X-ray Doppler tomography of Fe K$α$ emission in a low-mass X-ray binary 4U 1822-371 - a localized reflector at the accretion stream-disk overflow

Abstract

We present the X-ray Doppler tomography of the Fe K$α$ (6.4 keV) fluorescence line of the low-mass X-ray binary 4U 1822-371 obtained with XRISM. Eleven orbits of this short period (5.57 hr) binary were covered. The Doppler shift of the line shows clear modulation with the orbital period, motivating us to apply the Doppler tomography in the X-ray band for the first time. The resulting velocity map reveals a compact feature at ($v_{\mathrm{x}}$, $v_{\mathrm{y}}$) $\sim$ ($-$550, $+$125) km s$^{-1}$. This is inconsistent with the emission originating from a symmetric accretion disk, an extended corona around the neutron star, or the surface of the neutron or companion star. Instead, it suggests that the emission originates from the accretion stream-disk overflow. Remarkably, the Fe K$α$ velocity map closely resembles that of the O VI 3811 Å, indicating that both X-ray and optical lines arise from the same site irradiated by the central X-ray source. These results provide the first velocity-resolved X-ray map of the fluorescent line, directly localizing the major reflector in an X-ray binary and establishing X-ray Doppler tomography as a new probe of the structures of accreting systems.

Figures (10)

• Figure 1: X-ray light curves: (a) count rate (2--12 keV) and (b) hardness ratio (2--5.857 keV for the soft and 5.857--12 keV for the hard band). The origin of time is 2025-04-12 18:00:00 UT (60777.75 in MJD). The observation was interrupted by ADR recycles (red), SAA passages (blue), and Earth occultations (grey). The eclipses at the superior conjunction ($\phi_{\mathrm{orb}}=0$) are shown with green lines. Alt text: two scatter plots of X-ray lights for the Resolve count rate and hardness ratio.
• Figure 2: Folded light curve: (a) count rate (2--12 keV) and (b) count rate ratio between the Fe K$\alpha$ line band (6.35--6.45 keV) and the neighboring continuum band (6.0--6.3 keV) for two orbital cycles. The superior ($\phi_{\mathrm{orb}}=0$) and inferior ($\phi_{\mathrm{orb}}=0.5$) conjunctions are shown with the solid and dashed green lines, respectively. The eight bins for time-resolved spectroscopy are shown in different colors. Alt text: two scatter plots of X-ray lights folded by the orbital period for the Resolve count rate and the Fe K$\alpha$ line against continuum ratio.
• Figure 3: X-ray spectrum with a coarse binning. The RV shift was corrected for individual X-ray photons based on their arrival times following the RV curve in figure \ref{['f02b']}. Conspicuous spectral features are labeled. In the inset, continuum-subtracted spectrum of the Fe K band is shown with the dashed lines at the rest-frame energy. Alt text: X-ray spectrum.
• Figure 4: Fe K$\alpha$ line at eight orbital phase bins. The data (colored) as well as the Fe I K$\alpha_1$ and K$\alpha_2$ line energies (dashed grey lines) are shown. Alt text: Fe K$\alpha$ line at eight orbital phase bins.
• Figure 5: Radial velocity curve. The observed curve of the Fe K$\alpha$ line (black points for the data and black solid curve for the best-fit model), and those expected if the line was associated with the NS (grey dashed curve) or the companion star (grey dotted curve). In some phases, the data points are offset from the expected RV curve due to local noise in the spectrum. This is greatly alleviated by the use of Doppler tomography, in which the trail spectrogram is averaged along the orbital period. Alt text: Radial velocity curve of the Fe K$\alpha$.