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A strangely light neutron star within a supernova remnant

Abstract

To constrain the equation of state of cold dense matter, astrophysical measurements are essential. These are mostly based on observations of neutron stars in the X-ray band, and, more recently, also on gravitational wave observations. Of particular interest are observations of unusually heavy or light neutron stars which extend the range of central densities probed by observations and thus permit the testing of nuclear-physics predictions over a wider parameter space. Here we report on the analysis of such a star, a central compact object within the supernova remnant HESS J1731-347. We estimate the mass and radius of the neutron star to be \(M=0.7{7}_{-0.17}^{+0.20}\,{M}_{\odot }\) and \(R=10.{4}_{-0.78}^{+0.86}\) km, respectively, based on modelling of the X-ray spectrum and a robust distance estimate from Gaia observations. Our estimate implies that this object is either the lightest neutron star known, or a ‘strange star’ with a more exotic equation of state. Adopting a standard neutron star matter hypothesis allows the corresponding equations of state to be constrained.

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Fig. 1: Equation of state predictions and observational constraints as a function of the radius and mass of the compact star.

Data availability

XMM-Newton and Suzaku data used in the publication are publicly available at the respective missions’ data centres and HEASARC archives. The data reduction was carried out using the software and instructions provided by the respective missions’ science operation centres. The tabulated EOSs considered in this work are available as part of the original publications23,24,25. The posterior samples for neutron star mass and radius obtained in this work are available via https://doi.org/10.5281/zenodo.6702216 (ref. 47).

Code availability

Model atmospheres are included as part of HEASOFT package at https://heasarc.gsfc.nasa.gov/docs/software/heasoft/. BXA software is also in the public domain and available at https://johannesbuchner.github.io/BXA/index.html. Code for calculation of theoretically expected pulsed fraction for arbitrary local spectra is available upon reasonable request from the authors.

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Acknowledgements

This research made use of observations obtained with XMM-Newton, a European Space Agency (ESA) science mission with instruments and contributions directly funded by ESA Member States and NASA. For analysing X-ray spectra, we use the analysis software BXA33, which connects the nested sampling algorithm UltraNest34 with the fitting environment CIAO/Sherpa35. This research also made use of the astropy package36. The work was supported by the German Research Foundation (DFG) grant WE 1312/53-1 (VFS).

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Contributions

V.D. carried out data analysis and modelling and drafted the initial version of the manuscript. V.S. developed atmosphere models used in the work and calculated upper limits on theoretically allowed pulsation amplitudes. G.P. and A.S. contributed to the interpretation of the results. G.P. was also principal investigator for some of the XMM observations used in this work. All authors contributed to the writing of the manuscript.

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Correspondence to Victor Doroshenko.

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Nature Astronomy thanks Adriana Pires and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Comparison of the fit results for NS mass and radius from this work with those reported by 14.

The dotted line shows results from the latter work for a fixed distance of 3.2 kpc (1σ credible interval). The labeled contours show results obtained in this work: (1) - same model and energy range as 14, (2) - same as 1 but with data below 1 keV included, (3) - same as 2 but with the wabs model component substituted with tbabs, (3a) - same as (3) but also accounting for the scattering component, (4) same as (3a) but with distance fixed to 2.5 kpc, and (5) - same as 4 but with distance priors set to the Gaia estimate as described in the text.

Extended Data Fig. 2 Theoretically expected pulsed fraction limits as a function of angles defining the viewing geometry.

The expected pulsed fraction is calculated given the best-fit spectral parameters for each model as described in section ‘More complex temperature distributions’ of the Methods, and contours represent limits on possible angle values when upper limits on the observed pulsed fraction obtained in this work for various frequency ranges and reported in the Extended Data Table 2 are considered. The region to the lower left of the respective contours represents the range of angles allowed for a given model and corresponding upper limit on pulsed fraction. The shaded region corresponds thus to the weakest of the upper limits on the observed pulsed fraction (that is 9.7%), and thus represents the most conservative estimate.

Extended Data Fig. 3 Corner plots corresponding to the final fit with single temperature carbon atmosphere model including full distance priors and EOS constrain priors.

Unweighted samples from the BXA modeling described in the text for all relevant parameters are used to produce the plots, using the corner module for 1σ credibility intervals. The panels corresponding to the (well constrained) cross-normalization constants also included in the fit are omitted for clarity.

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Doroshenko, V., Suleimanov, V., Pühlhofer, G. et al. A strangely light neutron star within a supernova remnant. Nat Astron 6, 1444–1451 (2022). https://doi.org/10.1038/s41550-022-01800-1

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