Data recorded by NASA's Chandra X-ray Observatory of a neutron star as it passed through a dense patch of stellar wind emanating from its massive companion star provide valuable insight about the structure and composition of stellar winds and about the environment of the neutron star itself.
Stellar winds are the fast-flowing material, composed of protons, electrons, and metal atoms, ejected from stars. This material enriches the star's surroundings with metals, kinetic energy, and ionizing radiation. It is the source material for star formation. Until the last decade, it was thought that stellar winds were homogenous, but these Chandra data provide direct evidence that stellar winds are populated with dense clumps.
The neutron star observed is part of a high-mass X-ray binary system, the compact, incredibly dense neutron star paired with a massive 'normal' super-giant star. Neutron stars in binary systems produce X-rays when material from the companion star falls toward the neutron star and is accelerated to high velocities.
As a result of this acceleration, X-rays are produced that can in-turn interact with the materials of the stellar wind to produce secondary X-rays of signature energies at various distances from the neutron star. Neutral, uncharged iron atoms, for example, produce fluorescence X-rays with energies of 6.4 kilo-electron volts (keV), roughly 3000 times the energy of visible light.
Astronomers use spectrometers, like the instrument on Chandra, to capture these X-rays and separate them based on their energy to learn about the compositions of stars.
The researchers also used the Chandra's state-of-the-art engineering to identify a lower limit on the distance from the neutron star that the X-rays from neutral iron are formed. Their spectral analysis showed that neutral iron is ionized at least 2.5 light-seconds, a distance of approximately 750 million meters or nearly 500,000 miles, from the neutron star to produce X-rays.