Newly found neutron star could be made of strange quarks

Central compact objects (CCOs) are hot, bright, tiny sources that emit x rays and are found close to the centers of supernova remnants. Astronomers suspect that they’re a subclass of neutron stars, although less complex. CCOs don’t accrete material, for example, and they don’t emit pulsed radio waves. Their thermal x-ray emission may result from the object’s gradual cooling after its birth in a supernova explosion.

The relative simplicity of CCOs makes them suitable for studying the equation of state of dense matter. The equation expresses how temperature, pressure, density, and other variables in a star’s core are related and uses them to predict how the star, depending on its interior composition, will appear to an observer.

Back in 2016 Victor Doroshenko, of the Institute for Astronomy and Astrophysics at the University of Tübingen in Germany, and colleagues reported on a supernova remnant with not only a CCO at its center but also an optical star accompanying it. The false-color image below shows the x-ray and IR emissions surrounding the supernova remnant. The CCO and optical star were found in the central region bounded by the box.

Credit: V. Doroshenko et al., Mon. Not. R. Astron. Soc. 4582565 (2016)

Doroshenko and some of those same colleagues recently investigated the CCO further. They analyzed x-ray observations taken by the European Space Agency’s (ESA’s) XMM-Newton and determined the distance to the optical star by measuring its parallax—the apparent shift in the position of the optical star as related to the orbital motion of the ESA’s Gaia observatory. Gaia was used to observe that optical star and billions of others in the sky since its launch in 2013.

With those two pieces of data and some assumptions about the composition of the neutron star’s atmosphere, Doroshenko and colleagues conclude that the CCO could be an exceptionally light neutron star. Whereas most neutron stars are 1.4–2 solar masses, the CCO weighs in at just 0.77 solar masses.

But a neutron star isn’t the only possibility. The unusually light mass and the apparently normal radius of the CCO—deduced from modeling its x-ray spectra—indicate that the object may have a more exotic origin and could be a so-called strange star composed of strange quarks. Theorists have hypothesized about the existence of such objects for about 50 years. But distinguishing between strange stars and neutron stars has been difficult.

The unusually light star lends more credence to the possibility that the CCO in question could be a strange star. Normal neutron stars form from the gravitational collapse of iron cores, so an exceptionally light neutron star would require a heavy progenitor core. How such a light neutron star is made is unclear, and researchers suspect that the making of a strange light star could be easier. Although the strange star candidate isn’t conclusively verified, finding such an object would have major implications for how astronomers understand the properties of cold dense matter through the equation of state and the origin of neutron stars. (V. Doroshenko et al., Nat. Astron.2022, doi:10.1038/s41550-022-01800-1.)