Astronomers have discovered a vast cloud of high-energy particles called a wind nebula around a rare ultra-magnetic neutron star, or magnetar, for the first time. The find offers a unique window into the properties, environment and outburst history of magnetars, which are the strongest magnets in the universe.
A neutron star is the crushed core of a massive star that ran out of
fuel, collapsed under its own weight, and exploded as a supernova. Each
one compresses the equivalent mass of half a million Earths into a ball
just 12 miles (20 kilometers) across, or about the length of New York's
Manhattan Island. Neutron stars are most commonly found as pulsars,
which produce radio, visible light, X-rays and gamma rays at various
locations in their surrounding magnetic fields. When a pulsar spins
these regions in our direction, astronomers detect pulses of emission,
hence the name.
This illustration compares the size of a neutron
star to Manhattan Island in New York, which is about 13 miles long. A
neutron star is the crushed core left behind when a massive star
explodes as a supernova and is the densest object astronomers can
directly observe. Credits: NASA's Goddard Space Flight Center
Typical pulsar magnetic fields can be 100 billion to 10 trillion
times stronger than Earth's. Magnetar fields reach strengths a thousand
times stronger still, and scientists don't know the details of how they
are created. Of about 2,600 neutron stars known, to date only 29 are classified as magnetars.
The newfound nebula surrounds a magnetar known as Swift J1834.9-0846 -- J1834.9 for short -- which was discovered by NASA's Swift satellite
on Aug. 7, 2011, during a brief X-ray outburst. Astronomers suspect the
object is associated with the W41 supernova remnant, located about
13,000 light-years away in the constellation Scutum toward the central
part of our galaxy.
"Right now, we don't know how J1834.9 developed and continues to
maintain a wind nebula, which until now was a structure only seen around
young pulsars," said lead researcher George Younes, a postdoctoral
researcher at George Washington University in Washington. "If the
process here is similar, then about 10 percent of the magnetar's
rotational energy loss is powering the nebula’s glow, which would be the
highest efficiency ever measured in such a system."
A month after the Swift discovery, a team led by Younes took another look at J1834.9 using the European Space Agency's (ESA) XMM-Newton X-ray observatory,
which revealed an unusual lopsided glow about 15 light-years across
centered on the magnetar. New XMM-Newton observations in March and
October 2014, coupled with archival data from XMM-Newton and Swift,
confirm this extended glow as the first wind nebula ever identified
around a magnetar. A paper describing the analysis will be published by The Astrophysical Journal.
"For me the most interesting question is, why is this the only
magnetar with a nebula? Once we know the answer, we might be able to
understand what makes a magnetar and what makes an ordinary pulsar,"
said co-author Chryssa Kouveliotou, a professor in the Department of
Physics at George Washington University’s Columbian College of Arts and
Sciences.
The most famous wind nebula, powered by a pulsar less than a thousand years old, lies at the heart of the Crab Nebula supernova remnant in the constellation Taurus. Young pulsars like this
one rotate rapidly, often dozens of times a second. The pulsar's fast
rotation and strong magnetic field work together to accelerate electrons
and other particles to very high energies. This creates an outflow
astronomers call a pulsar wind that serves as the source of particles
making up in a wind nebula.
"Making a wind nebula requires large particle fluxes, as well as some
way to bottle up the outflow so it doesn't just stream into space,"
said co-author Alice Harding, an astrophysicist at NASA's Goddard Space
Flight Center in Greenbelt, Maryland. "We think the expanding shell of
the supernova remnant serves as the bottle, confining the outflow for a
few thousand years. When the shell has expanded enough, it becomes too
weak to hold back the particles, which then leak out and the nebula
fades away." This naturally explains why wind nebulae are not found
among older pulsars, even those driving strong outflows.
A pulsar taps into its rotational energy to produce light and
accelerate its pulsar wind. By contrast, a magnetar outburst is powered
by energy stored in the super-strong magnetic field. When the field
suddenly reconfigures to a lower-energy state, this energy is suddenly
released in an outburst of X-rays and gamma rays. So while magnetars may
not produce the steady breeze of a typical pulsar wind, during
outbursts they are capable of generating brief gales of accelerated
particles.
"The nebula around J1834.9 stores the magnetar's energetic outflows
over its whole active history, starting many thousands of years ago,"
said team member Jonathan Granot, an associate professor in the
Department of Natural Sciences at the Open University in Ra'anana,
Israel. "It represents a unique opportunity to study the magnetar's
historical activity, opening a whole new playground for theorists like
me."
ESA's XMM-Newton satellite was launched on Dec. 10, 1999, from
Kourou, French Guiana, and continues to make observations. NASA funded
elements of the XMM-Newton instrument package and provides the NASA Guest Observer Facility at Goddard, which supports use of the observatory by U.S. astronomers.
Editor: Ashley Morrow
