Stratified wind from a super-Eddington X-ray binary is slower than expected

Abstract

Accretion disks in strong gravity ubiquitously produce winds, seen as blueshifted absorption lines in the X-ray band of both stellar mass X-ray binaries (black holes and neutron stars)^1-4 and supermassive black holes⁵. Some of the most powerful winds (termed Eddington winds) are expected to arise from systems in which radiation pressure is sufficient to unbind material from the inner disk (L ≳ L_Edd). These winds should be extremely fast and carry a large amount of kinetic power, which, when associated with supermassive black holes, would make them a prime contender for the feedback mechanism linking the growth of those black holes with their host galaxies⁶. Here we show the XRISM Resolve spectrum of the galactic neutron star X-ray binary, GX 13+1, which reveals one of the densest winds ever seen in absorption lines. This Compton-thick wind significantly attenuates the flux, making it appear faint, although it is intrinsically more luminous than usual (L ≳ L_Edd). However, the wind is extremely slow, more consistent with the predictions of thermal-radiative winds launched by X-ray irradiation of the outer disk than with the expected Eddington wind driven by radiation pressure from the inner disk. This puts new constraints on the origin of winds from bright accretion flows in binaries, but also highlights the very different origin required for the ultrafast (v ~ 0.3c) winds seen in recent Resolve observations of a supermassive black hole at a similarly high Eddington ratio⁷.

Fig. 1. The Resolve/XRISM spectrum of GX 13+1.

This is dominated by multiple absorption lines from H- and He-like ions, blueshifted by about 330 km s⁻¹. All the fine structure transitions in these lines are resolved, showing that the lines are very narrow (velocity dispersion of around 150 km s⁻¹). All strong lines are labelled; the 1 − n transitions of Fe xxv and Fe xxvi are indicated in cyan and orange, respectively. Even the weakest line identified here (Ti xxii Lyα_1,2 around 4.95 keV) is highly significantly detected (Δχ² = 32 for 1 additional degree of freedom). The orange line shows the best-fit model described in the text, with an intrinsic continuum absorbed by the slow wind, but with a faster (700 km s⁻¹), broader (300 km s⁻¹) even more highly ionized component to fit the blue wing seen in Fe xxvi Lyα_1,2 (Fig. 3). The model also includes diffuse emission from the wind (modelled using scattered intrinsic flux plus photoionized line and recombination continua from both wind components, all with some self-absorption in the wind). This fits the data fairly well overall (Methods and Extended Data Table 2), except around the Fe xxv (8.8 keV) and Fe xxvi (9.25 keV) edges, in which the photoionization model used here is incomplete (only including transitions up to n = 16). The total electron scattering optical depth in the slow wind is τ_es ~ 1, and both winds is τ_es ~ 1.8, attenuating the intrinsic flux (Fig. 2).

Fig. 2. Historical X-ray variability of GX 13+1.

The archival NuSTAR spectrum of GX 13+1 (green) is similar to most of the archival RXTE data (grey). Instead, the XRISM-coordinated NuSTAR spectrum (orange) has lower flux and shows a much deeper K-edge from Fe xxv at 8.8 keV. On closer inspection, 5–10% of the archival RXTE spectra are similar to this recent NuSTAR observation, indicating that this dense wind/super-Eddington phase is recurring in the source. The blue band shows a range of possible continuum spectra of GX 13+1 after correcting for attenuation due to electron scattering in the wind. The lower end of the envelope corresponds to τ_es = 1 from the slow wind alone, whereas the upper end corresponds to τ_es = 1.8 as inferred from the best-fit model for the slow plus fast wind. The source is intrinsically more luminous than normal, at or above Eddington.

Fig. 3. Magnification around the Fe Kα lines.

The dashed vertical lines show the restframe energies of (from left to right) the weak Fe xxiv doublet (6.652 keV and 6.661 keV), Fe xxv (intercombination: 6.667 keV and resonance: 6.700 keV) and the Fe xxvi doublet (6.952 keV and 6.973 keV). The blue line shows a single photoionized absorption model with parameters that fit the multiple narrow lines in the rest of the spectrum. This predicts that the lines are black in their centres, but the data show residual emission due to the presence of diffuse flux (most likely reprocessing and scattering from the wind itself). It also misses the blue wing in the Fe xxvi Lyα_1,2 absorption line at 7 keV, showing that there is higher velocity material at higher ionization state. The orange line shows our best model, including the additional components in both absorption and emission.

Fig. 4. Impression of the wind in GX 13+1 as seen by XRISM.

The bulk of the wind (green) is optically thick, highly ionized and slow, but it has a faster, even more highly ionized skin on its inner edge (blue). We see the central source directly through this heavy absorption, but the irradiated wind material forms a secondary source of diffuse X-rays from scattering and re-emission, which can be seen along multiple paths. Illustration by CfA/Melissa Weiss.

Extended Data Fig. 1. Mn Kα lines from the ⁵⁵Fe source in the filter wheel.

The black bins show the Hp spectrum extracted using two gain fiducial points, summing the 34 pixels. The blue line shows the intrinsic line profile, whereas the red one represents the best fit model, with additional Gaussian broadening of FWHM=4.43 eV. The lower panel shows the residuals between the data and the model, indicating that this is a good description.

Extended Data Fig. 2. Effective temperature of the calibration pixel versus time.

The effective temperature across the observation is shown as a solid black line, compared to a linear interpolation between the measurements at the start and end of the observation (blue dashed line). We introduce an ad-hoc gain point (red filled cicle, with a temperature ΔT_eff below the first gain point), to give a better match (red solid line).

Extended Data Fig. 3. Effective temperature variations in all pixels except 27.

Each pixel has an effective temperature estimate corresponding to the gain fiducial measurements at the beginning and end of the observation. We introduced an additional gain point by scaling the ad-hoc gain point from the calibration pixel (see Extended Data Fig. 2) to each individual pixel (see the middle point in each colored line). The black line shows the calibration pixel, which is tracked continuously, for reference.

Extended Data Fig. 4. Ion ratio as a function of ionisation parameter.

We computed the ground state populations for each ion using the pion code as in Methods. The ratio of these populations (equivalently, the ratio of the column densities in different ions) is sensitive to the ionisation parameter, as shown. Using the ratio of column densities taken from Extended Data Table 1, we estimate the ionisation parameter of the slow component in our ion-by-ion fits as $\log \xi ~3.9$ (shaded regions), and the fast component of Fe and Ni as $\log \xi =4.15-4.53$ (shaded regions with black frames).

Extended Data Fig. 5. Ion fractions of Fe versus the ionisation parameter.

This is computed using pion as described in Methods, assuming that the gas is photoionised by the continuum shape observed. We estimate the ionisation parameter from our ion-by-ion fits using Extended Data Fig. 4, then used the curves above to determine the column density of completely-ionised Iron (Fe xxvii).