Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21360303l&link_type=abstract
American Astronomical Society, AAS Meeting #213, #603.03
Astronomy and Astrophysics
Astronomy
Scientific paper
We have analyzed 866 RXTE observations of the 2006-2007 outburst of the accreting neutron-star XTE J1701-462, during which it evolves from super-Eddington luminosities to quiescence. It first resembles the Cyg X-2 subgroup of Z sources, with frequent excursions on the horizontal and normal branches (HB/NB). The source then decays and evolves to the Sco X-1 subgroup, with increasing focus on the flaring branch (FB) and the lower vertex of the ``Z''. Finally, the FB subsides, and the source transforms into an atoll source, with the lower vertex evolving to the atoll soft state. Spectral analyses show that the atoll stage is characterized by a constant inner disk radius, while the Z stages exhibit a luminosity-dependent expansion of the inner disk. We interpret it as the local Eddington limit effects in the disk. Contrary to the view that the mass accretion rate ( - m) changes along the Z, we find that changes in - m are instead responsible for the secular evolution of the Z and the subclasses. Motion along the Z branches appears to be caused by three different mechanisms that may operate at roughly constant - m. For the Sco X-1-like Z stage, we find that the FB is an instability track that proceeds off the lower vertex when the inner disk radius shrinks from the value set by the X-ray luminosity toward the value measured for the atoll soft state. Excursions up the NB occur when the apparent size of the boundary layer increases while the disk changes little. The HB is associated with Comptonization of the disk emission. The Z branches for the Cyg X-2-like stage are more complicated, and their origin are unclear. Finally, our spectral results lead us to hypothesize that the lower and upper Z vertices correspond to a standard thin disk and a slim disk, respectively.
Homan Jeroen
Lin Dacheng
Remillard Ron A.
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