Astrophysics: Neutron stars with no exotic matter in their core

The new findings now speak against the hypothesis. As strange as the objects are – they obviously don’t behave quite as bizarre after all. “It seems that these very exotic states of matter do not exist in the nucleus of a neutron star,” comments the theoretical physicist Jorge Piekarewicz from Florida State University. Then the phase transition to quark matter – if it exists at all – would only take place shortly before the critical limit from which a neutron star inexorably collapses into a black hole. The exact value for this is not known, it is believed to be around three solar masses. “The question is,” says Watts, “if a strange substance exists at high density, when exactly does it form?”

If J0740 had gone through this phase transition and contained more easily compressible quark matter, according to Watts it should be between nine and ten miles. But even if you take the measurement uncertainties into account, according to Miller, 22 kilometers in diameter would be a fairly clear lower limit.

This means that if neutron stars produce quark matter at all, then only at some point beyond 2.1 solar masses. Protons and neutrons could just as easily be preserved even on the most extreme scales. “In any case, it looks like some of the models are now excluded,” comments Watts.

Illustration of a radiation burst on a magnetarLaden…

© NASA’s Goddard Space Flight Center (Ausschnitt)

Breakout on a magnetar | The illustration shows an outbreak of radiation on a magnetar, a neutron star with a particularly strong magnetic field.

The fact that measurements of the size of neutron stars are basically possible is due to a peculiarity of the celestial bodies. They rotate quickly and with them rotate spots on their surface, from which strong magnetic fields emanate and which send X-rays into space. Due to the enormous gravitational effect of the compact objects, even radiation that arises on the other side of the neutron star is bent by gravity and directed in our direction. With NICER, the arrival times of the X-ray flashes can be recorded with high precision. This allows conclusions to be drawn about the diameter.

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