A comment on the colour-colour diagrams of low-mass X-ray binaries

Marek Gierliński (Lead / Corresponding author), Chris Done

Research output: Contribution to journalReview articlepeer-review

55 Citations (Scopus)


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 present analysis of three transient atoll sources showing that there is an additional branch in the colour-colour diagram of atoll sources which appears at very low luminosities. This new branch connects to the top of previously known C-shaped (atoll) path, forming a horizontal track where the average source flux decrease from right to left. This turns the C-shape into a Z. Thus both atolls and Z sources share the same topology on the colour-colour diagram and evolve in similar way, as a function of increasing averaged mass accretion rate. This strongly favours models in which the underlying geometry of these sources changes in similar ways. A possible scenario is one where the truncated disc approaches the neutron star when the accretion rate increases, but in the atolls the disc is truncated by evaporation (similarly to black holes), and in the Z sources it is truncated by the magnetic field.

Original languageEnglish
Pages (from-to)L47-L50
Number of pages4
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 21 Apr 2002


  • Accretion, accretion discs
  • X-rays: binaries

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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