This artist's concept shows a pulsar, which is like a lighthouse, as its
light appears in regular pulses as it rotates.
Image credit:
NASA/JPL-Caltech. › Full image and caption
Like anthropologists piecing together the human family tree,
astronomers have found that a misfit "skeleton" of a star may link two
different kinds of stellar remains. The mysterious object, called PSR
J1119-6127, has been caught behaving like two distinct objects -- a
radio pulsar and a magnetar -- and could be important to understanding
their evolution.
A radio pulsar is type of a neutron star -- the extremely dense
remnant of an exploded star -- that emits radio waves in predictable
pulses due to its fast rotation. Magnetars, by contrast, are rabble
rousers: They have violent, high-energy outbursts of X-ray and gamma ray
light, and their magnetic fields are the strongest known in the
universe.
"This neutron star wears two different hats," said Walid Majid,
astrophysicist at NASA's Jet Propulsion Laboratory, Pasadena,
California. "Sometimes it's a pulsar. Sometimes it's a magnetar.
This
object may tell us something about the underlying mechanism of pulsars
in general."
Since the 1970s, scientists have treated pulsars and magnetars as two
distinct populations of objects.
But in the last decade, evidence has
emerged that these could be stages in the evolution of a single object.
Majid's new study, combined with other observations of the object,
suggests that J1119 could be in a never-before-seen transition state
between radio pulsar and magnetar. The study
was published in the Jan. 1 issue of Astrophysical Journal Letters, and
was presented this week at the American Astronomical Society meeting in
Grapevine, Texas.
"This is the final missing link in the chain that connects pulsars
and magnetars," said Victoria Kaspi, astrophysicist at McGill University
in Montreal, Canada. "It seems like there's a smooth transition between
these two kinds of neutron star behaviors."
When this mysterious object was discovered in 2000, it appeared to be
a radio pulsar. It was mostly quiet and predictable until July 2016,
when NASA's Fermi and Swift space observatories observed two X-ray
bursts and 10 additional bursts of light at lower energies coming from
the object, as reported in a study in the Astrophysical Journal Letters led by Ersin Gogus. An additional 2016 study
in the same journal, led by Robert Archibald, also looked at the two
X-ray bursts, incorporating observations from NASA's NuSTAR (Nuclear
Spectroscopic Telescope Array) telescope. This study also suggested that
the pulsar was behaving rebelliously -- like a magnetar.
When the outbursts happened, Kaspi excitedly emailed astrophysicist
Tom Prince at JPL/Caltech in Pasadena, telling him this would be a good
object to study from the southern hemisphere. Prince, Majid and
colleagues used the NASA Deep Space Network 70-meter radio telescope in
Canberra, Australia -- the largest dish in the southern hemisphere -- to
see what was going on.
"We think these X-ray bursts happened because the object's enormous
magnetic field got twisted as the object was spinning," Majid said.
The stress of a twisting magnetic field is so great that it causes
the outer crust of the neutron star to break -- analogous to tectonic
plates on Earth causing earthquakes. This causes an abrupt change in
rotation, called a "glitch," which has been measured by NuSTAR.
Neutron stars are so dense that one teaspoon weighs as much as a
mountain. The star's crust, roughly 0.6 miles (1 kilometer) thick, with
higher pressure and density at greater depths, is a neutron-rich
lattice. This particular neutron star is thought to have one of the
strongest magnetic fields among the population of known pulsars: a few
trillion times stronger than the magnetic field of the sun.
Two weeks after the X-ray outburst, Majid and colleagues tracked the
object's emissions at radio frequencies, which are much lower in energy
than X-rays. The radio emissions had sharp increases and decreases in
intensity, allowing scientists to quantify how the emission evolved.
Researchers used an instrument, which they informally call a "pulsar
machine," that was recently installed at the same DSN dish in Australia.
"Within 10 days, something completely changed in the pulsar," Majid
said. "It had started behaving like a normal radio pulsar again."
The question remains: Which came first, the pulsar or the magnetar?
Some scientists argue that objects like J1119 begin as magnetars and
gradually stop outbursting X-rays and gamma rays over time. But others
propose the opposite theory: that the radio pulsar comes first and, over
time, its magnetic field emerges from the supernova's rubble, and then
the magnetar-like outbursts begin. But, just as babies grow to be adults
and not vice versa, there is likely a single path for these objects to
take.
To help solve this mystery, much as anthropologists study the remains
of human ancestors at different stages of evolutionary history,
astronomers want to find more "missing link" objects like J1119. This
particular object was likely formed following a supernova 1,600 years
ago. Monitoring similar objects may shed light on whether this
phenomenon is specific to J1119, or whether this behavior is common in
this class of objects.
Astronomers continue to monitor J1119 as well. Majid and colleagues
observed in December a marked brightening of emissions at radio
wavelengths, in a pattern consistent with other magnetars.
"Our recent observations show that this object contains a bit of the
'astrophysical DNA' of two different families of neutron stars," Prince
said. "We are looking forward to finding other examples of this type of
transitional object."
JPL, a division of Caltech, manages the Deep Space Network for NASA.
News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Source: JPL-Caltech
