In this artist's impression, clouds of charged particles move along the pulsar's magnetic field lines (blue) and create a lighthouse-like beam of gamma rays (purple).
Credit: NASA
LONDON: An uncharacteristically strong magnetic field in a globular cluster has been detected, confirming the presence of a previously unknown, extremely powerful luminous gamma-ray pulsar.
Publishing today in Science, researchers using the Fermi Large Area Telescope (LAT) in the U.S. said the detection of this millisecond pulsar (MSP), which whirls at 43,000 revolutions per minute, will improve our understanding of the physics of hard-to-detect dense matter and magnetic forces in the galactic field. The new MSP has been named PSR J1823-3021A.
"The glow from a globular cluster is like a chorus of singers each singing a different song in a different key, and we can't pick out the individual voices. But PSR J1823-3021A is different: it's acting more like a solo artist, singing all the gamma-ray songs for its host cluster. The fact that its song can be heard so clearly over such a great distance is amazing," said co-author Tyrel Johnson, National Research Council research associate resident at the Naval Research Laboratory in Washington.
"Understanding how this MSP 'diva' was formed, with its uncharacteristically strong magnetic field and bright gamma-ray pulsations, will improve our understanding of the physics of ... magnetic fields in regimes we can't explore in a laboratory on Earth."
The lighthouse effect
A pulsar (or pulsating star) is a highly magnetised neutron star that emits an electromagnetic radiation in its process of rotation. The discovery of pulsar J1823-3021A in the globular cluster NGC 6624 is the most significant finding since the launch of the Fermi-LAT in 2008, confirming that this particular millisecond pulsar (MSP) is a bright gamma-ray source.
Globular clusters appear to contain some of the first stars to be produced in the galaxy, although their origin and role remain unclear. Gamma-ray pulsars have been detected previously, indeed some as long ago as 1665, but until the NGC 6624 were not detected solely by the presence of gamma-rays.
Millisecond pulsars have a rotational period in the range of about 1-10 milliseconds, and are detected by the Fermi-LAT as a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointed towards Earth, a process that is called the lighthouse effect, mimicking the 'pulsed' effect of the pulsar.
Singing and dancing corpse
Many MSPs are found in globular clusters - spherical collections of stars tightly bound by gravity and orbiting around a galactic core. Found in the halo of the galaxy, they are much older and consist of far more stars than the less dense 'open cluster', which are found closer to Earth in the disk. There are around 158 currently known globular clusters in the Milky Way.
J1823-3021A is the youngest MSP ever detected, and, according to Bruce Allen, director at the Max Planck Institute for Gravitational Physics in Germany, who was not involved in the study, is particularly significant because, "globular clusters are supposed to be the graveyards for old, dead millisecond pulsars. Imagine the surprise of visiting the mausoleum and instead of finding dusty bones you find the 'corpse' singing and dancing."
The gamma-ray 'burst', usually detected by long duration gamma-ray pulses that range from 1.4 milliseconds to 8.5 seconds, were, until the launch of the Fermi-LAT, too faint to be detected individually.
