A new catalog produced by a French-led international team of astronomers shows that NASA’s Fermi Gamma-ray Space Telescope has discovered 294 gamma-ray-emitting pulsars, while another 34 suspects await confirmation. This is 27 times the number known before the mission launched in 2008.
This visualization shows 294 gamma-ray pulsars, first
plotted on an image of the entire starry sky as seen from Earth and then
transitioning to a view from above our galaxy. The symbols show different types
of pulsars. Young pulsars blink in real time except for the Crab, which pulses
slower than in real time because its rate is only slightly lower than the
video’s frame rate. Millisecond pulsars remain steady, pulsing too quickly to
see. The Crab, Vela, and Geminga were among the 11 gamma-ray pulsars known
before Fermi launched. Other notable objects are also highlighted. Distances
are shown in light-years (abbreviated ly). Download
high-resolution video and images from NASA's Scientific Visualization Studio. Credit: NASA’s Goddard Space Flight Center
“Pulsars touch on a wide range of
astrophysics research, from cosmic rays and stellar evolution to the search for
gravitational waves and dark matter,” said study coordinator David Smith,
research director at the Bordeaux
Astrophysics Laboratory in Gironde, France, which is part of CNRS (the French National Center for Scientific Research). “This new
catalog compiles full information on all known gamma-ray pulsars in an effort
to promote new avenues of exploration.”
The catalog was published on Monday, Nov. 27, in The Astrophysical Journal Supplement.
Narrow beams of energy emerge from hot spots on the
surface of a neutron star in this artist's concept. When one of these beams
sweeps past Earth, astronomers detect a pulse of light. Credit: NASA's Goddard
Space Flight Center Conceptual Image Lab
Pulsars are a type of neutron star,
the city-sized leftover of a massive sun that has exploded as a supernova.
Neutron stars, containing more mass than our Sun in a ball less than 17 miles
wide, represent the densest matter astronomers can study directly. They possess strong magnetic fields, produce streams
of energetic particles, and spin quickly – 716 times a second for the fastest
known. Pulsars, in addition, emit narrow beams of energy that swing
lighthouse-like through space as the objects rotate. When one of these beams
sweeps past Earth, astronomers detect a pulse of emission.
The new catalog represents the work
of 170 scientists across the globe. A dozen radio telescopes carry out regular
monitoring of thousands of pulsars, and radio astronomers search for new
pulsars within gamma-ray sources discovered by Fermi. Other researchers have
teased out gamma-ray pulsars that have no radio counterparts through millions
of hours of computer calculation, a process called a blind search.
"More than 15 years after its launch, Fermi remains an incredible discovery
machine, and pulsars and their neutron star kin are leading the way.
ELIZABETH HAYS
Fermi Project Scientist
Of the 3,400 pulsars known, most of them observed via radio waves and located within our Milky Way galaxy, only about 10% also pulse in gamma rays, the highest-energy form of light. Visible light has energies between 2 and 3 electron volts. Fermi’s Large Area Telescope can detect gamma rays with billions of times this energy, and other facilities have observed emission thousands of times greater still from the nearby Vela pulsar, the brightest persistent source in the sky for Fermi.
This movie shows the Vela pulsar in gamma rays
detected by the Large Area Telescope aboard NASA's Fermi observatory. A single
pulsar cycle is repeated. Bluer colors indicate gamma rays with higher
energies. Credit: NASA/DOE/Fermi LAT Collaboration
The Vela pulsar and its famous sibling in the Crab Nebula are young, solitary objects, formed about 11,000
and 970 years ago, respectively. Their emissions arise as their magnetic fields
spin through space, but this also gradually slows their rotation. The younger
Crab pulsar spins nearly 30 times a second, while Vela clocks in about a third
as fast.
The Old and the Restless
Paradoxically, though, pulsars that
are thousands of times older spin much faster. One example of these so-called
millisecond pulsars (MSPs) is J1824-2452A. It whirls around 328 times a second
and, with an age of about 30 million years, ranks among the youngest MSPs
known.
Thanks to a great combination of
gamma-ray brightness and smooth spin slowdown, the MSP J1231-1411 is an ideal
“timer” for use in gravitational wave searches. By monitoring a collection of
stable MSPs, astronomers hope to link timing changes to passing low-frequency gravitational waves – ripples in space-time – that cannot be
detected by current gravitational observatories. It was discovered in one of
the first radio searches targeting Fermi gamma-ray sources not associated with
any known counterpart at other wavelengths, a technique that turned out to be
exceptionally successful.
"Before Fermi, we didn't know
if MSPs would be visible at high energies, but it turns out they mostly radiate
in gamma rays and now make up fully half of our catalog," said co-author
Lucas Guillemot, an associate astronomer at the Laboratory
of Physics and Chemistry of the Environment and Space and the University of Orleans, France.
Along Come the Spiders
The presence of MSPs in binary
systems offers a clue to understanding the age-spin paradox. Left to itself, a
pulsar’s emissions slow it down, and with slower spin its emissions dim. But if
closely paired with a normal star, the pulsar can pull a stream of matter from
its companion that, over time, can spin up the pulsar.
“Spider” systems offer a glimpse of
what happens next. They’re classified as redbacks or black widows – named for spiders known for consuming their mates.
Black widows have light companions (less than about 5% of the Sun's mass),
while redbacks have heavier partners. As the pulsar spins up, its emissions and
particle outflows become so invigorated that – through processes still poorly
understood – it heats and slowly evaporates its companion. The most energetic
spiders may fully evaporate their partners, leaving only an isolated MSP
behind.
J1555-2908 is a black widow with a
surprise – its gravitational web may have ensnared a passing planet. An analysis of 12 years of Fermi data reveals
long-term spin variations much larger than those seen in other MSPs. “We think
a model incorporating the planet as a third body in a wide orbit around the
pulsar and its companion describes the changes a little better than other
explanations, but we need a few more years of Fermi observations to confirm
it,” said co-author Colin Clark, a research group leader at the Max Planck Institute for Gravitational Physics in Hannover, Germany.
Other curious binaries include the so-called transitional pulsars, such as J1023+0038, the first identified. An erratic stream of gas flowing from the companion to the neutron star may surge, suddenly forming a disk around the pulsar that can persist for years. The disk shines brightly in optical light, X-rays, and gamma rays, but pulses become undetectable. When the disk again vanishes, so does the high-energy light and the pulses return.
This artist's concept illustrates a possible model for
the transitional pulsar J1023. When astronomers can detect pulses in radio
(green), the pulsar's energetic outflow holds back its companion's gas stream.
Sometimes the stream surges, creating a bright disk around the pulsar that can
persist for years. The disk shines brightly in X-rays, and gas reaching the
neutron star produces jets that emit gamma rays (magenta), obscuring the pulses
until the disk eventually dissipates. Credit: NASA's Goddard Space Flight
Center
Some pulsars don’t require a
partner to switch things up. J2021+4026, a young, isolated pulsar located about
4,900 light-years away, underwent a puzzling “mode change” in 2011, dimming its
gamma rays over about a week and then, years later, slowly returning to its
original brightness. Similar behavior had been seen in some radio pulsars, but
this was a first in gamma rays. Astronomers suspect the event may have been
triggered by crustal cracks that temporarily changed the pulsar‘s magnetic
field.
Farther afield, Fermi discovered
the first gamma-ray pulsar in another galaxy, the neighboring Large Magellanic Cloud, in 2015. And
in 2021, astronomers announced the discovery of a giant gamma-ray flare from a different type of neutron star (called a
magnetar) located in the Sculptor galaxy, about 11.4 million light-years away.
“More than 15 years after its
launch, Fermi remains an incredible discovery machine, and pulsars and their
neutron star kin are leading the way,” said Elizabeth Hays, the mission’s
project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Explore the Fermi gamma-ray pulsar catalog on WorldWide Telescope
By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Source: NASA’s Fermi Mission Nets 300 Gamma-Ray Pulsars … and Counting - NASA Science
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