Extreme gravity effects revealed by oxygen for the first time in neutron star that 'eats' white dwarf

Although strong gravity near neutron stars and black holes has been studied before in a similar way, this result is unique. Until now, only blurred X-ray signatures of iron atoms have been observed in the X-rays from a neutron star. However, the characteristics of these so called 'iron lines' are disputed, which makes them less suited for extreme gravity field measurements.

The neutron star has been studied before but now Oliwia Madej, PhD student at Utrecht University and SRON Netherlands Institute for Space Research, has found blurred oxygen signatures in the X-rays from the star. She made this discovery in an archival observation performed by ESA's XMM-Newton observatory, which is equipped with the SRON reflection grating spectrometer (RGS) that is extemely sensitive in these particular wavelenghts. The research was carried out under supervision of SRON-researcher Peter Jonker.

The neutron star that the astronomers observed is part of a binary system called 4U 0614+091. In the binary, the neutron star and a white dwarf closely orbit each other in roughly 50 minutes. The white dwarf -- basically a burnt out star -- orbits at such a small distance from the neutron star that the oxygen-rich gas is pulled off the dwarf and starts closely swirling around the neutron star in a disk.

Extreme gravity

"Normally, hot oxygen atoms emit X-rays at a specific energy," Madej explains. "But because of the extreme gravity and the hot gas in the disk around the neutron star, this oxygen signature in the X-ray data is blurred." From the shape of the blur Madej tried to estimate the inner radius of the oxygen-rich disk around the neutron star, which should give an idea of the maximum radius that the neutron star could possibly have.

"Unfortunately, the current data are not yet good enough to give a definitive answer on the size of a neutron star," Peter Jonker admits. "To determine this in greater detail we need more observation time. And when we find the signature of iron molecules as well, we can now compare the characteristics of the two emission lines. Measured together, uncertainties about the measurements of the iron line can be taken away, which will guide the interpretation in other systems where only iron has been seen. All in all our observations are definitely an important step on the way towards a better understanding of the extreme conditions around and inside a neutron star."

Neutron stars -- shaped out of the collapsing cores of massive stars -- are the most compact objects with a surface in the universe. A neutron star has a slightly higher mass compared to a white dwarf, but the matter is squeezed into a ball of only 10-20 km in diameter. At these high densities, normal atoms cannot exist anymore. Anything denser would collapse into a black hole. Therefore, astronomers are very interested in the state of the matter inside a neutron star.

The results of the research appear in the Monthly Notices of the Royal Astronomical Society.