Analysing the atolls: X-ray spectral transitions of accreting neutron stars

Abstract

Неге we investigate disc accreting neutron stars with low magnetic fields (atoll sources) by systematically analysing all the available X-ray spectra of these sources from the Rossi X- ray Timing Explorer database, provided by the High Energy Astrophysics Science Archive Research Center. From these we show that all atoll sources show broadly similar spectral evolution as a function of mass accretion rate with hard spectra at low luminosities switching to soft spectra at high luminosities. This is similar to the well known hard/soft spectral dichotomy in black holes. However, we also show that there are subtle but significant differences in the hard/soft transitions for individual atoll sources. There are two different types of hard/soft transition, those where the spectrum softens at all energies, leading to a diagonal track on a colour-colour diagram, and those where only the higher energy spectrum softens, giving a vertical track. The luminosity at which the transition occurs is correlated with this spectral behaviour, with the vertical transition at L/L(_Edd) ~ 0.02 while the diagonal one is at ~ 0.1. Superimposed on this is the well known hysteresis effect, but we show that classic, large scale hysteresis occurs only in the outbursting sources, indicating that its origin is in the dramatic rate of change of mass accretion rate during the disc instability. We show that the long term mass accretion rate correlates with the transition behaviour, and speculate that this is due to the magnetic field being able to emerge from the neutron star surface for low average mass accretion rates. While this is not strong enough to collimate the flow except in the millisecond pulsars, its presence may affect the inner accretion flow through changing the jet properties.