Researchers Observe Formation of a Magnetar 6.5 Billion Light Years Away
Magnetars are some of the most extreme objects in the Universe. They are extremely compact objects with masses like our Sun, but with radii of only about 12 miles. One teaspoon of neutron star/magnetar matter weighs as much as Mount Everest. Magnetars generate extremely powerful magnetic fields -- the most intense magnetic fields observed in the Universe. When two neutron stars merge to become a magnetar, the resulting magnetic field is a quadrillion (that is, a million billion) times stronger than the magnetic field that deflects compass needles at the Earth's surface. The field strength is so intense that it heats the surface to 18 million degrees Fahrenheit. (Image Credit: NASA, CXC, M.Weiss)
Researchers Observe Formation of a Magnetar 6.5 Billion Light Years Away
A team of researchers from the University of Arkansas, Pennsylvania State University, and the University of Nevada collaborated with colleagues in China, Chile, and the Netherlands to identify an outburst of X-ray emission from a galaxy approximately 6.5 billion light years away, which is consistent with the merger of two neutron stars to form a magnetar -- a large neutron star with an extremely powerful magnetic field. Based on this observation, the researchers were able to calculate that mergers like this happen roughly 20 times per year in each region of a billion light years cubed.
The research team, which included Bret Lehmer, assistant professor of physics at the University of Arkansas, analyzed data from the Chandra X-ray Observatory, NASA’s flagship X-ray telescope. The Chandra Deep Field-South survey includes more than 100 X-ray observations of a single area of the sky over a period of more than 16 years, to collect information about galaxies throughout the Universe.
A neutron star is a small, very dense star, averaging around 12 miles in diameter. Neutron stars are formed by the collapse of a star massive enough to produce a supernova, but not massive enough to become a black hole. When two neutron stars merge to become a magnetar, the resulting magnetic field is a quadrillion (that is, a million billion) times stronger than the magnetic field that deflects compass needles at the Earth's surface.
“Neutron stars are mysterious because the matter in them is so extremely dense and unlike anything reproduceable in a laboratory,” Lehmer explained. “We do not yet have a good understanding of the physical state of the matter in neutron stars. Mergers involving neutron stars produce lots of unique data that gives us clues about the nature of neutron stars themselves and what happens when they collide.”
A previous discovery of two neutron stars merging, which used gravitational waves and gamma rays to make the observation, gave astronomers new insight into these objects. The research team used this new information to look for patterns in Chandra Observatory’s X-ray data that were consistent with what they learned about merging neutron stars.
The researchers found an outburst of X-rays in the data from the Chandra Deep Field-South survey. After ruling out other possible sources of the X-rays, they determined the signals came from the process of two neutron stars forming a magnetar.
“A key piece of evidence is how the signal changed over time,” said Lehmer. “It had a bright phase that plateaued and then dropped off in a very specific way. That is exactly what you’d expect from a magnetar that is rapidly losing its magnetic field through radiation.”
Similar calculations about the rate of neutron star mergers have been made based on the mergers detected by gravitational waves and gamma rays, strengthening the case for using X-ray data to find such exotic merger events in the Universe.
Pinning Down the Location of a Magnetar
In 2006, University of Arizona (UA) physicists traced the source of extremely high-energy bursts of radiation from deep space back to a magnetar, and they pinned down the exact location of the magnetar from which the burst emerge, solving a decades-old puzzle.
Magnetars are a type of neutron star - the leftover core of a supernova explosion that occurs at the end of a massive star's life - and are characterized by their intense magnetic fields, said Fulvio Melia, a physics and astronomy professor.
In one-tenth of a second, the bursts produce the equivalent energy output of the sun, said Feryal Ozel, a physics and astronomy professor.
The existence of magnetars was proposed in the early 1990s by Robert Duncan, an astronomer at the University of Texas at Austin, as a possible explanation for the source of the high-energy bursts, according to Duncan's Web site.
Until now, however, the precise location of the magnetar from which the bursts originate had been a mystery, Ozel said.
"It's the frontier of physics," she said. "We are dealing with the densest form of matter in the current Universe, and on top of that, we're dealing with the most energetic phenomena in the current Universe."
Neutron stars pack the mass of one and a half suns into a sphere with the radius the length of Chicago, Melia said.
One teaspoon of neutron star matter weighs as much as Mount Everest, Ozel said.
The stars also spin extremely fast, some executing a full revolution in a few thousandths of a second, Melia said.
With magnetic fields that are more than 100 trillion times stronger than that of Earth, magnetars are home to the highest magnetic fields in the Universe, Ozel said.
In a collaboration with two members of NASA's Marshall Space Flight Center in Huntsville, Alabama, Ozel and visiting graduate student Tolga Guver used data from the XXM-Newton Observatory and a computer model to determine the precise origin of the powerful blasts of radiation.
