A comment on the colour-colour diagrams of Low Mass X-ray Binaries
Abstract
Disc accreting neutron stars come in two distinct varieties, atolls and Z sources, named after their differently shaped tracks on a colour-colour diagram as the source luminosity changes. Here we show that the atoll systems are poorly named. On a colour-colour diagram they do not trace out an atoll (or C) shaped path. They trace out a Z. Thus they evolve with increasing mass accretion rate in very similar ways to the Z sources! This strongly favours models in which the underlying geometry of these sources changes in similar ways. A likely scenario is one where the transition between the upper and lower branch of the Z in both sets of sources is caused at the point where a truncated inner disc finally penetrates down to the neutron star surface, but that in the `atolls' the disc is truncated by evaporation whereas in the Z sources it is truncated by the magnetic field.Keywords
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