A highly ionised outflow in the X-ray binary 4U 1624-49 detected with XRISM

TL;DR

XRISM Resolve phase-resolved spectroscopy of the high-inclination NS LMXB 4U 1624-49 reveals a clearly detected highly ionised wind with $v\sim200$–$320$ km s$^{-1}$ and $N_H\gtrsim10^{23}$ cm$^{-2}$, characterized by narrow line widths of $\sim$50–100 km s$^{-1}$. Phase-resolved modelling across five orbital intervals shows blueshifted Fe XXV/XXVI absorption with phase-dependent ionisation and velocity, and requires photoionised plasma models (including a possible second absorber near dips) to reproduce the data; the wind launching radius is inferred to be $r\sim3.9\times10^{10}$ cm, consistent with a thermal-radiative wind from the disc outskirts. The outflow’s properties, together with the launching radius and modest line widths, strongly support a thermal-radiative origin rather than a magnetic drive, and the results are contextualised by comparisons to GX 13+1 and Cir X-1. While scattering and line-saturation effects complicate interpretation, the study demonstrates the power of XRISM’s high-resolution, phase-resolved spectroscopy in constraining disc-wind physics in neutron-star binaries.

Abstract

The origin of accretion disc winds remains disputed to date. High inclination, dipping, neutron star Low Mass X-Ray Binaries (LMXBs) provide an excellent testbed to study the launching mechanism of such winds due to being persistently accreting and showing a nearly ubiquitous presence of highly-ionised plasmas. We aim to establish or rule out the presence of a wind in the high inclination LMXB 4U 1624-49, for which a highly ionised plasma has been repeatedly observed in X-ray spectra by Chandra and XMM-Newton, and a thermal-radiative pressure wind is expected. We leverage the exquisite spectral resolution of XRISM to perform phase-resolved spectroscopy of the full binary orbit to characterise the highly ionised plasma at all phases except during absorption dips. An outflow is clearly detected via phase-resolved spectroscopy of the source with XRISM/Resolve. Based on analysis of the radial velocity curve we determine an average velocity of ~200-320 km/s and a column density above 10$^{23}$ cm$^{-2}$. The line profiles are generally narrow, spanning from ~50 to ~100 km/s, depending on the orbital phase, pointing to a low velocity sheer or turbulence of the highly ionised outflow and a potential increase of turbulence as the absorption dip is approached, likely due to turbulent mixing. The line profiles, together with the derived launching radius and wind velocity are consistent with a wind being launched from the outskirts of the disc and without stratification, pointing to a thermal-radiative pressure origin.

Figures (6)

• Figure 1: XRISM/Resolve and NuSTAR light curves in the 2--10 keV and 4--30 keV energy range, respectively, for observations 1 (top) and 2 (bottom) showing absorption dips at the start and end of the observations. The binning is 25 s in each panel. The thick tick marks at the top axis mark the start and end of phase-resolved intervals.
• Figure 2: XRISM/Resolve and NuSTAR spectra for interval p2 fitted with Model 1 ( hot*(bb+compt), see text)
• Figure 3: Residuals in the Fe K$\alpha$ energy range for intervals p0--p4 after fitting of the continuum. The y-scale has been shifted for p0-p3 for clarity.
• Figure 4: Spectra for intervals p0 to p4 (top to bottom, respectively) with models 2a, 2b and 2c overplotted in cyan, blue and red, respectively (see Tables \ref{['tab:specfits1b']}--\ref{['tab:specfits1c']} for fitted parameters). The insets show a detail of the Fe xxv and Fe xxvi K$\alpha$ regions highlighting the different prediction of the three models for the Fe xxvi spin-orbit doublet ratio.
• Figure 5: Spectrum for interval p0 with the model with two absorbers displayed in Table \ref{['tab:specfits2']} overplotted in magenta for comparison with models 2b and 2c with one absorber overplotted in blue and red, respectively (see Tables \ref{['tab:specfits2']}, \ref{['tab:specfits1b']}--\ref{['tab:specfits1c']} for fitted parameters). The insets show a detail of the Fe xxv and Fe xxvi K$\alpha$ regions highlighting the different prediction of the three models for the Fe xxvi spin-orbit doublet ratio.