Evidence for a compact jet dominating the broad-band spectrum of the black hole accretor XTE J1550-564

Dave Russell, D. Maitra, R. J.H. Dunn, S. Markoff

    Research output: Contribution to journalArticle

    Abstract

    The black hole X-ray binary XTE J1550-564 was monitored extensively at X-ray, optical and infrared wavelengths throughout its outburst in 2000. We show that it is possible to separate the optical/near-infrared (OIR) jet emission from the OIR disc emission. Focussing on the jet component, we find that as the source fades in the X-ray hard state, the OIR jet emission has a spectral index consistent with optically thin synchrotron emission (α≈-0.6 to -0.7, where Fν∝να). This jet emission is tightly and linearly correlated with the X-ray flux; suggesting a common origin. This is supported by the OIR, X-ray and OIR to X-ray spectral indices being consistent with a single power law (α=-0.73). Ostensibly the compact synchrotron jet could therefore account for ∼100 per cent of the X-ray flux at low luminosities in the hard state. At the same time, (i) an excess is seen over the exponential decay of the X-ray flux at the point in which the jet would start to dominate, (ii) the X-ray spectrum slightly softens, which seems to be due to a high-energy cut-off or break shifting to a lower energy and (iii) the X-ray rms variability increases. This may be the strongest evidence to date of synchrotron emission from the compact, steady jet dominating the X-ray flux of an X-ray binary. For XTE J1550-564, this is likely to occur within the luminosity range ∼(2 × 10-4- 2 × 10-3) LEdd on the hard-state decline of this outburst. However, on the hard-state rise of the outburst and initially on the hard-state decline, the synchrotron jet can only provide a small fraction (∼ a few per cent) of the X-ray flux. Both thermal Comptonization and the synchrotron jet can therefore produce the hard X-ray power law in accreting black holes. In addition, we report a phenomenological change in the OIR spectral index of the compact jet from possibly a thermal distribution of particles to one typical of optically thin synchrotron emission, as the jet increases in energy over these ∼20 d. Once the steady jet is fully formed and the infrared and X-ray fluxes are linearly correlated, the spectral index does not vary (maintaining α=-0.7) while the luminosity decreases by a factor of 10. These quantitative results provide unique insights into the physics of the relativistic jet acceleration process.

    Original languageEnglish (US)
    Pages (from-to)1759-1769
    Number of pages11
    JournalMonthly Notices of the Royal Astronomical Society
    Volume405
    Issue number3
    DOIs
    StatePublished - Jul 1 2010

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    broadband
    x rays
    near infrared
    synchrotrons
    outburst
    luminosity
    power law
    energy
    cut-off
    physics
    wavelength
    index
    decay

    Keywords

    • Accretion, accretion discs
    • Black hole physics
    • ISM: jets and outflows
    • X-rays: binaries

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

    Cite this

    Evidence for a compact jet dominating the broad-band spectrum of the black hole accretor XTE J1550-564. / Russell, Dave; Maitra, D.; Dunn, R. J.H.; Markoff, S.

    In: Monthly Notices of the Royal Astronomical Society, Vol. 405, No. 3, 01.07.2010, p. 1759-1769.

    Research output: Contribution to journalArticle

    Russell, Dave ; Maitra, D. ; Dunn, R. J.H. ; Markoff, S. / Evidence for a compact jet dominating the broad-band spectrum of the black hole accretor XTE J1550-564. In: Monthly Notices of the Royal Astronomical Society. 2010 ; Vol. 405, No. 3. pp. 1759-1769.
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