Stars prove their magnetic attraction
Sep 16, 2002
Astronomers have found new evidence for the existence of stars called "magnetars" that draw their energy from magnetic fields. Fotis Gavriil of McGill University in Canada and colleagues believe that anomalous X-ray pulsars are powered by the decay of an ultra-high magnetic field. These kind of pulsars are known to be neutron stars but the source of their energy has long remained a mystery. Gavriil and colleagues discovered that two bursts of X-rays from one such pulsar resemble the radiation bursts from another kind of neutron star thought to be a magnetar, a so-called soft-gamma-ray repeater (F Gavriil et al. 2002 Nature 419 142).
Physicists have known for many decades that ordinary stars are powered by nuclear fusion, and have discovered other types of star that rely on alternative sources of energy, such as gravity, radioactivity or rotation. In addition, many researchers now believe that soft-gamma-ray repeaters are powered by magnetism. It is thought that these spinning objects require a high magnetic field because their period of rotation rapidly increases over time and because they generate intense bursts of gamma-rays.
Researchers have suggested that anomalous X-ray pulsars are also powered by magnetic fields because, unlike other X-ray pulsars, they are not accompanied by companion stars from which they can accrete matter. They also do not have sufficient rotational energy to power their emission. They rotate at about the same rate as soft-gamma-ray repeaters, are also located in the galactic plane and have similar X-ray spectra. To date, however, they have not been observed to produce bursts of radiation as soft-gamma-ray repeaters do.
Now, after studying routine data from the Rossi X-ray Timing Explorer satellite, Gavriil and colleagues have discovered two X-ray bursts with properties similar to those of soft-gamma-ray repeaters that originate from the direction of an anomalous X-ray pulsar called 1E 1048.1-5937. They also found a feature in the spectrum of this object that is probably caused by the gyration of protons in an intense magnetic field. If confirmed, this and a similar feature recently found in the spectrum of a soft-gamma-ray repeater would provide direct evidence of strong magnetic fields surrounding these objects.
The researchers propose that the bursts from anomalous X-ray pulsars are harder to detect than those from soft-gamma-ray repeaters because the former are young stars with more malleable outer layers, or crusts. The more malleable the crust, they argue, the more difficult it is to support the strong magnetic fields that give rise to radiation bursts.
Magnetars are thought to have magnetic fields of at least 1014 Gauss, some billion times stronger than the largest sustained magnetic fields created artificially on Earth. Such strong fields could be a boon to physicists as they might give rise to strange effects predicted by quantum electrodynamics.
About the author
Edwin Cartlidge is News Editor of Physics World