On the nature of the first transient Z-source XTE J1701-462: its accretion disk structure, neutron star magnetic field strength, and hard tail

TL;DR

This study analyzes the spectral evolution of the transient Z source XTE J1701-462, revealing how accretion disk structure, magnetic field strength, and hard X-ray tails change during different spectral states.

Contribution

It provides new insights into the relationship between accretion rate, disk structure, and magnetic field in a transient Z source, supported by detailed spectral analysis.

Findings

Inner disk radius depends on mass accretion rate as predicted by models.

Magnetic field strength estimated at ~(1--3)×10^9 G, between normal atoll and Z sources.

Presence of a significant hard tail in the horizontal branch.

Abstract

Using the data from the Rossi X-Ray Timing Explorer satellite, we investigate the spectral evolution along a "Z" track and a "v" track on the hardness-intensity diagrams of the first transient Z source XTE J1701-462. The spectral analyses suggest that the inner disk radius depends on the mass accretion rate, in agreement with the model prediction, R_in \propto ((dM/dt)_disk)^{2/7}, for a radiation pressure dominated accretion disk interacting with the magnetosphere of a neutron star (NS). The changes in the disk mass accretion rate (dM/dt)_disk are responsible for the evolution of the "Z" or "v" track. The radiation pressure thickens the disk considerably, and also produces significant outflows. The NS surface magnetic field strength, derived from the interaction between the magnetosphere and the radiation pressure dominated accretion disk, is ~(1--3)X10^9 G, which is possibly between normal atoll and Z sources. A significant hard tail is detected in the horizontal branches and we discuss several possible origins of the hard tail.