X‐Ray Observations of XTE J1550−564 during the Decay of the 2000 Outburst. II. Timing

Abstract

We report on the evolution of the X-ray energy spectrum for the blackhole candidate (BHC) X-ray transient XTE J1550-564 during the decayof the 2000 outburst. The Rossi X-Ray Timing Explorer (RXTE) andChandra observations span nearly 5 orders of magnitude in luminosity.The RXTE spectra are dominated by a power-law component, and a comparativelyweak soft component was also detected for the first two observations.The source made a transition to the canonical hard state near a luminosityof 9×1036 ergs s-1 over several observations, as evidenced by a dropin the flux of the soft component in the RXTE energy band and a hardeningof the power-law component to a photon index near 1.6. The powerlaw did not exhibit this behavior for the previous XTE J1550-564outburst. For some observations, we detect a high-energy cutoff andfind that the cutoff is more significant and at lower energy duringthe transition than in the hard state. The cutoff in the hard stateis at higher energy than has been seen for most previous accretingBHCs. The Chandra spectrum provides evidence for spectral evolutionafter the hard-state transition. It is well, but not uniquely, describedby a power law with a photon index of 2.30+0.41-0.48 (90% confidence)and interstellar absorption. Advection-dominated accretion flow modelspredict gradual spectral softening as the luminosity drops, but ourobservations do not allow us to determine whether the spectral evolutionis gradual or sudden. The lowest luminosity we measure for XTE J1550-564with Chandra is 5×1032 ergs s-1 (0.5-7 keV, for a distance of 4 kpc).Although this is probably not the true quiescent luminosity, it representsa useful upper limit on this quantity.