An unexpected drop in the magnetic field of the X-ray pulsar V0332+53 after the bright outburst occurred in 2015

Abstract

How the accreted mass settling on the surface of a neutron star affects the topology of themagnetic field and howthe secular evolution of the binary system depends on the magnetic fieldchange is still an open issue.We report evidence for a clear drop in the observed magnetic fieldin the accreting pulsar V0332+53 after undergoing a bright 3-month long X-ray outburst. Wedetermine the field from the position of the fundamental cyclotron line in its X-ray spectrumand relate it to the luminosity. For equal levels of luminosity, in the declining phase wemeasurea systematically lower value of the cyclotron line energy with respect to the rising phase. Thisresults in a drop of ~1.7 × 1011 G of the observed field between the onset and the end of theoutburst. The settling of the accreted plasma on to the polar cap seems to induce a distortion ofthe magnetic field lines weakening their intensity along the accretion columns. Therefore, thedissipation rate of the magnetic field could bemuch faster than previously estimated, unless thefield is able to restore its original configuration on a time-scale comparable with the outburstsrecurrence time.