A long-sought-after emission line of oxygen, carrying the imprint of strong gravitational fields, has been discovered in the XMM-Newton spectrum of an exotic binary system composed of two stellar remnants, a neutron star and a white dwarf. Astronomers can use this line to probe extreme gravity effects in the region close to the surface of a neutron star.

Stellar remnants are the last evolutionary step of the life of stars which, after having burned their nuclear fuel, collapse into very compact and exotic objects - white dwarfs, neutron stars and black holes, depending on the mass of the stars.

With an enormous mass contained in a very restricted space, these objects are extremely dense; in particular, neutron stars and black holes give rise to very strong gravitational fields and thus prove to be excellent testbeds for Einstein's theory of general relativity.

Stars often come in pairs, and neutron stars and black holes are no exception, often being found as one component of a binary system. Due to the strong gravitational attraction that the compact remnant exerts on its companion, material from the latter flows onto the remnant forming an accretion disc.

As the material in the disc spirals around the remnant, it is heated up to millions of degrees - because of internal friction - and produces copious amounts of X-rays. These systems are thus referred to as X-ray binaries.

The object of this study, 4U 0614+091, is a very special X-ray binary, consisting of two remnants, namely a neutron star accreting mass from a white dwarf. The fact that the companion star is also a compact object is evident from the exceptionally short orbital period of the system: in fact, the two objects orbit around each other in about 50 minutes, which identifies the source as an Ultra-Compact X-ray Binary (UCXB).

Due to their compact nature, it is virtually impossible to directly image the immediate vicinity of a neutron star and its accretion disc. Fortunately, spectroscopy of these systems yields plenty of information to fill the gap and represents a unique tool to investigate the dynamics of the accretion process in X-ray binaries.

The material surrounding the neutron star, irradiated by X-rays, reflects this radiation and, during the process, ions of heavy elements, such as oxygen and iron, that are present in the disc leave their imprint on the spectrum of the reflected light as characteristic emission lines. The profile of these so-called 'fluorescent' lines is deeply influenced by the strong gravitational field of the compact remnant, hence their detection is extremely important for testing the strong regime of general relativity.

"The only line so far observed in X-ray binaries was the iron line, which corresponds to an energy of about 6.4 keV," explains Oliwia Madej, a PhD student at the Netherlands Institute for Space Research (SRON) and Utrecht University who led the study that detected, for the first time, a broad line of oxygen in the spectra of 4U 0614+091.

"This line is at a lower energy than the iron one - about 0.7 keV - and represents not only an additional diagnostic of the inner parts of the system, but actually a more powerful one. "The advantage is that instruments are able to collect more photons at the energy of the oxygen line than at the energy of the iron line, resulting in a better quality spectrum," she adds.

The outstanding result relies on both low- and high-resolution spectra of 4U 0614+091 collected by XMM-Newton. "The high-resolution of the spectra delivered by the Reflection Grating Spectrometers (RGS) was crucial for isolating the long-sought-after oxygen signature amongst the plethora of spectral features," comments Norbert Schartel, XMM-Newton Project Scientist.

The line, which is intrinsically narrow, appears broadened towards lower as well as higher energies. Relativistic effects are responsible for the broadening towards lower energies through a combination of gravitational redshift - as photons lose energy as they escape the strong gravitational field of the neutron star- and relativistic Doppler effect.

"The broadening towards higher energies is interpreted, instead, in terms of photons scattering off the very hot electrons present in the disc and gaining energy through this process," explains Peter Jonker from SRON, one of Madej's PhD supervisors.

By studying the profile of the oxygen line in very great detail, it is possible to infer a wealth of information about the accretion disc within a few to a few tens of neutron star radii, corresponding to a distance of only a few kilometres to several tens of kilometres from the neutron star's surface. Probing these regions allows us to test Einstein's general relativity in an extreme environment, where the gravity is immensely stronger than in our Solar System.

"It is amazing how Nature provides us with astronomical sources that are exceptional laboratories to study how matter behaves in such a strong gravitational field, so dense that one teaspoonful would weigh a billion tons on Earth," comments Schartel. "Our role is to figure out better and better tools to observe these sources and uncover all the information they conceal."