Astronomers from the University of Amsterdam, including Yuri Cavecchi and Anna Watts, have observed a neutron star that renders existing models obsolete when it comes to X-ray bursts on such extreme objects. In the case of the neutron star in X-ray binary IGR J17480-2446, the X-ray luminosity in certain parts of the star appears to be far more intensive than in others due to the star’s magnetic field. The results of the study by Yuri Cavecchi et al. (2011) will be published in Astrophysical Journal Letters.
Their research focuses on X-ray binary IGR J17480-2446 (hereafter J17480) in the Terzan 5 globular cluster. X-ray binaries are characterised by a neutron star and a so-called accretor, which revolve around one another. Neutron stars are 1.5 times heavier than the sun, but have a diameter of no more than 25 km. They have a strong gravitational field that attracts gas from the accretor. This gas can accumulate on the neutron star’s surface and explode in a rapid, high-energy nuclear explosion referred to as a Type 1 X-ray burst. In most cases, the entire stellar surface will explode with equal intensity. In about 10 percent of cases, however, some parts of the star will become far more luminous than others. The cause of this phenomenon is still unknown.
Old models rendered obsolete
Various theoretical models have been developed over the past few years in an effort to explain the phenomenon. According to one explanation, the star’s rapid rotation retards the flame front, in the same way that the earth’s rotation contributes to the formation of cyclones (the Coriolis effect). According to another theory, the explosion creates large waves in the star surface. The ‘ocean’ on one side of the star cools down and becomes less luminous, while the other side heats up and becomes more luminous.
These new observations of the J17480 neutron star render both models obsolete. Like other stars, J17480 displays unusually clear spots on its surface during nuclear explosions. However, the star rotates much more slowly than other stars with the same behaviour – a mere 10 times a second (the second slowest star rotates around its own axis 245 times a second). At this speed, the Coriolis effect will not be powerful enough to retard the flame front, preventing the occurrence of thermonuclear ‘cyclones’. The idea of large-scale wave action also fails to provide a solution.
Astronomers now attribute the uneven combustion on J17480 to the star’s magnetic field. As soon as the nuclear explosion starts, the burning gas expands. It moves upwards and outwards, stirring up the magnetic field which acts as a ‘rubber band’ and prevents the fireball from expanding any further. ‘We will need to carry out further follow-up studies to confirm our findings, but we believe this offers a highly plausible explanation for our observations in the case of J17480’, explains main author Yuri Cavecchi.
Co-author Anna Watts emphasises that the new model does not explain irregular combustion in all stars. ‘It only seems to apply to this star – and possibly several others – in which the magnetic field is strong enough to affect the flame front in this manner. The other models may still apply to other stars with the same abnormal behaviour.