Radius of neutron star PSR J0437-4715 determined

Millisecond pulsars are rotating neutron stars that emit regular electromagnetic signals, similar to lighthouses. The pulsar PSR J0437-4715 is the closest to Earth and thus the brightest. Researchers with the participation of TU Darmstadt have now gained new insights into the neutron star’s radius and its consequences for the equation of state of dense matter. The results are published in a series of papers in the renowned journal “The Astrophysical Journal Letters”.

PSR J0437 is a pulsar, a rotating neutron star that emits electromagnetic radiation. It is located about 510 light-years from Earth in the southern constellation of Pictor. PSR J0437 rotates 174 times per second around its axis and has a white dwarf, i.e., a compact, old star, as a companion. Like a lighthouse, the pulsar sends a beam of radio waves and X-rays toward Earth every 5.75 milliseconds. This makes it the closest millisecond pulsar to Earth. In part because it is so close, it is also the brightest millisecond pulsar.

Scientists led by Devarshi Choudhury and Anna Watts from the University of Amsterdam (Netherlands) have now determined its radius. For the radius measurement they used data from the NICER X-ray telescope aboard the International Space Station. They combined the X-ray data with a technique called pulse profile modeling. This led to an extraction of the star’s radius, with the help of mass measurements from Parkes Pulsar Timing Array in Australia. The resulting radius of PSR J0437 is 11.4 kilometers; it has a mass of 1.4 times that of our Sun.

Nuclear theorists from the TU Darmstadt including Melissa Mendes, Isak Svensson and Achim Schwenk teamed up with the astrophysicists led by Nathan Rutherford from the University of New Hampshire (USA) to study the consequences of the new radius measurement for the properties of dense matter in the star’s interior. Using a statistical inference framework with new calculations of the dense matter equation of state for the first few kilometers into the neutron star as well as the available NICER and gravitational wave observations of neutron stars, the researchers could extract more precise constraints on neutron star radii and on the properties of dense matter in the interior of neutron stars.

The TU Darmstadt work was led by Melissa Mendes and Isak Svensson. “Our results show that the new NICER measurement tightly constrains the equation of state at intermediate densities a few times those in atomic nuclei”, explains Mendes. This leads to a high pressure but with the smaller PSR J0437 radius the equation of state turns out to be relatively softer. As Svensson points out, “there is a particularly interesting interplay of 1.4 solar mass and heavy two solar mass neutron stars, which provides important constraints for dense matter interactions”.
“This work combines the state-of-the-art information we have from nuclear theory and astrophysics observations to constrain the physics of strongly interacting matter under extreme conditions”, says Achim Schwenk. “It will be interesting how future NICER and gravitational wave observations continue to pin down the equation of state of the densest observable matter in the Universe.”

The paper by Nathan Rutherford, Melissa Mendes, Isak Svensson et al. is part of a series of papers. In addition to this equation of state paper, there is the radius measurement of PSR J0437-4715 led by Devarshi Choudhury (University of Amsterdam) https://arxiv.org/abs/2407.06789, the mass determination led by Daniel Reardon (Swinburne University of Technology, Australia) https://arxiv.org/abs/2407.07132, and an update to the radius measurement for the heavy pulsar PSR J0740+6620 led by Tuomo Salmi (University of Amsterdam) https://arxiv.org/abs/2406.14466.

Originalpublikation:

Nathan Rutherford, Melissa Mendes, Isak Svensson, Achim Schwenk, Anna L. Watts, Kai Hebeler, Jonas Keller, Chanda Prescod-Weinstein, Devarshi Choudhury, Geert Raaijmakers, Tuomo Salmi, Patrick Timmerman, Serena Vinciguerra, Sebastien Guillot, and James M. Lattimer:
“Constraining the dense matter equation of state with new NICER mass-radius measurements and new chiral effective field theory inputs”
Accepted for publication in The Astrophysical Journal Letters, https://arxiv.org/abs/2407.06790.

http://www.tu-darmstadt.de/