A neutron star is the core left behind after a massive star explodes. Its matter is so compressed that a teaspoonful would weigh billions of tons. But the exact size and mass of such objects remain debated. A team of astronomers studied the system EXO 0748-676, where a neutron star pulls matter from a normal companion star. It occasionally flares up and then quiets down. During quiescent periods, its surface cools, and X-ray emission can reveal key parameters.
The researchers gathered 20 observations from 2008 to 2025—14 from Chandra and 6 from XMM-Newton. They ran the data through a hydrogen atmosphere model and applied a Markov Chain Monte Carlo method. They found the star's mass to be 1.77 solar masses and its radius 12.62 km. That's like squeezing one and a half Earths into a ball the size of a small city. These numbers favor a fairly stiff equation of state — meaning the matter inside doesn't compress easily.
They also compared two quiescent periods — before and after the 2024–2025 outburst. After the new flare, the star's crust began cooling again, and they could track the process. This is a rare chance to see a neutron star recover from violent accretion.
as written by the authors arXiv:2606.23466
We present new constraints on the mass and radius of the neutron star in the neutron star low-mass X-ray binary EXO 0748$$-$$676 obtained from a joint analysis of 20 quiescent X-ray observations obtained between 2008 and 2025, including 14 Chandra and 6 XMM-Newton exposures. These data sample two quiescent episodes separated by the 2024$$-$$2025 outburst. We model the 0.5$$-$$10 keV spectra with a hydrogen-atmosphere model, assuming a source distance of 7.1 kpc. In a global Markov Chain Monte Carlo analysis in which the hydrogen column density, neutron star mass, and radius are tied across all observations, we obtain a neutron-star mass of $$1.77^{+0.17}_{-0.22}\,M_\odot$$ and a radius of $$12.62^{+0.56}_{-0.74}$$ km ($$1\sigma$$ credible intervals). We further perform independent fits to the first and second quiescent epochs and find that the combined data set significantly reduces the low-mass tail in the posterior distribution, leading to tighter lower bounds on the neutron-star mass. Incorporating the distance uncertainty of $$7.1\pm1.2$$ kpc, we conservatively constrain the neutron-star mass and radius to $$M\simeq 1.41-2.11~M_{\odot}$$ and $$R\simeq 10.15-15.13$$ km, favoring relatively stiff dense-matter equations of state. We also trace the thermal evolution across two quiescent epochs and find evidence for renewed crust cooling following the 2024$$-$$2025 outburst, providing a unique opportunity to compare the thermal relaxation behavior after two distinct accretion episodes.
A neutron star spins so fast that its surface can move at tens of thousands of kilometers per second.
In Robert Forward's novel "Dragon's Egg," life is depicted on the surface of a neutron star, where time flows millions of times faster.