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Dual perspectives on GX 17+2: a simultaneous NICER and NuSTAR study

Malu Sudha, Renee M. Ludlam, Jeroen Homan, Dacheng Lin, Benjamin Coughenour, Edward M. Cackett

TL;DR

This paper addresses constraining the accretion geometry of GX 17+2, a Sco-like Z-source, by performing the first simultaneous NICER+NuSTAR spectral-timing analysis. The authors apply RELXILLNS relativistic reflection to joint NICER+NuSTAR data across the full HID track to measure the inner disk radius and inclination. They find the inner disk is relatively close to the NS, $R_{in} \\sim 1$–$4\\,R_{ISCO}$, with a more truncated disk on the HB (\\sim 4\\,R_{ISCO}) and inward migration toward the NB/SA/FB, where the FB shows the closest approach. The observed rms variability suggests the variability originates from the disk/boundary layer rather than the corona, and the inclination is constrained to about $i \\approx 25^{\\circ}$; these results illustrate how simultaneous high-energy spectroscopy can break degeneracies in NS LMXB geometry and inform constraints on NS radius proxies.

Abstract

We performed the first simultaneous NICER & NuSTAR spectral and timing study of the Sco-like Z source GX 17+2. The source traced the full Z track during four observations. We detect signatures of relativistic reflection in the broadband spectra and report results using a reflection framework. The disk is relatively close to the innermost stable circular orbit ($\sim$ 1-4 R$_{ISCO}$), which agrees with previous studies of GX 17+2, but the location of the inner disk is farther out in the horizontal branch (HB) and moves inward toward the flaring branch (FB). We find the FB to be the point of closest approach of the disk to the neutron star. We qualitatively conclude that the evolution of the source along the HID is that of a relatively truncated disk in the HB ($\sim$ 4 R$_{ISCO}$) that approaches the neutron star as it goes along the HID towards the normal branch (NB), soft apex (SA), and finally the FB. We attribute the source evolution along the Z track to varying mass accretion rate and disk instabilities. Rms variability increases from the NB towards the SA and then drops to a constant along the FB indicating that the observed variability likely originates from the disk/boundary layer rather than the corona.

Dual perspectives on GX 17+2: a simultaneous NICER and NuSTAR study

TL;DR

This paper addresses constraining the accretion geometry of GX 17+2, a Sco-like Z-source, by performing the first simultaneous NICER+NuSTAR spectral-timing analysis. The authors apply RELXILLNS relativistic reflection to joint NICER+NuSTAR data across the full HID track to measure the inner disk radius and inclination. They find the inner disk is relatively close to the NS, –, with a more truncated disk on the HB (\\sim 4\\,R_{ISCO}) and inward migration toward the NB/SA/FB, where the FB shows the closest approach. The observed rms variability suggests the variability originates from the disk/boundary layer rather than the corona, and the inclination is constrained to about ; these results illustrate how simultaneous high-energy spectroscopy can break degeneracies in NS LMXB geometry and inform constraints on NS radius proxies.

Abstract

We performed the first simultaneous NICER & NuSTAR spectral and timing study of the Sco-like Z source GX 17+2. The source traced the full Z track during four observations. We detect signatures of relativistic reflection in the broadband spectra and report results using a reflection framework. The disk is relatively close to the innermost stable circular orbit ( 1-4 R), which agrees with previous studies of GX 17+2, but the location of the inner disk is farther out in the horizontal branch (HB) and moves inward toward the flaring branch (FB). We find the FB to be the point of closest approach of the disk to the neutron star. We qualitatively conclude that the evolution of the source along the HID is that of a relatively truncated disk in the HB ( 4 R) that approaches the neutron star as it goes along the HID towards the normal branch (NB), soft apex (SA), and finally the FB. We attribute the source evolution along the Z track to varying mass accretion rate and disk instabilities. Rms variability increases from the NB towards the SA and then drops to a constant along the FB indicating that the observed variability likely originates from the disk/boundary layer rather than the corona.

Paper Structure

This paper contains 3 sections, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Observation and Data Reduction
  3. Data Analysis & Results

Figures (2)

  • Figure 1: Left: HID of GX 17+2 obtained using NuSTAR observations. Hard color is defined as the ratio of X-ray photon counts in the 8--20 keV and 5--8 keV energy bands. Intensity (cts/s) used is from the 5--20 keV energy band. Colored polygons and differently styled markers are used to indicate the separate branches of the Z track selected for further analysis. Right: NuSTAR lightcurves in the 3--20 keV energy band on the bottom panel (binned to 128 s) using the same styled markers as used in the HID and NICER lightcurves in the 0.5--10 keV energy band binned to 128 s in the top panel marked using the star symbol in pink color.
  • Figure 2: Ratio of NuSTAR FPMA data to continuum model indicating the presence of a Fe K line emission at $\sim$ 6.7 keV. The continuum is modeled by a diskbb+bbody+powerlaw model. Variation in the Fe K strength as it moves along the different branches can be clearly noted.