Correlation between the photon index and X-ray luminosity of black hole X-ray binaries and active galactic nuclei: Observations and interpretation

Qi Xiang Yang, Fu Guo Xie, Feng Yuan, Andrzej A. Zdziarski, Marek Gierliński, Luis C. Ho, Zhaolong Yu

    Research output: Contribution to journalArticle

    54 Citations (Scopus)

    Abstract

    We investigate the observed correlation between the 2-10 keV X-ray luminosity (in unit of the Eddington luminosity; lX ≡ LX/LEdd) and the photon index (G{cyrillic}) of the X-ray spectrum for both black hole X-ray binaries (BHBs) and active galactic nuclei (AGNs). We construct a large sample, with 10-9 ≲ lX ≲ 10-1. We find that G{cyrillic} is positively and negatively correlated with lX when lX ≳ 10-3 and 10-6.5 ≲ lX ≲ 10-3, respectively, while G{cyrillic} is nearly a constant when lX ≲ 10-6.5. We explain the above correlation in the framework of a coupled hot accretion flow-jet model. The radio emission always comes from the jet while the X-ray emission comes from the accretion flow and jet when lX is above and below 10-6.5, respectively. More specifically, we assume that with the increase of mass accretion rate, the hot accretion flow develops into a clumpy and further a disc-corona two-phase structure because of thermal instability. We argue that such kind of two-phase accretion flow can explain the observed positive correlation.

    Original languageEnglish
    Pages (from-to)1692-1704
    Number of pages13
    JournalMonthly Notices of the Royal Astronomical Society
    Volume447
    Issue number2
    DOIs
    Publication statusPublished - 21 Feb 2015

    Keywords

    • Accretion
    • Accretion discs
    • Galaxies: active
    • Quasars: general
    • X-rays: binaries
    • X-rays: general

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