A pulsar’s changing magnetic field

The Vela X-1 binary consists of a massive blue star and a spinning neutron star. The pair are close enough that the neutron star’s gravity draws in material from its companion’s powerful wind. Because of the neutron star’s intense magnetic field, the material is funneled onto the star’s poles. In the process, the material gets so hot that it emits x rays, which are pulsed because the magnetic and rotation axes are misaligned. NASA’s Swift has been monitoring Vela X-1 since the orbiter’s launch in 2004. That long train of data has given Valentina La Parola of Italy’s National Institute for Astrophysics and her colleagues the opportunity to investigate changes in the neutron star’s magnetic field. Specifically, they tracked a cyclotron absorption line at around 54 keV; the line arises, as in the quantum Hall effect, from the quantization of energy levels in a strong magnetic field. The researchers found that shifts in the line’s energy were positively correlated with fluctuations in x-ray luminosity. Two mechanisms could account for the correlation. When the wind is strong and more material falls onto the poles, the luminosity rises and the x-ray-emitting region is pushed down into a region of higher magnetic field. Alternatively, the higher the luminosity, the more radiation pressure slows the infalling material and the smaller the line’s Doppler redshift. La Parola also found that since 2004 the line’s energy and, therefore, the magnetic field at the poles have been steadily dropping, independently of changes in luminosity. Distortion or displacement of the field lines by piled up material is a possible cause. (V. La Parola et al., Mon. Not. R. Astron. Soc., in press, doi:10.1093/mnras/stw1915 .)