| Astronomers
at Columbia University used the Rossi X-ray Timing Explorer to find
the youngest pulsar yet - a hot, spinning, highly-magnetized infant
no more than ten miles across, born in a massive star explosion about
700 years ago.
by Imagine the Universe @ NASA.
Other known pulsars, in comparison, are thousands to millions of
years old. This pulsar, in the supernova remnant Kes 75, is about
300 years younger than the Crab pulsar, which used to be the youngest
known. And this baby is full of surprises too.
Scientists have long considered the Crab to be the archetypal young
pulsar. Yet, compared to the Crab, the Kes 75 pulsar spins ten times
more slowly; it is slowing down at a rate ten times faster; and
it has a magnetic field that is ten times greater - all unexpected
findings that may cause scientists to rethink the birth properties
and evolution of pulsars.
"People
have been searching for this pulsar for years," said Dr. Eric Gotthelf,
the Columbia astronomer who spotted the pulsar with NASA's Rossi X-ray
Timing Explorer (RXTE). "We have seen bright radio waves in the core
of Kes 75, telltale evidence for a pulsar. The problem was that we
were expecting to find a rapidly spinning 'Crab-like' pulsar. What
we found instead is quite different from our expectations."
A pulsar is a type of neutron star, the core remains of a giant
star that was once at least ten times more massive than the sun.
Such a star, upon depleting all of its nuclear fuel, explodes, ejecting
most of its outer shell and leaving only a core, which collapses
upon itself to form a hot ember about 10 miles in diameter. The
ejected material forms a beautiful supernova remnant of colorful
gas, visible for several millennia in visible light and at X-ray
and radio wavelengths.
NASA An
image of a pulsating neuton star
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Pulsars have strong magnetic fields that channel electrons along
the magnetic field lines away from the pulsar's polar regions. These
electrons, accelerated to nearly the speed of light, radiate at
the poles, particularly as radio waves, X rays and gamma-rays. Astronomers
see pulsars "pulse" on and off as the beams of radiation from the
rotating pulsar sweep across the Earth, like a beam from a lighthouse.
Locating the pulsar associated with a given supernova remnant is
not so straightforward. When the star explodes, the shock might
kick the resulting pulsar away from the site of the explosion. Other
times, the pulsar's beams of light might not sweep past the direction
of the Earth, so it is never seen. Or, as is suspected in recent
1987a supernova, the stellar core might collapse into a black hole.
"This pulsar is located right in the center of the supernova shell,"
said Gotthelf. "We only see this for a few other pulsars. So, the
Kes 75 pulsar provides rather strong observational evidence that
the neutron star is born in the stellar explosion that gives rise
to the supernova remnant shell."
X-ray data reveals that the Kes 75 pulsar spins upon its axis once
every 0.3 second, which is slow for young pulsars but fast compared
to older ones. The pulsar's age is 700 years, consistent with the
age of the supernova remnant that houses it.
NASA
The
Kes 75 pulsar
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The Kes 75 pulsar is also at least 10 times more magnetic than
ordinary pulsars but about 10 times weaker than magnetars, a mysterious,
newly identified class of highly magnetized objects. The Kes 75
pulsar may therefore be a missing link between these classes of
objects. The pulsar's magnetic field is 100 trillion times that
of the Earth's and would distort compass readings a half million
miles away. Such a strong magnetic force is actually rapidly slowing
down the spin of the pulsar.
Younger, highly magnetized, rapidly spinning pulsars provide astronomers
with a wealth of information about this strange class of objects,
first discovered only 33 years ago. Young pulsars are prone to starquakes,
which changes the spin rate and reveals clues to their internal
structure. Monitoring their change in spin rate allows astronomers
to measure the rotational energy loss and compare it to the observed
radiation. This provides a powerful probe of this central engine
powering bright radio cores of supernova remnants.
Gotthelf found the newly born pulsar, now named PSR J1846-0258,
through a deep observation with RXTE of the area around Kes 75 lasting
nearly three hours. He narrowed his search to a central region within
the Kes 75 supernova remnant using archived X-ray data from another
satellite, the Japan-US Advanced Satellite for Cosmology and Astrophysics.
The pulsar is about 60,000 light years from Earth on the far side
of the Milky Way galaxy.
The Rossi X-ray Timing Explorer is a resourceful X-ray telescope
with a large photon collecting area and ability to measure millisecond
fluctuations in X-ray signals. This makes it a prime and unique
instrument to detect and study rapidly spinning objects - pulsars,
neutron stars and black holes.
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