The results of the study showed that the bursts are emitted from small patches on the surfaces of magnetars called magnetic islands, Ozel said.
"By comparing our theoretical model (to the data), we can say, yes, there is a magnetic island on the neutron star that became activated during this initial phase of the outburst," she said.
"This island extends a few meters below the hard surface of the star, and that's where the energy is being stored," she added.
Now that the precise origins of the high-energy bursts are known, the next step is to determine how or why the bursts occur, Ozel said.
One possibility is that there are earthquakes on the surface of magnetars, Ozel said.
"These magnetic fields are producing so much stress on the crusts (of the magnetars), at some point it just gives - yields to the stress - and produces this massive earthquake, and you see the ringing of this earthquake for years," she said.
While the recent findings involve a magnetar 12 million light years from Earth, the study has implications closer to home.
"I like to see it as this nice interface between our understanding of the physical world overall, and our understanding of astronomical objects - using one to advance the other," Ozel said.
"It's a big explosion, and explosions are fun."
Sometimes Neutron Stars and Magnetars Get Too Close to a Black Hole
Magnetars are a special kind of neutron star. They are born rotating very quickly, which causes their magnetic fields to get amplified. But after a few thousand years, their intense magnetic field slows their spin to a more moderate period of one rotation every few seconds. The magnetic fields both inside and outside the star twist, however, and according to the theory, these intense fields can stress and move the crust much like shearing along the San Andreas Fault in California.
The shear moves the crust around along with the magnetic fields tied to the crust, generating twists in the magnetic field that can sometimes break and reconnect in a process that sends trapped positrons and electrons flying out from the star, annihilating each other in a gigantic explosion of X-rays and hard gamma rays.
Every couple of days or so a burst of gamma rays will appear randomly from any direction in the sky. Most of these events last a few tens of seconds. These "long bursts" are thought due to the collapse of a massive star which forms a black hole. Occasionally, however, a much shorter duration event is seen (lasting less than 2 seconds). The origin of these "short bursts" is one of the great unsolved mysteries in astronomy.
Observations made by Swift and other telescopes, including the European Southern Observatory's Very Large Telescope, indicate that some short bursts may be due to the merger of two neutron stars - dense cores of dead stars. These can combine after orbiting each other for perhaps hundreds of millions of years to form a black hole which powers a brief flash of gamma-rays. The light from such an event will decay away very quickly. The new observation suggests that another, rarer mechanism may be involved in the formation of some short bursts in which the objects that merge are a black hole and a neutron star.
Dr. Paul O'Brien from the University of Leicester says "This short burst emitted X-rays for over a day after the bright gamma-ray flash had faded. Multiple X-ray flares were also seen. This all suggests a binary system of a black hole and a dense neutron star was involved. The neutron star, around the mass of the Sun, was literally torn apart after coming too close to the black hole. At peak, the total power output was equivalent to about a million, billion Suns."
Professor Andrew King, also from the University of Leicester says "The black hole shredded the neutron star and either swallowed it in chunks or it formed a disc around the black hole which was then accreted. This could be the first ever observation of a black hole-neutron star binary system."
Data from previous missions is not forgotten though - astronomers from the University of Hertfordshire, have made a new and unexpected discovery. By statistically comparing the distribution of the nine years of short-duration bursts detected by the Compton Gamma Ray Observatory with the distribution of galaxies within about 300 million light years of the Milky Way, they conclude that around 15% originate from these relatively nearby galaxies. This is more than ten times closer than previously thought.
These nearby short bursts, could, like their more distant brethren, result from catastrophic collisions of neutron stars, but if so then their outbursts must be much weaker. Alternatively, they could be a fundamentally different kind of explosion. A prime candidate would be an exotic object called a magnetar, a lone neutron star with a magnetic field 100,000 billion times that of the Earth, tearing itself apart due to enormous magnetic stresses.
An example of such an explosion was seen in 2005 coming from a magnetar in our own galaxy, the Milky Way, so it seems reasonable to expect they should occur occasionally in other galaxies too, said Dr Nial Tanvir from the University of Hertfordshire. If so, they would look very much like short-duration gamma-ray bursts. He continues, "Although we still don't know for sure what produces the short duration gamma-ray bursts, this is a crucial breakthrough in astronomy as knowing where a phenomenon occurs is often the first step towards understanding it."
Since its launch on 20 November 2004, Swift has observed over 100 Gamma Ray Bursts. Swift's power lies in its ability to detect a fast-fading burst and then turn autonomously to point sensitive telescopes at the burst before it has faded. Dr. Julian Osborne, Lead Investigator for Swift at the University of Leicester says "Swift is unique in being able to observe the fading X-ray light from a GRB so quickly after the gamma-ray flash. The accurately determined position in this case was sent to observers on the ground who found the host galaxy for the burst within a few hours. This particular burst occurred four billion light years from Earth."
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