Swift observations of the cooling accretion disk of XTE J1817−330

Preprint, Article English OPEN
Rykoff, E. S. ; Miller, J. M. ; Steeghs, D. ; Torres, M. A. P. (2007)
  • Publisher: Institute of Physics Publishing Ltd.
  • Related identifiers: doi: 10.1086/520329, doi: 10.1086/520329
  • Subject: QB | Astrophysics
    arxiv: Astrophysics::Galaxy Astrophysics | Astrophysics::High Energy Astrophysical Phenomena | Astrophysics::Cosmology and Extragalactic Astrophysics | Astrophysics::Earth and Planetary Astrophysics | Astrophysics::Solar and Stellar Astrophysics

The black hole candidate X-ray transient XTE J1817-330 was observed by the Swift satellite over 160 days of its 2006 outburst with the XRT and UVOT instruments. At the start of the observations, the XRT spectra show that the 0.6-10 keV emission is dominated by an optically thick, geometrically thin accretion disk with an inner disk temperature of $\sim0.8$ keV, indicating that the source was in a high/soft state during the initial outburst phase. We tracked the source through its decline into the low/hard state with the accretion disk cooling to $\sim 0.2 \mathrm{keV}$ and the inner disk radius consistent with the innermost stable circular orbit at all times. Furthermore, the X-ray luminosity roughly follows $L_X \propto T^4$ during the decline, consistent with a geometrically stable blackbody. These results are the strongest evidence yet obtained that accretion disks do not automatically recede after a state transition, down to accretion rates as low as $0.001 L_{Edd}$. Meanwhile, the near-UV flux does not track the X-ray disk flux, and is well in excess of what is predicted if the near-UV emission is from viscous dissipation in the outer disk. The strong correlation between the hard X-ray flux and the near-UV flux, which scale as $L_X^{0.5}$, indicate that reprocessed emission is most likely the dominate contribution to the near-UV flux. We discuss our results within the context of accretion disks and the overall accretion flow geometry in accreting black holes.
  • References (2)

    These results are the strongest evidence yet obtained that accretion disks do not automatically recede after a state transition. Rather, the evolution of the disk temperature appears to be smooth across state transitions, and the inner disk appears to remain at or near the innermost stable circular orbit, at least down to LX /LEdd ∼ 0.001. We have made a major step forward in being able to demonstrate this result through robust trends, while prior work merely detected disks in the low/hard state. Cool disks with inner radii consistent with the ISCO have been found in every case where good quality soft X-ray spectra have been obtained with a CCD spectrometer, including GX 339−4, Cygnus X1 (Miller et al. 2006b), SWIFT J1753.5−0127 (Miller et al. 2006a), XTE J1817−330, and GRO J1655−40 (this work). However, we must note that geometrically thin accretion disks are likely impossible at the lowest accretion rates observed in black holes, and that an advective flow must take over at some point below LX /LEdd ∼ 0.001.

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