# A Massive-born Neutron Star with a Massive White Dwarf Companion

## TL;DR

This paper reports precise mass measurements of a binary pulsar system, revealing the neutron star's mass as the largest known at birth, and discusses its implications for gravity theories and stellar evolution.

## Contribution

The study provides the most accurate measurements of the masses in the PSR J2222-0137 system, highlighting its significance for testing gravity and understanding neutron star formation.

## Key findings

- Neutron star mass measured at 1.76 solar masses, the largest at birth.
- White dwarf companion mass determined to be 1.293 solar masses.
- System useful for constraining alternative gravity theories.

## Abstract

We report on the results of a 4-year timing campaign of PSR~J2222$-0137$, a 2.44-day binary pulsar with a massive white dwarf (WD) companion, with the Nan\c{c}ay, Effelsberg and Lovell radio telescopes. Using the Shapiro delay for this system, we find a pulsar mass $m_{p}=1.76,\pm\,0.06,M_\odot$ and a WD mass $m_{c}\,=\,1.293\,\pm\,0.025\, M_\odot$. We also measure the rate of advance of periastron for this system, which is marginally consistent with the GR prediction for these masses. The short lifetime of the massive WD progenitor star led to a rapid X-ray binary phase with little ($< \, 10^{-2} \, M_\odot$) mass accretion onto the neutron star (NS); hence, the current pulsar mass is, within uncertainties, its birth mass; the largest measured to date. We discuss the discrepancy with previous mass measurements for this system; we conclude that the measurements presented here are likely to be more accurate. Finally, we highlight the usefulness of this system for testing alternative theories of gravity by tightly constraining the presence of dipolar radiation. This is of particular importance for certain aspects of strong-field gravity, like spontaneous scalarization, since the mass of PSR~J2222$-0137$ puts that system into a poorly tested parameter range.