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| PRI (MPIfR) 11/2011 (1) | Press Release | November 03, 2011 |
Paulo Freire from the Max Planck Institute for Radio Astronomy in Bonn and his collaborators have discovered the first gamma-ray pulsar in a globular
cluster using the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The pulsar, labelled J1823-3021A, is located in the globular
cluster NGC 6624 in Sagittarius, not far from the direction to the Galactic Center. At a distance of approximately 8.4 kpc (or 27,000 light years) it is also
the most distant pulsar ever detected in gamma rays. Its extreme gamma-ray luminosity implies that is the youngest millisecond pulsar discovered to date
and that its magnetic field is
much larger than previously predicted by pulsar recycling theories. It suggests the existence of a whole new population of such extreme objects forming
at the same rate as the more normal millisecond pulsars.
This exciting result is reported in the latest issue of "Science Express".
Figure 1:
Fermi LAT gamma-ray image of the the globular cluster NGC 6624. Left: Pulsar J1823-3021A is ON; Right: Pulsar is Off. None of the other five known pulsars in this cluster are detectable in gamma rays because of its huge distance.
Credit: P Freire et al., Science Express (Click image for higher resolution).
When the cores of massive stars run out of nuclear fuel they collapse catastrophically, a phenomenon known as a supernova. This spectacular event marks the birth of a neutron star: a ball of neutrons, a single giant atomic nucleus with a radius of about 10-16 km and about half a million times the Earth's mass! A pulsar is a rapidly rotating neutron star for which we can detect pulsations (normally at radio, but now also at gamma-ray wavelengths), modulated by the rotation of the object - like a lighthouse. Ordinary pulsars have rotation periods between 16 milliseconds and 8 seconds. Even faster rotating are the so-called millisecond pulsars, which can have rotation periods as fast as 1.4 milliseconds - corresponding to 43,000 rotations per minute! They are thought to have been spun up by accretion of matter from a companion star, a theory which is supported by the observation that roughly 80% of MSPs are found in binary systems.
Millisecond pulsars possess extraordinary long-term rotational stability, which is in some cases similar to those of the best atomic clocks on Earth. They are basically giant flywheels in space, where nothing disturbs their rotation. They are being used to test Einstein's General Theory of Relativity, search for Gravitational waves and study the properties of the superdense matter at their center.
"We have discovered more than 100 of these objects in globular clusters with radio telescopes", says Paulo Freire from the Max Planck Institute for Radio Astronomy (MPIfR), the lead author of the paper. "Thanks to the sensitivity of the Large Area Telescope on the Fermi satellite we have been able, for the first time, to see one of them in gamma rays." The newly detected gamma-ray pulsar, labeled J1823-3021A, is shown in Fig. 1, taken with the Large Area Telescope onboard Fermi.
Globular clusters are ancient swarms of hundreds of thousands of stars bound together by their mutual gravity. They produce many binary systems of the kind that lead to the formation of millisecond pulsars. One of these clusters is NGC 6624 in Sagittarius (see Fig. 2). At a distance of approx. 8.4 kpc (about 27,000 light years) it is in the proximity of the Galactic center. A total of six pulsars have been discovered in this globular cluster to date, three of these to be announced soon. The first pulsar found in NGC 6624 was J1823-3021A. With a rotation period of 5.44 milliseconds (11,000 rotations/minute) it is the most luminous radio pulsar found in a globular cluster to date. It has been timed since discovery in 1994 with several large radio telescopes, in particular with the Lovell Telescope of the University of Manchester/England and with the radio telescope at Nançay/France.
"To our surprise we found the pulsar to be extremely bright in gamma rays as well", says Damien Parent from the George Mason University resident at Naval Research Laboratory, a leading member of the team that analysed the Fermi data. "Millisecond pulsars were not supposed to be that bright. This implies an unexpectedly high magnetic field for such a fast pulsar."
"This challenges our current theories for the formation of such objects", states Michael Kramer, director at MPIfR and head of the research group "Fundamental Physics in Radio Astronomy". "We are currently investigating a number of possibilities. Nature might even be forming millisecond pulsars in a way we have not anticipated."
"Whichever way these anomalous pulsars are formed; one thing appears to be clear", concludes Paulo Freire: "at least in globular clusters, they are so young that they are probably forming at rates comparable to the large known population of normal millisecond pulsars."
Figure 2:
Globular cluster NGC 6624 in the constellation Sagittarius. Six pulsars have been detected in that cluster.
Three of the discoveries are to be published soon.
Credit: NASA/ESA/I. King (Univ. of California, Berkeley).
(Click image for higher resolution).
Fermi Gamma-ray Space Telescope: NASA’s Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the U.S.
Fermi Large Area Telescope (LAT):
Gamma rays possess so much energy that they can pass straight through a lens or mirror, the main component of a conventional telescope. Instead, Fermi's main instrument, the Large Area Telescope (LAT), operates more like a particle detector. When a gamma ray enters the LAT, it strikes metal foil and converts to a pair of charged particles that continue through the detector. Using 880,000 silicon strips, the LAT tracks the moving particles and allows astronomers to reconstruct the path of the original gamma ray with unprecedented resolution and sensitivity. The LAT images the entire sky every three hours. The LAT detects gamma rays with energy from about 20 million to about 300 billion electron volts (20 MeV to 300 GeV) and sees about 20 percent of the sky at any moment.
Fermi Detection of a Luminous Gamma-ray Pulsar in a Globular Cluster,
P. Freire et al., 2011, Science Express,
Gravitational wave research helps hunt down an elusive new type of pulsar
, MPI for Gravitational Physics, 03 November 2011.
Max-Planck-Institut für Radioastronomie (MPIfR).
Fundamental Physics in Radio Astronomy (Research Group at MPIfR).
Fermi Gamma-ray Space Telescope (NASA).
Center for Earth Observing and Space Research (CEOSR).
Naval Research Laboratory (NRL).
Jodrell Bank Centre for Astrophysics.
Station de Radioastronomie de Nançay.
Dr. Paulo Freire,
Max-Planck-Institut für Radioastronomie.
Fon: +49(0)228-525-496
E-mail: pfreire (at)
mpifr-bonn.mpg.de
Prof. Dr. Michael Kramer,
Director and Head of Research Group "Fundamental Physics in Radio Astronomy",
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49(0)228-525-278
E-mail: mkramer (at)
mpifr-bonn.mpg.de
Dr. Norbert Junkes,
Max-Planck-Institut für Radioastronomie.
Press and Public Outreach,
Fon: +49(0)228-525-399
E-mail: njunkes (at)
mpifr-bonn.mpg.de