X-ray source in 4U 1822-371 found where the stream of matter from secondary star hits the neutron star's disk

Star system: 4U 1822-371 A | Distance from the Sun: 23195 light years

Scientists have used X-ray observations to create a detailed map of the Fe K emission in the low-mass X-ray binary system 4U 1822-371. This system consists of a compact object, likely a neutron star, and a companion star orbiting each other every 5.57 hours. The study used data from the XRISM satellite, covering eleven orbits of the system, to track how the X-ray emission changes with the orbital period.

By analyzing the Doppler shifts in the Fe K line, the researchers produced the first velocity-resolved X-ray map of this kind. The map revealed a compact region of emission that does not match what would be expected from a symmetric accretion disk or the surfaces of the stars. Instead, the findings suggest that the X-ray emission comes from the area where the stream of material from the companion star overflows onto the disk around the neutron star. This region also matches the location of optical emission seen in other studies, indicating both X-ray and optical light are produced at the same site.

This work demonstrates that X-ray Doppler tomography can directly pinpoint the main reflecting region in an X-ray binary system. It opens up a new way to study the complex structures around accreting objects like neutron stars.

Key findings
- X-ray Doppler tomography was used to map Fe K emission in 4U 1822-371.
- The main reflector is located at the accretion stream-disk overflow, not in the disk or on the stars.
- X-ray and optical emissions originate from the same region, providing new insights into accreting systems.

Scientific study

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