First X-ray and radio polarimetry of the neutron star low-mass X-ray binary GX 17+2

TL;DR

This study delivers the first joint X-ray and radio polarimetric analysis of the Sco-like Z-source GX 17+2 using IXPE, NICER, NuSTAR, and VLA. The source was in the NB state during IXPE observations, with the X-ray polarization measured as PD $=1.9\pm0.3\%$ and PA $=11\pm4^{\circ}$ in the $2-8\mathrm{keV}$ band, while joint spectroscopy requires disk, Comptonization, and reflection components. Radio polarimetry reveals $PD=2.2\%$ and a Faraday-corrected EVPA of $1^{\circ}$ (RM $\sim210\ \mathrm{rad\,m^{-2}}$), suggesting the jet axis is aligned with the X-ray polarization within uncertainties. Spectro-polarimetric modeling indicates the polarization is compatible with a combination of disk emission, a Comptonizing region, and disk reflection, though component disentanglement is limited by current data; the results favor a geometry where the X-ray polarization is connected to the jet axis and disk-related processes, highlighting differences between Sco-like and Cyg-like Z-sources and underscoring the need for broader energy polarimetry with future missions.

Abstract

We report the first polarimetric results of the neutron star (NS) low-mass X-ray binary (LMXB) Z-source GX 17+2 using the Imaging X-ray Polarimetry Explorer (IXPE) and the Very Large Array (VLA). We find that the X-ray source was polarized at PD = 1.9 +/- 0.3 % (1-sigma errors) with a polarization angle of PA = 11 +/- 4 degree (1-sigma errors). Simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) observations show that the source was in the normal branch (NB) during our IXPE observations. The X-ray spectro-polarimetry results suggest a source geometry comprising an accretion disk component, a Comptonization component, and a reflection component. The VLA radio polarization study shows a PD = 2.2 +/- 0.2 % with a Faraday-corrected intrinsic polarization angle of 1 +/- 5 degree, which is an indication of the jet axis. Thus, we find the estimated X-ray PA from the source is consistent with the radio PA. We discuss the accretion geometry of the Z-source in light of our X-ray spectro-polarimetry and radio findings.

Figures (5)

• Figure 1: First panel: IXPE (2--8 keV) light curve of GX 17$+$2. Time bins of 128 s are used. Second Panel: NICER (0.5--10 keV) light curve of GX 17$+$2. Time bins of 8 s are used. Third Panel: NuSTAR (3.0--79.0 keV) light curve during observation 1 and observation 2 of GX 17$+$2. Time bins of 60 s is used. The source was observed by NICER and NuSTAR simultaneously with the IXPE observations.
• Figure 2: Top: Color-color diagram constructed from all NuSTAR observations of GX 17$+$2 (ObsIDs starting with 30101023, 30902026, and 91001339). Time bins of 128 are used. The CCD indicates that the source was in the NB branch of its Z-track during our IXPE observations (black data points; gray are all previous NuSTAR observations of GX 17$+$2). Bottom: Hardness–intensity diagram constructed from the NICER observations simultaneous to our IXPE observation of GX 17$+$2. The NICER and NuSTAR CCDs indicate that the source was in the NB branch of its Z-track during the IXPE observations.
• Figure 3: Left panel: Contour plots of the polarization degree and angle, determined with the PCUBE algorithm, at the 68 %, 95 % and 99 % confidence levels, in the 2–8 energy band. Right panel: Contour plots of the polarization degree and angle, determined with the PCUBE algorithm, at the 68 %, 95 % and 99 % confidence levels, in the 2–4 keV (blue), 4-8 keV (red), 4-6 keV (black), and 6-8 keV (orange) energy bands. The contour plots show no strong variation of the source PA/PD with energy, considering the uncertainties.
• Figure 4: First panel: Model fitted joint spectra of GX 17$+$2 as observed by IXPE DU1 (spring green), IXPE DU2 (green yellow), IXPE DU3 (dark green), NICER (orange) and NuSTAR (plum). The spectra are fitted with the tbnew*(diskbb+nthcomp+relxillNS)*polconst*const model in the 1.5--30 keV energy band. The total model is shown with the dotted black (NICER, NuSTAR, and IXPE). The spectral components are consistent with the previously reported X-ray studies of the source. Second panel: The residuals between the data and the best fit model. Third Panel: The fraction of the different model components over the total model. The data for the IXPE, NICER, and NuSTAR spectra are rebinned only for plotting and representation purposes.
• Figure 5: A comparison of the X-ray PA and radio jet position angle obtained from our study of GX 17$+$2. The X-ray polarization degree and angle are determined with the PCUBE algorithm, at the 68%, 95% and 99% confidence levels, in the 2-8 keV energy band (in blue). The PA ($+1$$^\circ$) and thus potential radio jet/jet axis position angles ($+1$$^\circ$ if parallel or $-89$$^\circ$ if perpendicular) are shown in maroon and black, with the gray lines representing the uncertainties on the radio jet position angle ($\pm5\hbox{$^\circ$}$). The lower limit (-5$^\circ$) on the uncertainty of the jet-axis position angle (-94$^\circ$ or 86$^\circ$) goes beyond the plot. Our study shows the consistency between the X-ray PA and radio jet position angles in the case of GX 17$+$2.