The neutron star "eats" its companion. The matter first accumulates in an accretion disk around the neutron star. (Source: NASA)
Reading aloud US scientists have achieved the most accurate calculation of neutron star data so far. The investigated neutron star, which is located 30, 000 light-years from Earth in the constellation Flying Fish and is part of the binary star system EXO 0748-676, has 1.75 solar mass more mass than some theories had previously assumed. Death Strohmayer of NASA's Goddard Space Flight Center and Adam Villarreal of the University of Arizona believe that they have come a step closer to their result of the long sought neutron star equation of state. The physical equation of state describes the relationship between the temperature, the pressure and the density of an object. The relationships of these three magnitudes to each other is therefore so important to the scientists because, in the case of the neutron star, they can infer what kind of matter the star consists of. In addition, these relationships allow conclusions to be drawn on the fundamental theories of particle physics.

"The stuff in the center of the neutron star I would really like to get my hands on, " says Strohmayer. According to his calculations, this "stuff" really, as the name "neutron star" suggests, consists of neutrons. Although the matter density is huge in this neutron star? one cubic centimeter of matter has a mass of about 500 million tons? Is not it enough, as some scientists have guessed, to include the neutrons in their components? these are three quarks? disassemble.

"With our results, we have roughly circled the possible equations of state, " says Villarreal. "It looks like an equation of state will make the race that predicts neither very large nor very small neutron stars. But more exciting is that with our observation technique we should now be able to determine the mass-to-radius ratios of other neutron stars as well. "

The two astronomers first "observed" the speed with which the neutron star revolves around its own axis. Of course, this can not be observed directly. The decisive clue for determining the rotational speed is provided by thermonuclear explosions, which occur several times per hour on a neutron star and then last for one to two minutes each. display

These explosions result from the small neutron star slowly "eating" its giant companion. They occur when the companion's matter falls on the neutron star at near the speed of light. During the rotation of the neutron star, these explosions become noticeable as flickering? much as a spinning light on a lighthouse appears to an observer as a flicker.

It turned out that the neutron star rotates about its axis 45 times per second. Next, the researchers measured the red or blue shift of the spectral lines of light emitted by the incident material on the neutron star. Because of the fast rotation of the star, the light is shifted to lower or higher frequencies because of the Doppler effect? depending on whether the emission point is moving away from us or towards us. A similar effect is subject to sound waves of a passing car.

Because of the alternating red and blue shift, the emitted spectral lines are broadened. In addition, an additional shift of the spectral lines due to the strong gravitational force of the neutron star must be considered. From the data thus obtained, Strohmayer and Villarreal were finally able to determine the radius and mass of the neutron star.

Axel Tilleman

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