half of millisecond pulsars occur in binary systems, which suggested an explanation for their rapid spin.

"Astronomers have long suspected millisecond pulsars were spun up through the transfer and accumulation of matter from their companion stars, so we often refer to them as recycled pulsars," explained Anne Archibald, a postdoctoral researcher at the Netherlands Institute for Radio Astronomy (ASTRON) in Dwingeloo who discovered J1023 in 2007.


Zoom into an artist's concept of AY Sextantis, a binary star system whose pulsar switched from radio emissions to high-energy gamma rays in 2013. 

This transition likely means the pulsar's spin-up process is nearing its end. 

Credit: NASA FERMI

During the initial mass-transfer stage, the system would qualify as a low-mass X-ray binary, with a slower-spinning neutron star emitting X-ray pulses as hot gas raced toward its surface.

A billion years later, when the flow of matter comes to a halt, the system would be classified as a spun-up millisecond pulsar with radio emissions powered by a rapidly rotating magnetic field.

To better understand J1023's spin and orbital evolution, the system was regularly monitored in radio using the Lovell Telescope in the United Kingdom and the Westerbork Synthesis Radio Telescope in the Netherlands.

These observations revealed that the pulsar's radio signal had turned off and prompted the search for an associated change in its gamma-ray properties.

A few months before this, astronomers found a much more distant system that flipped between radio and X-ray states in a matter of weeks.

Located in M28, a globular star cluster about 19,000 light-years away, a pulsar known as PSR J1824-2452I underwent an X-ray outburst in March and April 2013. As the X-ray emission dimmed in early May, the pulsar's radio beam emerged.

While J1023 reached much higher energies and is considerably closer, both binaries are otherwise quite similar. What's happening, astronomers say, are the last sputtering throes of the spin-up process for these pulsars.

In J1023, the stars are close enough that a stream of gas flows from the sun-like star toward the pulsar. The pulsar's rapid rotation and intense magnetic field are responsible for both the radio beam and its powerful pulsar wind.

When the radio beam is detectable, the pulsar wind holds back the companion's gas stream, preventing it from approaching too closely but now and then the stream surges, pushing its way closer to the pulsar and establishing an accretion disk.

Gas in the disk becomes compressed and heated, reaching temperatures hot enough to emit X-rays. Next, material along the inner edge of the disk quickly loses orbital energy and descends toward the pulsar.

When it falls to an altitude of about 50 miles (80 km), processes involved in creating the radio beam are either shut down or, more likely, obscured.

The inner edge of the disk probably fluctuates considerably at this altitude. Some of it may become accelerated outward at nearly the speed of light, forming dual particle jets firing in opposite directions, a phenomenon more typically associated with accreting black holes.

Shock waves within and along the periphery of these jets are a likely source of the bright gamma-ray emission detected by Fermi.

The findings were published in the July 20 edition of The Astrophysical Journal. The team reports that J1023 is the first example of a transient, compact, low-mass gamma-ray binary ever seen.

The researchers anticipate that the system will serve as a unique laboratory for understanding how millisecond pulsars form and for studying the details of how accretion takes place on neutron stars.

"So far, Fermi has increased the number of known gamma-ray pulsars by about 20 times and doubled the number of millisecond pulsars within in our galaxy," said Julie McEnery, the project scientist for the mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"Fermi continues to be an amazing engine for pulsar discoveries."

More information: Paper: "A State Change In The Missing Link Binary Pulsar System PSR J1023+0038" iopscience.iop.org/0004-637X/790/1/39 - Paper: "A Radio Pulsar/X-ray Binary Link" arxiv.org/abs/0905.3397

NASA's Fermi Gamma-ray Space Telescope: Completes five years in space - mission extended

This view shows the entire sky at energies greater than 1 GeV based on five years of data from the LAT instrument on NASA's Fermi Gamma-ray Space Telescope. Brighter colors indicate brighter gamma-ray sources. 

Credit: NASA/DOE/Fermi LAT Collaboration

During its five-year primary mission, NASA's Fermi Gamma-ray Space Telescope has given astronomers an increasingly detailed portrait of the universe's most extraordinary phenomena, from giant black holes in the hearts of distant galaxies to thunderstorms on Earth.

But its job is not done yet. On Aug. 11, Fermi entered an extended phase of its mission—a deeper study of the high-energy cosmos. This is a significant step toward the science team's planned goal of a decade of observations, ending in 2018.

"As Fermi opens its second act, both the spacecraft and its instruments remain in top-notch condition and the mission is delivering outstanding science," said Paul Hertz, director of NASA's astrophysics division in Washington.

Paul Hertz

Fermi has revolutionized our view of the universe in gamma rays, the most energetic form of light.

The observatory's findings include new insights into many high-energy processes, from rapidly rotating neutron stars, also known as pulsars, within our own galaxy, to jets powered by supermassive black holes in far-away young galaxies.

The Large Area Telescope (LAT), the mission's main instrument, scans the entire sky every three hours.

The state-of-the-art detector has sharper vision, a wider field of view, and covers a broader energy range than any similar instrument previously flown.

Peter Michelson

"As the LAT builds up an increasingly detailed picture of the gamma-ray sky, it simultaneously reveals how dynamic the universe is at these energies," said Peter Michelson, the instrument's principal investigator and a professor of physics at Stanford University in California.

Fermi's secondary instrument, the Gamma-ray Burst Monitor (GBM), sees all of the sky at any instant, except the portion blocked by Earth.

This all-sky coverage lets Fermi detect more gamma-ray bursts, and over a broader energy range, than any other mission.

These explosions, the most powerful in the universe, are thought to accompany the birth of new stellar-mass black holes.

Bill Paciesas

"More than 1,200 gamma-ray bursts, plus 500 flares from our sun and a few hundred flares from highly magnetized neutron stars in our galaxy have been seen by the GBM," said principal investigator Bill Paciesas, a senior scientist at the Universities Space Research Association's Science and Technology Institute in Huntsville, Ala.

One of Fermi's most striking results so far was the discovery of giant bubbles extending more than 25,000 light-years above and below the plane of our galaxy.

Scientists think these structures may have formed as a result of past outbursts from the black hole—with a mass of 4 million suns—residing in the heart of our galaxy.