Swift Observations of the Cooling Accretion Disk of XTE J1817-330

Abstract

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.