The hard X-ray emission along the "Z" track in GX 17+2

TL;DR

This study analyzes the evolution of hard X-ray emission in GX 17+2 along its Z track, revealing a hard X-ray tail likely produced by bulk-motion Comptonization near the neutron star surface.

Contribution

It demonstrates the presence and behavior of hard X-ray tails along the Z track and links their origin to bulk-motion Comptonization near the neutron star.

Findings

Hard X-ray tail detected in HEXTE spectra.

Hard tail hardens from horizontal to flaring branch.

Bulk-motion Comptonization explains the hard tail.

Abstract

Using the data from the Proportional Counter Array (PCA) and the High-Energy X-ray Timing Experiment (HEXTE) on board it Rossi X-Ray Timing Explorer for Z source GX 17+2, we investigate the evolution of its PCA spectra and HEXTE spectra along a "Z" track on its hardness-intensity diagram. A hard X-ray tail is detected in the HEXTE spectra. The detected hard X-ray tails are discontinuously scattered throughout the "Z" track. The found hard tail hardens from the horizontal branch, through the normal branch, to the flaring branch in principle and it contributes ~(20-50)% of the total flux in 20-200 keV. Our joint fitting results of the PCA+HEXTE spectra in 3-200 keV show that the portion of Comptonization in the bulk-motion Comptonization (BMC) model accounts for the hard X-ray tail, which indicates that the BMC process could be responsible for the detected hard tail. The temperature of the seed photons for BMC is ~2.7 keV, implying that these seed photons might be emitted from the surface of the neutron star (NS) or the boundary layer between the NS and the disk and, therefore, this process could take place around the NS or in the boundary layer.