NICER observes the full Z-track in GX 13+1

TL;DR

This study uses NICER data to analyze the neutron star low-mass X-ray binary GX 13+1, successfully tracing its entire Z-track and identifying distinct spectral states and noise features for the first time.

Contribution

It provides the first complete observational mapping of GX 13+1's Z-track, clarifying its spectral states and timing properties.

Findings

Detected the full Z-track in GX 13+1 for the first time.

Identified a peaked noise component at ~5.4 Hz in the power density spectra.

Observed a positive slope of the horizontal branch in the HID.

Abstract

We present the temporal analysis of the persistent neutron star low-mass X-ray binary (NS LMXB) GX 13+1 using NICER data. Classification of this source has been ambiguous so far. We investigate the evolution of the source in its hardness-intensity diagram (HID) and power density spectra (PDS) of the 0.5-10 keV NICER archival data. For the first time, we detect the source tracing out the entire Z-track, distinctly identifying the horizontal branch (HB), normal branch (NB) and flaring branch (FB). We also detect a peaked noise component in the PDS at $\sim$ 5.4 Hz, which appears to be present when the source is either in the NB or FB. We note a positive slope of the HB in the HID which could be due to either the high intrinsic absorption of the source or the stronger contribution of the soft spectral components in the soft energy domain.