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These artist's renderingsshow 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. Image Credit: NASA's Goddard Space Flight Center
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.
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
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 for short -- 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.
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
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
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." Related Links: