Explore Fermi's discovery of the first gamma-ray pulsar detected in a galaxy other than our own.
Credits: NASA's Goddard Space Flight Center.
The pulsar lies in the outskirts of the Tarantula Nebula in the Large
Magellanic Cloud, a small galaxy that orbits our Milky Way and is
located 163,000 light-years away. The Tarantula Nebula is the largest,
most active and most complex star-formation region in our galactic
neighborhood. It was identified as a bright source of gamma rays, the
highest-energy form of light, early in
the Fermi mission. Astronomers initially attributed this glow to
collisions of subatomic particles accelerated in the shock waves
produced by supernova explosions.
"It's now clear that a single pulsar, PSR J0540-6919, is responsible
for roughly half of the gamma-ray brightness we originally thought came
from the nebula," said lead scientist Pierrick Martin, an astrophysicist
at the National Center for Scientific Research (CNRS) and the Research
Institute in Astrophysics and Planetology in Toulouse, France. "That is a
genuine surprise."
NASA's Fermi Gamma-ray Space Telescope has detected
the first extragalactic gamma-ray pulsar, PSR J0540-6919, near the
Tarantula Nebula (top center) star-forming region in the Large
Magellanic Cloud, a satellite galaxy that orbits our own Milky Way.
Fermi detects a second pulsar (right) as well but not its pulses. PSR
J0540-6919 now holds the record as the highest-luminosity gamma-ray
pulsar. The angular distance between the pulsars corresponds to about
half the apparent size of a full moon. Background: An image of the
Tarantula Nebula and its surroundings in visible light.Credits: NASA's Goddard Space Flight Center; background: ESO/R. Fosbury (ST-ECF). Hi-res image
A gamma-ray view of the same region shown above in visible wavelengths. Lighter colors indicate greater numbers of gamma rays with energies between 2 and 200 billion electron volts. For comparison, visible light ranges between 2 and 3 electron volts. The two pulsars, PSR J0540−6919 (left) and PSR J0537−6910, clearly stand out.
Credits: NASA/DOE/Fermi LAT Collaboration. Hi-res image
When a massive star explodes as a supernova, the star's core may survive
as a neutron star, where the mass of half a million Earths is crushed
into a magnetized ball no larger than Washington, D.C. A young isolated
neutron star spins tens of times each second, and its rapidly spinning
magnetic field powers beams of radio waves, visible light, X-rays and
gamma rays. If the beams sweep past Earth, astronomers observe a regular
pulse of emission and the object is classified as a pulsar.
The Tarantula Nebula was known to host two pulsars, PSR J0540-6919 (J0540 for short) and PSR J0537−6910 (J0537), which were discovered with the help of NASA's Einstein and Rossi X-ray Timing Explorer (RXTE) satellites, respectively. J0540 spins just under 20 times a second, while J0537 whirls at nearly 62 times a second -- the fastest-known rotation period for a young pulsar.
The Tarantula Nebula was known to host two pulsars, PSR J0540-6919 (J0540 for short) and PSR J0537−6910 (J0537), which were discovered with the help of NASA's Einstein and Rossi X-ray Timing Explorer (RXTE) satellites, respectively. J0540 spins just under 20 times a second, while J0537 whirls at nearly 62 times a second -- the fastest-known rotation period for a young pulsar.
Nevertheless, it took more than six years of observations by Fermi's
Large Area Telescope (LAT), as well as a complete reanalysis of all LAT
data in a process called Pass 8, to detect gamma-ray pulsations from
J0540. The Fermi data establish upper limits for gamma-ray pulses from
J0537 but do not yet detect them.
Martin and his colleagues present these findings in a paper to be published in the Nov. 13 edition of the journal Science.
"The gamma-ray pulses from J0540 have 20 times the intensity of the
previous record-holder, the pulsar in the famous Crab Nebula, yet they
have roughly similar levels of radio, optical and X-ray emission," said
coauthor Lucas Guillemot, at the Laboratory for Physics and Chemistry of
Environment and Space, operated by CNRS and the University of Orléans
in France. "Accounting for these differences will guide us to a better
understanding of the extreme physics at work in young pulsars."
J0540 is a rare find, with an age of roughly 1,700 years, about twice
that of the Crab Nebula pulsar. By contrast, most of the more than
2,500 known pulsars are from 10,000 to hundreds of millions of years
old.
Despite J0540's luminosity, too few gamma rays reach the LAT to
detect pulsations without knowing the period in advance. This
information comes from a long-term X-ray monitoring campaign using RXTE,
which recorded both pulsars from the start of the Fermi mission to the
end of 2011, when RXTE operations ceased.
"This campaign began as a search for a pulsar created by SN 1987A,
the closest supernova seen since the invention of the telescope," said
co-author Francis Marshall, an astrophysicist at NASA's Goddard Space
Flight Center in Greenbelt, Maryland. "That search failed, but it
discovered J0537."
Prior to the launch of Fermi in 2008, only seven gamma-ray pulsars were known. To date, the mission has found more than 160.
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and
particle physics partnership, developed in collaboration with the U.S.
Department of Energy and with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.
