New method designed by UA researchers to identify neutron stars "in disguise"
Alicante. Monday 19 February 2018
University of Alicante researchers have led a study from which a new method has been developed by an international team to identify neutron stars “in disguise” and differentiate them from black holes, thus unravelling another mystery of the universe.
Neutron stars are born when a massive star or celestial object (eight or more times the mass of the Sun) dies and explodes as a supernova, according to José Miguel Torrejón, director of the UA Research Group in X-Ray Astrophysics and one of the designers of the method, developed from a study conducted by UA PhD student María Martínez.
“What was once the massive star’s core becomes an ultracompact sphere, about 12 km in radius, rotating at an extremely high speed and with a very powerful magnetic field”.
The matter around the neutron star falls on its magnetic poles, at relativistic speeds, driven by the lines of its intense magnetic field. “When that matter collides, the poles reach extremely high temperatures and shine. With rotation, distant observers (astronomers and scientists) can see this radiation, pulsating like a lighthouse”, which is known as a pulsar, Torrejón explains.
The UA researcher points out: “Only if a pulsar is detected can it be determined, without a doubt, that a neutron star exists, it is like its fingerprint”. In some cases, though, this pulsation is never detected and it is impossible to determine the nature of the compact object. It is as if the neutron star were, so to speak, “in disguise”.
However, this barrier has now been overcome thanks to this new method, developed at the UA in cooperation with expert Konstantin Postnov, from Moscow’s Sternberg Astronomical Institute, PhD Lida Oskinova, from the University of Postdam's Institute for Physics and Astronomy, and US scientists.
“To develop the method, we drew on observations proposed by us from NASA's Chandra X-ray Observatory to detect the existence of neutron stars in non-pulsating binary systems”, José Miguel Torrejón says.
“The new method is based on analysing the variation of the spectral parameters of emitted light and the emission lines of highly ionised iron during an X-ray flare in the neutron star”.
This behaviour points to the existence of a highly effective cooling mechanism, which can only be explained by what is known as “inverse Compton scattering” (the hot matter conveys energy to the light photons and these “go” to the X-rays, which cools down the plasma). In turn, as Torrejón says, this mechanism requires a very powerful magnetic field, which only occurs in neutron stars.
The expert highlights that this new method “helps restrict the initial conditions which can give rise to a black hole or a neutron star, and it is also useful to compile a census of the objects or the systems from which they originated. The most important thing would be to find out the conditions under which they are formed, as these are theoretically predicted, but experimentally testing the theory is not that easy”.
Studying neutron stars and black holes is important to test theoretical predictions on star evolution and the final phases of massive stars. Among others, it can allow experts to test the theory of general relativity when gravity is strong and restrict “nuclear matter equations of state” from observation, according to the UA researcher.